bunch length dependence of plasma wakefield accelerators

1

Bunch Length Dependence ofPlasma Wakefield Accelerators

A Proposal for a User Experimenton the ATF User Facility at the

Brookhaven National Laboratory

Submitted byThomas Katsouleas (PI) and Patrick Muggli (Co-PI)

University of Southern California, Los Angeles

P. Muggli, BNL/CAP 1-2/6/2000

2

OUTLINE


- Motivation for this work

- Plasma Wakefield Accelerator (PWFA) scaling law

- Why ATF?

- Description of the experiment & plan

- Diagnostics and measurements

- Proposed schedule

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0

1

2

3

4

5

6

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1930 1940 1950 1960 1970 1980 1990 2000 2010

LivingstonCurrent statusfuture

Log

of E

nerg

y in

MeV

Year

RAL/IC/UCLA/LULI NRL, UM

UCLA

UCLA Port J. LLNL/UCLA

?

SLAC/USC/UCLA/LBL

LEP


MOTIVATION

PWFA?

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- Transverse dynamics (E-157)- Betatron oscillations- e- beam tail motion

- Hose instability- Limits the propagation/acceleration length- Limits the accelerating gradient

- Long plasma production- Meter long, homogeneous plasmas (E-157)

- Acceleration scaling law


PWFA KEY ISSUES

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WHAT DO WE PROPOSE?

We propose to measure the parametric dependenciesof the PWFA gradient:

eE ≅ 70MeV / mNb

1010

1.0

σ z (mm)

2

λp ≅ 4σ zwith

Q=0-1nC, Nb=0-7·109 e-

τ=σz/c=1-10ps

n0=1013-1015

Never measured before!

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Physical Principles of the PlasmaPhysical Principles of the Plasma Wakefield Wakefield Accelerator Accelerator

• Space charge of drive beam displaces plasma electrons

•• Transformer ratio

•• Wake Phase Velocity = Beam Velocity (like wake on a boat)

•• Plasma ions exert restoring force => Space charge oscillations

E Ezacc dec beam. .

• Wake amplitude ∝ N b zσ 2 ( for 4σ z ≈ λp ∝1

no

)

++++++++++++++ ++++++++++++++++

----- --- ----------------

--------------

--------- ----

--- -------------------- - --

---- - -- ---

------ -

- -- ---- - - - - - ------ - -

- - - - --- --

- -- - - - - -

---- - ----

------

electronbeam

+ + + + + + + + + + ++ + + + + + + + + + + + + + ++ + + + + + + + + + + + + + +

+ + + + + + + + + + + + + + +-

- --

--- --

EzEz


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WAKE FIELD AMPLITUDE

eE(eV / cm) = n0 (cm−3 )nb

n0

kpσ ze− kp

2σ z2 2

1+ 1 kp2σ r

2Linear Theory:*

The maximum gradient is given by:

with a plasma density corresponding to: 2σ z ≅π

cω p

or λp ≅ 4σ z

eE ≅ 70MeV / mNb

1010

1.0

σ z (mm)

2

and scale as 1/σz2 ! -50

0

50

100

150

200

250

0 5 1014 1 1015 1.5 1015 2 1015 2.5 1015

eE (

MeV

/m)

n0 (cm-3)

*S. Lee et al., Phys. Rev. E, May 2000

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ACCELERATING STRUCTURE

2-D non-linear PIC numerical simulation

E-157 PWFA : ne=2·1014 cm-3-> f=127GHz

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WAKE FIELD AMPLITUDE

0

0.2

0.4

0.6

0.8

1

106

107

108

109

1010

0 5 10 15

PeakField&Npart(PlasDens).data

Peak

Acc

eler

atin

g Fi

eld

(GeV

/m)

Num

ber of Particles

Plasma Density (× 10-14 cm-3)

2-D non-linear PIC simulation for E-157:

Nb=4·1010 e-

σz=0.6mmσr=70µmγ=60000

no≈2.1·1014 cm-3

λp≈4σz

Check!

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σz [mm]

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6

E1 [

GeV

/m]

N = 4e10 e-

σr = 75 µm

n0 optimum


ACCELERATING FIELD (E1)vs. BUMNCH LENGTH

2-D cylindrical, non-linear simulation

Numerical simulations confirm linear result: eE∝1/σz2

Check!

