Download Beam diagnostics

Beam diagnostics
What to measure
• Intensity
– From very weak to very intense beams
• aA to mA
• Profile
– From very low energy to high energy
– From very weak to very intense beams
• Timing
– Noise from the accelerator RF
• Same frequency as for the beam pulses
Beam transformers
DC-Transformer for the SIS at GSI
In order for the transformer to see the magnetic field
produced by the beam,
•it must be mounted over a ceramic insert in the metallic
vacuum chamber.
•The ferromagnetic core is wound of high permeability
metal tape or made of ferrite, to avoid eddy currents.
•Bandwidths exceeding 100 MHz can thus be achieved.
•An idealized transformer with a secondary winding of
inductance L and connected to an infinite impedance
would deliver as signal a voltage
The signal now shows a much more useful behaviour
(Fig. 4). Provided the length of a beam bunch is longer
than the transformer's rise time and shorter than its droop
time, the signal will be a good reproduction of the bunch
shape.
For a beam circulating in a machine, the succession of bunches
seen by the transformer will be much longer than its droop time.
Therefore, to obtain a signal representing the beam intensity,
one has to electronically treat the transformer's signal such that the
effective droop time is much longer than the time that the beam
circulates. At the same time, this increases the signal rise time, so
that the bunch structure will disappear. Such a treatment is often
called a "low pass" or "integration". Figure 6 shows three
commonly used methods.
Beam current transformers are not very sensitive
Wall-current monitors
One may want to observe the bunch shape at frequencies far
beyond the few 100 MHz accessible with beam transformers.
The bunches may be very short, as is often the case with
electrons or positrons, or they may have a structure in their line
density, caused by intentional processes or by instabilities.
Wall-current monitors with a bandwidth of several GHz have been
built. Their principle is quite simple (Fig. 8a) :
•A modulated beam current Ib is accompanied by a "wall current", IW,
which it induces in the vacuum chamber, of equal magnitude and
opposite direction.
•An insulating gap forces the wall current to pass through the
impedance of a coaxial cable. The gap may also be bridged with
resistors, across which a voltage is picked up.
•To avoid perturbation through circumferential modes, the wall
current (or the gap voltage) is picked up at several points around the
circumference and summed. When the beam is not at the centre of the
vacuum chamber, the wall current will be unequally distributed
around the circumference of the chamber. Separate pick-up and
separate observation (Fig. 8b) will thus also show the beam position
with GHz bandwidth.
A conducting shield must be placed around a wall-current
monitor.
•Without it, troublesome electromagnetic radiation from the
beam would leak out through the gap and the monitor itself
would be perturbed from the outside.
•The shield constitutes a short-circuit at low frequencies and
thus severely limits the lower end of the monitor's bandwidth.
•Loading the volume of the shield with ferrite increases the
inductance and the cut-off can be lowered to some 100 kHz,
sufficient for undifferentiated observation of bunch shape in
most accelerators.
Position pick-up monitors (PU)
Transverse beam position
electrostatic
magnetic
electromagnetic
The beam will induce electric
charges on the metallic electrodes,
more on the one to which it is
closer, sum remaining constant.
The induced charges can be carried
away for measurement into a lowimpedance circuit or be sensed on a
high impedance as a voltage on the
capacity between the electrode and the
surrounding vacuum chamber.
“Shoe box” type position monitor for SIS
and ESR at GSI
In electron and positron machines, no electrodes can be
tolerated in the mid-plane : there they would be hit by the
synchrotron radiation and the resulting secondary electron
emission would perturb the signal. So-called "button"
electrodes are used, housed in recesses
Faraday cup
Beam intensity measurement (electric current)
•Stop the beam and measure the current
The beam must be STOPPED in the cup
The range of the particle must be less than the thickness of
the cup bottom.
Range of protons in Cu:
Very important: Do not let secondary electrons escape from the
FC nor let secondary electrons from e.g. a collimator hit the FC!
COL
e- suppressor
Note the diameters!
ebeam
FC
-U
A
Secondary-emission monitors
(SEM)
Under the impact of the beam particles on some solid
material electrons are liberated from the surface, thus
producing a flow of current.
The provision of a "clearing field" of a few 100 V/cm is
essential to ensure that the liberated electrons are rapidly
cleared away. Otherwise, an electron cloud may form over the
foil surface and impede further emission.
Wire scanners
Fast Wire Scanner
at TRIUMF
Very thin wires
Multi-wire chambers
Electrons produced in the gas by the passing beam particles will
travel towards the nearest wire. In the high gradient close to the
wire they experience strong acceleration and create an avalanche. A
wire chamber can be used in counting or in proportional mode. The
distribution of counting rate or signal height over the wires
represents the beam profile.
More about this by Grigori Tiourine
Ionization chamber
This is a gas-filled, thin-walled chamber with a collector
electrode inside. Particles passing through it will ionize the gas,
the ions will travel towards the cathode, the electrons towards
the anode and a current can be measured (Fig. 20). The voltage
should be in the "plateau" region where all charges are collected
but no avalanche occurs.
Residual-gas monitors
When neither the residual gas pressure nor the beam
intensity are too low, ionization of the "natural" residual gas
may supply electrons in sufficient number and a gas curtain
is not needed.
1D projection or 2D profile
2D profile
The Ionization Beam Scanner (IBS)
is a further device relying on residual gas. It employs a timevarying electric and a static magnetic field, at right angles to
each other and to the beam, to guide the ionization electrons
towards a collector or electron multiplier. Although a precise
instrument for low intensity beams, the IBS is too easily
perturbed by the space charge fields of intense beams.
Instead of collecting electrons from the ionization, one can also
observe the light from de-excitation of the residual gas atoms.
This is achieved more easily at the low energies of a pre-injector
(500-800 keV) combined with the prevalent modest vacuum.
Scintillator screens
Scintillators were the first particle detectors, a century ago.
The most common scintillator used to be ZnS powder which,
with some binder, was painted onto a metal plate. Such
screens deliver green light and have high efficiency but are
unfit for use in high vacuum and are burnt out at some 1014
protons/cm2 at GeV energies.
A great step forward was the formation of thick Al203 layers on
aluminium plates under simultaneous doping with Cr.
Chemically, this is the same as ruby and the light emitted is red.
These screens are fit for ultra high vacuum and have a long
lifetime (1020 to 1021 p/cm2 at 50 MeV).
Choise of the TV camera important. Often it needs to be
radiation resistant. The model developed at CERN uses
nuvistors and stands 108 Rad.
Ordinary lenses turn brown under radiation. Catadioptric optics do
a bit better but when radiation is really a problem, one has to buy
expensive lenses developed for use in reactors.
For very weak beams a combination image
intensifier - Vidicon is used.
Also, CCD-cameras offer high sensitivity, but are little
resistant to radiation.
Help:
•Use telescopic lens and cameras inside radiation shield
•Use fiber optics
Comments/hints for current
measurement
• Remember grounding
– Sometimes the beam tube has been insulated
from the main beam line
insulator
Electrically connected to
Cyclotron/ion source
To current
meter
– How non-grounded FC behaves?
– FC is floating
• It shows current when it is charging/discharging, i.e.
when the beam is switched on and off
• Very small currents with particle detectors
– aA
• Otherwise with a Faraday cup (proper amplifiers)
– 10 pA and more
– Small currents
• problem with noise
– Very high currents
• Remember to cool the FC (and preceding collimator) with
water
• The region between particle detectors and a
current meter is difficult.
– Be sure that high current does not hit (kill) the particle
detector
Emittance measurement
Emittance ellipse describes the
area/volume in the phase
space, which the beam
occupies. Emittance E is the
area of the ellipse (E = pe).

