a 6 gev compact x-ray fel (cxfel) driven by an x-band linac
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A 6 GeV Compact X-ray FEL(CXFEL)
Driven by an X-Band Linac
Zhirong Huang, Faya Wang, Karl Bane and
Chris Adolphsen
SLAC
Parameter symbol LCLS CXFEL unit
Bunch Charge Q 250 250 pC
Electron Energy E 14 6 GeV
Emittance gex,y 0.4-0.6 0.4-0.5 µm
Peak Current Ipk 3.0 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 30 m
Sat. Power Psat 30 10 GW
FWHM Pulse Length DT 80 80 fs
Photons/Pulse Ng 2 0.7 1012
Compact X-Ray (1.5 Å) FEL
Linac-1250 MeV
Linac-22.5 GeV
Linac-36 GeV
BC1 BC2
UndulatorL = 40 mS
rfgun
X
undulator
LCLS-like injectorL ~ 50 m
250 pC, gex,y 0.4 mm
X-band Linac Driven Compact X-ray FEL
X X
X-band main linac+BC2G ~ 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
Design Issues• X-band Components• Cost• Performance• Emittance Preservation• Tolerance
Beam Parameters Units CXFEL 2004 NLCFinal Beam Energy GeV 6 250Bunch Charge nC 0.25 1.2RF Pulse Width* ns 150 400Linac Pulse Rate Hz 120 120Beam Bunch Length μm 7 110
Operation Beam Parameters
* Possible for multi-bunch operation with separation > 10ns
Layout of Linac RF Unit50 MW XL4
50 MW 1.5us
400 kV 1.6us
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 17.9Power Needed for <Ea> = 70 MV/m
MW 48 73
Two Accelerator Structure Types
Units T53VG3RA H60VG3R
Average RF Phase Offset+ Deg 2.6 2.6
Power Gain 4.83 4.83
Klystrons per Unit 2 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 m 122 108
Total Linac Length# m 192 178
RF Unit for Two Structure Types Operating at 70 MV/m*
*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
NLC RF Component Costs(2232 RF Units)
Per Item Cost (k$)LLRF 26.1
Modulator 83.7
Klystron 56.6
TWT 13.3
SLED-II 242.3
Structures 21.5
For RF unit quantities less than 50, assume the rf item cost will be 4 times the NLC cost
6 GeV X-Band Main Linac Cost
Using Structure Type:
T53VG3R, Total Cost = 56 M$ (3.1 M$ Per RF Unit)
H60VG3R, Total Cost = 62 M$ (2.6 M$ Per RF Unit)
50 60 70 80 90 1001
1.05
1.1
1.15
1.2
1.25
1.3
1.35
1.4
Gradient (MV/m)
Re
lativ
e 6
Ge
V M
L C
ost
H60VG3RT53VG3R
Assuming 1) Tunnel cost 25 k$/m, AC power + cooling power 2.5 $/Watt 2) Modulator efficiency 70%, Klystron efficiency 55%.
Gradient Optimization
60 65 70 75 8010
-5
10-4
10-3
10-2
10-1
100
Gradient (MV/m)
Bre
akd
ow
n R
ate
at 1
50
ns
12
0 H
z (#
/hr)
H60VG3RT53VG3R
Structure Breakdown Rates with 150 ns Pulses
1) H60VG3R scaled at 0.2/hr for 65 MV/m,400 ns, 60Hz2) T53VG3R scaled at 1/hr for 70 MV/m, 480 ns, 60 Hz3) Assuming BDR ∞G26, ∞ τ6
At 70 MV/m, Rate Less
Than 1/100hr at 120 Hz
Design Issues• X-band Components• Cost• Performance• Emittance Preservation• Tolerance
H60VG3 Dipole Wakes
K. Bane, SLAC-PUB-9663, 2003
Fitted Equation
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
K. Bane, SLAC-PUB-9663, 2003
Wake Averaged over a Gaussian Bunch
Average wake for Gaussian bunch as function of bunch length. Note that in Linac-2, _z= 56 m; in Linac-3, _z= 7 m
Linac-2
Linac-3
Emittance GrowthStrength parameter:
Emittance growth due to injection
jitter xo if small:
Chao, Richter, Yao
• 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-> xrms
2/Mp
• lcu= a2/2z= 1.6 m (Linac-3)—catch-up distance, estimate of distance to steady-state
(for ~ E)
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.
Wakefield Damping and Detuning for Multibunch Operation
Time of Next Bunch
Time After Bunch (ns)
Wak
efiel
d Am
plitu
de (V
/pC/
m/m
m)
Measurements
Ohmic Loss Only
Damping and Detuning
Detuning Only
Dipole Mode Density
Frequency (GHz)
High Gradient Structure Development
Traveling-Wave Structure 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.
RF Unit Test in2003-2004
FromEight-Pack
3 dB3 dB
3 dB
3 dB3 dB
Beam
FromStation 2
FromStation 1
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.
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).
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