LCLS Diagnostics and Commissioning Workshop

1
LCLS Diagnostics and Commissioning Workshop

• Injector, Linac and Undulator Diagnostics and
  Beam Position Monitors
• P. Krejcik

2
Context of Diagnostics in Commissioning

• Review scope of proposed diagnostics
• Emphasize that diagnostics themselves need
  commissioning
• Consider if full features (resolution,
  automation) are needed at beginning of
  commissioning
• Implicit sequence of commissioning e.g.
  feedbacks after BPMs commissioned slice
  parameters need prof. monitors and TCAVs

3
Readiness of diagnostic systems

• Which SLC diagnostics should be preserved?
• Technology choices still being made on some new
  systems
• BPM modules trying to attain desired resolution
• Prof monitor cameras resolution, controls
  integration, data rate
• Some diagnostics are turnkey systems, others are
  RD projects
• RD still required for ultrafast diagnostics
• CSR THz power bunch length monitors
• EO bunch profiling

4
Dynamic Aspects of Commissioning

• Initially diagnose a wildly mis-tuned and
  unstable machine
• Yet the same diagnostics should ultimately have
  finesse to optimize SASE operation
• Deal with imperfect and uncalibrated settings
• Detective work for finding hardware faults
• Quantify magnitude and sources of jitter

5
Diagnostics Roadmap for electrons

• Setup
• Tuning

• Beam size
• resolution
• Emittance
• Energy spread

Bunch charge
F E E D B A C K

• Trajectory
• resolution
• Position
• Angle
• Energy

• Slice parameters
• resolution
• Emittance
• Energy spread

• Bunch length DT
• development
• Longit. profile
• Single shot rms

Noninvasive
Invasive

• Stabilization
• response
• Jitter characterization

120 Hz
6
Accelerator System Diagnostics

• 180 BPMs at quadrupoles and in each bend system

• 8 Energy (BPM) ?E?, energy spread (Prof) sE
  measurements

• 2 Transverse RF deflecting Cavities for slice
  measurements

• 5 Bunch length monitors

RF gun
L0
L3
L1
X
L2
upstream linac
BC1
BC2
DL1
DL2
undulator
LTU
Dump E, DE
7
Beam Position Monitoring requirements
8
Beam Position Monitors

• Stripline BPMs in the injector and linac
  (existing) and in the LTU
• Differencing large numbers
• Mechanical precision
• Fabrication by printing electrodes on ceramic
  tubes
• Drift in electronics
• Digital signal processing
• Cavity BPMs in the undulator, LTU launch
• Signal inherently zero at geometric center
• C-band (inexpensive) signal needs to be mixed
  down in the tunnel

9
Stripline versus Cavity BPM Signals

• noise (resolution) minimized by removing analog
  devices in front of ADC that cause attenuation
• drift minimized by removing active devices in
  front of ADC

10
Simplistic View of Digital BPMs

• Is the purely digital approach the best way to
  go?
• Must always maximize signal to noise for best
  resolution
• So eliminate any cause of attenuation couplers,
  hybrids, active devices etc.
• This also eliminates drift which causes offsets
• Other approaches also try to do this e.g. AM to
  PM conversion with a hybrid and then digitize
• Might as well digitize first, eliminate the
  middle men, and do the conversions digitally
• Ultimately left with calibrating the drift in the
  BPM cables, because ADCs are now very stable.

11
Linac stripline BPMs

• Need to replace old BPM electronics
• Commercially available processing units look
  promising
• Beam testing of module as soon as funding
  available
• Test new BPM fabrication techniques

http//www.i-tech.si
12
Analysis of Test Signals in the Libera module
S. Smith

• Measured signal to noise ratio implies resolution
  of 7 mm in a 10 mm radius BPM
• Identified fixable artifacts in data processing

13
Cavity beam position monitors for the undulator
and LTU
RD at SLAC S. Smith
Coordinate measuring machine verification of
cavity interior

• X-band cavity shown
• Dipole-mode couplers

• X-band cavity shown
• Dipole-mode couplers

NLC studies of cavity BPMs, S. Smith et al
14
C-band beam tests of the cavity BPM S. Smith
cavity BPM signal versus predicted position at
bunch charge 1.6 nC
25 mm

• Raw digitizer records from beam measurements at
  ATF

200 nm

• plot of residual deviation from linear response
• ltlt 1 mm LCLS resolution requirement

• C-band chosen for compatibility with wireless
  communications technology

15
LCLS BPM Testing

• Testing is planned at the Controls Test Stand
  to be located at the FFTB, 2005.
• Evaluation of processor electronics
• Resolution determined by comparing several
  adjacet BPMs
• Possibility to test new striplines
• Copy the design of NLC C-band cavity BPMS

16
Beam Size Measurement

• Wire scanners, based on existing SLAC systems
• Measures average projected emittance
• But is minimally invasive and can be automated
  for regular monitoring
• Profile monitors
• Single shot, full transverse profile
• YAG screen in the injector for greater intensity
• OTR screens in the linac and LTU for high
  resolution
• 1 mm foils successfully tested in the SPPS
• Small emittance increase disrupts FEL,
• but no beam loss
• -11 imaging optics gt 9 mm resolution
• Used in combination with TCAV
• for slice energy spread and emittance
• CTR for bunch length measurement

OTR image taken in the SPPS Courtesy M. Hogan, P.
Muggli et al
17
Profile Monitor Camera Specification

• Digital camera technology
• Not TV camera that subsequently needs a frame
  grabber
• External trigger supplied to the camera by
  control system
• 30 fps at 1280x960 pixels, 10 bit resolution
• Digital image read out over ethernet or firewire
• Inexpensive, commercially available 1k Z.
  Salata.

