1. Breakout Sessions SC1/SC2 – Accelerator Physics
E-Beam Diagnostics Overview
P. Krejcik

2. Diagnostics Roadmap Setup Beam size Bunch charge Tuning F resolution EK
Beam size
resolution
Emittance
Energy spread
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

3. Recent Workshop on Diagnostics and Timing:
ICFA Future Light Sources Subpanel Miniworkshop on XFEL Short Bunch Measurement and Timing Stanford Linear Accelerator Center, July , 2004

4. 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
3 prof. mon.’s
(Dyx,y = 60°)
* See also P. Emma talk how
optics is optimized for diagnostics RF
gun L0
upstream linac L1 X L2 L3
DL1
BC1
BC2
DL2
undulator
LTU
Dump

5. Beam Position Monitoring requirements

6. 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

7. Cavity beam position monitors for the undulator and LTU
R&D 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

8. 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
<< 1 mm LCLS resolution requirement
C-band chosen for compatibility with wireless communications technology

9. 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

10. Bunch length diagnostic comparison
Device Type
Invasive
measurement
Single shot
Abs. or rel.
Timing
Detect
m bunching
RF Transverse
Deflecting
Cavity
Yes: Steal 3 pulses
No: 3 pulses
Absolute No
Coherent
Radiation
Spectral power
No for CSR
Yes for CTR
Yes
Relative
Autocorrelation
(2nd moment only)
Electro Optic
Sampling
Energy
Wake-loss

11. Bunch Length Measurements with the RF Transverse Deflecting Cavity
Cavity on
Cavity off
- 180°
Bunch length reconstruction
Measure streak at 3 different phases
sz = 90 mm
(Streak size)2 s y
2.4 m
30 MW
Asymmetric parabola indicates incoming tilt to beam

12. Calibration scan for RF transverse deflecting cavity
Bunch length calibrated in units of the wavelength of the S-band RF
Beam centroid
[pixels]
Further requirements for LCLS:
High resolution OTR screen
Wide angle, linear view optics
Cavity phase [deg. S-Band]

13. OTR Profile Monitor in combination with RF Transverse Deflecting Cavity
Simulated digitized video image
Injector DL1 beam line is shown
Best resolution for slice energy spread measurement would be in adjacent spectrometer beam line.

14. BC1 Bunch Length Monitor - based on coherent spectral power detection
400 GHz
1.2 mm
CSR Power spectral density signal for bunch length feedback
Spectral lines accompanying micro-bunching instability
– Z. Huang.

15. 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

16. Interferometer for autocorrelation of CTR - tests at SPPS
12 mm rms
Transition radiation is coherent at
wavelengths longer than the bunch length,
l>(2p)1/2 sz
Limited by long wavelength cutoff and absorption resonances
P. Muggli, M. Hogan

17. Jitter in bunch length signal over 10 seconds ~10% rms
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

18. Bunch length and arrival time from Electro Optic measurements at SPPS
A. Cavalieri
Principal of
temporal-spatial correlation
single pulse
EO xtal
Line image camera
analyzer
polarizer Er
width
centroid
30 seconds, 300 pulses: sz = 530 fs ± 56 fs rms Dt = 300 fs rms

19. Electro-Optical Sampling at SPPS – A. Cavalieri et al.
<300 fs
Single-Shot
200 mm thick ZnTe crystal
Ti:Sapphire laser e-
Timing Jitter
170 fs rms
e- temporal information is encoded on transverse profile of laser beam

20. Energy and Bunch Length Feedback Loops
DL1
Spectr.
BC1
BC2 L2 L3
BSY 50B1
DL2
Vrf(L0)
Φrf(L2)
Vrf(L1)
Φrf(L3) E sz
Φrf(L1)
4 energy feedback loops
2 bunch length feedback loops
120 Hz nominal operation, <1 pulse delay
More detail given in breakout session SC5 talk on Controls

21. Closed Loop Response of Orbit Feedback
Undulator trajectory launch loop to operate at 120 Hz, <1 pulse delay
Damps jitter below 10 Hz
Linac orbit loops to operate at 10 Hz because of corrector response time
Antidamp
Damp
Gain bandwidth shown for different loop delays
- L. Hendrickson

22. Summary
Diagnostics integrated into the LCLS design
All systems require attention to achieve LCLS resolution requirements
New diagnostics are being developed for bunch length and timing
Developmental work at SPPS is critical
Diagnostics being developed hand-in-hand with controls and feedbacks