Dielectric Wakefield Accelerator for an X-ray FEL User Facility

C. JingC. Jing11, R. Lindberg, R. Lindberg22, J. Power, J. Power33, A. Zholents, A. Zholents22

11 Euclid Techlabs Euclid Techlabs22 Advanced Photon Source, ANL Advanced Photon Source, ANL33 High Energy Physics, ANL High Energy Physics, ANL

Future Light Source Workshop, Jlab, March 5-9, 2012Future Light Source Workshop, Jlab, March 5-9, 2012

Assessment of opportunities

Bunch compressor

Energy gain 13 MeV/m

Spreader40 MeV 2.4 GeV

~ 50 m~ 50 m ~ 350 m~ 350 m

~ 250 m~ 250 m

~ 100 m

~ 100 m

~ 50 m~ 50 m

Multi-user soft x-ray FEL facility based on SRF linac (talk by J. Corlett)

Motivation

• Reduce construction and operational costs of a high bunch Reduce construction and operational costs of a high bunch rep. rate FEL facility: rep. rate FEL facility: – accelerating gradient > 100 MV/m, accelerating gradient > 100 MV/m, – peak current > 1KA, peak current > 1KA, – bunch rep. rate of the order of 1MHz, bunch rep. rate of the order of 1MHz, – electron beam energy of a few GeVelectron beam energy of a few GeV

Dielectric Wakefield Accelerator•Simple geometry

•Capable to high gradients

•Easy dipole mode damping

•Tunable

• Non expensive

Recent impressive results (obtained along development of a Recent impressive results (obtained along development of a Linear Collider):Linear Collider): - - 1000 MV/m level in the THz domain (UCLA/SLAC group)1000 MV/m level in the THz domain (UCLA/SLAC group) - 100 MV/m level in the MHz domain (AWA/ANL group)- 100 MV/m level in the MHz domain (AWA/ANL group)

2

2

21( ) exp cos( )

2z

Zn

QW z kz

a

Wake field in dielectric tube induced by a short Gaussian beam

b a

Q

CuCu

-300

-200

-100

0

100

200

300

-0. 25 0. 25 0. 75 1. 25 1. 75 2. 25 2. 75

Di stance (mm)

Wz(M

V/m/

1nC)

Wakefield Amplitude Dependence onAperture or 1/f

1

10

100

1000

10000

100000

0.01 0.1 1 10

Inner Radius a (mm)

Ez(

MV

/m/1

0n

C)

a=240 um; Q=1 nC; bunch length=0.5 ps (FWHM), f=650 GHza=240 um; Q=1 nC; bunch length=0.5 ps (FWHM), f=650 GHz

Increase Transformer Ratio, i.e., a Increase Transformer Ratio, i.e., a ratio of the maximum energy gain ratio of the maximum energy gain experienced by witness bunch to experienced by witness bunch to maximum energy loss experienced by maximum energy loss experienced by drive bunch or train of bunches. drive bunch or train of bunches.

Beam based Beam based RB, RBT RB, RBT

Structure based Structure based two channels two channels

Ramped BunchRamped Bunch

Ramped Bunch TrainRamped Bunch Train

Reference: Schutt et. al., Nor Ambred, Armenia, (1989)Reference: Schutt et. al., Nor Ambred, Armenia, (1989)

Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985)Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985)

c (z)

W+

W-z

zd d

W -

W+

d

(z)

Road map to a high energy gain acceleration

7

Euclid Quartz Euclid Quartz DWA (before DWA (before metalization) metalization) ID=400 umID=400 um

A schematic of a x-ray FEL user facility based on a 2.4 GeV DWA

FEL10FEL10

FEL2FEL2

FEL1FEL1

1 MHz,1 MHz,P=320 kWP=320 kW

Key technology: bunch shaping enhances transformer ratio

Triangular bunchTriangular bunch

Double triangular Double triangular bunchbunch

TR~10TR~10

TR~17TR~17

Double EEX technique: a convenient tool for bunch shaping

z →x emit. exch.z →x emit. exch. x → z emit. exch.x → z emit. exch.

QFQD QD QF

Emittance exchange

T

QDQF

B

QDQF

B

B

B

-I -I

QD QF

QDQF

B

QDQF

B

B

B

-I -I

QD QF

TMTM110110 TMTM010010TMTM010010

Deflecting cavity

Emittance exchangeFODO

MaskMaskBunch shaping manipulations

Low charge witness (main) bunch can also be Low charge witness (main) bunch can also be made out of drive bunch at the same timemade out of drive bunch at the same time

Key technology: DWA structure design

ID, OD, Length 400 m, 464.7 m, 10 cm

, tan 3.75, 0.6x10-4

Freq. of TM01, TM02, TM03 850 GHz, 3092 GHz, 5749 GHz

Q of TM01, TM02, TM03 1260, 3173,4401

r/Q of TM01, TM02, TM03 94.1 k/m, 3.2 k/m, 0.5 k/m

g of TM01, TM02, TM03 0.592c, 0.794c, 0.813c

Thermal load and cooling

Average power load 50 W/cmAverage power load 50 W/cm2 2

at a 100 kHz rep. rate mostly at a 100 kHz rep. rate mostly dissipates in Cudissipates in Cu

The pulse temperature rise from the The pulse temperature rise from the wake field pulse is estimated to be only wake field pulse is estimated to be only ~ 20 ºC ~ 20 ºC

The structure overheating problem is The structure overheating problem is much less severe in the DWA much less severe in the DWA comparing to S-band Cu linac because comparing to S-band Cu linac because of a small amount of energy used to of a small amount of energy used to excite the wake fields and a short period excite the wake fields and a short period of time that the wake field remains of time that the wake field remains inside of the structure.inside of the structure.

Drive bunch charge 1.6 nC/ drive bunch

Drive bunch profile Double triangular

Drive bunch length (total), T 3.3 ps (1mm)

Unloaded Gradient 114 MV/m

Transformer Ratio 16.5

Wakefield generation

Beam loading

dzzzWE

dzzzW EE2

12

10 MeV in 10 cm

150 KeV (~1.5%)

Electron bunch is strongly chirped in energy

14

Accelerated currentAccelerated currentWakefieldWakefield

Strongly chirped beams for FEL applications: preliminary results• For short beams (<10 um rms) the energy chirp is approximately linear in time• Accelerated beam is strongly chirped (little FEL gain)• Using the chirp to compress the beam does not seem to be useful for radiation

(although it is at the limit of various typical FEL approximations)• Tapering of undulator strength or period can counteract large energy chirp and

maintain gain

15

Linear Linear gaingain

Nonlinear Nonlinear regimeregime

For example, chirping the undulator strength K we have

Power evolution of DWA beam + undulator taper Power profile near

saturation z/LG = 20Chirped SASE spectrum near saturation z/LG = 20

Some applications favors wide bandwidthSome applications favors wide bandwidth

Summary

• Several DWAs driven by a single SRF linac can be Several DWAs driven by a single SRF linac can be used to serve several FEL undulator lines, each at a used to serve several FEL undulator lines, each at a 100 kHz rep. rate.100 kHz rep. rate.

• Energy chirped electron bunch coming from DWA will Energy chirped electron bunch coming from DWA will produce a powerful broad band x-ray light.produce a powerful broad band x-ray light.

• A proposed facility is energy efficient and may have a A proposed facility is energy efficient and may have a relatively low operational cost. relatively low operational cost.

• Much more studies are needed to prove the Much more studies are needed to prove the feasibility of DWA and to solicit new ideas.feasibility of DWA and to solicit new ideas.