Electron beam diagnostic methods

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Electron beam diagnostic methods
William S. Graves MIT-Bates Laboratory
Presented at 2003 ICFA S2E Workshop DESY-Zeuthen A
ugust, 2003
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Experimental Methods

• Hardware/software controls.
• Thermal emittance measurement using solenoid
  scan.
• Cross-correlation of UV photoinjector drive laser
  with 100 fs IR oscillator.
• Streak camera time resolution
• Electron beam longitudinal distribution measured
  using RF zero phasing method.
• Slice transverse parameters are measured by
  combining RF zero phase with quadrupole scan.
• Will not address undulator diagnostics. See
  Shaftan, Loos, Doyuran.

Enables injector optimization and code
benchmarking.
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DUVFEL Facility at BNL
1.6 cell gun with copper cathode
Coherent IR diagnostics
NISUS 10m undulator
Bunch compressor with post accel.
70 MeV
Bend
Bend
Undulators
Linac tanks
5 MeV
Triplet
Triplet
70-200 MeV
RF zero phase screen
Dump
Dump
30 mJ, 100 fs TiSapphire laser
Photoinjector 1.6 cell BNL/SLAC/UCLA with copper
cathode 4 SLAC s-band 3 m linac sections Bunch
compressor between L2 and L3 Approximately 60 CCD
cameras on YAG screens.
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Control system
Automated measurements very important for
gathering large amounts of data and repeating
studies.
All sophisticated control is done in the MATLAB
environment on a PC. Physicists quickly
integrate hardware control with data
analysis. Low level control is EPICS on a SUN and
VME crates.
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Thermal Emittance
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Projected emittance vs charge and FWHM
HOMDYN simulations estimate limits on maximum
bunch length and charge. Choose working
parameters of 2 pC, 2 ps FWHM.
Simulation
Simulation
YAGCe screen very useful for low charge, high
resolution profiles. Screen thickness, surface
quality, multiple reflections, and camera lens
depth-of-focus and resolution are all important
issues.
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• Can measure
• charge
• energy
• x and y centroid
• x and y beamsize
• px and py

Solenoid Scan Layout
65 cm
12 cm
33 cm
YAG screen
Mirror
Dipole trim
Telecentric lens magnif. 1
1.6 cell photoinjector
Solenoid
CCD Camera

• YAGCe screen very useful for low charge, high
  resolution profiles.
• Screen thickness, surface quality, multiple
  reflections, and camera lens depth-of-focus and
  resolution are all important issues. See SLAC
  GTF data.

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Low Energy Beam Measurements
pixels

• Video processing
• 3x3 median filter applied.
• Dark current image subtracted.
• Pixels lt few of peak are zeroed.

Error estimates Monte Carlo method using measured
beam size jitter.
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Emittance vs laser size
Emittance shows expected linear dependence on
spot size. Small asymmetry is always present.
FWHM 2.6 ps Charge 2.0 pC Gradient 85 MV/m RF
phase 30 degrees
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Beam size and divergence vs laser spot size
High
Size
Divergence
High
Low
Low
Error bars are measured data. Blue lines are from
HOMDYN simulation using RF fields from SUPERFISH
model and measured solenoid B-field. Upper blue
line has 1/2 cell field 10 higher than full
cell. Lower blue line has 1/2 cell field 10
lower than full cell.
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Emittance vs RF phase
Error bars are measured data points. Curve is
nonlinear least squares fit with ßrf and Fcu as
parameters ßrf 3.10 /- 0.49 and Fcu 4.73
/- 0.04 eV. The fit provides a second estimate
of the electron kinetic energy Ek 0.40 eV, in
close agreement with the estimate from the radial
dependence of emittance.
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Time profile of UV laser pulse
Cross-correlaton difference frequency generation
experiment by B. Sheehy and H. Loos
Phase matching angle of harmonic generation
crystals used to produce UV affects time and
spatial modulations.
Note Sub-ps streak camera is inadequate for
this measurement
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Laser masking of cathode image
Above Laser cathode image with mask removed
showing smooth profile. Below Resulting
electron beam showing hot spot of emission.
Above Laser cathode image of air force mask in
laser room. Below Resulting electron beam at
pop 2.
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Streak Camera

