Physics of laser-driven and beam-driven plasma accelerators

Carl B. Schroeder

USPAS 2011

Hampton, VA

Office of Science

Supported by the Office of High Energy Physics, Office of Science, U.S. DOE under Contract No. DE-AC02-05CH11231

(similarities & differences, comparable & contrasting features, lasers or beams?)

Plasma-based accelerators for future colliders

2

•  PWFA-linear collider: •  two-beam accelerator geometry •  25 GeV drive beams •  19 plasma stages (1 TeV) •  n=1017 cm-3 (set by 30 um driver

bunch length)

•  LPA-linear collider: •  50 stages (1 TeV collider) •  10 GeV/stage •  requires ~10 J laser (at tens of kHz, hundreds of kW) •  n=1017 cm-3 (set by laser

depletion)

Leemans & Esarey, Physics Today (2009)

Schroeder et al., PRSTAB (2010)

Seryi et al., Proc. PAC (2009)

  Plasma wave excitation   Transverse wake structure

  Beam-driver - space-charge fields: extends plasma skin depth   Laser-driver - local ponderomotive force: extends laser spot size

  Regimes of operation: quasi-linear and non-linear   Energy gain: operational plasma density

  Driver propagation in plasma   Driver diffraction/divergence, self-guiding, and head-erosion   Plasma wave phase velocity ~ driver propagation velocity

  Slippage - taper for laser-driven plasma waves   Self-trapping for low phase velocities

  Driver-plasma coupling   Staging for high-energy physics

3

Outline

€

∂ 2

∂t 2+ω p

2⎛

⎝ ⎜

⎞

⎠ ⎟ nn0

= −ω p2 nbeamn0

+ c 2∇2 a2

2

Plasma wave: electron density

perturbation

Ponderomotive force (radiation pressure)

Common features:   Wave excitation efficient for driver duration ~ plasma period   Bucket size ~ plasma wavelength:   Large waves excited for nbeam/n0 ~1 or a~1

  Characteristic accelerating field:

  Phase velocity of wave determined by driver velocity

λp =2πc/ωp= (πre-1/2 ) np

-1/2 ~10-100 µm

€

E ~mcω p

e⎛

⎝ ⎜

⎞

⎠ ⎟ ≈ 96V/m( ) n0[cm

-3]

Plasma acceleration: ultrahigh accelerating gradients

Space-charge force of particle beam

€

nn0

€

nbeam

n0, a

2

2

4

€

a =eAmc 2

∝ λI1/ 2

€

ω pt

Tajima & Dawson, PRL (1979) Chen et al., PRL (1985)

Transverse wakefield structure

Beam driver (a=0)

Wakefields of a narrow bunch (kprb<<1) will extend to skin depth ~kp

-1

€

Ez

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')[ ] r'dr'∫ I0(kpr<)K0(kpr>)nbn0

kpr

kp(z-ct)

r~1/kp

kprb=0.05

Keinigs & Jones, Phys. Fluids, (1987)

rb

Transverse wakefield structure

Laser driver (nb=0)

Wakefields determined by local laser intensity gradient: extend to laser spot ~rL

€

Ez

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')[ ]∂ξa2

2

kpr

kp(z-ct)

kprL=1

Sprangle et al., APL (1988)

Beam driver (a=0)

Wakefields of a narrow bunch (kprb<<1) will extend to skin depth ~kp

-1

€

Ez

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')[ ] r'dr'∫ I0(kpr<)K0(kpr>)nbn0

kpr

kp(z-ct)

r~1/kp

kprb=0.1

Keinigs & Jones, Phys. Fluids, (1987)

rb

Transverse wakefield structure

Laser driver (nb=0)

Wakefields determined by local laser intensity gradient: extend to laser spot ~rL

€

Ez

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')[ ]∂ξa2

2

kp(z-ct)

kprL=2

Sprangle et al., APL (1988)

kpr

Beam driver (a=0)

