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Thomson scattering
Roberto Pasqualotto
11 February 2009
European Joint Ph.D Programme on Fusion Science and Engineering
2° Advanced Course in Lisboa, February 2009,
On Diagnostics and Data Acquisition
[email protected]
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OUTLINE
Theory:
TS from single electron
TS from plasma Æ Te & ne
TS measurement: experimental issues
TS diagnostic: main components
Examples of existing TS systems:
RFX
TCV
Textor
HRTS
LIDAR JET
ITER core LIDAR issues & design
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LASER-AIDED PLASMA DIAGNOSTICS
Laser-aided diagnostics are widely applied in the field of high-temperature plasma diagnostics for
a large variety of measurements.
Various types of laser-aided plasma diagnostics exist, all based on different physical interactions
between the electromagnetic wave from the laser and the plasma.
In general one can distinguish interaction based on:
(a)
(b)
(c)
(d)
absorption and/or reemission,
changes in the refractive index,
changes in the polarization ellipse,
scattering.
Incoherent Thomson scattering is used for highly localized measurements of the electron
temperature and density in the plasma.
Coherent Thomson scattering yields information on the fast ion population in the plasma and/or
depending on the geometry and wavelength chosen electron density fluctuations.
Interferometry and polarimetry are often combined in a single diagnostics setup to measure the
electron density and the component of the magnetic field parallel to the laser chord.
Density fluctuations can be measured by means of phase contrast imaging, scattering, and various
other laser-aided techniques.
A. J. H. DONNÉ, C. J. BARTH, H. WEISEN - FUSION SCIENCE AND TECHNOLOGY VOL. 53 FEB. 2008, p.397
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LASER-AIDED PLASMA DIAGNOSTICS
Active diagnostics with lasers as the probing source have a number of distinct merits:
(a) the laser beam can be focused in the plasma, resulting in good spatial resolution;
(b) the measurements do not perturb the plasma because of the relatively small
interaction cross sections;
(c) lasers have a high spectral brightness Æ good signals @ t,x,λ;
(d) both with pulsed and continuous wave laser systems a good temporal resolution
can be obtained;
(e) the lasers (and in many applications also the detectors) can be positioned far
from the plasma, where they can be more easily maintained.
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(direct method: no models, assumptions,..)
ve
or
• Straightforward stand alone
measurement
De
te
ct
• What is it?
– Laser beam scatters off of
electrons in the plasma
– doppler effect gives wavelength
shift
Laser beam
Why Thomson Scattering?
• Electron velocity distribution
directly observed (ne, Te)
• Accurate spatial location via
imaging or time of flight
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Thomson Scattering
• Scattering of electromagnetic radiation by a charged particle.
• The electric and magnetic components of the incident wave
accelerate the particle, which in turn emits radiation in all
directions.
• Phenomenon was first explained by J.J.Thomson.
• It can be split into coherent and incoherent scattering (more later).
• The experimental application of TS as a diagnostic tool had to wait
for the development of high power light sources, e.g., the Qswitched ruby laser in the early 1960s.
Since then, various plasma parameters have been measured by
means of this technique.
• The first demonstration of TS as a suitable diagnostic tool for hot
plasmas was given by Peacock et al. in 1969 when they
measured the electron temperature and density in the Russian T3
tokamak.
• Further developments: Te and ne along the full plasma diameter,
resolving up to ~ 100 spatial elements with time separations of
~10 µs to 10 ms.
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Role of Thomson scattering in
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Thomson scattering spectrum
In this lesson the focus will be on:
Logic of steps to derive TS spectrum (less on math)
What can be measured
Under which conditions
John Sheffield, “Plasma scattering of electromagnetic radiation”, Academic Press 1975
S.E. Segre, “Thomson scattering from a plasma”
Course on Plasma diagnostics and data acquisition systems, Varenna 1975,
P Nielsen, “Thomson scattering in high temperature devices” , Varenna 1986,
Some PhD Thesis: Rory Scannel (MAST), Alberto Alfier (RFX), R. Pasqualotto (RFX)
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Thomson scattering from a single electron
(classical limit of the Compton scattering)
Incident
electric field
- scattering of an incident photon
by a moving electron (β=v/c)
- electron energy is constant
(Ee>>ħω)
Incident
photon
electron
r
Ei
iˆ
θ
k̂
Propagation
&
scattering
directions
ŝ
scattering
angle
Observer
The scattered radiation is frequency shifted as a double Doppler effect takes place, one
in the reception, one in the emission of radiation by the electron:
1. the photon approaching the
moving electron
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2. the photon leaving the moving
electron
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TS as limit of Compton scattering
Conservation of energy and momentum
ħωi + mic2 = ħωs + msc2
ħki + mivi = ħks + msvs
where: mi,s = m0/(1-βi,s2)½
When incident wave has frequency ω such that
The solution to these equations is:
ωs = ωi (1-βi • êi) / (1-βi • ês + (1-cos(θ))ħωi/mic2)
ħωi << mec2
ÆThomson scattering,
limit effect of Compton Scattering,
Ignoring the term ħωi/mec2 we get
ωs – ωi = ∆ω = (ks - ki) • ve = k • ve
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering
in which the quantum effect may be neglected:
the electron moves at same velocity as before
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TS as limit of Compton scattering
(some math)
Seen by the electron, initially stationary (vi =0):
With simple algebra:
In the TS limit
Transforming back to the lab reference system:
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TS as limit of Compton scattering
(some math)
Not relativistic
Compton scattering
ħωi << mec2
1 eV << 0.5 MeV
1 eV energy of a photon with λ = 1 µm
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TS from single electron
Incident wave electric field:
and associated magnetic field:
Force on the electron by the e.m wave:
with
Acceleration produced by this force
negligible if v<<c
and m’=m
with
Such an accelerated electron produces an e.m. field.
At an observation distance ρ large compared to electron displacement during measurement,
electrostatic part (∝ 1/ρ2) is negligible.
Radiative part (∝ 1/ρ) is the scattered e.m. wave:
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TS from single electron
distance electron – point of observation
unit vector in propagation direction
quantities in bracket evaluated
at retarded time
retarded time: delay between the photon emission and the moment at which it reaches the observer
Phase of scattered field = phase of incident field
(evaluated at ret- time)
if v = const (influence of e.m. wave on
electron is ignored and no static B field):
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ρ
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TS from single electron
The time dependent part of the phase indicates that the scattered wave is monochromatic, with a
frequency ωs:
Scattered radiation is still monochromatic
Displacement in frequency proportional to the component of the e velocity in the k direction
This expression is valid also at relativistic velocity.
If we want to observe the drift velocity of a plasma, scattering geometry must be such that k · vd ≠ 0
When
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TS from single electron
Only a flavour of full math formulation
Incident wave electric field:
and associated magnetic field:
Equation of motion of the electron accelerated by the e.m wave:
with
Acceleration produced by this force
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TS from single electron
Only a flavour of full math formulation
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TS from single electron
Low electron velocity: non-relativistic approximation
Standard geometry:
90° scattering
ss the classical radius of electron
Intensity of scattered wave does not depend on ψ
and it is zero in the direction of the polarization (ξ=0) of the incident wave (not true if β finite)
Max intensity when ξ = π/2
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TS from single electron
Measured quantity is scattered power per unit solid angle:
Is the Poynting vector
Averaging over many periods, and using
Incident intensity
Is the Thomson scattering cross section
Scattered power ∝ 1/m2 Æ in a plasma contribution from the ions is negligible :
m_e = 10-32 kg
m_p = 10-27 kg
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TS from single electron
non relativistic
from Sheffield
relativistic
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TS from a plasma
Total E given by contribution from each electron:
Average scattered power
First term: sum of power scattered by each electron independently of others
Second term: contribution due to correlation between electron positions.
