Beam Diagnostics for Ion Sources CERN Accelerator School 2012 Uli - - PowerPoint PPT Presentation

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Beam Diagnostics for Ion Sources CERN Accelerator School 2012 Uli Raich CERN BE/BI CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 1 The LHC and its injectors High particle density, small emittance -> high luminosity CAS Slovakia

slide-1
SLIDE 1

Beam Diagnostics for Ion Sources

CERN Accelerator School 2012 Uli Raich CERN BE/BI

1 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 2

The LHC and its injectors

2 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI High particle density, small emittance

  • > high luminosity
slide-3
SLIDE 3

CERN accelerator chain for Hadrons

3

  • Source:

up to 100 KeV

  • RFQ:

up to some MeV

  • Linac:

50 Mev – few GeV

  • Synchrotons: up to some TeV

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-4
SLIDE 4

Source and RFQ

4

Source and LEBT determine beam properties later in the accelerator chain Need to measure beam parameters before entering the RFQ

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 5

LEBT

  • Transport beam from the source to the RFQ

5 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-6
SLIDE 6

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire Harps and Wire Scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

6 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 7

LEBT Commissioning Stages

7

Source Source Source Source Emittance meter Emittance meter Emittance meter Faraday Cup

Faraday Cup + SEM Grid

Faraday Cup

  • Sol. 1

Sol. 1 Sol. 1 Sol. 2 Slit SEM grid Beam Current Transformer (BCT) Faraday Cup RFQ Input

1 2 3 4 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 8

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

8 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-9
SLIDE 9

Faraday Cup

9

  • Source intensity measured by a retractable Faraday

Cup

  • Secondary electron emission is suppressed by

polarization voltage which also eliminates parasitic electrons created in the source

  • Pneumatic in/out mechanism on PLC is used to

enter and retract the cup into/from the beam

  • Oscilloscope is used for signal observation
  • A ~ 1 MHz sampling ADC may be used to acquire

the Faraday Cup signal

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 10

Faraday Cup pieces

10 active electrode guard ring Faraday Cup body CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 11

Electro-static Field in Faraday Cup

11

In order to keep secondary electrons within the cup a repelling voltage is applied to the polarization electrode Since the electrons have energies of less than 20 eV some 100V repelling voltage is sufficient

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 12

Energy of secondary emission electrons

12

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0.1 1 10 100 1000

Itotal vs. eV

90keV 50keV 30keV I(µA) V

  • With increasing repelling voltage

the electrons do not escape the Faraday Cup any more and the current measured stays stable.

  • At 40V and above no decrease in

the Cup current is observed any more

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 13

Faraday Cup with water cooling

13

For higher intensities water cooling may be needed

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 14

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

14 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-15
SLIDE 15

Current Transformers

Beam current

l c qeN t qeN Ibeam β = =

Magnetic field ri

i r

r r lN L

2

ln 2π µ µ =

ro Fields are very low Capture magnetic field lines with cores

  • f high

relative permeability (CoFe based amorphous alloy Vitrovac: μr= 105) CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 15

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SLIDE 16

The ideal transformer

Beam signal Transformer output signal

dt dI L

beam

= U

Inductance L of the winding

droop

t beam

e N R t I t U

τ −

= ) ( ) (

RF RL CS A

s s rise

C L = τ

L droop

R R L + = τ

L L f droop

R L R A R L ≈ + = τ

Ls RL CS R

The passive AC transformer The active AC transformer

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 16

slide-17
SLIDE 17

Principle of a fast current transformer Image Current BEAM Calibration winding

  • 500MHz Bandwidth
  • Low droop (< 0.2%/µs)

Ceramic Gap

80nm Ti Coating ⇒20Ω to improve impedance

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 17

slide-18
SLIDE 18

Current Transformers

18 Good magnetic shielding avoids interference from nearby pulsing magnets Shielding simulation and test measurements have been done CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-19
SLIDE 19

Typical Transformer Signal

19 Calibration signal before after beam pulse Digitization of 400 µs pulse at 10 MHz Measures

  • total intensity
  • intensity per Booster ring

Background suppression by software CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 20

The DC current transformer AC current transformer can be extended to very long droop times but not to DC Measuring DC currents is needed on DC ion sources Must provide a modulation frequency Takes advantage of non/linear magnetisation curve

B H CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 20

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SLIDE 21

Principle of DCCT

beam

Compensation current Ifeedback=-Ibeam

modulator

V=RIbeam

Power supply

R Synchronous detector Va-Vb Vb Va CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 21

slide-22
SLIDE 22

Modulation of a DCCT without beam

Modulation current has only

  • dd harmonic frequencies

since the signal is symmetric

dt dB NA U =

B NA Udt B + = ∫

B=f(t) B H 1 2 5 3 4 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 22

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SLIDE 23

Modulation of a DCCT with beam

B=f(t) B H 1 1 2 5 3 4 Sum signal becomes non-zero Even harmonics appear CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 23

slide-24
SLIDE 24

Modulation current difference signal with beam

  • Difference signal has 2ωm
  • ωm typically 200 Hz – 10 kHz
  • Use low pass filter with

ωc<< ωm

  • Provide a 3rd core, normal

AC transformer to extend to higher frequencies

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 24

slide-25
SLIDE 25

Photo of DCCT internals

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 25

slide-26
SLIDE 26

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

26 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 27

SEMGrids for Profile Meas.