Wakefields of a narrow bunch (kprb<<1) will extend to skin depth ~kp

-1

€

Ez

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')[ ] r'dr'∫ I0(kpr<)K0(kpr>)nbn0

kpr

kp(z-ct)

r~1/kp

kprb=0.1

Keinigs & Jones, Phys. Fluids, (1987)

rb

8

  Blow-out/Bubble/Cavitated regime:   Highly nonlinear   Expulsion of plasma electrons and formation of co-moving ion cavity:

  Focusing forces for electrons linear (determined by ion density)   Accelerating fields for electrons transversely uniform

Nonlinear regime: ion cavity formation

Beam driver:

density

nb/n0=5

conditions for cavitation: nb>n0 kpL<1 kpRb<1

Rosenzweig et al., PRA (1991) Lu et al., PRL (2006)

condition for cavitation:

€

kprL < a

density

a0=5

Laser driver:

€

kp−2∇

⊥

2 1+ a2 /2( )−1/ 2 ~ kp−2∇⊥

2φ ~ n n0 −1

Mora & Antonsen PRE (1996) Pukhov & Meyer-ter-Vehn APB (2002)

kpx

kpz

11

-11 -12 5

9

  Bubble/Blow-out/Cavitated regime:   High field (a2>>1)   Highly asymmetric and nonlinear

  Increasing intensity increases asymmetry   ion cavity:

  Focuses electrons   Defocuses positrons

  positron acceleration on density spike   Nonlinear focusing forces   Non-uniform accelerating forces

  Self-trapping may be present for laser driver (low phase velocity of wake) with a>4

staging difficult

density

accelerating field

radial field

Ultra-high laser intensity: ion cavity formation

a=3.5 k/kp=20 kpL=1 kpR=5

phase region for e+

C. Benedetti (INF&RNO)

10

  Quasi-linear/weakly-relativistic regime   a ~ 1   Nearly-symmetric regions for electron/position

acceleration/focusing   Dark-current free (no self-trapping)   Stable propagation in plasma channel   Allows shaping of transverse fields

Quasi-linear laser intensity regime: allows for e+ acceleration

density

accelerating field

radial field

a=1 k/kp=20 kpL=1 kpR=5

€

a2 1+ a2 /2( )−1/ 2 << kp2r2 4

condition for quasi-linear regime:

C. Benedetti (INF&RNO)

Shaping transverse laser intensity allows tailored transverse wakefield (focusing force)

y/r0  

Gaussian  1st-­‐order  

HG0  

HG1  

y/r0  

a2  

0.7  0.5  0.4  0  

a1/a0  

a2  =  a02  HG02  +  a12  HG1

2  

Ey/E0  

y/r0  

11

€

Er

E0= −kp

3 dξ'∫ cos kp (ξ − ξ')( )∂ra2 /2∝∇⊥a2

Add Gaussian modes: (all modes guided in parabolic plasma channel)

  Allows additional (independent) control of focusing forces (and matched beam spot)

Cormier-Michel et al., submitted for publication

  Subject to filamentation instability:

  Growth rate:

Broad beam-driver allows shaping transverse fields of beam-driven wake

12

  Shaping transverse field of beam driver requires beam transverse size to be many plasma skin depths: kprb>>1

  Return current flows in beam:

kprb>>1

kpLb~1

electron return current

€

kpΓ ~nbn0γ b

⎛

⎝ ⎜

⎞

⎠ ⎟

1/ 2

Keinigs & Jones, Phys. Fluids, (1987); Bret (2009)

kprb=2π kpLb=21/2

40

-40 5 -5

C. Benedetti (INF&RNO) kpz

kpr

€

ω pt = γ bn0 np

Operational plasma density for nonlinear PWFA

€

Ez ∝Nb

L2∝ Nbn €

nb n0 >>1

€

kpLb ~ 2

n p (c

m-3

)

bunch length, L (micron)

Lu et al., PRL (2006) Lotov (2005)

  Higher gradient achieved for ultra-short drive bunches (operating at higher plasma densities).