= 0 if electrons randomly distributed
For a plasma, typical correlation length is
Phase very large and changes rapidly from electron to
an other, when summing over distances of order λd
Æ 2° term = 0, incoherent scattering
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TS from a plasma
•
In (a) the phases do not add up
while in (b) the opposite is true
• The scattering parameter is
α = 1/kλd
α >> 1 coherent scattering
α << 1 incoherent scattering
incoherent
2π
λ=
k
coherent
2π
λ=
k
For hot plasma with medium density (Te = 1 keV; ne= 1019 – 1020 m-3)
using visible or NIR laser and with scattering angle 90°:
α < 0.001
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Incoherent TS
Electron velocity distribution function f(v)
Electron contained in
Contribute to total scattered power per unit volume,
in frequency range
If we define differential scattering cross section
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Incoherent TS
Thermal equilibrium: Maxwel distribution
Assuming relativistic effects negligible
Scattered spectrum is a gaussian centred on input frequency
for a ruby laser
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Incoherent TS
Te
- electrons are in thermal equilibrium (Maxwellian distribution)
- relativistic effects are negligible
the scattered spectrum has a Gaussian shape
∝
3
2
ne
1
Visible or IR laser
0
-10
θ ~ 90° geometry
-5
0
λLaser-λ
5
The total power, integrated over frequency collected from volume
∆Ps_plasma = ∆Ps_e n ∆V
where
A
l
W0 = <I0> A:
area of beam cross section
length of scattering volume observed
Power of laser beam
If the laser is pulsed, we consider the energy of the pulse
and the total collected energy ∆Es
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10
Incoherent TS: relativistic effects - depolarization
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Incoherent TS: relativistic effects – blue-shift
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Incoherent TS: relativistic effects – blue-shift
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Incoherent TS: relativistic spectrum
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Incoherent TS: effect of B
Spectrum scattered from single
electron: series of lines centred on the
line at frequency
and separated by ωc
From a maxwellian plasma:
Modulation
distinguishable
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TS measurement: experimental issues
TS attractive as diagnostic tool for plasmas:
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TS measurement: experimental issues
Incoherent TS: spectrum depends on
7
Scattered Spectra
Spectral Intensity
6
•Electron density, ne
•Scattering angle, θscat
•Laser wavelength, λ0
•Electron temperature, Te
Selden-Matoba, θ=180o
5
0.5keV
5keV
10keV
20keV
40keV
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Normalised Wavelength
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Imaging Thomson Scattering
•
Laser
(pulse)
The scattered light is
Plasma
imaged from the plasma
•
A ’spectrometer’
disperses the light
•
Collection
optics
A set of detectors
collects the light
•
The data is digitised and
analysed
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Experimental issues
Defines the time
resolution
(50Hz)
Critical aspects:
- low cross section
- low collection angle
- detection of the scattered radiation
Define the
spatial resolution
(<1 cm)
N TS
photons
N inc
photons
= 10−13
Background noise:
- plasma light: broadband radiation
- stray light from baffles and dumps:
monochromatic radiation (at the laser
wavelength)
One of the most fundamental but critical diagnostics in fusion experiments
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Detected signal
Detected power, over entire spectrum:
Photons entering the spectrometer
0.025
0.1 keV
0.5 keV
1 keV
2 keV
10 keV
0.02
a.u.
0.015
l = 10 mm
0.01
# photoelectrons
detected/channel:
Fraction of spectrum detected by
i-th spectral channel: 10-20 %
0.005
0
800
850
900
950
1000
wavelength (nm)
1050
1100
102 – 5x103
Balanced # of spectral channels: low to maximise signal, high to maximise resolution (min 2)
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1150
Signal errors
Poisson statistic of photoelectrons (p.e)
Detector noise: dark noise Æ noise equivalent number of p.e.
multiplication noise Æ noise factor F Æ equivalent number of p.e. N* = N /F
Background plasma light (most dangerous at high frequency: plasma fluctuations, ELMs):
It can be 100-1000 x Bremmstrahlung contribution:
Stray light: monochromatic λi
from diffusion from inner wall, windows and optics
rejection R = 104-5 usually required in spectrometers to sufficiently reduce it
however it is quite reproducible
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0.025
Signal simulation
0.1 keV
0.5 keV
1 keV
2 keV
10 keV
0.02
a.u.
0.015
0.01
Npe
Npe
0.005
0
ch.3
800
850
900
950
1000
wavelength (nm)
1050
1100
1150
ch.2
ch.1
ch.4
CORE
EDGE
σ (%)
ch.4
3
σ (%)
Te
21
Te
ne
ne
keV
keV
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Te and ne from relative & absolute calibration
relative spectral response of
a spectrometer
Nd:YLF
1053n
m
Ch 4
Normalized transmission
Te: from the relative
sensitivity of the 4 spectral
channels
Ch 3
Ch 2
Ch 1
TS
spectrum
ne: from the absolute
sensitivity of the 4 spectral
channels
3.5·1019m-3
50 eV
19
3·10 m
-3
Rotational Raman Scattering
in N2
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Te: relative calibration
Measure the relative transmission function of spectral
channels in each spectrometer
Standard technique: CW light source +
monochromator +
calibrated energy
monitor
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Te: relative calibration
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ne: absolute calibration
Rotational Raman Signal
induced by the laser in N2 gas
Torus filled with 50-500 mbar
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ne: absolute calibration
Two main issues affect its
validity and make it very
difficult:
The dependence
of signal on the
pressure gives the
abs. cal.
- Laser misalignment
- plasma deposition on
collection window (may
influence also relative
calibration)
Rayleigh
scattering
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Calculation of Te & ne
Yi = measured signal of channel-i
yi = theoretical signal of channel-i,
given Te, ne
σi = experimental errors
B ∝ 1/Te
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Calculation of Te & ne
only depends on Te
non linear minimization
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Realising a TS System
Lasers
Spectral Calibration
Plasma Measurement
Density Calibration
Scattering
Collection of Light
Spectral Analysis
Data Acquisition
Data Analysis
Results
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TS diagnostic: main components
We’ll look now at main components of a TS diagnostic:
Laser
Collection optics
Spectrometer
Detector
Data acquisition
Analysis
Then we’ll see examples of working systems
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laser
Most present TS experiments employ Q-switched ruby or Nd:YAG lasers as the source.
The ruby laser operating at 694.3 nm produces outputs up to 25 J in 15 ns
However, their repetition rate is usually rather low: 5 Hz (1 J / pulse).
When more than several pulses per minute are required, an intracavity ruby laser can produce a burst of
high-energy pulses (~15 J/pulse, ∆t ~ µs) with a repetition rate of ~10 kHz (see Textor).
Ruby lasers are usually employed in systems where good spatial resolution is preferred above a high
time resolution.
The most frequently used system for periodic TS measurements is based on the application
of Nd:YAG lasers operating at 1064 nm, with outputs of ~1 J, 15 ns and a repetition rate of 20 to 50 Hz.
Combining a set of lasers the repetition rate can be increased.
The beam divergence of both types of lasers is ~0.3 to 1.0 mrad. The polarization of the laser beam is
chosen perpendicular to the scattering plane. The high laser powers require special precautions for the
used optics. Laser beam diameters should be kept large enough such that for 15-ns pulses the energy
density is below the damage threshold of ~5-10 J/cm2.
Transmitting surfaces need to be coated and tilted with respect to the beam propagation to
Prevent losses and back-reflected light entering the laser system again. Furthermore, curved
transmission optics should have concave entrance surfaces, to prevent focusing of the back-reflected
beams (which might lead locally to very high power densities).
Other types of pulsed lasers (e.g., Ti:Sapphire and Alexandrite lasers) have been proposed for TS (e.g.,
for ITER). Nevertheless, there are not yet applications of these sources to present devices.
