1. Beam dynamics for an X-band LINAC driving a 1 keV FEL
R. Bartolini
Diamond Light Source Ltd
and
John Adams Institute, University of Oxford
XB10 Workshop
Daresbury, 02 December 2010

2. Outline
Introduction
A brief look at the NLS project and users’ requirements
Can we do the same with an X-band Linac?
First optimisation of the 2.25 GeV Linac and AP challenges
layouts and performance
a comparative look at L-band vs X-band
collective effects (CSR and wakefields)
control of the e– phase space distribution
Conclusions
XB10 Workshop
Daresbury, 02 December 2010

3. Layout of NLS
photoinjector
3rd harmonic cavity
BC1
BC2
BC3
laser heater
accelerating modules
collimation
diagnostics
spreader
FELs
IR/THzundulators
gas filters
experimental stations
3 free-electron lasers covering the photon energy range 50 eV–1 keV,
- GW peak power
- 20 fs pulses
- laser HHG seeded for
longitudinal coherence
high brightness electron gun, operating (initially) at 1 kHz
2.25 GeV cw superconducting linac
Operates with pC
L-band gradient 15 MV/m (shallow minimum of cost optimisation)
Total length from gun to final tube = 290 m

4. Can we do the same with an X-band Linac?
NLS science case had pressing requirements for high rep rate
Phase 1 – 1 kHz
Phase 2 upgrade – 100 kHz and 1MHz  SC L-band technology
15 MV/m  150 m active length to reach 2.25 GeV
Assuming lower gradient in X-band structures for higher repetition rate
35 MV/m can operate at 1 kHz (see C. Christou’ talk)
35 MV/m  65 m active length to reach 2.25 GeV
XB10 Workshop
Daresbury, 02 December 2010 4

5. Accelerator Physics challenges
Soft X-ray are driven by high brightness electron beam
2.25 GeV n  1 m ~ 1 kA  /   10–4
This requires:
a low emittance gun (norm. emittance cannot be improved in the linac)
acceleration and compression through the linac keeping the low emittance
Requirements for the operation of seeded FELs
Assuming a 20 fs FWHM seed laser pulse we need an electron bunch with constant slice parameters over 20 fs plus the relative time jitter between the electron bunch and the laser seed pulse.
constant slice parameters on a length of 100 fs – or longer
no residual energy chirp (or very limited)
low sensitivity to jitter
The slice parameters to control are not only slice current, emittance, energy spread but also slice offset and angle and Twiss parameters

6. Design and Optimisation of LINACs driving FELs (WIP)
Parameters used in the optimisation
Accelerating section and 3HC amplitude and phase,
Bunch compressors strengths (R56)
Validation with full start-to-end simulation Gun to FEL (time dependent) including space charge in the gun, CSR, longitudinal space charge, wake-fields in RF cavities (elegant)
Astra
Elegant
GENESIS
Gun
S01 LH
X4H
S02
S03
S04
BC1
X01
X02
X03
X04
X05
X06
BC2 DL
undulators
tubes S band operating at 12 MV/m + 96 tubes X-Band T53
First compression at 450 MeV – total length from gun to last tube  190 m
Gun
S01 LH
X4H
S02
S03
BC1
X01
X02
X03
X04
X04
X05
X06
BC2 DL
undulators
4 + 8 tubes S band operating at 12 MV/m tubes X-Band T53
First compression at 350 MeV – total length from gun to last tube  180 m

7. Linac design
Additional S band cavities operating at 12 MV/m and transition to X-band cavities as soon as possible
Two compression stages at 450 MeV and 1.3 GeV made of two C-chicane with 4 dipoles;
Linearise 4HC before S-module at 135 MeV – reduced gradient requirements (blow 35 MV/m)
96 T53 X-band cavities (0.53 m) – 1.8 GeV in X band MeV in S band
Doublet focussing structure every 16 tubes – 1.2 m length for diagnostics
Spreader design based on a dog leg: further upgrade based on NLS spreader (no issues expected)

8. NLS S-band gun (WIP)
A new S-band gun has been optimised to operate at higher rep rate (1 KHz)
Modified BNL-ATF type design operating at 100 MV/m to reach 1 kHz
(instead of 120 MV/m)
Emittance below 0.2 um
8 ps FW bunch
< 5 keV en. spread
7 A peak
Courtesy J.H. Han 8

9. X-band cavities wakefields for lower gradient
SLAC X-band cavity has a/ = 0.18 – 45MV/m
Wakefield used in this simulations is scaled from the SLAC X band structure using the analytical formula (K. Bane Z. Huang private communication)
Assuming a/ = 0.13 – 35MV/m
Changing a, keeping g and L fixed (readjusting b to keep the resonant frequency)

10. Ratio of wakes (a/ = 0.187 to a/ = 0.13)
LCLS - NLC design
SLAC high rep rate design
Transverse wakefields with a low rep rate design are expected to be 3-4 time stronger
This factor was included in the elegant simulations

11. 50 pC operation For SASE with short pulses – 20 fs FWHM
8 ps FW at start
0.8 ps FW at 460 MeV compression ratio 10
60 fs FW at 1.3 GeV compression ratio 13

12. Slice analysis of electron beam at the FEL
Current distribution
Norm emittance
delta gamma
XB10 Workshop
Daresbury, 02 December 2010

13. FEL time dependent simulations (50 pC)
FEL SASE time dependent simulations confirm the good slice beam quality achieved in the optimisation
3 GW power at 40 m
5 m FWHM (17 fs) 1 keV pulse

14. 200 pC operation For seeded FEL with short HHG seed – 20 fs FWHM
8 ps FW at start
1.5 ps FW at 460 MeV (C 5)
100 fs FW at 1.3 GeV (C 15)

15. Slice analysis of electron beam at the FEL (WIP)
Current distribution
Norm emittance
3D Xie gain length
60 fs flat region
XB10 Workshop
Daresbury, 02 December 2010

16. Comparative Optimisation L-band vs X-band
Can use 2 compressors instead of 3
the bunch from the S-gun is shorter (higher gradient at gun S-band gun even if operated at higher rep. rate 100 MV/m used)
Clash between the requirements
start with X-band structure asap (use more X-band structure asap)
which implies first compressor asap
but do not compress too much too early (space charge issues)
changing to X-band requires a short bunch in order not to reintroduce a curvature from the RF acceleration (which is bad for compression in second compressor)
Reduction of energy chirp easier with X-band than with L-band
Usual conditions for the control of the optics functions at the compressors

17. Conclusions and ongoing work Thank you for your attention
We have started the analysis of an X-band linac to drive a Soft X-ray FEL with a repetition rate of 1 kHz
for ultrashort pulses (20 fs) in SASE mode
and
for seeded FEL (20 fs seed on flat top of 100 fs)
using scaled version of the SLAC X-band cavity to (a/ = 0.13)
Beam dynamics optimisation at low charge (50 pC) for SASE appears to be straightforward.
Higher charge seeded operation is more demanding but appears to be feasible. Cheaper option for NLS (see R. Walker’s talk tomorrow)
Thank you for your attention