  For large accelerating gradient, operate in the nonlinear blow-out regime:

  Operational density determined by length of (unshaped) bunch (for fixed charge):

13

€

kpRb <1

Linear regime of beam-driven wakefields

Conditions for linear regime: kpL<1 kpRb<1 kpr

kp(z-ct)

Ez/E0

€

1≥Ez

E0= 2π nb

n0(kpL)(kpR)

2 ln(1/kpR)[ ]∝ Nbn1/ 2

  Linear regime accessible for low plasma density (for fixed bunch charge)

€

Ez = 2E0(kpre )Nb ln(1/kpR)[ ]∝ Nb

L2∝ Nbn ∝

1Nb

Nb Nb

n p (c

m-3

)

E z (V

/m)

€

n ∝1 Nb2

€

Ez ∝1 Nb€

kpR = 0.1

€

kpR = 0.1

linear regime linear regime

nb/n0

PWFA: Energy gain and transformer ratio

15

  Energy gain in beam-driven plasma wave given by transformer ratio: •  Drive beam losses energy after distance:

•  Energy gain of witness bunch:

  General considerations (e.g., symmetric bunches):

€

R = E+ E−

€

Ld ~ γ bmc2 eE−

€

Δγmc 2 ~ eE+Ld ~ R γ bmc2( )

€

R ≤ 2

€

kp (z − ct)

€

E−

€

E+€

nb n0

  Higher transformer ratios can be achieved using shaped (asymmetric bunches) •  Triangular longitudinal bunch

•  Ramped bunch train

•  Nonlinear blow-out regime: ramped bunches for high R

Lu et al., PAC (2009)

€

E−

€

E+

€

nb n0

€

kp (z − ct)

€

R ~ LbRb

n0nb

Chen et al., PRL (1986)

Drive beam hose instability

16

  Hose instability:

  Long bunches (or train of bunches) subject to electron-hose instability €

Γhose ~ choseγ b−1/ 6 ω pt( )

1/ 3kpL( )

2 / 3

Huang et al., PRL (2007)

Instability growth:

Operational plasma density for laser-driven plasma accelerators

  Laser-plasma interaction length limited by laser depletion length:

  Excited wake:

  Energy gain (single-stage):

Shadwick et al., Phys. Plasmas (2009)

€

Ld = 2.8 1+ a2 /2a2

⎛

⎝ ⎜

⎞

⎠ ⎟

⎡

⎣ ⎢

⎤

⎦ ⎥ λp3

λL2 ∝ n−3 / 2

€

Ez = 0.38 a2

1+ a2 /2

⎛

⎝ ⎜

⎞

⎠ ⎟

⎡

⎣ ⎢

⎤

⎦ ⎥ E0 ∝ n1/ 2

€

Δγmc 2 ~ Ld Ez ∝1 n

plasma density, np (cm-3)

Bea

m e

nerg

y (M

eV)

€

Δγ ∝1n

LBNL 2006

RAL 2009

MPQ 2007

LLNL 2010

LBNL 2004 RAL 2004

LOA 2004

LOA 2006 APRI 2008

U.Mich 2008

Laser-driven plasma accelerators: triggered-injection for low densites

  Phase velocity of laser-driven plasma wave function of density:

  Plasma electron self-trapping threshold increases as plasma wave phase velocity increases

€

γ p ≈ λp λ0 ∝1 n

€

vp ≈ vg = c 1−ω p2 ω 0

2( )1/ 2

  Low densities require triggered-injection techniques:   Density gradient injection   Ionization injection   Colliding pulse injection

  generate ultra-short (fs) bunches

γ

kp (z-ct)

laser"Δγ

trapped orbits

untrapped orbits

laser"e-

Esarey et al., PRL (1997); Schroeder et al., PRE (1999)

Faure et al., Nature (2006)

  Plasma wave excitation   Transverse wake structure

  Beam-driver driven by space-charge fields: extends plasma skin depth   Laser-driver driven by local ponderomotive force: extends laser spot size

