Multi-user, High Repetition-Rate,Soft X-ray FEL User Facility

(based on a Collinear Dielectric Wakefield Accelerator)

Euclid Techlabs LLC: C.Jing, A.Kanareykin, P.SchoessowArgonne National Laboratory, HEP: W.Gai, G.Ha, C.Li, J.G.PowerArgonne National Laboratory, APS: R.Lindberg, A.ZholentsNorthern Illinois University: P.Piot

John Power, Argonne

Assessment of Opportunities

High Brightness Beams Workshop, San Juan, Puerto Rico, March 25, 2013

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Multi-user, High Rep Rate, Soft X-ray FEL User Facility

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1 MHz ACCELERATOR(BLACK BOX)

2 GeV50 MeV

Low-emittance injector:• 1 MHz bunch rep. rate

Flexible x-ray beamlines• Tunable pulse length• Seeded• 2 color seeded• SASE

Lasers linked with a fiber-optics time

distribution network

Beam spreader• 100 kHz bunch rep. rate

Capable of serving~2000 scientists/year

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Multi-user soft x-ray FEL facility based on: SRF linac

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Capable of serving~2000 scientists/year

Low-emittance injector:• 1 MHz bunch rep. rate

Flexible x-ray beamlines• Tunable pulse length• Seeded• 2 color seeded• SASE

Lasers linked with a fiber-optics time

distribution network

Beam spreader• 100 kHz bunch rep. rate

2 GeV50 MeV

~50 m~100 m

~ 300 m

~ 250 m

~ 50 m

750m

CW superconducting linac~1MHz bunch rep. rate~2 GeV beam energy~1 kA peak current

Multi-user soft x-ray FEL facility based on: DWFA linac

~50 m

~25 m

CompactBeam

Spreader

Facility Footprint350m x 250m

~50 m

4

~50 m

350m750m

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~30 m

Compact DWFA linac~1MHz bunch rep. rate~2 GeV beam energy~1 kA peak current

~100 m ~50 m

2 GeV

BeamShaper

Dielectric Wakefield Acceleration (DWFA) linac

200 MeV

ConfigurableFEL Array

1 keVX-rays

End S

tations

1.2 GeV100 pC

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Flexible x-ray

beamlines

Flexible accelerator beamlines

……

……

ConfigurableDWFA Accelerator

0.5 keVX-rays

2.4 GeV50 pC

Ultra-flexible facility

Dielectric Wakefield Acceleration (DWFA) linac

Motivation for DWFA for the High Rep Facility Low energy spreader Accelerating gradient > 100 MV/m Room temperature quartz fibers Tunable electron beam energy of a few GeV Tunable peak current > 1KA Bunch rep. rate of the order of 1MHz

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Compact

Inexpensive

Flexible

Many hurdles to overcome as you will see…

Is it possible to replace some of the SRF linac with a DWFA linac??

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CollinearDielectric Wakefield Acceleration

FUNDAMENTALS:

Simple geometry Capable of high gradients Easy dipole mode damping Tunable Inexpensive

Recent results (obtained for Linear Collider development):− 1000MV/m level in the THz domain (UCLA/SLAC group)− 100 MV/m level in the MHz domain (AWA/ANL group)

Cylindrical Dielectric Wakefield Accelerator

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2

2

21( ) exp cos( )2

zZ

n

QW z kza

2b 2a

eQ

Cu

-300-200-100

0100200300

-0. 25 0. 25 0. 75 1. 25 1. 75 2. 25 2. 75Di 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(M

V/m

/10n

C)

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

Wake field in dielectric tube induced by a short Gaussian beam

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The Wakefield Theorem and the Transformer Ratio

(Maximum energy gain behind the drive bunch)(Maximum energy loss inside the drive bunch) < 2R = W+

W-=

W-

W+

The R< 2 limit has kept interest in

collinear wakefield accelerators to a

minimum.1.5 0.25 1 2.25 3.5 4.75 60.2

0.1

0

0.1

0.2

Wak

efie

ld (M

V/m

/nC

)

