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 monitorsOne 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 low-impedance 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 cupBeam 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!

A-U

beam

COL e- suppressor

FC

e-

Note the diameters!

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 monitorsWhen 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 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.

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 lineinsulator

Electrically connected to

Cyclotron/ion source

To current meter

• 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 measurementEmittance ellipse describes the area/volume in the phase space, which the beam occupies. Emittance E is the area of the ellipse (E = ).

cossin

cos

'

0

1

1

yy

Twiss parameters

21

s

sdss

00 )(

)(

GSIEmittance scanner (LBNL/JYFL)

lxxx '001

l

x0

x1a

x1b

x’0a

x’0b

•Measure x0, x1a and x1b

•Get points x’0a and x’0b

•Scan x0 and get the ellipse contour

x1a x1b

x1

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

l

x0

Scan Scan

FCx1

l

x0

Scan

FCE

Scan

E

2

l

r

d

+U

-U

For simplicity, assume that vE

Then we get circular motion:

UdU

dUq

qUqEE

qEmvr

qEr

mv

accacck

2222

2 dUE 2

accdUlU

r

lx

22sintan'

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

•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)

x

y

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

• B-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!!!!

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

Protons in Germanium (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

Protons in Silicon (SRIM-output)

Analyzing dipole

qpB

mqB

mpE

2)(

2

222

20

2220 EcpEE

Time-of-flightCOL

COL

Time signal

L(ength)

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 frequencies – 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)