LCLS Diagnostics and Commissioning
Workshop
Injector, Linac and Undulator Diagnostics
and Beam Position Monitors
P. Krejcik
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
R&D projects
R&D still required for ultrafast diagnostics
CSR THz power bunch length monitors
EO bunch profiling
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Diagnostics Roadmap for electrons
Bunch charge
Trajectory
resolution
•Position
•Angle
•Energy
• Setup
•Tuning
Beam size
resolution
F
E
E
D
B
A
C
K
•Emittance
•Energy spread
Slice parameters
resolution
Bunch length & DT
development
•Longit. profile
•Single shot rms
Noninvasive
120 Hz
• Emittance
• Energy spread
Invasive
Stabilization
response
•Jitter characterization
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Accelerator System Diagnostics*
180 BPMs at quadrupoles and in each bend system
8 Energy (BPM) E, energy spread (Prof) sE measurements :
5 Emittance gex,y measurements (Profs, Wire Scanners) :
2 Transverse RF deflecting Cavities for slice measurements
5 Bunch length monitors
RF
gun
* See also P. Emma talk how
optics is optimized for diagnostics
3 prof. mon.’s
(Dyx,y = 60°)
upstream linac
L1
DL1
L2
X
BC1
L3
BC2
DL2
undulator
LTU
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Dump
E, DE
Beam Position Monitoring requirements
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Stripline versus Cavity BPM Signals
P
l/4
Stripline
f
700 MHz
ADC
x4
RF in
l
500 MHz
BP filter
C-band
cavity
~5 GHz
Dipole
mode
coupler
Mixer
LO
sync’ed to RF
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
IF
Digital
processing
Control
system
119 MHz
Clock
24th harmonic
• noise (resolution) minimized
by removing analog devices in
front of ADC that cause
attenuation
• drift minimized by removing
active devices in front of ADC
P. Krejcik
pkr@slac.stanford.edu
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.
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Cavity beam position monitors for the undulator and LTU
R&D at SLAC – S. Smith
NLC studies of cavity
BPMs, S. Smith et al
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
Coordinate measuring
machine verification of cavity
interior
• X-band cavity
shown
• Dipole-mode
couplers
P. Krejcik
pkr@slac.stanford.edu
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
• C-band chosen for
compatibility with wireless
communications technology
200 nm
• plot of residual
deviation from linear
response
• << 1 mm LCLS resolution
requirement
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
-1:1 imaging optics => ~ 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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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.
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Profile Monitor Camera Dynamic Range
How many bits are necessary to see the tails?
saturation
3s needs 10 bits
4s needs 12 bits
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Bunch length diagnostic comparison
Device Type
Invasive
Single shot
measurement measurement
Abs. or rel.
Timing
Detect
measurement measurement m bunching
Yes: Steal 3
pulses
No: 3 pulses
Absolute
No
No
No for CSR
Yes for CTR
Yes
Relative
No
Yes
Coherent
No for CSR
Radiation
Yes for CTR
Autocorrelation
No
Absolute
No
No
Electro Optic
Sampling
No
Yes
Absolute
Yes
No
Energy
Wake-loss
Yes
No
Relative
No
No
RF Transverse
Deflecting
Cavity
Coherent
Radiation
Spectral power
(2nd moment
only)
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Bunch Length Measurements with the RF Transverse Deflecting Cavity
2.4 m
30 MW
Bunch length reconstruction
s y Measure streak at 3 different phases
X10
Y = A * (X - B)**2 + C
A =
1.6696E-02
STD DEV =
1.3536E-03
B =
28.23
STD DEV =
3.084
C =
1328.
STD DEV =
8.235
RMS FIT ERROR
=
23.63
(Streak size)2
1.7
sz = 90 mm
Cavity on
*
1.6
Cavity on
- 180°
*
*
*
1.5
Cavity off
*
*
*
*
*
E
*
*
1.4
*
*
E
*
*
1.3
-80
MANUAL STEPPING.
-40
0
40
SBST LI29 1 PDES (S-29-1)
STEPS =
80
30
1-APR-03 20:21:16
0
180
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
Asymmetric parabola indicates
incoming tilt to beam
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Calibration scan for RF transverse deflecting cavity
•
Beam
centroid
[pixels]
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
Cavity phase [deg. S-Band]
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
OTR Profile Monitor in combination with
RF 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.
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Diagnosing Coherent Radiation
1. spectral power
Smooth Gaussian bunch
spectrum from BC1
Dl
Fixed BW detector
Signal prop. 1/sz
s z  190 mm
- J. Wu
Bunch length signal
for RF feedback
• Measure
bunch length
With 5%
microbunching
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
• Detect
microbunching
P. Krejcik
pkr@slac.stanford.edu
BC2 Bunch length monitor spectrum
- based on coherent spectral power detection
BC2 bunch length feedback
requires THz CSR detector
Demonstrated with CTR at
SPPS
4 THz main peak
Spikes in the distribution
now have same spectral
signature as microbunching
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Diagnosing Coherent Radiation
2. autocorrelation
1.6
sz  9 mm
1.2
0.8
0.4
0
-100
Transition radiation is coherent at
wavelengths longer than the bunch length,
l>(2p)1/2 sz
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
CombinedCTRInterferogramsSm
-50
0
50
100
Limited by long wavelength cutoff
and absorption resonances
SLAC SPPS measurement:
P. Muggli, M. Hogan
P. Krejcik
pkr@slac.stanford.edu
Transport issues for THz radiation
Simple model: Gaussian, sz=20 µm, d=12.7 µm, n=3 Mylar window+splitter
100
0.1
10-1
0.001
10-5
10-7
10-2
10-9
10-11
Mylar
resonances
-13
10
10-15
10-17
10-3
Filter Amplitue (a.u.)