CollinearDielectric Wakefield

Acceleration

DRIVE WITNESS

Methods to increase R>2 in a collinear wakefield accelerator

Ramped Bunch

Ramped Bunch Train(demonstrated at ANL)

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

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

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A case study of an x-ray FEL user facility based on a 2.4 GeV DWFA

EXAMPLE:

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High rep. rate, X-ray FEL user facilitybased on a 2.4 GeV DWFA

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FEL10

FEL2

FEL1

P=320 kW, 1 MHz

~30m

Quartz

DWFA

1.6 nC

TR = 16.5

ID=400 umfreq = 850 GHz

ID, OD, Length 400 m, 464.7 m, 10 cme, 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/mng of TM01, TM02, TM03 0.592c, 0.794c, 0.813c

Key technology:DWFA RF structure design

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Quartz DWFA ID=400 um

RF pulsed heating • DT ~ 20 ºC

Average thermal heating • Average power load 50 W/cm2

@100 kHz rep rate

How can a small DWFA can handle High Rep Rate????

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RF packet ~333 ps

e

Collinear DWFA• Ultra-short RF pulse (~333 ps)• Heating is much less severe than

microwave accelerator

Quartz DWFA ID=400 um--cooling--

Triangular bunch

Double triangular bunch

TR~10

TR~17

Key technology: drive bunch shaping enhances transformer ratio

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dzzzWE 10 MeV in 10 cm

Key technology: witness bunch generation

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Drive and Witness from the same source bunch minimal timing jitter

QFQD QD QF

Emittance exchangeT

QDQF B

QDQF

B

B

B

-I -I

QD QFQD

QF B

QDQF

B

B

B

-I -I

QD QF

TM110 TM010TM010

Deflecting cavity

Emittance exchangeFODO

Double EEX technique: a convenient tool for drive and witness bunch shaping

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Before mask After mask At EEX exitmask

(c)

2 1 0 -1 -2time (ps)

witn

ess

120010008006004002000

curr

ent (

A)

z →x emit. exch. x →z emit. exch.

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Accelerated currentWakefield

Key technology: How to handle beam loading:

Eacc=115 MV/m

~DE=30 MV/m

Gaussian Electron bunch• Large energy spread• Strongly chirped in energy

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Key Technology: Undulator BAD: Accelerated beam is strongly chirped (little FEL gain) BAD: Using the chirp to compress the beam does not seem to be useful for radiation GOOD: For short beams (<10 m rms) the energy chirp is approximately linear in time

Longitudinal GradientTapering the undulator strength or period can counteract large energy chirp and maintain gain

Transverse GradientVarying the undulator strength transversely can counteract large energy chirp and maintain gain

𝑡

Δ𝛾

Δ𝛾𝛾 0

𝑥

N

S

Smaller undulator strength K

Larger undulator strength K

Strongly chirped beams for FEL applications

Strongly chirped beams for FEL applications: preliminary results

Linear gain

Nonlinear regime

Tapering the undulator strength K

Power evolution of DWFA beam + undulator taper Power profile near

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

Some applications favor wide bandwidth21

Example: Longitudinal Gradient witness beam chirp

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Can we reduce energy spread due to beam loading?

Gaussian witness bunch 15 10 5 0 5 10 15 z (um)

110

100

90

80

70

Ener

gy (M

eV)

∙Q=50 pC∙Edec=13.6 MV/m∙Eacc=81.7 MV/m∙sigmaE=5.3%∙R=6

Gaussianbunch

Key idea: Match the curvature of the self-wake to the drive wake

Witness self-wake

Drive-wake

~20x reduction in energy spread23

Reversetriangularbunch ∙Q=50 pC∙Edec=6.3 MV/m∙Eacc=86.3 MV/m

20 10 0 10 20 z (um)

110

100

90

80

70

Ener

gy (M

eV)

Reverse triangular witness bunch

=0.3%R=14

Beam pipe OD, 2b 1.14 mmDielectric tube OD, 2a 1.24 mm

Waveguide cutoff 298 GHzCharge of the drive bunch 5 nCLength of the drive bunch 2.127 ps

Charge of the witness bunch 250 pC

Length of the witness bunch 75 fs

Time between the bunches 9.4 ps

Transformer ratio 3.16ΔG/G 1.5*10-5

By additionally customizing the shape of the main bunch we designed the configuration which minimizes the wakefield-induced energy spread in the main bunch. The energy spread may be made as low as 0.001%.