  • SEMGrid resolution: up to 0.5mm,

up to 36 wires

  • New analogue electronics for 36

under design

  • Needs time resolved

measurements (200 kHz)

  • New VME readout card has been

developed (36 channels), series of 50 cards have been produced

  • In/out mechanism by motor with

PLC control

27 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-28
SLIDE 28

Wire Scanners

28 Slowly drives the wire through the beam Measures wire position and collected current on the wire Reconstructs the beam profile CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-29
SLIDE 29

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

29 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-30
SLIDE 30

Ionisation Profile Monitor

  • An Ionization Profile

Monitor (IPM) measures beam profile by collecting rest gas molecules/electrons ionized by the beam.

  • The ions/electrons are

guided by electric field to MCP

  • Gas injection may be

needed to increase yield

  • Micro-channel plates age,

and need to be replaced.

  • P. Forck GSI
slide-31
SLIDE 31

Luminescence Monitor

  • Gas fluorescence monitor

measures light emitted by atoms/molecules excited by the beam.

  • Cross sections much lower than

for ionization

  • Light emittance isotropically.
  • What is the rest gas pressure?
  • F. Becker et al, GSI
slide-32
SLIDE 32

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

32 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-33
SLIDE 33

Emittance measurements

  • If for each beam particle we

plot its position and its transverse angle we get a particle distribution who’s boundary is an usually ellipse.

  • The projection onto the x axis

is the beam size

x’ x Beam size

33 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-34
SLIDE 34

The slit and grid method

  • If we place a slit into the beam we cut
  • ut a small vertical slice of phase

space

  • Converting the angles into position

through a drift space allows to reconstruct the angular distribution at the position defined by the slit

x’ x slit

34 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-35
SLIDE 35

Transforming angular distribution to profile

  • When moving through a

drift space the angles don’t change (horizontal move in phase space)

  • When moving through a

quadrupole the position does not change but the angle does (vertical move in phase space)

x’ x slit x’ x slit Influence of a drift space Influence of a quadrupole x’ x slit

35 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-36
SLIDE 36

The Slit Method

36 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-37
SLIDE 37

Emittance Meter

37

  • SEM grid read-out

250kHz

  • Stepping motors to allow

coarse + fine position tuning CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-38
SLIDE 38

Transverse Emittance Measurement

38 Slit and grid phase space scanner L-shaped 0.1mm slit moves under 45 degrees Slit and grids move independently Positioning precision: 50 µm Movement PLC controlled Slit and grids mounted in 2 independent vacuum boxes which can be separated Horizontal and vertical SEMGrid

  • wire distance .75 mm
  • 30 signal wires
  • readout with home built 36 channel

250 kHz ADC

  • time resolved profiles

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

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SLIDE 39

Emittance Evaluation

39 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-40
SLIDE 40

Pseudo Scubexx evaluation

40 Histogram of signal levels Background for each slit Position Emittance Plot Emittance when taking less and less channels around peak CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-41
SLIDE 41

Emittance plot Solenoid

41 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-42
SLIDE 42

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

42 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-43
SLIDE 43

Allison Scanner

The whole detector is passed through the beam Slit defines position Deflection plates with ramped electric field determine particle angles Angle distribution is measured with a Faraday Cup CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 43

  • M. Stöckli, ORNL
slide-44
SLIDE 44

Allison Scanner results SNS

Apply 10% threshold to get rid

  • f background

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 44

slide-45
SLIDE 45

Emittance results along time axis

  • First plot: no beam yet
  • Big changes during the first 2 time

slices (source plasma not stabilized yet)

  • Then only small changes
  • Last time slice: Big change due to

decaying plasma when RF is switched

  • ff.

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 45

slide-46
SLIDE 46

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

46 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-47
SLIDE 47

Pepperpot Emittance Measurement

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 47 Pepperpot: 15x15 holes on copper plate Luminescent screen Data acquisition: high resolution CCD Example from GSI Darmstadt

Advantage: Single shot measurement

  • P. Forck GSI
slide-48
SLIDE 48

Pepperpot Results

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 48 Needs calibration of the screen to determine Orientation of the emittance ellipse

slide-49
SLIDE 49

Parameters to be measured

  • Beam Intensity

– Faraday Cup (destructive) – Transformer (non destructive)

  • Transverse Profile

– Wire harps and scanners – Residual Gas Monitors

  • Transverse Phase space

– Slit/Grid device – Allison Scanner – Pepperpot

  • Energy and Energy Spread

– Spectrometer

49 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-50
SLIDE 50

Energy Spread

  • Slit: reduces space charge

effects and beam divergence

  • Slit and wire grid are positioned

at focal points of the optics

  • Calibration by modification of

the source extraction voltage (50 eV/mm)

  • Profile width is determined by

energy spread

50 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-51
SLIDE 51

Setup for charge state measurement

  • The spectrometer

magnet is swept and the current passing the slit is measured

beam Ion source

Scan of Bending magnet Current with extraction voltage 20.5kV - 11/04/03 -JCh

  • 0.05

0.05 0.1 0.15 60 65 70 75 80 85 90 95 100 Bending Magnet Current (A) Average Current from Fararday cup 2 (mA)

Faraday Cup Spectrometer Magnet

51 CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-52
SLIDE 52

Select Charge States

52 Faraday Cup Slit Spectrometer Magnet Charge States measured at the FaradayCup when ramping the spectrometer magnet CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI

slide-53
SLIDE 53

Conclusions

  • Beam diagnostics tells you how well your ion source

performs

  • Needed to understand LEBT optics to adapt source

beam to RFQ characteristics

  • Typical measurements:

– Beam current and total intensity (no of charges) – Current stability over the beam pulse – Transverse Profile – Longitudinal Profile – Transverse emittance

CAS Slovakia 25.5 - 8.6. 2012 Uli Raich CERN BE/BI 53