1. Short pulse, low charge LCLS operation
ARD R&D meeting
Zhirong Huang
on behalf of the LCLS commissioning team
Nov. 10, 2009
LCLS

2. Outline Introduction Low charge operation Outlook and summary
General considerations
Machine setup
Compression studies
Soft x-rays results
Hard x-rays results
Outlook and summary
Sub-fs possibility
Bunch length diagnostics
Summary

3. Promise of LCLS
Ultra-bright
Ultra-fast
LCLS

4. Single Molecule Imaging with Intense fs X-ray
R. Neutze et al. Nature, 2000

5. LCLS gain and saturation at 1.5 Å (250 pC)
gex,y  0.4 mm (slice)
Ipk  3.0 kA
sE/E  0.01% (slice)
(25 of 33 undulators installed)
Lg  3.3 m

6. Typical Measured LCLS Parameters
P. Emma

7. Outline Introduction Low charge operation Outlook and summary
General considerations
Machine setup
Compression studies
Soft x-rays results
Hard x-rays results
Outlook and summary
Sub-fs possibility
Bunch length diagnostics
Summary

8. Lower charge LCLS operation*
FEL gain depends on peak current, not charge
X-ray SASE has many random spikes, each spike ~1 fs
Less charge, same peak current  shorter x-ray pulses
Less charge, smaller laser spot size on cathode smaller thermal emittance
Less peak current in the accelerator (until the last step of compression)
Less linac wakefield and CSR
Less microbunching
Longitudinal space charge of a very short bunch should be considered and minimized if possible (compression prior to the entrance of undulator?)
* Suggested by J. Frisch; Also by J. Rosenzweig et al., NIMA2008

9. Bunch Compression & CSR Measured after BC2 (0.25 nC)
sz < 5 mm
nominal
sz  2 mm
old screen used
sz > 25 mm
BC2
(4.3 GeV)
BSY
(14 GeV)
TCAV
(5.0 GeV)
550 m L2
4 wires
PRSTAB, 12, (2009)

10. Time-sliced x-Emittance at Very Low Charge
TAIL
0.14 µm
(not same data)
20 pC, 135 MeV, 0.6-mm spot diameter, 400 µm rms bunch length (5 A)

11. Measurements and Simulations for 20-pC Bunch at 14 GeV
Y. Ding et. al, PRL 2009
Photo-diode signal on OTR screen after BC2 shows minimum compression at L2-linac phase of deg.
weaker CSR emittance blowup
L2 at deg
(under-compress)
L2 at -35 deg
(over-compress)
Horizontal projected emittance measured at 10 GeV, after BC2, using 4 wire-scanners.

12. Simulated 20-pC LCLS FEL performance
1.5 Å
15 Å
z = 25 m
@ 25 m,
15 Å,
2.41011 photons,
Ipk = 2.6 kA,
ge  0.4 µm
1.5 Å,
31011 photons
Ipk = 4.8 kA
ge  0.4 µm
1.2 fs
LCLS FEL simulation at based on measured injector beam and Elegant tracking, with CSR and LSC, at 20 pC.
(power profile at z = 25 m varies from shot to shot due to noisy startup)

13. Resistive wall wake in undulator
Transient effect
~10 m 2a vz c
wake catch-up distance ~ a2/2sz ~ 10 m for a=2.5 mm, sz=0.5 mm
Steady-state wake for very short bunch (K. Bane)
For 20 pC, 5 GeV beam,
take 0.7 correction factor for flat vacuum chamber
 sE/E = 5x10-4 at z = 60 m < FEL bandwidth
Undulator wake appears relatively weak at low charge

14. Low charge machine setup
UV laser ~1 ps (rms), 0.6-mm spot diameter, 15 deg to gun rf
injector projected norm. emit. ~0.2 mm (x/y)
injector bunch length 220~250 mm (rms)
Laser heater off
L1S&X are the same as 250 pC configuration
Vary L2 chirp to find maximum compression

15. Bunch length monitor signal
J. Frisch
H. Loos
LiTrack simulation assumes
20 pC bunch charge
3 keV initial rms slice energy spread
0.23-mm initial rms bunch length

16. FEL gas detector signal
photon 840 eV
Half of undulators inserted (to prevent pulse lengthening due to slippage after FEL saturation)
BL signal
-1 deg
+1deg
LiTrack simulation assumes
20 pC bunch charge
3 keV initial rms slice energy spread
0.23-mm initial rms bunch length
FWHM bunch length (LiTrack)
X-ray pulse duration should be <10 fs, but no direct measurement yet possible
4-fs