Minimization of the energy spread in a witness bunchCourtesy of E. Simakov, LANL

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General (nonlinear) shapes are possible

leaf

Multi-leaf collimator:• Used in medical linacs to shape the x-rays • Each vertical leaf moves independently

Multi-leaf collimatorVarian's 120-leaf multileaf collimator

Varian's high-definition multileaf collimator

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Feedback on desired witness and drive shape

http://varian.mediaroom.com/index.php?s=31899&mode=gallery&cat=2473

QD QF QDQF B

QDQF

B

B

B

-I -I

QD QF

Emittance exchange

20 10 0 10 20 z (um)

110

100

90

80

70

Ener

gy (M

eV)

Multi-leaf mask Measured Spectrum

FEEDBACK

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Demonstrate EEX based bunch shaping at the Argonne Wakefield Accelerator

BEGINNING EXPERIMENTAL STUDIES 1:

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Demonstrate bunch shaping using a double-dog leg EEX beamline

RF Photocathode Gun

Linac Quads

Mask

20 deg

14 MeV

B1 B2 TDC

8 MeV

B1B2 B3 B4

at the AWA Facility

Demonstrate bunch shaping and compare measured shape to 1st order theory Measure EEX transfer matrix Study 2nd order effects in beamline Study space charge effects in beamline

Initial experimental goals:

TheArgonne Wakefield Accelerator Facility Low Energy (14 MeV) beamline

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Demonstrate bunch shaping using a double-dog leg EEX beamline

chirp

RF Photocathode Gun

Linac Quads

multiple masks on motorized actuator

20 deg

14 MeV

B1 B2 TDC

8 MeV

B1B2 B3 B4

at the AWA Facility

Key tunable parameters

x’ slopex, y beam size

TheArgonne Wakefield Accelerator Facility Low Energy (14 MeV) beamline

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Demonstrate bunch shaping using a double-dog leg EEX beamline

Example: Experiment I - Shaping capability

Multiple masks will be used to study the bunch shaping capability of the double dog-leg EEX beamline

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Propagation of drive beam through a 10 meter DWFA linac at APS

BEGINNING EXPERIMENTAL STUDIES 2:

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Drive bunch through a ID=400 m fiber !!! ID=400 um

Goal: Propagate drive bunch through meter scale DWFA• With no focusing

• Beam size will triple in one meter! • External focusing channel around dielectric

• ~10-20 cm focal length• Control SBBU with BNS damping

Drive bunch:• Charge = 1.6 nC• Normalized emittance = 2 m• Beam energy = 50 MeV (close to the accelerator end)• Beam size = 50 m (Beta function ≈ 10 cm)

10 m long structure test in APS LEUTL tunnel1. APS will install LCLS type e-gun in 2013

• 0.5 nC, 500 fs, 1 m bunches • Beam into the LEUTL tunnel in 2014

2. Propagate beam through 10 m long DWFA at APS• Single Bunch Beam Break Up (SBBU)• Vacuum pumping• Cooling design• etc.

LEUTL tunnel is ~ 40 m long and is ready to accept the beam

Some equipment exists, new equipment and diagnostics will be needed

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Summary The concept: High Repetition-Rate, Soft X-ray FEL User Facility

– 10 DWFAs linacs driven by a single SRF linac – 10 FEL lines @ 100 kHz rep. rate.– Compact, Inexpensive, and Flexible

A working group has started feasibility studies– Parameter studies of the overall concept– Bunch shaping studies at the AWA facility– Beam propagation through a 10m DWFA linac at APS– Modeling of the large energy spread in the FEL – Many more:

• Drive and witness jitter• Dielectric breakdown limitation testing• Etc.

We welcome collaborators and new ideas!