18
Profile Monitor Camera Dynamic Range

• How many bits are necessary to see the tails?

saturation
3s needs 10 bits
4s needs 12 bits
19
Profile monitor commissioning

• Can be tested off the beamline at the Controls
  Test Stand
• Evaluate data acquisition and integration into
  the control system
• test a complete optical setup and measure
  optical resolution and wavelength response

20
Bunch length diagnostic comparison
21
Bunch Length Measurements with the RF Transverse
Deflecting Cavity
30 MW
2.4 m
22
Commissioning of the Transverse Cavities

• Calibration of the deflection strength in units
  of pixels on the profile monitor
• Also requires beam trajectory feedback to
  stabilize the RF phase of the deflecting cavity
• Prof monitor image acquisition fully integrated
  into the control system

23
Calibration scan for RF transverse deflecting
cavity

• Bunch length calibrated in units of the
  wavelength of the S-band RF

• Further requirements for LCLS
• High resolution OTR screen
• Wide angle, linear view optics

24
OTR Profile Monitor in combination withRF
Transverse Deflecting Cavity- detailed
applications in P. Emma talk
Simulated digitized video image Injector DL1
beam line is shown Best resolution for slice
energy spread measurement would be in adjacent
spectrometer beam line.
25
Coherent radiation from the electron bunch

• Frequency domain
• Spectral power in a fixed bandwidth
• Spectrometry
• Autocorrelation
• Time domain
• Electro optic sampling
• Measured directly near the bunch
• Or transported out of the beam line

26
Diagnosing Coherent Radiation1. spectral power
Smooth Gaussian bunch spectrum from BC1
- J. Wu

• Measure bunch length
• Detect microbunching

With 5 microbunching
27
BC2 Bunch length monitor spectrum - based on
coherent spectral power detection
BC2 bunch length feedback requires THz CSR
detector Demonstrated with CTR at SPPS Spikes
in the distribution now have same spectral
signature as microbunching
4 THz main peak
28
Diagnosing Coherent Radiation2. autocorrelation
sz ? 9 mm
Limited by long wavelength cutoff and absorption
resonances
Transition radiation is coherent at wavelengths
longer than the bunch length, lgt(2p)1/2 sz
SLAC SPPS measurement P. Muggli, M. Hogan
29
Transport issues for THz radiation
Simple model Gaussian, sz20 µm, d12.7 µm, n3
Mylar windowsplitter
Modulation/dips in the interferogram

• Fabry-Perot resonance l2d/m, m1,2,
• Signal attenuated by Mylar (RT)2 per sheet

Smaller measured width sAutocorrelation lt
sbunch !
P. Muggli, M. Hogan
30
Developments in autocorrelation techniques

• Investigate other detector types for wavelength
  dependance
• Golay cell
• Beam splitters without wavelength dependance
• Single shot autocorrelator
• Camera records fringes on single shot
• Use CSR from chicane bed

31
Bunch length scan performed while observing
spectral power with THz detector
Comparison of bunch length minimized according to
wakefield loss and THz power
foil
Wake energy loss
LINAC
Linac phase
THz power
FFTB
Coherent transition radiation wavelength
comparable to bunch length
Pyroelectric detector
GADC
32
Dither feedback control of bunch length
minimization at SPPS - L. Hendrickson
Bunch length monitor response
Feedback correction signal
ping
optimum
Linac phase
Jitter in bunch length signal over 10 seconds
10 rms
Dither time steps of 10 seconds
33
Diagnosing Longitudinal phase spaceEnergy
spectrum versus Bunch length signal
- Muggli, Hogan et al
Jitter in the compressor phase
Resuting energy profile
Corresponding bunch length signal
jitter
signal
Single shot measurements
34
SPPS Four Dipole Chicane
9 GeV
Momentum compactionR56 75 mm
LB1.80 m B1.60 T
s
Linac chirp
LT14.3 m
SR background
35
Measured and predicted energy spread from
wakefield chirp in SPPS
Special setup to give 100 mm bunch length with
more charge at the head of the bunch
Measured at end of linac
36

• Wakefields change not only the energy spread in
  the bunch
• But also the centroid energy of the bunch
• Fast means of determining relative bunch length

37
Relative bunch length measurementbased on
wakefield energy loss scan
Energy change measured at the end of the linac
as a function of the linac phase (chirp) upstream
of the compressor chicane
Shortest bunch has greatest energy loss
Predicted wakeloss___ For bunch length s z __
38
Coherent radiation from the electron bunch