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Q-switched ruby laser in RFX
Pockels cell
R = 5 m rear mirror
Brewster angle TEM00 oscillator
8 x 1/4" ruby
polarizer
2 flashlamps
mode selection
aperture
etalon output
coupler R =
45° steering mirror
E = 35 mJ (2 x 20 mJ)
E = 1 J (2 x 0.75 J)
E = 15 J (2 x 12 J)
E = 10 J (2 x 7 J)
Amp. 2
Amp. 3
8 x 5/8" ruby
4" x 22.5 mm ruby
4 flashlamps
4 flashlamps
•
•
•
Roberto Pasqualotto
Amp. 1
8 x 3/8" ruby
4 flashlamps
spatial
filter
300 µm
pinhole
F = –67 cm lens
7 mW HeNe
45° steering mirror
F = 30 cm lens
Ruby laser (λ = 694.3 nm)
TEM00 oscillator (35 mJ, 25 ns, single pulse)
3 amplifiers (15 J, 25 ns, θ ≤ 0.4 mrad )
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laser
•
Modern TS systems use multiple lasers
(typically up to 8)
•
These can be bunched to tackle
different physics and provide
redundancy
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Stray light reduction
The laser beam enters and leaves the plasma vessel through
vacuum windows.
Passing these windows— especially the entrance one—generates
stray light, which can reach the collection optics.
Without precautions this stray light level can be six to eight
orders of magnitude larger than the TS light.
Reduction of the vessel stray light can be achieved:
by tilting the windows (placing them under the Brewster angle is
very effective),
by positioning them relatively far from the plasma,
by using baffles in both entrance and exit ducts,
and by mounting a viewing dump on the vessel wall opposite to the
collection optics.
A very effective light trap (reduction up to 100 times) can be made
from a stack of knife-edge blades. Carbon tiles on the inner wall of
the plasma vessel can give a reduction by a factor of ~ 20.
Finally, the laser beam is dumped on e.g. a piece of absorbing glass
placed under the Brewster angle.
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Collection optics
Scattered light is collected after passing a vacuum window and subsequently relayed to a
spectrometer. Because of the low scattering yield, the transmission of the collection and relay
optics should be of course as high as possible.
In devices with hot plasmas, a shutter is required between the plasma and the window to reduce
deposition of all kinds of materials on the inner window surface during the times the diagnostic is
not in use.
Various kinds of optics are used to collect the scattered light. These systems are used to guide the
TS light to the spectrometer. Basically, there are two possible ways to guide the scattered light
from the plasma to the detection system: via flexible fibers and via conventional optics
(lenses and mirrors).
The main advantage of fibers above conventional optics is that the linear etendue of the source can
be matched to that of the detector, albeit at the cost of reduced spectral resolution. For this purpose
the fiber array is rearranged such that the slit height is reduced and the slit width increased, for
example, by a factor of 2. As a result, the usable solid angle of the collection lens increases by a
factor of 4.
However, for TS diagnostics at small-sized plasma devices where the detection system can be
positioned relatively close to the plasma (≤10m), conventional optics gives a better transmission
(up to a factor of 3) than fiber optics. For systems with a comparable length of the optical path
from collection lens to spectrometer, e.g., the multiposition TS systems of JFT-2M and RTP, the
overall transmission is larger for systems using conventional relay optics (RTP: 25%) than for
those using fiber optics (JFT-2M: 7%).
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fibres
The major contributions to the losses in fiber-optic systems are the core-cladding ratio,
the packing fraction, the attenuation, input and output reflection losses, and an increase
of the exit cone. For fiber-optic arrays, transmissions of ~55% and even higher have
been reported.
Despite the lower transmission, fiber-optic systems have to be preferred when the
scattered light needs to be relayed over longer distances (e.g., to get outside the
biological shield of the plasma device). To bridge long distances with conventional
optics would require many large-sized lenses and mirrors, resulting in a low transmission
as well.
NA = 0.37
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Spectral analysis
Mainly two different techniques to disperse the scattered light are in use for TS systems: filter and
grating spectrometers.
In filter spectrometers, the scattered light is separated into different wavelength bands by means of
a cascade of interference filters. The number of separate wavelength channels in these systems is
usually rather limited (three to eight channels), and therefore, the interpretation of the data relies on
the assumption of a Maxwellian electron velocity distribution in the plasma.
In grating spectrometers a grating is used for dispersing the scattered light. Both mechanically
and holographically ruled gratings are used for this purpose. In this case, the number of independent
spectral channels can be quite large: up to 80 for the TVTS system on TEXTOR. In case of good
photon statistics, this enables one to determine the shape of the Maxwellian distribution.
To prevent the residual of the vessel stray light from disturbing the TS spectrum, the laser
wavelength should be carefully filtered out after dispersion has taken place. This can be done by
blocking the laser wavelength, by reflecting light at this wavelength away from the detector, or by
focusing it onto a special detector. Both filter and grating spectrometers have typical stray light
rejection ratios of 10-4 to 10-5.
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Spectral analysis
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grating Littrow spectrometer in RFX
interference notch filter
(30° incidence)
achromatic doublet
a.r. @ 694.3 nm
holographic
grating
(φ = 120 mm, f/3.4)
input fiber
optic bundle
entrance
slit
7600 Å
5400 Å
•
•
•
•
•
flat
spectral
plane
interference notch filter
(normal incidence)
input: 10 bundles of optical fibres
Concave holographic grating with flat field (F/3.4)
Interferential notch filters at λ = 694. 3 nm (R= 4x 10-3)
Stops on the focal plane at λ = 656.3 nm (Hα) and λ = 694.3 nm.
Rejection: RHα= 2x10-4 , RRuby = 10-6, T = 30%
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grating Littrow spectrometer in RTP & Textor
Scattered light is collected by an F/19 achromatic doublet (item 3) and guided to a Littrow polychromator
where the light is detected after dispersion. A field lens (item 4) and a spherical mirror (item 8) serve for pupil imaging.
The Littrow lens ~F/12.5 (item 6) collimates the incoming light beams and focuses the dispersed light at the two-part
mirror (item 8), giving a two dimensional image (λ, z). This image is projected onto the GaAsP cathode ~18% tube
efficiency of a 25 mm image intensifier by means of a Canon 50 mm, F/0.95 TV objective. Finally, the phosphor screen
of the intensifier is imaged at the cathode of two ICCD cameras (item 13) by a coupling lens system that consists
of three F/1.2 Rodenstock objectives (item 11) and a beam splitter (item 12). Double pulse operation is feasible with this
system. Light emitted by the phosphor screen of the GaAsP image intensifier (item 10) and generated by the first
laser pulse is recorded by one ICCD camera by gating it open during 20 µs. The second ICCD camera is gated open
at the moment of the second laser pulse, again during 20 µs. Time separation is typically 100–800 µs. The ICCD
integration time is made 20 µs to keep the overlap of the two pulses as small as possibel (~ 7% for 100 µs separation).
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Detection and data acquisition
In general, two different types of detection systems can be distinguished: time-resolving singleelement or multielement detectors, and signal-integrating multielement detectors. The first
category includes avalanche photodiodes (APDs) employing the high quantum efficiency of Si
between 500 and 1000 nm, photomultiplier tubes (PMTs), photodiode arrays, and multianode
PMTs.
These systems enable time resolutions of the order of the laser pulse duration of 15 ns. As a result
plasma light can easily be sampled just before or after the laser pulse. TS systems using periodic
Nd:YAGlasers mostly apply APDs for detection of scattered and plasma light. The signals of
photodiode detectors are recorded with charge-integrating analogue-to-digital convertors (ADCs)
or by means of fast transient recorders.
Time-integrating detectors have a lower time resolution and are called TV systems because the
detection principles are similar to those of a television camera able to receive a two-dimensional
image.Vidicon, charge coupled device (CCD), complementary metal oxide semiconductor CMOS, and
streak cameras belong to this category. These cameras have large numbers of image pixels, e.g., 106 to
107. The low readout time can vary for different types of cameras. For a 16-bit CCD camera, the
readout time can be ~1 s, while ultrafast CMOS cameras sample with frame rates of 104 images/s at a
12-bit dynamic Range.
The scattered light of the short laser pulse is captured with a gated image intensifier coupled to the TVlike recording system using a lens system. Data detected with TV systems are usually stored in internal
memories and after termination of the plasma discharge are sent to a computer for analysis.