• Hammamatsu FESCA 500
• 765 fs FWHM measured resolution
• Reflective input optics (200-1600 nm)
• Wide response cathode (200-900 nm)
• Optical trigger (lt500 fs jitter)
• Designed for synchroscan use. Also good
  single-shot resolution.
• 6 time ranges 50 ps - 6 ns

50 ps window
Streak image
Very helpful for commissioning activies and for
timing several optical signals. Limited time
resolution below 1 ps.
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Streak camera time profiles of laser pulses
360
340
320
300
280
signal (arb units)
260
240
220
200
180
160
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streak delay (picoseconds)
Amplified IR 796 nm FWHM 1.01 ps
single shot
UV 266 nm FWHM 2.40 ps singleshot
Oscillator 796 nm FWHM 765 fs
Time resolution depends on photon energy
energetic UV photons create photoelectrons with
energy spread that degrades time resolution
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RF zero-phase time profile
L3 phase 90, amp. set to remove chirp
L4 phase /-90, amp. set to add known chirp
L2 phase varies, amp. fixed
L1 phase 0, amp. fixed
Chicane varies from 0 cm lt R56 lt 10.5 cm
L1
L2
L3
L4
Pop 14 YAG screen
YAG images at pop 14
L3 corrects residual chirp, L4 is off
L4 phase -90 degrees
L4 phase 90 degrees
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Quad scan during RF zero phase
Movie of slice emittance measurement.
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Right side Circle is matched, normalized phase
space area at upstream location. Ellipse is phase
space area of slice at same location. Straight
lines are error bars of data points projected to
same location.
Left side Beam size squared vs quadrupole
strength. Each plot is a different time slice of
beam.
Collaboration with Dowell, Emma, Limborg, Piot
Software is used to time-slice beam.
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Slice emittance and Twiss parameters
z is parameter that characterizes mismatch
between target and each slice. z ½ (b0 g 2 a0
a b g0) a0, b0, g0 are target Twiss param. a,
b, g are slice Twiss param.
Beam Parameters 200 pC, 75 MeV, 400 fs slice
width
Note strongly divergent beam due to solenoid
overfocusing at tail, where current is low. Space
charge forces near cathode caused very different
betatron phase advances for different parts of
beam.
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Different slices require different solenoid
strength
Current projection
Head
Tail
Head
Tail
Head
Tail
Time
Head
Tail
Head
Tail
Head
Tail
Vertical dynamics Lattice is set to image end of
Tank 2 to RF-zero phasing YAG. Particles in tail
of beam are diverging, and in head
converging. Head has higher current and so
reaches waist at higher solenoid setting.
Increasing solenoid current
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Solenoid 98 A
Slice emittance vs solenoid strength. Charge
200 pC.
Data
Parmela
Projected Values (parmela in parentheses)
Solenoid 108 A
Solenoid 104 A
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Slice parameters vs charge
10 pC
50 pC
Low charge cases show low slice emittance and
little phase space twist.
200 pC
100 pC
High charge cases demonstrate both slice
emittance growth and phase space distortion.
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Longitudinal structure
Analysis of RF zero phasing data can be
complicated by modulations in energy plane. See
contribution from T. Shaftan for detailed
description.
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RF zero phasing vs RF deflectors

• RF zero phasing
• Method uses accelerating mode to streak the
  beam by increasing the energy spread (chirping).
• Uncorrelated energy spread is 5 keV and coherent
  modulations can be 20 keV. Streak chirp must be
  much larger than this.
• Time and energy axes are difficult to
  disentangle.
• RF deflector cavity
• Transverse momentum is 1 eV/c. Relatively
  small deflecting field gives excellent time
  resolution.
• Less sensitive to coherent energy modulations.
• Can obtain simultaneous time/energy/transverse
  beam properties when combined with dipole in
  other plane.

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Concluding Remarks

• Experience seems to indicate that most
  differences between experiment and simulation are
  due to experimental inaccuracies.
• Beam can be used to diagnose many
  hardware/applied field difficulties.
• Easy to use control system integrating realtime
  analysis and hardware/beam control very
  important.
• With adequate diagnostics, meeting beam quality
  and FEL performance goals is straightforward.
• See work by Shaftan, Loos, Doyuran of BNL on
  undulator diagnostics and trajectory analysis.