• Frequency domain
• Spectral power
• Spectrometry
• Autocorrelation
• Time domain
• Electro optic sampling
• Measured directly near the bunch
• Or transported out of the beam line

39
SPPS Electro Optic Bunch Length Measurement with
in-vacuum crystal
Geometry chosen to measure direct electric field
from bunch, not wakefield Modelled by H. Schlarb
40
Features of the SPPS Electro Optic Setup

• Compressed pulse from the users pump-probe TiSa
  laser oscillator
• Transported low power pulse over 150 m fiber to
  the electron beam line
• OTR provides coarse timing

Fiber launch
TiSa oscillator
Stretcher
Shaper
150 m fiber
EO xtl
p
imaging optics
e-
polarizing beamsplitter
s
OTR
41
Features of the SPPS Electro Optic Setup

• Fiber incorporated in pulse compression setup
  including compensating fiber dispersion with a
  spatial light modulator
• Cavalieri et al, FOCUS Group U. Michigan

Grating pair
To fiber
From stretcher
42
Features of the SPPS Electro Optic Setup

• Crystal mounted close to electron beam
• Avoid wakefields from smaller apertures
• ZnTe crystal
• 200 um thick
• EO coefficient,
• phase match,
• phonon resonances

43
Electro-Optical Sampling at SPPS A. Cavalieri
et al.
EO crystal
Line image camera
Pol. Laser pulse
analyzer
polarizer
Er
Electron bunch
44
Electro optic resolution limits

• Spatial imaging resolution limits time resolution
• Crossing angle determines width of time window
  and temporal resolution
• Resolution limit then set by crystal thickness
  and the phase velocity mismatch
• Crystal material chosen to minimize phase mismatch

45
Electro optic resolution limits

• Future experiments
• Smaller crossing angle
• Smaller angle magnifies time coordinate on
  spatial axis
• But reduces the time window to accommodate beam
  jitter
• EO polymer films
• Strong EO coefficient
• May not last long
• Higher laser power cross correlation techniques
  (Jamison et al)
• Laser amplifier located near beamline

46
Synchronization of the Laser timing

• Jitter in the laser timing effects
• Electro optic bunch timing measurement
• Pump-probe timing for the users
• Enhancement schemes using short pulse lasers

47
SPPS Laser Phase Noise Measurements R. Akre
476 MHz M.O.
fiber 1 km
MDL 3 km
TiSa laser osc
EO
VCO
diode
2856 MHz
x6
Phase detector
2856 MHz to linac
scope
48
Energy and Bunch Length Feedback Loops
4 energy feedback loops 2 bunch length feedback
loops 120 Hz nominal operation, lt1 pulse
delay Progressive commissioning schedule
49
Closed Loop Response of Orbit Feedback

• Undulator trajectory launch loop to operate at
  120 Hz, lt1 pulse delay
• Damps jitter below 10 Hz
• i.e. need stability above 10 Hz!
• At lower rep. rates, less damping
• Linac orbit loops to operate at 10 Hz because of
  corrector response time

Antidamp Damp
Gain bandwidth shown for different loop delays -
L. Hendrickson
50
Remaining intra-undulator diagnostics from
Bingxin Yang, Lehman Review August 04

• Location every long break (905 mm)
• Diagnostics chamber length 425 mm

• Functional components
• RF BPM, Cherenkov detector, OTR profiler, wire
  scanner, x-ray (intensity) diagnostics

51
FY04 accomplishments from Bingxin Yang, Lehman
Review August 04

• Layout of diagnostics chamber
• OTR profiler
• Camera module designed
• Wire scanner
• Scanner design in progress
• Wire card adapt SLAC design
• X-ray diagnostics design
• Beam intensity double crystal
• Beam profile imaging detector

52
Major issues at UCLA workshop from Bingxin
Yang, Lehman Review August 04

• Beam damage of optical components
• Example from Marc Ross coupon test, LINAC 2000
• Saturated FEL beam deposit higher energy density

• Desirable information
• Trajectory accuracy (Dx1mm)
• Effective K (DK/K 1.510-4)
• Relative phase (Df10º)
• Intensity gain (DE/E0.1, z-)
• Undulator field quality

53
Rethink x-ray diagnostics (Galayda) from
Bingxin Yang, Lehman Review August 04

• Intra-undulator diagnostics
• Electron beam position monitor (BPM)
• Electron beam profiler (OTR wire scanner)
• Low power x-ray Intensity measurements (RD)
• Beam loss Monitor
• Far-field low-power x-ray diagnostics (RD)
• Clean signature from spontaneous radiation
• Space for larger optics / detectors
• Single set advantage (consistency, lower cost)
• Goal obtain desirable information

54
Final Beam Dump

• Sensitive measurement of beam energy
• Optimized for energy spread resolution of 410-5
  (P.Emma)
• Bends smear out microbunching
• Dispersion hides emittance measurement
• Might be possible in the vertical plane

55
Summary

• Diagnostics integrated into the LCLS design
• All systems require commissioning time to achieve
  LCLS resolution requirements
• New diagnostics still require RD for bunch
  length and timing
• Developmental work at SPPS is critical
• Diagnostics being developed hand-in-hand with
  controls and feedbacks

56
Appendix