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Detection and data acquisition
Both PMT’s and TV detectors employ different kinds of photocathode materials to improve the photon
to electron conversion process. PMT and image intensifiers are equipped with GaAsP, S25, extended S20
cathodes to reach high conversion efficiencies in the visible and near infrared wavelength ranges.
The signal-noise S/N ratio of a detector directly depends on this conversion (quantum) efficiency:
where Npe denotes the number of photoelectrons generated by the incoming photons (Nph) and
η_conversion denotes the efficiency of the conversion from photons to electrons. However, the S/N ratio
of the complete detector will be lower because of the noise added by the amplification and readout
processes. More useful for evaluation of a detector is the effective detector efficiency, which includes
the noise factor: η_det = η_conversion /noise factor. The noise factor refers to noise increase in the
detector caused by the amplification process.
The S/N ratio of a complete detection system is determined by the statistical noise, the dark current of
the detector and background signals, as plasma light and stray light. Plasma light due to
bremsstrahlung and line radiation can be easily corrected for when photodiode detectors are used. Using
TV systems in combination with fiber optics for light relay offers the ability to sample plasma light from
an area just next to the laser beam and guide this to the same detector for simultaneous recording.
Alternatively, one can measure the plasma light just before and after each laser pulse. The contribution of
plasma light can be kept negligibly low when the laser energy is high (>10 J) and the sampling interval is
almost as short as the laser pulse (40 ns, for a pulse with 15 ns full width at half-maximum (FWHM)) .
Grating spectrometers combined with TV systems result in a large number of spectral channels, which
enables line radiation to be suppressed..
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Gated acquisition
delay
Plasma light +
TS pulse + Stray pulse
Plasma light only
delay
Plasma light fluct. +
TS pulse + Stray pulse
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Plasma light
Fluctuation only
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position (1-10)
Multianod MCP photomultipliers in RFX
spectral channel (1−10)
VBias
•
•
•
•
•
40 mm S20 photocathode (Q.E. = 7 % @700 nm).
V-stack MCP (G= 105 @ 1800 V, js = 230 µA/cm2,
recovery time τ = 10 µs).
Array of 10 x 10 anodi
Insensitive to B (3mT in spectrometer).
100 parallel channels in one detector.
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering
VMCP
ee- eee eee
e-
ePhotons
ee-
Photochathode
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MCP
Anode
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Photocathodes
R&D
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Examples of working systems:
Details of experimental setup
Measurements
Physics studies
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main TS at RFX
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main TS at RFX: layout
TS signal collected
through 3 ports
Fiber optic delay line multiplex
(∆l=15m Æ ∆t=70ns):
28 filters+APD
polychormators
4 spectral channels
3 positions/spectrometer
ND:YLF laser (λ=1053nm):
• E ~ 3J
• Pulse length ~20ns FWHM
• 10 pulses, 50 Hz
84 scatt. volumes on equatorial
diameter (-0.95<r/a<0.85)
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main TS at RFX: filter polychromators & APDs
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Polychromator
Objects
Z-pos
adjustable
Field
Imaged on filters
Imaged on detectors
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Upgraded TS diagnostic
TS su RFX
TS su RFX-mod
Improvement
Time
resolution
1 profile per
discharge
10 profile per
discharge (<50Hz)
from Rb laser (693nm) to custom built
Nd:YLF laser (1053nm)
Spatial
resolution
10 points
8cm
84 points
1cm
Optical delay lines
(3 points per spectrometer)
gated signals
raw signals
0.5GHz
from gated MCPs
to interference filters spectrometers
with digitizers
Acquisition
system
RFX
RFX
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RFX-mod
RFX-mod
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Results obtained on RFX-mod
Te profile during different plasma states and in various scenarios:
800 T (eV)
e
19532 @ 95ms
19532 @ 45ms
600
400
stochastic
plasma core
partially
ordered
plasma core
200
0
-400
-200
0
200
Radius (mm)
400
Tomographic
reconstruction of
SXR emissivity
(pressure) profile in
a poloidal section
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RFX: Te from TS & double filter
Double-foil Te
1. the entire
profile is
pumped up for
all the QSH
cycle;
Double foil and TS Te profiles
#22159 @ 199ms
Double-foil Te in 2D
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HRTS at JET
Similar to the Main
TS on RFX-mod
- Te and ne profiles along external radius (R=2.9-3.9m);
- maximum spatial resolution of 15 mm in 63 positions on 21
spectrometers with optical delay lines;
- time resolution of 20Hz (Nd:YAG laser - 5J );
- partially share the laser path of the other TS system (red path)
- interference filters spectrometers and digitizers
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2. HRTS layout in torus hall
HRTS system currently operational:
- outer radius covered
- 61 points, 1.5 cm sampling resolution
- 20 Hz repetition rate, full JET pulse
lens
Vacuum
window
Paraboloidal
mirrors
& fibres
(126)
Laser beam
Scattering
volume
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2. Imaging optics
• double vacuum window, 192 mm diam, F/25
• imaging lens and 2 motorised mirrors
• the lens images scattering volumes to West
Wall
first
mirro
r
Fiber
Paraboloidal
mirror
second
mirror
lens
• 5m optical bench on West Wall holds
126 paraboloidal mirrors (3x4 cm)
• each mirror images the lens onto one fiber
optic
• one fibre corresponds to 8 mm scattering
volume
fibres
Vacuum window
192 mm diam
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2. Laser
Nd:YAG (λ=1064 nm) laser from Quantel (France):
2 beams vertically displaced
E = 2.5 J / beam
Repetition rate 20 Hz
Full remote control
Beams profile
Burnspot on paper
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2. Polychromators
• 21 filter polychromators with avalanche photodiodes (APD)
from GA / PPPL
• 4 spectral channels
• Two sets of filters: 7 for the edge (Te = 30 eV - 3 keV)
14 for the core (Te = 0.2-15 keV)
750
850
950
APD + amplif
Lens +
interf filter
Amplifiers from PPPL:
• AC output for TS signal:
- lower frequency cutoff (τ = 5 µs)
filters plasma background light,
- upper frequency limit allows to separate
3 time-delayed signals, 50 ns apart
• DC signal for plasma background light
core
650
7 fibers bundle
1050
nm
edge
800
900
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1000
1100 nm
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2. Acquisition system
Waveform recorders (oscilloscopes):
AC output (TS) into 1 GS/s, 150 MHz, 8 bit.
DC output (plasma background) recorded at 1kHz, 12 bits.
Data acquired between laser shots : real time acquisition
3 positions / polychromator with optical delay lines:
2 fibres/position (15-20 mm spatial resolution)
20 m
20 + 30 m (150 ns)
20 + 60 m (300 ns)
Fiber bundle into each polychromator
ns
TS pulses from 3 positions into same channel
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4. Project schedule
2001 2002
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2003 2004 2005
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2006
2007 2008
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4. Project evolution: 2005
First TS measurements on 4th October 2005 (JPN 63804)
Green: with plasma; blu: without plasma
JPN 63863 (dry run) & 63865
Spectrometer 16 (core)
ch1
TS signals
Roberto Pasqualotto
White stray
from inner wall
(dump)
Spectrometer 7 (edge)
Raman in
air
ch2
TS signals
ch1
ch2
ch3
ch3
ch4
ch4
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4. Project evolution: 2006
Improvements in 2006
End 2005: good sensitivity demonstrated, but spurious pulses pollute TS signal
During 2006, general upgrade of the system (operation restarted in October)
- Broadband stray light nearly cancelled by enlarging the laser beam on the dump
- Monochromatic stray light nearly cancelled by tilting last filter in spectrometers
- Edge spectrometers realigned and all recalibrated with more accurate procedure
- Fiber optic delay lines installed with long delays to avoid problems with spurious
pulses, but with smaller number of positions (37 instead of 60)
- Raman calibration improved
- Analysis programs finalised
- Protection system for laser beam risking to damage optics:
burst max duration 20/10 pulses
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4. Project evolution: 2006
TS improved measurements in November 2006 (1)
Main improvement: both monochromatic and broadband stray light nearly
cancelled7
Spectrometer
Spectrometer 5
blu: with plasma; green: without plasma
Delay lines
configurations
20 m
20 + 30 m (150 ns delay)
Roberto Pasqualotto
ch1
ch1
ch2
ch2
ch3
ch3
ch4
ch4
20 m
20 + 30 m (150 ns delay)
20 + 60 m (300 ns delay)
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4. Project evolution: 2007
Te and ne profiles in March 2007
The new HRTS system compared to existing electron diagnostics at JET
- The systems are in fair agreement
- All systems show single profiles, but core LIDAR averaged over 5 profiles (1 s)
ne
Te
Nucl. Fusion 48 (2008) 115006
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4. Damaged optics
Laser produces
2 beams,
vertically displaced.
