Beam diagnostics

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Beam diagnostics
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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

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Beam transformers
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DC-Transformer for the SIS at GSI
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• 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

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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.
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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.
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Beam current transformers are not very sensitive
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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)
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• 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.

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• 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.

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Position pick-up monitors (PU)
Transverse beam position
electrostatic magnetic electromagnetic
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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 low-impedance circuit or be
sensed on a high impedance as a voltage on the
capacity between the electrode and the
surrounding vacuum chamber.
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Shoe box type position monitor for SIS and ESR
at GSI
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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
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Faraday cup

• Beam intensity measurement (electric current)
• Stop the beam and measure the current

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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!
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COL
e- suppressor
Note the diameters!
e-
beam
FC
A
-U
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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.
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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.
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Wire scanners
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Fast Wire Scanner at TRIUMF
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Very thin wires
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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
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Ionization chamber
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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.
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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
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2D profile
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The Ionization Beam Scanner (IBS) is a further
device relying on residual gas. It employs a
time-varying 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.
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Scintillator screens
Scintillators were the first particle detectors,
a century ago.
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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).
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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.
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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

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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
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• 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

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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).
Twiss parameters a, b, g
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Emittance scanner (LBNL/JYFL)
GSI
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x0b
x1b
x0a
x1a
x0
l
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• Measure x0, x1a and x1b
• Get points x0a and x0b
• Scan x0 and get the ellipse contour

x1a
x1b
x1
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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

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Scan
Scan
FC
x1
x0
l
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Scan
a
E
Scan
x0
FC
l
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U
a
d
E
-U
l
r
2a
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For simplicity, assume that
Then we get circular motion
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• 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

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Pepper pot method
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• 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
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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

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Sources of beam spot broadening (un-wanted)
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Measuring energy

• Br-value from an analyzing dipole magnet
• 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!!!!

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Protons in Germanium (SRIM-output)
Ion dE/dx dE/dx
Projected Longitudinal Lateral Energy
Elec. Nuclear Range
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 1.09 mm 1.35 mm
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Protons in Silicon (SRIM-output)
Ion dE/dx dE/dx
Projected Longitudinal Lateral Energy
Elec. Nuclear 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
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Analyzing dipole
r
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Time-of-flight
COL
COL
g
g
L(ength)
Time signal
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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

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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
• Youll always get RF-background
• Short beam pulses large higher harmonic
  frequencies RF has only h1 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)