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0

0.05

0.1

0.15

0.2

0.25

0.3

-150

-100

-50

0

50

100

150

200

0 5 10 15 20

ATF-Wake.data

n0/n

beE

z (MeV

/m)

(z-ct)c/ωp


SIMULATION FOR ATF2-D fully non-linear simulation (NOVA code)

Q=1nC, σz=300µm, σr=100µm, n0=2·1014 cm-3

eE≈160MeV/m, δn/n0=0.12

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- High quality e- beam

- High current e- beam

- Variable e- bunch length (σz)

- TW synchronized CO2 laser for plasma production/diagnosis


WHY ATF?

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EXPERIMENTAL PLAN

ATF e- beam0.1,1ps ≤ τ ≤ 10ps

Q ≤ 1nC

Multi-TWATF CO2 laserI>1014 Wcm-2

over a few cmH gas cell

0.01T≤P≤1Tor

Li sourceArF laser

Diagnostics:- Thomson scattering- Plasma light- Radiation at fp

Q=1nC eE (MeV/m) ne (cm-3)

τ=1ps

τ=10ps

160

1.6

7.8·1014

7.8·1012

Plan: Vary ne for each σz (or τ), and Q (or Nb)

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PLASMA SOURCE

- Field ionized H or He static fill

- requires CO2 laser pulse with I>1014 W cm-2 over a few cm (TW ATF upgrade)

- fully ionized

- plasma density determined from gas pressure

- Lithium plasma source

- requires ArF Excimer laser

- large dimensions plasma

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Pscatt

Pinc .

=π2

n0

ncrit

δn

n0

L

λ

2

- Scattered power:

- Compare to the UCLA Mars case:

δn/n0=0.3 (eE=3GeV/m, ne=1016 cm-3), λ=0.5µm, n0/ncrit≈2.2·10-6, L≈200µm

δn/n0=0.1 (eE=160MeV/m, ne=2·1014 cm-3), λ=10.6µm , n0/ncrit ≈7.8·10-5, L≈2cm

Pscatt

Pinc .

≈ 30Pscatt

Pinc . UCLA

- Laser light scatters on the plasma wake (grating)


THOMSON SCATTERING

(at k-matching)

- Frequency shift: ωs=ω±ωp or 130GHz @ ne=2·1014 cm-3

Spectrograph can resolve ∆ν ≈ 10GHz

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THOMSON SCATTERINGMars Laboratory, UCLA:

space and time resolved Thomson scattering

ne≈1016 cm-3

λ=0.5µmL≈200µm

0.00 0.23 0.33 0.40 0.46 0.52

4

0

-4

-8

-10 -5 0 5 10

Position (mm)

20

100

120

40

60

80

(a)

(b)

0 100 200 300 (ps)

Beat Wave Amplitude (n1 /no )

Am

plitu

dePo

sitio

n (m

m)

Parameters:

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e- BeamOnly

Laser/PlasmaOnly

e- Beam+

Plasma


PLASMA LIGHTFROM WAKE DISSIPATION

(Preliminary E-157 result*)

* P. Catravas et al., in prep.

UV

e-

view

Li Vapor

Plasma Light

5

10

15

20

25

30

35

40

45

0.1 1 10

PlasmaLight.dataPlas

ma

Lig

ht D

iode

Sig

nal (

mV

)

Incident UV Energy (mJ)

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PLASMA RADIATION AT fp

- With ne in the 1014 cm-3 range, fp≈100GHz range

- Large amplitude plasma waves are excited

- Plasma waves do not couple efficiently to vacuum modes

- Detect leakage of plasma radiation at fp

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CO2 Light toThomson Scattering

Radiationat ωp

TW CO2

Laser Pulse

FocusedATF

e- Beam

FocusingOAP

Gas Cell

Plasma LightMeasurement

Plasma


SCHEMATIC OF THE EXPERIMENT

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PROPOSED SCHEDULE

- Experiment extends over 2-3 years, with 1 or 2 runs/year

- Each run is 3-4 weeks

- Each run is preceded by >4 weeks of off line preparation

- However, will accommodate ATF schedule

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- Use unique ATF capabilities to measure the wake amplitude scaling with respect to the e- bunch length (eE∝1/σz

2)

- Use a field-ionized H plasma or a lithium plasma source

- Measure wake amplitude as a fct of σz, Nb, and n0 using:

- Thompson scattering of CO2 laser pulse- Plasma light produced by dissipation of the wake- Radiation at ωp from the plasma (µwave)

- Parameters:- Q≤1nC- σz=1ps eE=160MeV/m n0≈7.8·1014 cm-3

- σz=10ps eE=1.6MeV/m n0 ≈7.8·1012 cm-3


EXPERIMENTAL PLAN

bunch length dependence of plasma wakefield accelerators

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