e

cos  0  

 y1  
   
e

sin    cos  
 y1 '   



Twiss parameters , , g
g
1  2

s
ds
 ( s)  0  
 ( s)
0
Emittance
• Usually we mean 2-sigma of the distribution for
ions
• In electron machines 1-sigma or rms-emittance is
used
• Must be determined in both transverse directions
– The primary axis of the beam ellipse (xy) should be in
the direction of measuring – otherwise the values are
larger than the true emittance
GSI
Emittance scanner
(LBNL/JYFL)
x’0b
x1b
x’0a
x1  x0  x0 'l
x1a
x0
l
•Measure x0, x1a and x1b
•Get points x’0a and x’0b
e x ,rms 
x
•Scan x0 and get the ellipse contour
x1a
x1b
x1
2
x'  xx'
2
2
How to measure x1?
• Scan a slit at x1 and measure current with a
Faraday cup
– Mechanical (both x0 and x1)
– Slow
• Scan the beam direction by bending the beam
– With E or B
– Faster (only x0 mechanical)
– May need high voltages for a high energy beam/large
divergence
Scan
Scan
x1
x0
l
FC
Scan

x0
E Scan
FC
l
+U

d
E
-U
l
r
2
 
For simplicity, assume that E  v
Then we get circular motion:
mv2
 qE
r
mv2 2 Ek 2qU acc U acc d
r



qE
qE q  2U
U
d
l
2U
E
d
lU
2
x'  tan   sin  

r
2dU acc
For large divergences, calculate transverse
acceleration/deceleration:
•Divide velocity into longitudinal and transverse components
•Transverse energy zero at l/2 due to deceleration
•Longitudinal velocity/energy does not change
•For small divergences same result as with circular motion
Pepper pot method
“For a particular hole radius, Rh, the output
distribution was propagated to a downstream
screen a distance Lscreen = 50 cm away. Since the
input divergence was manually set to have a
Gaussian distribution, the spot on the downstream
screen was first projected along the x axis (not
sliced) and fit to a Gaussian. From this projection,
the 1-sigma beam width of the spot was extracted,
which, for the case of a Gaussian, is also equal to
the standard deviation of the distribution. The width
of the spot due to the divergence of the beamlet
is added in quadrature with the r.m.s. width of the
pepper pot hole to give the width of the spot at the
screen. From this, the divergence is calculated
2
according
to,”
1
R 
Rms-divergence
 '
2  h
Lscreen
screen
 
