1. A 6 GeV Compact X-ray FEL (CXFEL) Driven by an X-Band Linac
Zhirong Huang, Faya Wang, Karl Bane and Chris Adolphsen
SLAC

2. Compact X-Ray (1.5 Å) FEL Parameter symbol LCLS CXFEL unit
Bunch Charge Q
250 pC
Electron Energy E 14 6
GeV
Emittance
gex,y µm
Peak Current
Ipk
3.0 kA
Energy Spread
sE/E
0.01
0.02 %
Undulator Period lu 3
1.5 cm
Und. Parameter K
3.5
1.9
Mean Und. Beta
β 30 8 m
Sat. Length
Lsat 60
Sat. Power
Psat 10 GW
FWHM Pulse Length DT 80 fs
Photons/Pulse Ng 2
0.7
1012

3. X-band Linac Driven Compact X-ray FEL
250 MeV
Linac-2
2.5 GeV
Linac-3
6 GeV
BC1 X
BC2 X S
Undulator
L = 40 m X
undulator rf
gun
LCLS-like injector
L ~ 50 m
250 pC, gex,y  0.4 mm
X-band main linac+BC2
G ~ 70 MV/m, L ~ 150 m
Use LCLS injector beam distribution and x-band H60VG3R structure (<a>/l=0.18) after BC1
LiTrack simulates longitudinal dynamics with wake and obtains 3 kA “uniform” distribution
Similar results for T53VG3R (<a>/l=0.13) with 200 pC charge

4. Design Issues X-band Components Cost Performance
Emittance Preservation
Tolerance

5. Operation Beam Parameters
Units
CXFEL
2004 NLC
Final Beam Energy
GeV 6
250
Bunch Charge nC
0.25
1.2
RF Pulse Width* ns
150
400
Linac Pulse Rate Hz
120
Beam Bunch Length μm 7
110
* Possible for multi-bunch operation with separation > 10ns

6. Layout of Linac RF Unit
50 MW XL4
400 kV 1.6us
50 MW 1.5us

7. Two Accelerator Structure Types
Units
T53VG3R
H60VG3R
Structure Type
Constant E_surface
Detuned
Length cm
0.53
0.6
Filling time ns
74.3
105
Phase Advance/ Cell π
2/3
5/6
<a>/λ %
13.4
17.9
Power Needed for <Ea> = 70 MV/m MW 48 73

8. RF Unit for Two Structure Types Operating at 70 MV/m*
Units
T53VG3RA
H60VG3R
Average RF Phase Offset+
Deg
2.6
Power Gain
4.83
Klystrons per Unit 2
Acc. Structures per Unit 9 6
RF Unit Length (Scaled to NLC) m
6.75
4.5
Total RF units 18 24
Main Linac Length
122
108
Total Linac Length#
192
178
*Assume 13% RF overhead for waveguide losses
+ scaled to NLC for single bunch loading compensation
# including ML, 50m injector and 20 m BC2 at 2.5 GeV

9. NLC RF Component Costs (2232 RF Units)
Per Item Cost (k$)
LLRF
26.1
Modulator
83.7
Klystron
56.6
TWT
13.3
SLED-II
242.3
Structures
21.5
For RF unit quantities less than 50, assume the rf item cost will be 4 times the NLC cost

10. 6 GeV X-Band Main Linac Cost
Using Structure Type:
T53VG3R, Total Cost = 56 M$ (3.1 M$ Per RF Unit)
H60VG3R, Total Cost = 62 M$ (2.6 M$ Per RF Unit)

11. Gradient Optimization
Assuming 1) Tunnel cost 25 k$/m, AC power + cooling power 2.5 $/Watt
2) Modulator efficiency 70%, Klystron efficiency 55%.

