Synergistic Direct/Wakefield Acceleration In
the Plasma Bubble Regime Using Tailored
Laser Pulses
Gennady Shvets, The University of Texas at Austin
John Adams Institute for Accelerator Science, Oxford, UK, May 25, 2016
History of Accelerators: Higher
Energies from Bright Ideas
βFermi predicted that future
accelerators would grow in power and
size. They would not be built on the
earth but around it, and physics
laboratories would be in outer spaceβ¦
You may expect that at some future
time accelerators will change the aspect
of the earth and make it resemble the
planet Saturnβ, Laura Fermi, 1974.
Prediction: 20 TeV CM energy by 1994
at a cost of $170B
NB: SSC would have been 40 TeV CM if
it was not cancelled in 1993 (!!)
βWhat can we learn with High-Energy Acceleratorsβ,
Retiring Presidential Address of APS, Columbia, 1954
How big are todayβs accelerators?
*Tevatron: 1 TeV/6km proton/antiproton
β’Hadrons are made of quarks need high energy/proton
huge radius for reasonable magnetic field strength
β’Rings donβt work for high energy e-p need linacs
24
4
Rm
E
dz
dE
Major problem: synchrotron radiation
LHC: 7 TeV/27km proton-proton
*Bad picture: 3km Main Injector Ring
looks larger than the Tevatron!
Linacs are for Leptons: International Linear
Collider (ILC), Next HEP Project (?)
β’SLC 2-mile linac: 50 Gev x 50 GeV collider with L = 1030 cm-2 sec-1
β’Conventional linacs are very long: 30km for 500 GeV
β’Accelerating gradient in SC cavities: 32 MV/m
β’High gradient acceleration enables miniaturization
High accelerating gradients high frequencies
cm
eE
1trap Etrap = 10 MeV/m x f [GHz]
Use lasers
whenever
you can
highest
frequency
Courtesy of Dave Whittum
The Basics of Laser Acceleration
Linear in electric field acceleration in vacuum is
impossible (Lawson-Woodward-Palmerβs theorem)
Slow electron
photon
Fast electron p
p
)0,2( pk
Cannot stop a
photon in vacuum!
Near-field accelerators: possess non-radiative field
components due to boundaries
(inverse Smith Purcell, PBG,
surface wave, plasma wakefield, β¦)
Far-field accelerators: electrons execute transverse motion
in external DC fields (IFEL, inverse
CARM, inverse Ion Channel Laser)
??
Plasma wave as a near-field accelerator
T. Tajima & J. M. Dawson, PRLβ79
Ultimate nonlinear wake: plasma bubble
Plasma bubble: the workhorse
plasma bubble
Particle advances inside bubble gains energy
from low-frequency electric field energy gain is
limited by dephasing
π―π΄π βππ
ππΈππβπΏ β π«π©π± = ππΈπ
ππ«πΏ
Can we do better??
Far-field Accelerators
Far-field accelerators (no boundaries, plasmas, etc):
β’Inverse free-electron laser (IFEL): ππΏ β ππΏπ£π₯ = ππ€π£π₯
β’Cyclotron resonance laser accelerator: ππΏ β ππΏπ£π₯ = Ξ©π/πΎ
β’Inverse ion-channel laser (a.k.a. DLA): ππ³ β ππ³ππ = ππ·
Drawbacks: (a) accelerating gradient reduces with πΈ, (b) large transverse
undulating motion, (c) difficult to maintain resonance condition
π πΈ/π π β π/πΈ IFEL curse
DLA History: From Plasma Channelsβ¦
D. H. Whittum et. al., PRLβ90;
Phys. Plasmas β92
Dave Whittum, Andy
Sessler, and John Dawson
invent an ion channel laser
A. Pukhov et. al., PoPβ99; C. Gahn et. al., PRLβ99
The MPQ team proposes and realizes the inverse ion channel laser
Electrons motion inside the bubble
and Direct Laser Acceleration
Electrons execute
betatron motion with
frequency ππ·
Transverse energy πβ₯is
reduced due to the
conservation of the
action π°β₯ = πβ₯/ππ·
ππ½ = ππ 2πΎ 1/2
πβ₯ β‘ πβ₯π/ππΈππ +ππ
ππππππ/π
Betatron frequency
Transverse energy
Betatron motion
Break the adiabatic invariant
by introducing an additional
resonant laser pulse DLA
ππΏ β ππΏπ£ = (2π + 1)ππ
2πΎ
laser pulse (ππ³, ππ³)
ΞπΎ =Ξπβ₯/ππ2
1 β π/π£πβ
Earlier indications?
