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Lecture 6.1
ADVANCED PLASMA
DIAGNOSTIC TECHNIQUES
Presented by Dr Ian Falconer
[email protected]
Room 101
LANGMUIR PROBES
Selected ITER diagnostics
Diagnostic
Measures
Magnetic diagnostics
Plasma current, position, shape, waves ..
Spectroscopic & neutral
particle analyser systems
Ion temperature, He & impurity
density, ..........
Neutron diagnostics
Fusion power, ion temperature profile, ….
Microwave diagnostics
Plasma position, shape, electron density,
profile, …..
Optical/IR(infra-red) systems
Electron density (Line-average & profile,
electron temperature profile, ….
Bolometric diagnostics
Total radiated power, ….
Plasma-facing components &
operational diagnostics
Temperature of, and particle flux
to First Wall, …..
Neutral beam diagnostics
Various parameters
Selected low temperature
plasma diagnostics
Diagnostic
Measures
Langmuir probes
Plasma potential, electron temperature
& density
Magnetic diagnostics
Plasma current, plasma waves, ….
Spectroscopic
Plasma composition, ion temperature
& drift velocity, …….
Microwave diagnostics
Plasma electron density, density profile, …..
Mass / energy analyser
Identifie sspecies of ions, and measures
their charge state and energy
Laser diagnostics
Density etc. of various species in the plasma:
density, distribution, and even in the plasma.
PLASMA DIAGNOSTICS

Electrostatic probes (Langmuir probes)

Magnetic probes

Microwave and optical interferometry

Spectroscopic techniques

Particle analysis

Thomson scattering

Nuclear radiation detection

Laser diagnostics of processing plasmas
General characteristics of a useful plasma diagnostic
•
The diagnostic must not perturb the plasma –
i.e. it must not change the conditions within
the plasma
•
Plasma diagnostics generally do not give the
parameters) directly. An understanding of the
physics of the processes involved in interpreting
diagnostic results is essential
Electrostatic probes (Langmuir probes)
A short length of wire, inserted in a plasma can give valuable information of the
plasma properties at a point in the plasma.
A Langmuir probe consists of such a short , thin wire inserted into the plasma:
the current to/from the probe is measured as its potential is changed.
A sheath forms around the probe of thickness ~ Debye length

Current to sheath
jr AS
where jr
 random current density
AS
 surface area of sheath
For a Maxwellian velocity distribution
12
jr

1 nve
4

1 n e  2kTe 


2
m

e

But this applies ONLY if the potential of the
probe is the same as that of the plasma.
How will the current to a Langmuir probe change if we use an
external voltage source to change the probe’s potential?
A “typical” Langmuir probe characteristic
Typical probe characteristic: 1
A. VS is the space or plasma potential
(the potential of the plasma in the
absence of a probe). There is no E.
The current is due mainly to the
random motion of electrons (the
random motion of the ions is much
slower).
B. If the probe is more positive
than the plasma, electrons
are attracted towards the probe
and all the ions are repelled.
An electron sheath is formed
and saturation electron current
is reached.
X
Typical probe characteristic: 2
C. If the probe is more negative
than the plasma, electrons are
repelled (but the faster ones still
reach the probe) and ions are
attracted. The shape of this part
of the curve depends on the
electron velocity distribution. For
a Maxwellian distribution with Te
> Ti, the slope of ln Ip
plotted against Vs is
e
kTe
D. The floating potential VF
(an insulated electrode would
assume this potential)
The ion flux = the electron flux
so Ip = 0.
Typical probe characteristic: 3
E. All the electrons are repelled. An ion sheath is formed
and saturation ion current is reached.
Sheath and presheath
There is a region adjacent to the sheath – the presheath – where the plasma is
imperfectly shielded from the probe potential. In region A ions are accelerated
through the resulting small potential to reach a velocity comparable with the
electrons’ thermal velocity. This must be taken into account when using this
region of the probe’s characteristic to estimate ion density in the plasma.
Probe
surface
Magnetic probes
A voltage is induced by the changing
magnetic field through this coil
V

NA
dB
dt
Integrating this voltage gives
V0

NAB
RC
Rogowski coil: measures plasma current
Voltage induced in this toroidal coil by the magnetic field
passing through area A
dI
V  NA0
dt
Integrating
VI

 V dt

NA0 I
Voltage loop: typically used to give the voltage induced in the
plasma by the Ohmic heating transformer
A voltage is induced between the (open) ends of a (usually)
single-turn loop adjacent to the plasma current. This voltage
gives the voltage induced in the plasma by the transformer.
Measurement of induced voltage in
plasma enable calculation of
plasma conductivity – and hence
temperature
Monitoring plasma position.
Coils inside and outside the plasma in a tokamak, and voltage loops above and
below the plasma, give the position of the plasma within the toroidal vacuum
vessel. Signals from these sensors are used for feedback control of the plasma
position.
(But only for toroidal plasmas with a circulating current – tokamaks.)
Interferometry
Consider these two beams of electromagnetic radiation
E1
 E0 sin  t  and
E2
 E0 sin  t   
When combined with a phase difference  they give a resultant electric field
Et
 2 E0 sin  t   2  cos  2 
When these combined beams fall on a square-law detector the output
of the detector
Vout
 2 E02 1  cos  

higher-order terms
The phase shift of a beam of EM radiation passing through a plasm a
 

0


kd
0


c
d
where
k

2

The phase difference measured by an interferometer


 k
0
plasma
 k0  d


   1 c d
0
Now for a plasma

2
now for
 1   2p  2
 1  ne e 2  2 m 0
ne e 2  2 m 0
1
  1  ne e
2
2
 1
 m 0
2
(usual case for this diagnostic)
so that


1
2
ne e 2  2 m 0
Thomson scattering
Thomson scattering is scattering off free electrons in the plasma. The electrons are set
oscillating by the incoming laser beam, and then radiate as dipole radiators.
The intensity of the scattered radiation gives the electron density, the double-Doppler
broadening of the scattered radiation gives the electron temperature.
The Thomson scattering cross-section for individual electrons is minute: 6.65 x 10-29 m2.
Thus for a plasma of density ~ 1022 m-3 only ~ 6.65x10-9 of the incident radiation will be
scattered from a scattering volume of 1 cm3 and only a small fraction of this will enter
the spectrometer/detection system. (The electrons are dipole radiators.) Thus a powerful
laser is required to obtain sufficient photons to detect the scattered radiation, and stray
light from the laser and other sources presents difficulties in observing the scattered
radiation.
(A 0.05 joule pulse from a frequency-doubled Nd:YAG laser at 532 nm - a powerful laser
pulse – contains ~1017 photons, so that only ~109 photons will be scattered from this
volume, and many fewer than 10% of these will enter the detection system.)
Layout of a typical Thomson scattering experiment
The ITER LIDAR Thomson scattering system
Here the spectrum of the laser radiation scattered back along the laser beam
is recorded as a function of time. The width of the spectrum gives the electron
temperature at a point within the plasma, and time of arrival of the scattered
radiation gives the position at which the temperature was measured.
As the Doppler shift for radiation that is backscattered through a very large
angle is small this technique is only feasible for very hot plasmas. It is best
suited fas a diagnostic for a large plasma, so that the incident laser pulse and
the weak scattered beam are well-separated in time.
Conclusion
•
An array of non-perturbing diagnostic techniques has been
developed to probe both fusion and “processing” plasmas
•
Selection of an appropriate diagnostic depends on the nature of the
plasma – and the relative cost of the diagnostics available
•
Effective use of a diagnostic technique depends on a thorough
knowledge of the physics of both the plasma and the diagnostic
technique adopted