Burnspots:
2° lens, damaged
If last amplifier doesn’t work:
beam divergence is changed
and beam focuses on 2nd lens
because each amplifier has a
thermal lensing effect on the
beam
Roberto Pasqualotto
Laser
NORMAL
Laser
FAULTY
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5. Position calibration
• Scaled ruler positioned along laser beam path by remote handling
• Each fibre is back-illuminated: an image of collection mirror is produced
• Circular spot with diameter:
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average separation=8 mm
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5. HRTS single profile measurements
LIDAR
HRTS
Single profiles are now of good quality
1.5 cm sampling resolution
across full profile
Temperature
LIDAR
HRTS
Æ
Pedestals are very steep!
Æ
Only one point in barrier
Density
Spatial profile
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5. Time evolution
Time evolution of Te and ne by KE11 at fixed position (R = 3.2m),
compared with core LIDAR (ne) and ECE (Te)
Pulse #73634
HRTS
LIDAR
Electron Density
HRTS
LIDAR
ECE
Electron Temperature
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5. Profile comparison
ELM mitigation through impurity seeding:
the pedestal pressure stays about constant
during the type I ELM phase and then
collapses
after Frad ~ 50 % during the type III ELMs
Average over 3-4 measured profiles
Nucl. Fusion 48 (2008) 095004, M.Beurskens et al.
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5. ROG sweep to improve spatial sampling
1.5 cm ROG sweep
ROG sweep and pre-ELM data selection
Increases data sampling:
Æ Pedestal width analysis is possible
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5. Diagnostics comparison
ne: HRTS vs reflectometer
Te: HRTS vs ECE
Agreement with
preliminary data from KG8A
is also good
Agreement with ECE is often very good
However a shift of the ECE profile is required
1.7MA/1.8T
r/a
KK3 pre-ELM (shifted 8 cm)
(see movie)
HRTS pre-ELM
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5.
RLCFS from HRTS
A problem we encountered is that the absolute position calibration got lost
(presumably the position stepper motors moved during shutdown)
Æ Before position loss a good agreement was found between EFIT LCFS
and HRTS pedestal foot in 2007. (Nucl. Fusion 48 (2008) 115006, A.Alfier et al.)
Æ Use EFIT LCF position as reference for absolute calibration
Rlcfs= C+1/2δ
Pe (kPa)
20
10
0
3.7
3.8
3.9
discrepancy (cm)
Pedestal fit: RLCF=C+1/2δ
R_LCFS (m)
EFIT and HRTS agree in LCFS within +/- 0.5 cm
3.84
EFIT
3.83
3.82
3.81
0.5
HRTS
(single
profile fits)
ROG sweep
3.8
1.0
73344
0
-0.5
-1.0
55
56
57
58
59
60
61
62
63
64
65
Time (s)
At start of campaign the position was not right and the profile was shifted to match LCFS.
But technique was validated when we had absolute calibration in 2007
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5. Edge filaments
Filaments in the edge
during the
ELMy H mode phase.
Evidence confirmed by the
fast visible camera
paper IAEA-CN- EX/P3-4 (2008), M.Beurskens et al.
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The TS diagnostic(s) on TCV
Main TS (blue):
- 25 spatial channels (3 cm)
- 40Hz time resolution (2 x 20Hz 1J Nd:YAG)
Edge TS on loan from Consorzio RFX:
- 9 spatial channels (1cm)
- sharing the same laser
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H-mode and ELMs
H-mode (high confinement
regime): a transport barrier
develops at the edge, called
“pedestal”
Æ enhanced confinement
properties
TS laser
Released energy and particle
may damage plasma facing
components
ELMs: MHD instabilities
appear at the edge when the
edge pedestal gradient
overcome a stability threshold
(Hα lines)
∆tEL
M
Æ their control is crucial
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Analitical fit of the edge profile
The edge profiles is fitted with a
modified tanh fit (5 parameters)
X ⋅ e− X
F ( X ) = a (5) − a (1) ⋅ tanh X − a (1) ⋅ a (4) ⋅ X
e + e− X
where X is the normalized coord.:
R − a (2)
X=
a (3)
Pedestal parameters
2·a(3) : width
Roberto Pasqualotto
a(1)+a(5) : height
a(1)/a(3) : slope
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Type-III ELM on TCV
In the narrow time window ±150µs around the ELM peak :
- Relaxed : monotonic slope, no clear sign of a pedestal a significantly
smaller gradient than normal profiles;
- Bumpy : bumps at the LCFS;
- Normal.
normal
bumpy
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relaxed
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Results from single profile fit method
Time evolution of pedestal height and width during an ELM phase:
- drop of Te(15%) and ne(35%) – Te drop 500µs after the ELM peak
- sub-ms recovery time scale
- transient region not available, profile deviates from tanh fit
Transient phase of the ELM
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Time evolution during ELM cycle from coherent averaging
1. Single profiles are grouped in bins with respect to
their ∆tELM (quasi-stationari condition);
2. the tanh-fit is then avereged out
From several bins
From two bins
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Eur.PhD Fusion: Thomson scattering
Results obtained with
the single fit method
are confirmed
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RFX: edge TS
Why an edge TS on
RFX-mod?
Main TS
Outer edge region:
Edge TS
- edge physics is influenced by the
active MHD control system
- not covered by the main TS
- Time resolution: 1 shot per pulse with a Ruby laser (7J @ 694nm, 30ns at FWHM);
- Spatial resolution: 1 cm resolution on 12 measuring points;
- Dispersion system: Intensified CCD spectrometer measuring from few eV to 500eV;
- Novelty: the input system and collection window are on the same mechanical structure:
easier alignment
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RFX-mod: TS systems (1/5)
Main TS
RFX-mod
- Profiles @ 50Hz;
- 84 spatial points @ 10mm;
- Te = 20 – 1500eV and ne > 1019m-3.
RFX
Plasma
Edge TS (is being commissioned)
- Single profile;
- 12 spatial points @ 10mm resolution;
- Te = 3 – 300eV and ne > 0.3·1019m-3.
Vessel wall
Ruby laser
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Input system
The entrance port hosts the
input system & the collection
window Æ stable alignment.
Vacuum
A ruby laser is focused on a
3mm pin-hole in vacuum.
2
3
chamber
Pin-hole
4
1
Ruby laser
beam
Sapphire prism deflects the beam
by 30°; a sapphire lens images the
pin-hole in the vacuum vessel.
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Collection system
Schematic top view
Image of the back illuminated fibers with the
extracted structure during the alignement
process
4 points for measuring BKG (13-16)
12 points for measuring
TS spectrum (1-12)
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He-Ne laser to
trace the path
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The spectrometer
E
D
A
B
I.I.