A great deal of theoretical work: Nemeth, et al., PRLβ07, PRL; Phuoc, et al., PoPβ08, J. L.
Shaw et. al., PPCFβ14
Dino Jaroszynski produces MeV Gamma rays,
possibly via DLA mechanism inside a bubble!! S. Cipiccia et. al., Nature Physicsβ2011
βIn fact, this observation of high harmonic
generation could provide the first (albeit somewhat
indirect) experimental evidence of DLA, which has
so far been elusive.β G. Shvets, Nature Physicsβ2011
Big questions: (a) monochromatic beam? (b) best
laser pulse format? (c) best injection approach?
(d) major paradigm shift of LPAs in the making??
Outline of the Talk
β’ How LWFA and DLA can work together, delay dephasing,
and bifurcate the phase space
β’ How to inject electrons into the plasma bubble and have them
experience synergistic DLA/LWFA
β’ Constant gradient DLA in the decelerating phase of the wake
β’ Mix-and-match: combining multiple lasers for DLA + LWFA
Can LWFA and DLA work together?
β’DLAβs resonance condition can be undone by rapid wakefield
acceleration: ππ³ π β ππ/πππ = ππ/ ππΈ
β’DLA requires large πβ₯because π¨π³ β πβ₯ β π¨π³, but the conservation
of π°β₯reduces |πβ₯|during acceleration!
LWFA is
bad for
DLA
X. Zhang, V. Khudik, and GS,
PRL 114, 184801 (2015)
X. Zhang, V. Khudik, A. Pukhov,
and GS, PPCF 58, 034011 (2016)
But the benefits of combining
the two could be substantial!
DLA is
bad for
LWFA
β’DLA laser pulse can distort the bubble and impede LWF
acceleration or electron injection into the bubble
β’Large amplitude of betatron oscillations may reduce the
accelerating gradient experienced inside the bubble
Benefits of Synergistic Laser Wakefield &
Direct Laser Acceleration
β’Cumulative energy gain from LWFA and DLA
β’Potentially higher energy gain from LWFA due to
delayed dephasing
β’Large transverse momentum π² = πβ₯/ππ efficient
source of X-rays and πΈ βrays up to π²πharmonic of ππ³
β’Combining multiple laser pulses (mid-IR + near-IR)
X. Zhang et. al., PRL 114, 184801 (2015);
PPCF 58, 034011 (2016)
π π»
π (ππ)β
π
ππΈππβ
π + πβ₯π/ππ
πππ
πΈπ
π» = π β πππ
accel decel
Synergistic DLA/LWFA: single-particle
simulations of a particle swarm
Necessary ingredients of DLA/LWFA synergy:
(a) electron injection with large transverse energy
(b) strong overlap between electrons and the laser
(c) betatron resonance between electrons and the laser
ππ = ππΏ
1 + ππ§2 π2π2
2πΎ2+
1
2πΎπβ2
ππ (π)
ππ·(π)
Resonance
condition:
ππ β ππ·
Swarm of initial
conditions (πβ₯, πβ₯)
DLA electron
non-DLA
electron
Can DLA happen in a plasma bubble?
Pump pulse creates a bubble
Density bump βshakesβ the
bubble side-injection with
large πβ₯ facilitates DLA
n0=1.8Γ1018cm-3; n1=5.4Γ1018cm-3
π = π. πππ
I0=6Γ1019w/cm2;
I1=6Γ1018w/cm2
L2 =1.6mm, L3 = L4 = L5 β πππππ
delay:80fs
Density
ramp
injection
scenario
Self-injected electrons interact
with the weaker laser pulse
delayed by Dt=80fs
X. Zhang et. al. PRL 114,
184801 (2015)
DLA inside a plasma bubble after 1cm propagation
Electrons separated into two
groups DLA electrons with
large πβ₯gain more energy and
fall behind the non-DLA ones
Pump: ππ³ = π. π, ππ³ = ππππ, ππ = ππππ
DLA : ππ³ = π. π, ππ³ = ππππ, ππ = ππππ
DLA
with DLA pulse
w/o DLA pulse
DLA
non-DLA
X. Zhang et. al. PRL 114,
184801 (2015)
Phase space bifurcation
Two-peak spectrum
separated by 400 MeV
Bifurcation is absent
without DLA pulse
Phase Space Correlations: Key to Synergy
DLA
non-DLA
DLA electrons
strong correlation
between total
energy πΎππ2 and
transverse energy
πβ₯ =πππ
ππΈπ+
ππππππ
π
Strong bifurcation in (πβ₯, πΈ) phase space
Synergy between DLA and LWFA higher energy
gain from the wake for the DLA population
delayed dephasing!
π π»
π (ππ)β
π
ππΈππ β
π + πβ₯π/ππ
πππ
πΈπ
DLA electrons gain extra 200 MeV
from the wake and extra 400MeV
from the laser (DLA)
DLA is compatible with ionization injection!
π0 = 4 Γ 1018ππβ3
πΌpump = 2.3 Γ 1019π/ππ2
πion = 870ππ from π7+to π8+
Off-axis or off-peak phase ionization produces DLA electrons!
Electrons after 3mm
πΌDLA = πΌpump/2
πpump = 96 ππ
Off-peak phase ionization and βricochetβ
DLA electrons: real atoms meet meta-atoms
π¬β₯
π¨β₯
Off-peak ionization phase:
electrons leave the laser
pulse with finite
transverse momentum
πβ₯ + ππ¨β₯/π = ππ¨β₯ ππ /π
Ricochet electron starts out
with large πβ₯, interacts with
the DLA pulse gains even
larger πβ₯ and more energy
Phase
One Step Back, Two Steps Forward: Laser
Wakefield Decelerator + DLA
cmct 8 π΅ = π
π΅ = π
π΅ = π
Transverse
energy
growth
ππΏ
1 + ππ§2 π2π2
2πΎ2=
π΅ = π
π΅ = π
π΅ = π
= πππ
2πΎ
Model: constant
decelerating field π¬πΎ
Multiple DLA harmonics:
ππΈπ = β40πΊππ/π ππΏ = 2, π£π = π
Initial conditions
Who needs LWFA if DLA is so great?
ππ = π. πππ pulse:
π·π = πππTW (ππ = π)
ππ = ππfs, ππ = πππm
π0 = 4 Γ 1018ππβ3
The wake decelerates the electrons, but
the DLA accelerates them at more than
twice the deceleration rate!
Loss to wake
Gain from laser
External injection into
the decelerating phase ππ₯0 = 25πππ
π₯ β ππ‘ /ππΏ
π₯ β ππ‘ /ππΏ
Relativistic πΎ
The mix-and-match approach to LA: the
case for combining near- and mid-IR lasers
β’Mid-IR lasers produce a large bubble ππ βΌ ππ ππ³ because less dense
plasma is used large-amplitude betatron oscillations are not a problem
β’Vector potential ππ³ βΌ ππ³ π°π³ is large for modest laser intensity
β’External electron injection into a large bubble is easy
β’Unique opportunity for combining a mid-IR laser pulse (βwork horseβ that
makes a bubble) with an ultra-short solid-state laser pulse (βsurgical toolβ
that injects electrons, excites betatron oscillations, provides DLA)
Electric field or vector potential?
ππ³π βΌ ππ³
ππ°π³
Ponderomotive
potential:
Ionization rate of
neutral gasses:
π¬π³ βΌ π°π³
Direct Laser
Acceleration gradient:
π¬π³ β ππ· βΌ π°π³
Injection, LWFA, and DLA using a sequence
of π. πππ and a π. πππ laser pulses
ππ = πππ pulse:
π·π = ππTW (ππ = π. π)
ππ = ππfs, ππ = πππm
ππ = π. πππ pulse:
π·π = ππTW (ππ = π. π)
ππ = ππfs, ππ = πππm
Time delay: π«π = πππfs
π0 = 8 Γ 1017ππβ3
π₯ = 5.5mm Electrons gain
400MeV from wake
and 200MeV from
π = 0.8ππ laser: 1st
harmonic DLA
ππΏ 1 β π£π§ π£π = ππ½
DLA on a budged: 10 TW-scale laser systems
Pump: ππ = π. πππ, π·π = ππTW,
ππ = ππfs, ππ = πππm
DLA: ππ = π. πππ, π·π = ππTW,
ππ = ππfs, ππ = πππm
non-DLA
electrons
DLA
electrons
DLA electrons
non-DLA
Large number of DLA electrons can be
observed at much lower laser powers and higher
plasma densities (π0 = 1.5 Γ 1019ππβ3)
Time delay: Ξπ = 24ππ
Problem: time delay jitter!
Rel
ativ
isti
c πΎ
Sensitivity to the time delay jitter
Destructive interference: πΏπ = βπ/2π
The bubble is not distorted large number
of trapped non-DLA electrons
Few DLA electrons
non-DLA
electrons
DLA
electrons
Sensitivity to the time delay jitter
βAverageβ interference: πΏπ = β3π/4π
The bubble is slightly distorted smaller
number of trapped non-DLA electrons
More DLA electrons
non-DLA
electrons
DLA
electrons
Sensitivity to the time delay jitter
Constructive interference: πΏπ = βπ/π
The bubble is strongly distorted small
number of trapped non-DLA electrons
Many more DLA electrons
non-DLA
electrons
DLA
electrons
Frequency doubling of the DLA pulse
Pump: ππ = π. πππ, π·π = ππTW,
ππ = ππfs, ππ = πππm
DLA: ππ = π. πππ, π·π = ππTW,
ππ = ππfs, ππ = πππm
DLA: ππ = π. πππ, ππ = π. ππm
Reduced π: no
bubble distortion
improved DLA
and non-DLA
electron yields
No interference
reduced jitter
sensitivity
How the entire LPA paradigm may be
changed by Direct Laser Acceleration
β’Synchronization of externally injected
beam is the key to injecting into the
decelerating phase
β’The main role of the bubble is not
accelerate but to provide focusing field to
undulating electrons
β’Excellent source of X-ray and Gamma-
ray radiation because of the large
undulator parameter πΎ = πβ₯/ππ
ππ βΌ 2πΎ2ππ½
πΎ3
1 + πΎ2βΌ πΎ3ππΏ
πΎ
Conclusions and Outlook
β’The synergy between DLA and LWFA acceleration mechanisms
can be realized using novel pulse formats (e.g. trailing bump, near-
IR laser trailing a mid-IR laser, etc.)
β’New physics: delayed dephasing due to electronsβ betatron motion
electrons advance slowly inside the bubble
β’Side-injection maximizing transverse electron momentum can be
realized using a sharp density bump or ionization injection
β’Unique acceleration opportunities for externally injected
electrons Constant Gradient Direct Laser Acceleration by
injecting into the decelerating phase of the bubble