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).