F
G
8°
Fiber
bundle
K
CCD
C
Energy
monitor fiber
A: 4x4 fiber pattern
B – G – K : camera lenses
C: four square lenses f = 400mm
I.I: Image Intensifier, ∅25mm
I.I.
controller
CCD
controller
D: three square spherical mirrors f = 200mm
F: one square lens f = 400mm
J: energy monitor fiber
CCD : 578x385 pixels, 22µm x 22µm pixel size, 1.5ms frame transfer
Transmission
functions
2eV
500eV
Expected accuracy of Te and
ne measurements
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10 kHz Repetitive High-Resolution
TV Thomson Scattering System for TEXTOR
* Intracavity laser with three bursts
of 50 – 100 pulses with 15 J each
* Ultra fast detector with CMOS camera
* expected performance: errors on Te ~ 8%
and ne ~ 4% @ 0.05 ≤ Te ≤ 2 keV &
ne = 2.5×1019 m-3
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•
•
•
•
•
•
•
•
•
•
•
•
•
•
Double-pass system
Number of bursts
Number of pulses
Number of back and
forth passes per pulse
Lens-spher. mirror space
Effective cavity length
Pulse probing energy
Pulse probing power
Total probing energy
Pulse duration
Pulse interval
Beam divergence
Beam chord in plasma
Probing region diameter
Roberto Pasqualotto
Achieved
1
10-12
10
8.5 m
18 m
12-23 J
6-12 MW
150-220 J
0.002 ms
0.200 ms
0.7 mrad
900 mm
2-5 mm
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plasma light image
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Plasma light and TS spectra
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TS spectrum
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Sequence of profiles in a burst
Temperature profiles
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Temperature profiles through 2 phases
of an m=2 island
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LIDAR
Plasma
• LIght Detection And Ranging
• ‘Point and shoot’ method
• So required access minimised
r
Lase pulse)
rt
(sho
• Short pulse of light transmitted to
the JET - ITER plasma.
Mirror
labyrinth
• The back-scattered light is
collected and analysed.
• Note the spatial extent is
recovered by the time delay.
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LIDAR
•
Spatial resolution means short pulse lasers and fast detectors are
required e.g. ITER requires ~7cm
Laser
Pulse
Plasma,
Length L
Scattered
Light
Scattered
Light
Note that the profile length in time is dt=2L/C.
Effectively 15cm/ns!
Detector and laser response defines spatial resolution
•
7cm is equivalent to ~460ps combined laser and detector response time
(so det/laser response ~300ps FWHM)
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LIDAR
Thomson Scattering
Principle
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Scattered signals
at different times
Gives Te and ne at
different positions
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Advantages over more conventional 90 degree scattering geometry
• One set of (6) detectors for all spatial positions – easy calibration
• 180 degree geometry makes alignment simple – easy to maintain
• All sensitive components can be outside biological shield – easy access
• Because of time localisation, stray laser light pulses can be traced
to particular objects – easy (ish) to remove
• Fast detectors means background plasma light level is low –
(can’t think of anything easy about that – easy to subtract perhaps?)
• BUT
• Spatial resolution? – not so easy
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LIDAR
er
s
a
L
ne
Te
x
λ
Collection
optics
.t
c
=
Space resolution:
∆x2= c2.( tlaser2+ tdet2+ tdaq2)/4
tlaser=300ps
tdet= 600ps
x=c.t/2
Roberto Pasqualotto
tdaq= 400ps
∆x= 12 cm
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LIDAR at JET
KE1 was the first TS on JET, ready nearly from the
start of operation in 1983.
The temperature measurement on JET were based
mostly on ECE. TS was required to keep it honest.
The KE1 system was designed with this in mind
and the fact it was only single point improved S/N
in any case as the laser could be more focussed.
First time that all essential components were
outside Torus Hall. Limited access. Long
distances.
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LIDAR at JET
The idea of LIDAR was around at the
Varenne meeting i 1982
Fortuitously it required only minor changes of
KE1 to implement the system
LIDAR improves S/N by a factor >100 from
the shorter integration time. This factor is
reduced by ~ 10 due to a larger etendue
required.
Weakness: limited resolution
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JET LIDAR laser
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JET LIDAR polychromators
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JET LIDAR detectors
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JET LIDAR raw data
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JET LIDAR profiles
Pedestals as measured with ECE, Li
beam, LIDAR and CXS
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Divertor LIDAR at JET
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Mapping on flux surfaces
If we can assume that Te and ne are constant on a flux surface
and
If we can align our LIDAR system so that the angle it’s line of sight
makes with the flux surfaces, instead of being perpendicular, is much
closer to tangential
then
Although the spatial resolution is still 12 cm along the line of sight,
perpendicular to the flux surfaces it can be ~4 - 5 times better giving
∆L = 2 –3 cm
However, the radial extent over which this resolution is achieved is limited
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Divertor LIDAR at JET: polychromator
4 channel filter spectrometer
• Optical path lengths to detectors are the same.
• Three filters at 12 degree incidence (F1 – F3) are shown
• A fourth filter limits the channel nearest the laser line.
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Divertor LIDAR at JET: raw data & profiles
Flux surfaces
ne (x1019m-3)
10
Mapping
increases spatial
resolution
0
Rmid
ser
La ht
D
ig
9
KE -of-s
e
lin
ne
0
(x1
-3 )
19 m
10
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JET boundary
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ITER requirements for Te & ne
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ITER requirements for Te & ne
Electron temperatures in ITER of up to 40keV and densities of up to several times 1020 m-3 are
expected. Thomson Scattering is a proven technique for making these measurements.
Successful deployment of such a system requires that all components maintain adequate
performance throughout the lifetime of the experiment. The parameters accessed by ITER lead
to very different operating conditions from existing devices. These range from a high dose
neutron environment to in-vacuum mirrors and the extremely long plasma discharges.
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ITER LIDAR
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Laser
Beam
dump
Mirrors
Large mirrors
collect suitable
amount of light
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The Relay Mirror
A possible
solution for
ITER LIDAR
2007, ~92inch
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The Relay Mirror
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Bird’s eye view
Laser diagnosis unit
New proposed laser beam
test area
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The Neutron/Radiation Challenge
•Influence of optical labyrinth
•High level of detail obtainable
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Laser Options/Requirements
•
Needs reasonable energy and short pulse simultaneously
•
Options to chose from:
– Nd:YAG (1064nm)
– Nd:YAG second harmonic (532nm)
– Ruby(694nm)
– Ti:Sapphire (~800nm)
– Nd:YLF (1056nm)
•
Wide temperature range
•
Time repetition expected from laser(s) – 100Hz
•
Also need to consider
– Space envelope/ Maintainability/ Power consumption/ Data quality
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Laser System
•
Laser specifications
– wavelength~ ~1.06microns (1ω +2ω +cal )
– laser energy ~5J/pulse
– laser pulse ~250-300ps
•
Proposing 7 lasers at ~15Hz
– More achievable technology
– Compact footprint
– Measurement capability maintained even if 1,2,3... lasers
malfunction
– Burst mode available to exploit plasma physics e.g. very fast MHD
events
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Options to combine lasers
• Hexagonally pack lasers
– no moving parts
• Use a scanning mirror
– all beams can overlap
• Rotating wheel with encoder
– all beams can overlap
Above shows a 7 laser hexagon pack at
machine vacuum boundary
In this option beams are expanded as they
go to the machine area to minimise risk of
damage to windows
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Scattered Spectrum
7
Scattered Spectra
Spectral Density
6
Selden-Matoba, θ=180o
5
0.5keV
5keV
10keV
20keV
40keV
4
3
2
1
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Normalised Wavelength
0.5
Note getting to λ/λ0~0.35 gets past the peak for
40keV
For 532nm laser, this means getting to 186nm
For 800nm laser, this means getting to 280nm
For 1064nm laser, this means getting to 370nm
Roberto Pasqualotto
Quantum Efficiency
GaAsP
NIR region
(λ > 850 nm)
0.4
GaAs
0.3
0.2
S-25
0.1
0
200
Eur.PhD Fusion: Thomson scattering
400
-
600
800
Wavelength [nm]
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1000
Nd:YAG
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1200
Core detectors
Upper: The Thomson scattering spectrum for an input wavelength of 1064 nm and a scattering angle q =
180˚, calculated at 5 different plasma temperatures.
Photon spectral density (a.u.)
Lower: examples of the spectral quantum efficiency of visible photo-cathodes available for LIDAR TS.