12. Structure Breakdown Rates with 150 ns Pulses
At 70 MV/m, Rate Less Than 1/100hr at 120 Hz
H60VG3R scaled at 0.2/hr for 65 MV/m,400 ns, 60Hz
T53VG3R scaled at 1/hr for 70 MV/m, 480 ns, 60 Hz
Assuming BDR ∞G26, ∞ τ6

13. Design Issues X-band Components Cost Performance
Emittance Preservation
Tolerance

14. H60VG3 Dipole Wakes
K. Bane, SLAC-PUB-9663, 2003

15. Fitted Equation K. Bane, SLAC-PUB-9663, 2003
Fit equation for wakefield of disk loaded structure. For average cell of h60vg3, a= 4.7 mm, g= 6.9 mm, L= 10.9 mm

16. Wake Averaged over a Gaussian Bunch
Linac-3
Linac-2
Average wake for Gaussian bunch as function of bunch length. Note that in Linac-2, _z= 56 m; in Linac-3, _z= 7 m

17. Emittance Growth (for ~ E) Strength parameter:
Emittance growth due to injection jitter xo if  small:
Chao, Richter, Yao
(for ~ E)
For CXFEL, eN= 250 pC, N= .4 m, = 0, and
Linac-2: E0= .25 GeV, Ef= 2.5 GeV, z= 56 m, l= 32 m, 0= 10 m (x0= 90 m) => = .14
Linac-3: E0= 2.5 GeV, Ef= 6 GeV, z= 7 m, l= 50 m, 0= 10 m (x0= 29 m) => = .01
For random misalignment, let x02-> xrms2/Mp
lcu= a2/2z= 1.6 m (Linac-3)—catch-up distance, estimate of distance to steady-state

18. Single Bunch Wake and Tolerance Summary
In both Linac-2 and Linac-3, << 1, => short-range, transverse wakefields in H60VG3 are not a major issue in that:
An injection jitter of x0 yields 1% emittance growth in Linac-2 and .003% in Linac-3
Random misalignment of 1 mm rms, assuming 50 structures in each linac, yields an emittance growth of 1% in Linac-2, 0.1% in Linac-3
With the T53VG3R structure, the jitter and misalignment tolerances are about three times smaller for the same emittance growth.
The wake effect is weak mainly because the bunches are very short.

19. Wakefield Damping and Detuning for Multibunch Operation
Dipole Mode Density
Wakefield Damping and Detuning for Multibunch Operation
Ohmic Loss Only
Frequency (GHz)
Detuning Only
Measurements
Wakefield Amplitude (V/pC/m/mm)
Time of
Next Bunch
Damping and Detuning
Time After Bunch (ns)

20. High Gradient Structure Development
Since 1999:
Tested about 40 structures with over 30,000 hours of high power operation at NLCTA.
Improved structure preparation procedures - includes various heat treatments and avoidance of high rf surface currents.
Found lower input power structures to be more robust against rf breakdown induced damage.
Developed ‘NLC/GLC Ready’ design with required wakefield suppression features.
Traveling-Wave Structure

21. RF Unit Test in
Powered eight accelerator structures in NLCTA for 1500 hours at 65 MV/m with 400 ns long pulses at 60 Hz: the structure breakdown rate was less than 1 per 10 hours.
Also accelerated beam.
From
Eight-Pack
3 dB
Beam
Station 2
Station 1

22. RF System Readiness (Black – Comments from 2004, Red - Comments from 2010)
Accelerator Structures
Continue efforts to improve high gradient performance (now includes the US High Gradient program and the CERN/SLAC/KEK collaboration).
Well developed fabrication procedures to achieve wakefield and energy performance.
Three production groups churning out structures (still three with CERN replacing FNAL).
System Integration
Accelerating beam with eight (three) structures at NLCTA.
Summary
Ready for industrialization (still ready – see X-band Workshop tomorrow)
Plan to expand NLCTA and GLCTA (now Nextef) to test industrially-built components (hope to build an improved 8-structure rf unit at NLCTA aimed at light source applications).