  Regimes of operation: quasi-linear and non-linear   Energy gain: operational plasma density

  Driver propagation in plasma   Driver diffraction/divergence, self-guiding and head-erosion   Plasma wave phase velocity ~ driver propagation velocity

  Slippage - taper for laser-driven plasma waves   Self-trapping for low phase velocities

  Driver-plasma coupling   Staging for high-energy physics

19

Outline

Driver propagation

  Focused e-beam diverges   Characteristic distance ~β   Beam body may be self-guided in blow-out regime

  Head of beam outside cavity, continues to diverge beam head erosion:

  Solution: Low emittance beam: long beta-function ~ beam-plasma interaction

length:

density

€

β =σ r2 ε e.g., β=1 m for ε =10-10 m and σr=10 um €

rate∝ε n

  Focused laser diffracts   Characteristic distance ~ Rayleigh range:

  Beam body may be self-guided in ion-cavity   Head of beam outside cavity, continues

to diffract laser head erosion   Emittance fixed by laser wavelength €

ZR = πσ r2 λ

laser

density

e.g., ZR = 2 mm for λ=1 um and σr=25 um

€

Laser diffraction controlled by plasma channel

21

RZ

ZR"

r"

Plasma density, n(r)"

€

dηdr

=ddr1−

ω p2

2ωL2

⎛

⎝ ⎜

⎞

⎠ ⎟ < 0Guiding: "

Laser diffraction: (L ~ZR) Solution: tailor plasma profile to form

plasma channel

Durfee & Milchberg PRL (1993) Geddes et al., PRL (2005)

electrode

gas in

0V+V

bellows

sapphire

channel

laser

in

electrode

gas in

0V+V

bellows

sapphire

channel

laser

in

gas in

0V+V

bellows

sapphire

channel

laser

in

Capillary discharge plasma waveguides:   Plasma fully ionized for t > 50 ns   After t ~ 80 ns plasma is in quasi-

equilibrium: Ohmic heating is balanced by conduction of heat to wall

  Ablation rate small   ne ~ 1017 - 1019 cm-3

Hooker et al.

Experimental demonstration: 1 GeV beam using Laser Plasma Accelerator

H-discharge capillary technology: plasma channel production (~1018 cm-3)

22 Leemans et al., Nature Physics (2006); Nakamura et al., Phys. Plasmas (2007)

3cm

1012 MeV 2.9% 1.7 mrad

1.5 J 46 fs 3x1018 cm-3

Beam driver propagation velocity

density

vp vb

  Phase velocity of the wake approximately driver propagation velocity

  Beam driver velocity typically ultra-relativistic:   Eg. 10 GeV, γb = γp ~ 104   No trapping of background plasma electrons

(dark current free)   Negligible slippage between drive and

witness bunch   Stiff driver stable propagation

23

Laser driver propagation velocity

  Laser driver velocity approximately the laser group velocity (function of plasma density):

  For typical underdense plasmas using 1-micron laser:

  Trapping of background plasma electrons (beam generation) present for sufficiently large plasma waves:

  1D theory:   Bubble regime:

  Slippage (between beam and wake) can limit energy gain:

€

vg = c 1−ω p2 ω 0

2( )1/ 2

€

γ p ~ γ g =ω 0 ω p ~ 10 −100

€

Ez E0 ~ a > 2γ p

bunch

€

vphase wave ≈ vgroup laser

vbeam laser

€

Δγ ∝ γ p2

24

€

a > γ p2 2 Kostykov et al., PRL (2009)

Taper to phase-lock beam to wake

Katsouleas, PRA (1986) Bulanov et al., (1997)

Sprangle et al., PRE (2001) Rittershofer et al., Phys. Plasmas (2010)

-15 -10 5

-15 -10 -5 5

vb

vb

vg

vg

(a) n0

(b) 2n0

kp0!

kp0!