12
10
•Long wavelength laser (e.g. NdYAG)
•Wide spectral range
•Shorter wavelengths efficient fast
detectors exist
‰ Recently proven at JET
(GaAsP)
‰ Modest improvement required
0.2 keV
8
6
1 keV
4
2
10 keV
40 keV
5 keV
0
200
0.5
400
600
800
1000
Nd:YAG
1200
Quantum Efficiency
GaAsP
NIR region
( λ > 850 nm)
0.4
GaAs
0.3
0.2
S-25
0.1
0
200
400
Roberto Pasqualotto
600
800
Wavelength [nm]
1000
Nd:YAG
Eur.PhD Fusion: Thomson scattering
•Detectors in the >850nm required
‰ Ternary alloy InxGa1-xAs could
produce a QE of the order of 5% up
to a cut-off wavelength of l~ 1000
nm.
‰ Transferred electron (TE)
detector. Externally biased,
InGaAsP/InP photocathode with a
possible QE in excess of 25% up to
λ=1.33 µm
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Attenuation in LIDAR Windows on ITER, due to ionising dose
Light Collection Window (Double – total thickness 3 cm)
Over ITER lifetime, equivalent ionising dose is 1.7 x 10-2 MGy
λ (nm)
Absorption (%)
400 – 800
0.5
350
0.9
300
2.9
250
8.3
Laser Input Window (Double – total thickness 3 cm)
Over ITER lifetime, equivalent ionising dose is 1.0 MGy
λ (nm)
Absorption (%)
400 – 800
20
350
35
300
76
250
98.5
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Radio-luminescence in KU1 Silica Windows
The value for the radio-luminescence intensity from the LIDAR light-collection window, falling
within the étendue of the detection optics, is around seven orders of magnitude lower than that due
to plasma bremsstrahlung collected within the same étendue. Consequently, the radio-luminescence
signal can be ignored in the assessment of the parasitic light that will be collected along with the
laser light scattered from the ITER plasma.
Variation of
Luminescence with
Wavelength for Various
Glasses
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Motivation
First mirror
100
Rhodium is a very attractive
option for first mirror material:
¾ Good reflectivity
¾ High melting point (1966 °C)
¾ Low sputtering yield (high Z)
Reflectivity (%)
90
80
Cu
Mo
Rh
70
SS
60
W
50
Calculated with (n, k)
from [1]
40
30
500
1000
1500 2000
Wavelength (nm)
High price of the raw material calls for developing thin film technology:
Magnetron sputtering (Vacuum deposition technique)
1Handbook
of optical constants of solids, ed. E.D. Palik, Acad. Press, 1985 and 1991
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Dielectric mirrors
Broadband Dielectric
Max size now <100mm
Protected Aluminium
to compare
Laser Mirror, max size 530mm!
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Lasers
• Study of short pulse high rep rate Nd:YAG lasers for scattering needs
to be carried out (both 1st and 2nd harmonic)
• 2nd Harmonic will generally have half the energy and half the
photons!
• Ruby has been demonstrated to work but the repetition rate is a
problem
• Now to have a brief look at the TiS optionÆ
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Use of TiS lasers
• TiS lasers have never been used for LIDAR TS in fusion
experiments
• Potential issues need to be studied and analysed: bandwidth, ASE
(amplified stimulated emission), maintenance, stability,
functionality.
• May be desirable to set up a test experiment (perhaps look at
scattering off a gas) using an existing TiS facility after a feasibility
study
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LIDAR Detector sensitivity?
•
For LIDAR, presently use gated 20 mm photocathode dual
chevron-MCP photomultipliers (10-12mm may be acceptable)
•
Photocathodes such as S20, Gallium Arsenide phosphide,
Gallium Arsenide, etc can cover the region to ~850nm
•
What about a detector in the 850-1060nm region? (sensitive
detectors in this region would beneficial)
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Detector Response time?
•
Spatial resolution in the LIDAR system is directly related to the
system response with 400ps (complete response when
convoluted with detector, digitiser and laser) corresponding to 6
cm (ITER specification)
•
Laser pulse can be ~200ps
•
Detectors currently in use have about 650ps response time
(800ps when response of complete system is included)
•
But very recent detector developments are encouraging:
Detector ~10mm diameter(between 10 and 20mm required):
response time between 110 and 133ps.
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Laser coupling efficiency
•
To optimise laser energy coupling efficiency, one should make
use of high reflectivity mirrors where possible.
•
This is not possible if broadband metal mirrors are used for
simultaneously transporting-in and collection-of the scattered
light. For example, if 5 rhodium mirrors were used in the duct
area, then immediately a transmission of 20% could be the
result (mirrors and windows)
•
Can we get around this?
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Can use separate laser and collection
First laser mirror
could be here
Bio Shield
Port Plug
F/18
F/12
F/6
Separate Laser path
(Fold not shown)
Detector ø18mm
Schematic straight through
optical path shown for clarity
Vacuum window ø110mm
4.2m
6.2m
8.2m
11.2m
14.8m
Using this approach, one can optimise the mirrors for the laser and collection separately.
This would require a small hole in the back of the First mirror (<5cm diameter)
Can we have dual laser dielectric mirrors situated at the back of the port duct that will be robust?
Note: Expect 6x1011neutrons/cm2/s First mirror, this would be down by at least a factor of 5 at
first laser mirror (window position is being studied)
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High Temperature Thomson Scattering Theory Review
•
Reliable electron temperature and density diagnostic for
modern tokamaks
All across the operating range
Current devices have temperatures exceeding 10 keV
But up to 40 keV predicted for ITER.
Then electron velocities are a substantial fraction of the
velocity of light
Large blue shift’ in the scattered spectrum
Change in the polarisation of the scattered light
•
•
•
•
•
•
•
The incoherent Thomson scattered power per
unit solid angle per unit angular frequency can be written
Roberto Pasqualotto
2
e
2
2
1 − βi
×
1 − β 2 f (β )δ (k ⋅ v − ω )d 3
1 − βs
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)
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β
(1 − cosθ )β
d P
= re2 ∫ Si d 3r ∫ 1 −
(1 − β i )(1 − β s )
d Ω s dω s
2
•
•
•
•
Depolarisation term important at high temperatures
Theory is solid but experiments challenged this due to fact TeTS can be up to 15-20%
lower than TeECE in some experiments
However the presence of high energy electron tail would cause TS to overestimate
Urgency to investigate the cause of the temperature discrepancy
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Laser Reliability
•For the TS systems, reliability is of paramount importance and redundancy
in design must be incorporated where possible.
•Typical operation of a multi-laser system on the MAST device has shown
that out of 4 lasers, at least 1 laser was available almost 100% of the time
while all 4 lasers were available >70% of the time (this corresponds to an
individual laser availability of about 92% per plasma shot).
•Translating this simply to ITER for a 7 laser system would give 5 or more
lasers available more than 98% of the time, and all 7 lasers > 56% of the
time.
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EU-Core TS (LIDAR)
Electron
Temperature
Electron Density
Spatial
range
r/a<0.9
Parameter range
Space
resolution
a/30
Accuracy
0.5-40keV
Time
resolution
10ms
r/a<0.9
3x1019 to 3x1020m-3
10ms
a/30
5%
10%
Electron Temperature (keV)
35
30
25
20
15
10
5
0
4.0
4.5
5.0
5.5
6.0
6.5
R (m)
7.0
7.5
8.0
7.5
8.0
Electron Density (1019 m-3)
3.5
•An example scenario (5n) that is expected in ITER.
•The required measurement resolution is a/30.
•This is equivalent to approximately 7 cm in real
space.