17. Charge dependence
Can easily go to 40 pC (laser iris diameter 0.8 mm) or 10 pC (laser iris diameter 0.5 mm)
FEL energy is approximately proportional to charge
X-ray pulse length probably also increases with charge (no direct measurement yet)

18. Measured Energy spread (@ 4.5 GeV)
Energy spread measured on the vertical dump OTR screen
(FEL suppressed)
full-compression
under-compression
over-compression
Difference in under/over-compression shows up in FEL bandwidth

19. 20pC, 4.3 GeV, L2 = -33 deg (Elegant simulation by Y. Ding)
What happens at full compression?
20pC, 4.3 GeV, L2 = -33 deg (Elegant simulation by Y. Ding)
BC2 END
Undulator entrance
σδ = 0.16%
σδ = 0.4%
Increase E spread
due to longitudinal
space charge
DL2 compression
At full compression, energy spread is too large, and bunch is too short. X-ray slips out of the beam before significant gain occurs.

20. Peak of compression shifted by DL2
gun
4 wire
scanners +
4 collimators
TCAV0
L1X
old
screen L0
3 wires
2 OTR
4 wire
scanners
3 OTR
vert.
dump
heater
L1S
m wall
3 wires
3 OTR
sz1
L2-linac
sz2
L3-linac
DL1
135 MeV
BC1
250 MeV
stopper
BC2
4.3 GeV
TCAV3
5.0 GeV
BSY
DL2
GeV
undulator
R56-DL2=130 m
L1S = degS
DE/E(DL2) = 0
BC2 Ipk
OTR33
20 MeV = 0.2 degS shift due to additional chirp
from LSC & Linac wake

21. From P. Emma

22. From P. Emma

23. Compression study vs L1S phase
20 pC studies confirm this physical picture
Bunch length signal
FEL signal at 4.5 GeV
J. Wu
Hor. shifts of peaks removed in this plot
100 MeV L2 chirp voltage = 1 degS L2 phase

24. 20 pC hard x-ray results
Results shown here from June, recent results not as good
Full 33 undulator with fixed und. taper (13.6 GeV, 1.5 Å)
YAGXRAY
Full compression?
BL signal
Full compression rms jitter ~25%
Over compression rms jitter ~11%
Under compression rms jitter ~13%

25. Gain length and FEL energy at Full compression
Unlike soft x-rays, FEL can perform full-compression
for the hard x-ray case (~140 mJ with 25% fluctuation)
Undulator K taper
Gain length =2.74 m
140 mJ FEL energy

26. L1S = -22 deg, BC2 full compression, 13.6 GeV
~150 mJ
1 fs
Elegant to Genesis simulations (Y. Ding)

27. Laser heater effects at full compression
Low charge, low current beam yields less microbunching
Not necessary to heat up E-spread to damp the instability
At full compression, initial E-spread becomes bunch length, reduces the final peak current and decreases FEL power sd
sz =sdR56
Indicate sub-mm bunch length w/o heater
Changed by laser heater, measured on injector spectrometer

28. Outline Introduction Low charge operation Outlook and summary
General considerations
Machine setup
Compression studies
Soft x-rays results
Hard x-rays results
Outlook and summary
Sub-fs possibility
Bunch length diagnostics
Summary

29. Sub-fs hard x-rays?
L1S = -17 deg, BC2 full compression, 13.6 GeV simulations (Y. Ding)
@ 70 m
~10 mJ
300 as

30. J. Frisch, FEL2009

31. AMO indicates 5-10 fs x-rays
20/40 pC short pulse mode accounts for ~1/3 AMO run time so far
AMO experiments yield valuable FEL information (data consistent with FWHM x-ray duration 5-10 fs)
Ne charge state distribution vs x-ray pulse duration (2 keV)
“Mismatched intensity of 2+ and 3+
is because the model leaves out shake off and Double Auger processes.”
Courtesy of Linda Young and Simulations by Robin Santra (ANL)

32. FEL bandwidth
AMO also saw significant difference in bandwidth from under to over 40 pC
L. Young et al.
Multi-shot measurements
include energy jitters
7eV or 0.8 % fwhm
17eV or 2 % fwhm

33. Summary
LCLS low charge beams deliver short x-ray pulses (<10 fs) to soft x-ray users (hard x-rays also available)
These studies illustrate an interesting mode of running SASE FELs
Future x-ray FEL designs may benefit from low charge configurations
Smaller emittance  lower beam energy for the same FEL
Less charge  less wake, more compact accelerators (x-band?) and more bunches
Much diagnostic challenge, especially the need for reliable bunch length measurements with fs resolution