Power Spectrum (a.u.)
10
10-4
1000
CTRFSpecSigmaz20My lar12.5_3
10
100
Wavelength (µm)
• Fabry-Perot resonance: l=2d/m, m=1,2,…
• Modulation/dips in the interferogram
• Signal attenuated by Mylar: (RT)2 per sheet
• Smaller measured width:
P. Muggli, M. Hogan
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
sAutocorrelation < sbunch !
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
Coherent transition radiation
wavelength comparable
to bunch length
FFTB
Pyroelectric
detector
GADC
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Dither feedback control of bunch length
minimization at SPPS - L. Hendrickson
Bunch length monitor
response
Feedback correction
signal
“ping”
optimum
Dither time steps of 10
seconds
Linac phase
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
Jitter in bunch length signal
over 10 seconds ~10% rms
P. Krejcik
pkr@slac.stanford.edu
Diagnosing Longitudinal phase space:
Energy spectrum versus Bunch length signal
- Muggli, Hogan et al
Jitter in the compressor phase:
Resuting energy profile
Corresponding bunch length signal
jitter
Single shot measurements
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
signal
P. Krejcik
pkr@slac.stanford.edu
9 GeV
SPPS Four Dipole Chicane
Momentum
compaction
R56= –75 mm
LB=1.80 m
B=1.60 T
BPM - energy
Prof. Monitor - DE
s
Linac chirp
Measured energy spread
LT=14.3 m
s
1.6%
Correlated
sz0
September 22-23, 2004 energy
1.2 mm
LCLS Diagnostics and Commissioning
Workshop
spread
sz
SR background
50 mm
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
MeasuredP. at
end of linac
Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Relative bunch length measurement
based 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
Predicted shape
due to wakeloss
plus RF curvature
P. Emma, K. Bane
Shortest bunch has
greatest energy loss
Predicted wakeloss___
For bunch length s z __
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
SPPS Electro Optic Bunch Length Measurement with
in-vacuum crystal
Defining
aperture
M1
EO
xtal
M2
Beam axis
Geometry chosen to
measure direct
electric field from
bunch, not wakefield
Modelled by H. Schlarb
Probe
laser
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Features of the SPPS Electro Optic Setup
Compressed pulse from the users pump-probe Ti:Sa laser
oscillator
Transported low power pulse over ~150 m fiber to the
electron beam line
OTR provides coarse timing
Ti:Sa
oscillator
Stretcher
Shaper
Fiber
launch
~150 m
fiber
EO xtl
e-
p
polarizing
beamsplitter
s
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
imaging
optics
OTR
P. Krejcik
pkr@slac.stanford.edu
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
SLM 640-pixel
f
f
From
stretcher
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
f
f
To
fiber
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Electro-Optical Sampling at SPPS – A. Cavalieri et al.
Single-Shot
Bunch length scan
<300 fs
EO crystal
Line image
camera
analyzer
Pol. Laser pulse
Timing Jitter
170 fs rms
polarizer
Er
Electron bunch
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
New chamber
T. Montagne
Ti:Sapphire
laser
e-
EO polymer films
Strong EO coefficient
May not last long
Higher laser power cross correlation
techniques (Jamison et al)
200 mm thick ZnTe crystal
Laser amplifier located near beamline
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
SPPS Laser Phase Noise Measurements
476 MHz
M.O.
MDL
3 km
fiber
~1 km
Ti:Sa
laser osc
VCO
x6
2856 MHz
2856 MHz
to linac
EO
diode
Phase
detector
scope
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
– R. Akre
P. Krejcik
pkr@slac.stanford.edu
Energy and Bunch Length Feedback Loops
E
Vrf(L1)
Φrf(L2)
E
Φrf(L3)
E
DL1
Vrf(L0)
L0
E
Φrf(L1) sz
DL1
Spectr
.
L1
sz
Φrf(L2)
BC2
BC1
L2
L3
BSY
50B1
DL2
4 energy feedback loops
2 bunch length feedback loops
120 Hz nominal operation, <1 pulse delay
Progressive commissioning schedule
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Closed Loop Response of Orbit Feedback
Undulator trajectory launch loop
to operate at 120 Hz, <1 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
Gain bandwidth
shown for different
loop delays
Antidamp
Damp
- L. Hendrickson
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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 (Dx~1mm)
Effective K (DK/K ~ 1.5×10-4)
Relative phase (Df~10º)
Intensity gain (DE/E~0.1%, z-)
Undulator field quality
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
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 (R&D)
Beam loss Monitor
Far-field low-power x-ray diagnostics (R&D)
Clean signature from spontaneous radiation
Space for larger optics / detectors
Single set advantage (consistency, lower cost)
Goal = obtain “desirable information”
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Final Beam Dump
Sensitive measurement of beam energy
Optimized for energy spread resolution of
4*10-5 (P.Emma)
Bends smear out microbunching
Dispersion hides emittance measurement
Might be possible in the vertical plane
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Summary
Diagnostics integrated into the LCLS design
All systems require commissioning time to
achieve LCLS resolution requirements
New diagnostics still require R&D for bunch
length and timing
Developmental work at SPPS is critical
Diagnostics being developed hand-in-hand
with controls and feedbacks
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu
Appendix
September 22-23, 2004
LCLS Diagnostics and Commissioning Workshop
P. Krejcik
pkr@slac.stanford.edu