•Note: the full profile from -0.9r/a to 0.9r/a is required
3.0
2.5
2.0
1.5
1.0
Assumes IR detectors are possible
0.5
0.0
4.0
4.5
5.0
Roberto Pasqualotto
5.5
6.0
6.5
R(m)
7.0
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High Importance Generic Topics to be Addressed
•
•
•
•
•
•
•
•
•
•
•
•
•
•
First/second Mirror surface recovery (MSR) techniques
Deposition prevention
First dielectric laser mirror
Background light calculations (need to get much better modelling)
Wide-band in-situ calibrations
Detectors (previously discussed at ITPA-need more physics
assessment)
Laser development
Shutter/calibration combination specification/outline
Alignment systems
Beam Dumps (common issue)
Reliability (should define what is expected)
TS/ECE issue resolution
Measurement requirements-still consistent- (detailed physics case)
Diagnostic exploitation
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ITERCore LIDAR vignetting
The ITER TS design is based principally on
the design of the JET LIDAR.
JET has the only LIDAR systems in the
world. It was generally acknowledges that
LIDAR was the only way of introducing TS
on ITER. To a large extent the ITER system
is a copy of KE3.
• The reliability is very high > 90%
• Alignment is stable, no components on
the Vacuum vessel
• BUT, The Jet systems are not using one
window but a cluster of 7 windows with the
laser in the center Vignetting!
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ITER Core LIDAR vignetting (old design)
•
Eliminate the central window, i.e. use
the full aperture for collection.
•
Use the collection window/mirror for
laser beam as well
•
•
•
Signal from inside double cone has no vignetting
Laser beam can be anywhere in this cone
Simple calculation of solid angle if the two apertures are
relayed to the detectors
Ddet / Fdet = Dblanket / Fblanket = Dblanket x Dmirror/(L2 - L1)
Dmirror/Dblanket = L2/L1
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ITERCore LIDAR optical design
The aim is to re-image the entrance pupil on the detector surface. The advantage of this idea is a
field-independent image diameter. The entrance pupil size is given by the size of the first
surface of the Collection Optics, M1. The diameter of M1 follows from the field definitions and
F/#’s.
M1 and M4 are spherical mirrors, M2 and M3 are identical toroidals.
M1 is imaged onto M4
Field position [m] + 2100 [m] before M1
Field [mm] at field position
0
10
F/6
2100
4200
50
F/17
110
For the relay-optics there is a balance between size-of-the-components and the total number-ofcomponents. Task is to re-image M4 by a relay and keeping every link in the chain identical.
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ITERCore LIDAR detectors
The TS spectrum in ITER will range from NIR (low Te) down to UV (high Te).
IR laser will be used only if IR detection is available Æ this influences also
calibration techniques.
•Long wavelength laser (e.g. NdYAG)
•Wide spectral range
•Shorter wavelengths
efficient fast detectors exist
‰ Recently proven at JET (GaAsP)
‰ Modest improvement required
•Detectors in the >850nm required
‰ Ternary alloy InxGa1-xAs could
produce a QE of the order of 5% up to
a cut-off wavelength of λ~ 1000 nm.
‰ Transferred electron (TE) detector.
Externally biased, InGaAsP/InP
photocathode with a possible QE in
excess of 25% up to λ=1.33 µm
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ITERCore LIDAR detectors
The main requirements for the ITER LIDAR TS detectors are :
Active area diameter D ≥ 11 mm
Equivalent quantum efficiency EQE ≥ 6%
Pulse response time t ≤ 330 ps FWHM.[1]
gating shutter ratio ~ 106.
gating on-off time ≤ 5ns.
EQE = QE/kF
kF is the excess noise factor that accounts for any additional
noise introduced after the primary detection.
At present these specifications can be met only by photoemissive
detectors. The above specifications are at the limit of the present
technology for the detectors operating in the visible. To extend
them to the NIR is a real challenge.
Two types of detectors available for the above spectral range: the
transferred electron (TE) hybrid photodiode and the InxGa1-xAs
microchannel plate (MCP) image intensifier
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NIR detectors
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NIR detectors
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NIR detectors
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Signal simulation with NIR detectors
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ITER core LIDAR project Work Breakdown Structure
Thomson Scattering
Core (LIDAR)
5.5.C.1.0.0.0.0
LIDAR Project
Management
1.1.0.0.0
LIDAR System
Concepts
1.2.0.0.0
Laser
Systems
1.3.0.0.0
Collection
Optics
1.4.0.0.0
Laser Path
Optics
1.5.0.0.0
Control &
Acquisition
1.6.0.0.0
LIDAR
Port Engineering
1.7.0.0.0
LIDAR
Services
1.8.0.0.0
Interfaces &
Integrated Testing
1.9.0.0.0
Key Project
Milestones
1.1.1.0.0
Overall Cluster
Co-ordination
1.2.1.0.0
Lasers
1.3.1.0.0
Collection
Optical Design
1.4.1.0.0
Laser Path
Optical Design
1.5.1.0.0
Control System
Interface Definition
1.6.1.0.0
Shutters
1.7.1.0.0
Water
Services
1.8.1.0.0
LIDAR
Interfaces
1.9.1.0.0
Key Project
Deliverables
1.1.2.0.0
Performance
Analysis
1.2.2.0.0
Laser
Layout
1.3.2.0.0
Collection
Windows
1.4.2.0.0
Laser
Windows
1.5.2.0.0
Control
System
1.6.2.0.0
Labyrinth
1.7.2.0.0
Interspace
Vacuum
1.8.2.0.0
Mock-up
Facility
1.9.2.0.0
Key ITER
Milestones & IPL
1.1.3.0.0
LIDAR
Neutronics
1.2.3.0.0
Laser Beam
Combiner
1.3.3.0.0
In-Vacuum
Collection Mirrors
1.4.3.0.0
Plasma Facing
Laser Mirrors
1.5.3.0.0
Acquisition
System
1.6.3.0.0
Extension Tubes &
Mirror Mounting
1.7.3.0.0
LIDAR
Power
1.8.3.0.0
Basic Mock-up
Tests
1.9.3.0.0
Overall
Management
1.1.4.0.0
Scattering
Theory
1.2.4.0.0
Ex-Vacuum
Collection Optics
1.4.4.0.0
Other Laser
Mirrors
1.5.4.0.0
LIDAR
Instrumentation
1.6.4.0.0
External Port Optics
Mounting
1.7.4.0.0
Spectrometer
Area
1.8.4.0.0
Tokamak
Tests
1.9.4.0.0
Safety & HP
Management
1.1.5.0.0
R&D
Tasks
1.2.5.0.0
Collection Optics
Mechanical Design
1.4.5.0.0
Laser Path
Mechanical Design
1.5.5.0.0
Safety
Interlocks
1.6.5.0.0
Bioshield
1.7.5.0.0
Laser
Room
1.8.5.0.0
Final System
Testing
1.9.5.0.0
Risk
Management
1.1.6.0.0
Radiation
Effects Data
1.2.6.0.0
Spectrometer
System
1.4.6.0.0
Beam
Dump
1.5.6.0.0
Safety
System
1.6.6.0.0
BSM
Penetrations
1.7.6.0.0
Port Cell/
Interspace
1.8.6.0.0
System Assembly
& Dis-assembly
1.9.6.0.0
Quality
Management
1.1.7.0.0
Remote
Handling
1.2.7.0.0
Detectors
1.4.7.0.0
Alignment
System
1.5.7.0.0
Item
Test Unit
1.2.8.0.0
Alignment
System
1.4.8.0.0
Calibration
System
1.5.8.0.0
Engineering
Analysis
1.2.9.0.0
Calibration
System
1.4.9.0.0
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering -
EM Analysis for
In-Port Comp.
1.7.7.0.0
11th February 2009
178/181
ITER divertor TS’s
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering -
11th February 2009
179/181
ITER divertor TS’s
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering -
11th February 2009
180/181
ITER divertor TS’s
The life-time of optical components is expected to be limited due to contamination with
carbon and beryllium-based material eroded from the beryllium wall and carbon tiles.
As well as significantly reduced optical transmission, thin layers can dramatically change the
slope of the spectral reflectivity of rather low reflectivity mirrors, especially like W or Mo.
Roberto Pasqualotto
Eur.PhD Fusion: Thomson scattering -
11th February 2009
181/181