 2 
•Analyze the spot size
•In both directions
•Hole size may be
significant
•You get also correlations
•Note the major axis in
beam ellipse (xy-plane)
y
x
Notes
• Light production should be linear
– No saturation
• For slow beams (heavy ions, injection line)
– Ions stop within a few molecular layers and the
active surface gets worse
– KBr is on possible material
• Use frame grabber and appropriate software
Sources of beam spot broadening
(un-wanted)
Measuring energy
• Br-value from an analyzing dipole magnet (B)
– Get momentum p. Charge state and mass known from
elsewhere. Calculate energy
• Time-of-flight
– Get velocity v. Mass known from elsewhere. Calculate
energy
• Measure with a particle detector (Si, Ge)
– Remember the range
– Use very low beam intensity!!!!
Protons in Germanium (SRIM-output)
Ion
Energy
dE/dx
Elec.
dE/dx
Nuclear
Projected
Range
Longitudinal Lateral
Straggling Straggling
----------- ---------- ---------- ---------- ---------- ---------10.00 MeV 2.598E-02 1.251E-05 422.64 um 21.45 um 30.12 um
11.00 MeV 2.424E-02 1.151E-05 496.38 um 24.82 um 35.03 um
12.00 MeV 2.274E-02 1.066E-05 575.21 um 28.25 um 40.24 um
13.00 MeV 2.144E-02 9.939E-06 659.02 um 31.75 um 45.74 um
14.00 MeV 2.030E-02 9.311E-06 747.75 um 35.31 um 51.53 um
15.00 MeV 1.929E-02 8.762E-06 841.32 um 38.96 um 57.61 um
60.00 MeV 6.782E-03 2.548E-06 9.36 mm 429.05 um 575.68 um
65.00 MeV 6.390E-03 2.371E-06 10.77 mm 485.73 um 658.54 um
70.00 MeV 6.048E-03 2.218E-06 12.27 mm 543.10 um 745.79 um
80.00 MeV 5.483E-03 1.967E-06 15.49 mm 733.46 um 932.94 um
90.00 MeV 5.033E-03 1.768E-06 19.02 mm 913.47 um
1.14 mm
100.00 MeV 4.667E-03 1.608E-06 22.85 mm
1.35 mm
1.09 mm
Protons in Silicon (SRIM-output)
Ion
Energy
dE/dx
Elec.
dE/dx
Nuclear
Projected Longitudinal Lateral
Range
Straggling Straggling
----------- ---------- ---------- ---------- ---------- ---------10.00 MeV 3.479E-02 1.786E-05 709.23 um 32.70 um 31.70 um
11.00 MeV 3.233E-02 1.641E-05 837.16 um 38.04 um 37.14 um
12.00 MeV 3.023E-02 1.518E-05 974.42 um 43.42 um 42.94 um
13.00 MeV 2.841E-02 1.414E-05 1.12 mm
48.89 um 49.09 um
14.00 MeV 2.682E-02 1.323E-05 1.28 mm
54.44 um 55.59 um
15.00 MeV 2.542E-02 1.244E-05 1.44 mm
60.08 um 62.44 um
60.00 MeV 8.596E-03 3.566E-06 16.85 mm 723.38 um 669.30 um
65.00 MeV 8.085E-03 3.316E-06 19.42 mm 819.57 um 767.89 um
70.00 MeV 7.641E-03 3.100E-06 22.16 mm 916.43 um 871.91 um
80.00 MeV 6.909E-03 2.746E-06 28.07 mm
1.26 mm
1.10 mm
90.00 MeV 6.328E-03 2.467E-06 34.56 mm
1.58 mm
1.34 mm
100.00 MeV 5.857E-03 2.241E-06 41.62 mm
1.89 mm
1.60 mm
Analyzing dipole
p
Br 
q
r
p 2 ( Br ) 2 q 2
E

2m
2m
E  E0 
2
 p c E
2 2
2
0
Time-of-flight
COL
g
COL
L(ength)
Time signal
g
Time-of-flight
• Time signal from gamma-rays
– Standard timing electronics
– Fast detectors (scintillators)
– Fixed target (collimator or beam dump)
• Time signal from capacitive pick-ups
– Fast amplifiers
– Simultaneous signals (= different beam bunches) with
moving pick-up
• Effectively measure of the distance (n x ) of beam pulses (with
RF-frequency)
• Known: Time and distance. Get: velocity
Important notes
• The signal propagates in the cable at a speed of
(approximately) 0.6c
– Use cables that have the same electrical length!
• Beam pulses appear at RF-frequency
– You’ll always get RF-background
– Short beam pulses: large higher harmonic amplitudes –
RF has only h=1 frequency
• If gammas come from a collimator
– Stopped particles may have different energy than those
who reach the target (due to dispersion at the
collimator)
• Beam time structure may change when going
through collimators (dispersion)