-5

Ez/E0

Ez/E00 1 2 3 4 5 61.0

1.52.02.53.03.54.0

k p(z

)/kp(0

)kp3 z /k02

(a)(b)

€

λp ∝1 n

  To lock phase of accelerating field, plasma density must increase (plasma wavelength decrease) as beam slips with respect to driver:

25

Tapering yields enhanced energy gain and efficiency in weakly-relativistic regime

€

n(z)n(0)

€

Nbucket =1

€

Nbucket = 2

€

z Ld€

Δγ€

Nbucket = 2

€

n =1018cm−3

no taper

linear taper

optimal taper

  In weakly-relativistic regime:   dephasing length << depletion length:

  Significant energy gains can be realized with plasma tapering:

Optimal longitudinal density profile: Energy gain:

€

kp3z /k0

2

€

a2 <<1

€

Ldephase ~λp3

λ02 << Ldeplete ~

λp3

a2λ02

Rittershofer et al., Phys. Plasmas (2010)

  In plasma channel, focusing and accelerating wakes have different phase velocities: varying density and channel radius to phase lock both.

26

  Plasma wave excitation   Transverse wake structure

  Beam-driver driven by space-charge fields: extends plasma skin depth   Laser-driver driven by local ponderomotive force: extends laser spot size

  Regimes of operation: quasi-linear and non-linear   Energy gain: operational plasma density

  Driver propagation in plasma   Driver diffraction/divergence, self-guiding and head-erosion   Plasma wave phase velocity ~ driver propagation velocity

  Slippage - taper for laser-driven plasma waves   Self-trapping for low phase velocities

  Driver-plasma coupling   Staging for high-energy physics

27

Outline

Plasma-based accelerators for future colliders

28

•  PWFA-linear collider: •  two-beam accelerator geometry •  25 GeV drive beams •  20 stages (1 TeV collider) •  1017 cm-3 (set by 30 um driver

bunch length)

•  LPA-linear collider: •  50 stages (1 TeV collider) •  10 GeV/stage •  requires 10 J laser (tens of kHz, hundreds of kW) •  1017 cm-3 (set by laser

depletion)

Seryi et al., PAC09 (2009)

Leemans & Esarey, Physics Today (2009)

Schroeder et al., PRSTAB (2010)

High-energy physics applications: Staging plasma-based accelerators

  For fixed driver energy, increasing beam energy will require staging

  Accelerator length will be determined by staging distance (technology) Lstage

plasma Driver

Laccelerator Lcouple

€

Nstage ~Wfinal WdriverNumber of stages:

€

Ltotal = NstageLstage

L tot

al (m

)

Lcouple (m)

Nstage = 50

coupling ~10 GeV

drive beam coupling ~10 J laser couple laser

with plasma mirrors

Laccelerator = 2 m

29

Laser in-coupling using plasma mirrors allows compact staging

  Conventional optics approach: stage length determined by damage on conventional final focus laser optics

Laser

Laser ~10 m

30

  Plasma mirror in-coupling:   “Renewable” mirror for high laser intensity  Relies on critical density plasma production  Thin liquid jet or foil (tape)   Laser contrast crucial (>1010) ~10 cm

plasma mirror

Plasma density, n (cm-3)

Lcouple =0.3 m

Lcouple =1 m Lcouple =10 m

L tot

al (m

)   Short in-coupling distance for plasma wave driver [high average (geometric) gradient]

Laser driver: Laccelerator~ n-3/2

Summary

31

  Laser or beams use different excitation mechanisms   Transverse field structure   Access to linear/non-linear regimes   Wake phase velocity

  Driver propagation:   Driver divergence   Driver-plasma interaction length and coupling length

  Driver technology:   High power, high efficiency, high rep rate beam-drivers available.   High average laser drivers under development   Laser footprint small: <10mx10m for 10’s J delivering 1-10GeV beams   Beam-driver footprint potentially small: e.g., use X-band technology with

high transformer ratio (asymmetric bunch)

  Many of these physics issues will be addressed at existing and future facilities: