Summary Report of Working Group 6: LaserPlasma Acceleration
A. Ting,1 R. F. Hubbard,1 and G. Shvets2
Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375-5346
2
Illinois Institute of Technology, Chicago, IL 60616
Abstract. There has been considerable progress on laser-plasma acceleration. Presentations on
laser-plasma acceleration concepts at the 2002 Advanced Accelerator Concepts Workshop are
reviewed, and the status and future directions of research in this area are discussed.
INTRODUCTION
Laser-plasma acceleration has received considerable attention because of the
extraordinarily high accelerating gradients that can be attained. Plasma is also the
ultimate 'disposable' structure since it can be regenerated as often as needed. Many
successes have come to light in recent years in this field. These include the
experimental demonstration of accelerating gradients exceeding 100 GV/m, energy
gains exceeding 100 MeV, optical guiding of high intensity laser pulses in plasma
channels over many tens of Rayleigh ranges, the development of all-optical injectors,
and an extensive modeling and simulation capability. The primary laser-plasma
accelerator concepts are the standard (short pulse) and self-modulated (long pulse)
laser wakefield accelerators (LWFA) and the plasma beatwave accelerator (PBWA).
Sprangle (NRL) offered a roadmap in his plenary talk for achieving a table-top,
multi-GeV high quality laser accelerator with existing laser technology. The approach
is based on the standard LWFA with phased, mono-energetic optical injection to
produce high electron beam quality and guided acceleration in a plasma channel to
overcome diffraction. Further gains in energy beyond 1 GeV are possible by tapering
the plasma channel density to extend the dephasing length and employing additional
laser stages to overcome energy depletion of the drive laser pulse. Most of the major
experimental groups have designed their future experiments around some variation of
this basic approach. This is also reflected in much of modeling and simulation effort.
First generation acceleration experiments have mostly been in the self-modulated
(SM) LWFA regime. This regime is attractive experimentally because it does not
require external injection of electrons, and the laser is self-guided due to relativistic
effects. However, electron beam quality is generally poor, and the propagating laser
pulse is highly unstable. The second generation experiments, which will employ
external electron injection and long range optical guiding, are much more challenging.
Modeling and simulation also plays a major role in analyzing current experiments and
CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli
© 2002 American Institute of Physics 0-7354-0102-0/02/$19.00
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new concepts. There continues to be a vigorous effort to develop new schemes for
injection, acceleration, and optical guiding and new experimental diagnostics.
PROGRESS AT MAJOR EXPERIMENTAL FACILITIES
The first part of the working group was devoted to reviews of some the major
experimental facilities in the U.S., Europe, and Japan. More details are given in later
sections on injection, guiding, acceleration, modeling, and future plans.
Plasma Beatwave Accelerator
Clayton (UCLA) described progress on the plasma beatwave accelerator (PBWA)
that is being developed at UCLA's Neptune facility. The major goal is to demonstrate
acceleration to 100 MeV with phase locking and small energy spread. The
acceleration process involves the beating of CC>2 laser pulses at 10.3 and 10.6 jim
wavelengths at 1 TW and 40-100 psec pulse duration. A low emittance 11 MeV
electron beam is to be injected into the PBWA. Recent experiments employing a
collinear Thomson scattering diagnostic have demonstrated the excitation of a strong
plasma beatwave. Clayton also summarized work on phased injection using double
beatwave and inverse free electron laser (IFEL) approaches, terahertz radiation
sources, and electron trapping at density transitions.
Laser Wakefield Accelerator
Although the long term interest is primarily devoted to the standard LWFA, most
experiments continue to operate in the self-modulated (SM) regime.
Results from a major new SM-LWFA at LOA were reported by Fritzler. Energies
up to 200 MeV were reported with the characteristic broad energy spectrum seen n
other experiments. Electron beam emittance improved when the higher energy
component of the spectrum was selected. This experiment used a short laser pulse that
may have transitioned from the self-modulated to the standard LWFA regime. This
experiment produced considerable excitement and was the subject of several
simulation studies at other laboratories.
Leemans (LBNL) demonstrated control of electron production in a SM-LWFA
experiment by precise alignment and position of the laser focusing and by shaping the
laser pulse envelope. This offers a new path for enhanced control of wake excitation.
Kaganovich (LET/NRL) reported initial results from the upgraded T3 laser in which
the pulse is split into two beam lines with separate compressors. One of the beam
lines has been operated as a SM-LWFA, producing an electron spectrum that peaks at
3 MeV. Saleh (U. Michigan) reviewed work at that laboratory, emphasizing a proof
of principle experiment for the LILAC all-optical injector.
Y. Kitagawa (U. Osaka) reported SM-LWFA experiments with a 60 TW pulse
operating at a plasma density of 1019 cm"3, including forward Raman scattering (FRS)
measurements. Suk and Lee (KERI) reported on LWFA and photon acceleration
simulations and experiments in Korea.
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Kando provided an overview of the programs at JAERI/APR. This facility has a
100 TW laser, a 150 MeV microtron for electron beam injection, and the capability to
produce z-pinch plasma channel waveguides. A recent experiment at APR with a 2
TW, 50 fs laser and a gas jet density of VxlO17 cm"3 detected ~20 GV/m plasma waves.
For these parameters, the experiment should operate in the short pulse or standard
LWFA regime. Wakefield amplitudes and wavelengths were consistent with ID PIC
simulations and linear theory.
Other major laboratory overview talks were given in later sessions. These included
talks by Downer (U. Texas) and Milchberg (U. Maryland) that emphasized guiding in
plasma channels and Pogorelsky (BNL) that discussed guiding and x-ray sources.
ELECTRON BEAM INJECTION
In the SM-LWFA, electrons are self-trapped from the laser-produced plasma, so
external electron beam injection is not required. However, the PBWA and standard
LWFA require phased, external injection to produce a high quality beam. Much of the
recent interest has centered on all-optical injection schemes since these offer the
possibility of high beam quality and phased injection. These schemes generally
require some mechanism to perturb the phase space of the plasma so that some
electrons acquire sufficient axial velocity to be trapped and accelerated to high
energies.
Single Pulse Plasma-Based Injectors
Perhaps the simplest all-optical injector is an SM-LWFA. Both NRL and LBNL
are considering using such an injector for future LWFA experiments. One problem
with this scheme is that the energy spectrum of the accelerated electrons is broad.
Kaganovich (LET/NRL) described using a SM-LWFA with a magnetic energy
selector to overcome this difficulty. The system passes electrons from the SM-LWFA
source through a focusing solenoid that is followed by a low dispersion steering
magnet. Energy selection is achieved by aperturing the electron beam. Compared
with NRL's LIPA injector (Laser lonization and Ponderomotive Acceleration), this
scheme requires substantially less laser power for multi-MeV injection. However,
LIPA is expected to produce a much shorter bunch, more suitable for phased injection.
Leemans (LBNL) reported plans for using a SM-LWFA injector in a future
channel-guided LWFA experiment. Model calculations indicate a high charge in the
bunch, a broad energy spectrum mostly in the 1-3 MeV range, and small angular
spread. Electrons are injected directly into the channel.
Kinoshita (U. Tokyo) reported on the generation of relativistic electrons using the
usual SM-LWFA configuration, a 4 TW laser, and a gas jet.
An alternative plasma-based injection scheme is plasma density transition trapping
(PDTT). This scheme passes a drive electron beam through a sharp density transition
to produce phased injection of a short, low emittance bunch. Thompson (UCLA)
described a proposed PDTT experiment using the 14 MeV drive beam on the PWFA
experiment. Simulations were carried out using the widely-used MAGIC code.
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Suk (KERI), who originated the electron beam-driven version of the PDTT concept
while at UCLA, has considered using a laser as the drive beam. He reported OSIRIS
simulation results and described possible future experiments. Best results are obtained
when the density transition is shorter than the plasma wavelength.
Multiple Pulse All-Optical Injectors
A second class of optical injectors utilizes one or more "injection" pulses in
addition to the primary drive pulse. The first of these concepts to be proposed was the
U. Michigan LILAC scheme. In the original LILAC concept, the drive pulse operated
in the standard LWFA regime, and a transverse injection pulse perturbed the wake
plasma, kicking some plasma electrons into a phase space region where they could be
trapped and accelerated. Saleh (U. Michigan) described recent experiments in which
the drive pulse was in the self-modulated regime. Parameters were chosen so that in
the absence of the injection pulse, the SM-LWFA drive pulse produced little
acceleration. The addition of the injection pulse produced a substantial increase in the
quantity and characteristic energy of the accelerated electrons. In addition, the
injection pulse produced a factor of two reduction in the transverse emittance. This is
believed to be the first demonstration of a multiple pulse all-optical injector.
However, phased injection will probably require operating at lower plasma densities
or shorter pulses so that acceleration is in the standard LWFA regime.
The colliding (CP) pulse optical injector is being pursued at LBNL. The original
colliding pulse scheme involved a large amplitude drive pulse and two directly
counterpropagating lower intensity injection pulses. Leemans (LBNL) presented
plans for upcoming colliding pulse experiments, including preliminary results from a
simpler two pulse scheme with a larger amplitude injection pulse replacing the two
counterpropagating pulses. To simplify the experimental configuration, this pulse was
injected at a 30 degree angle with respect to the drive pulse. The two-pulse
configuration produced a significant increase in the yield of accelerated electrons.
Esarey (LBNL) described model calculations for both the two pulse and three pulse
configurations. He also reported model calculations of electron trapping from density
transitions.
Cary (Tech-X) reported extensive simulations of both LILAC and CP optical
injectors. The object-oriented VORPAL code was used for these studies. Particular
emphasis was given to the regime where the size of the wakefield focusing region in
phase is increased due to nonlinear effects.
Tochitsky and Clayton (UCLA) reported on the double beatwave injection scheme.
This involves using both 10.3 and 10.6 jim pulses from the UCLA Neptune laser that
are further split into two beamlines, an "injector arm" and an "acceleration arm".
Experiments to date have not produced significant acceleration. Later analysis
identified the use of different focusing optics in the two beamlines as the likely source
of the problem.
Pitthan (SLAC) described injector emittance and bunch particle requirements for
plasma-based injection schemes from the point of view of the eventual user. Plasmabased injectors offer the possibility of significantly higher brightness than can be
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obtained from conventional injectors. There is significant potential payoff both for
future linear colliders as well as advanced radiation sources such as FELs.
RF injectors
There have also been significant advances in applying conventional RF-based
injectors to laser-plasma accelerators. The typical strategy employs a photocathode for
timing and emittance control and a chicane or inverse free electron laser (IFEL) to
shorten the pulse. This approach is producing high quality bunches with very short
bunch lengths that may make it unnecessary to rely on all-optical injectors.
JAERI has a 150 MeV microtron available as an injector for future experiments.
Tochitsky (UCLA) reported on a THz IFEL prebuncher that they are using on the
photocathode gun in their PWFA facility. Wang (BNL) described how 10 fs bunches
with 50-200 MeV energy could be generated by chirping the energy in the bunch and
compressing in a chicane.
GUIDING AND ACCELERATION IN PLASMA CHANNELS
Acceleration to high energy generally requires some form of optical guiding.
Relativistic guiding is believed to play an important role in the SM-LWFA
experiments. However, the guiding effect is not well-controlled in this regime, and
relativistic guiding is expected to be ineffective for the short-pulse (standard) LWFA
regime. Plasma channels, which are long plasma columns with an on-axis density
minimum, have successfully guided laser pulses at many laboratories. The subject
was reviewed in the plenary talk by Zigler (Hebrew U.).
Recent channel guiding experiments
The first technique developed for guiding intense pulses in channels utilized the
axicon-focused ("Bessel") laser, in which the laser produces a shock that evolves to
the desired plasma density profile. This technique is currently employed in
experiments at U. Maryland, U. Texas, and LBNL. Milchberg (U. Md.) presented
results from a recent experiment that showed that the efficiency of coupling from the
laser to the plasma could be "resonantly" enhanced by a self-trapping mechanism.
This coupling leads to shortened trapped pulses and parametric instability that
modulates the channel. The use of end funnels on the channel improved the coupling.
Downer (U. Texas) emphasized the importance of producing a fully-ionized, low-Z
channel so that the guided laser pulse does not produce additional ionization that
interferes with the guiding process. Geddes (LBNL) reported improvements in the
nozzle design and plans for future channel-guided LWFA experiments with a SMLWFA injector.
The other major approach to producing plasma channels involves the use of
capillary discharges. Ablative wall discharges have been used at several laboratories,
including Hebrew U., NRL, BNL, and Oxford. These discharges have a limited
lifetime but are the easiest plasma channel generation device to field experimentally.
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Zigler (Hebrew U.) reported recent work on using segmented capillary discharges to
extend the length of the capillary and produce an axial variation or taper in the density.
Tapered density channels can partially overcome the LWFA energy gain limitations
arising from dephasing.
Pogorelsky (BNL) reported the first demonstration of channel guiding of a short
pulse CC>2 laser. These experiments are geared towards x-ray source applications as
well as accelerators.
Laser guiding has also been demonstrated in gas-filled capillary discharges. These
devices have a ceramic wall, so they have much longer lifetimes than the ablative wall
discharges and can produce a fully-stripped, low-Z plasma. Kando (JAERI) described
guiding experiments using a fast z-pinch discharge. Jaroszynski (U. Strathclyde)
described plans to use a slow, gas filled discharge.
Cros (U. Paris) described ongoing guiding experiments in capillary tubes. In this
scheme, the guiding is due to the interaction between the laser and the capillary walls
and is not due to a plasma channel. Guiding at intensities above 1017 W/cm2 over
distances of up to 12 cm has been achieved. Preliminary experiments performed at
Rutherford Appleton Laboratory in gas-filled capillaries operating in the SM-LWFA
regime have produced forward Raman scattering but no accelerated electrons.
Plasma channel diagnostics
Much of the effort in recent experiments has been in improving diagnostics.
Capillary discharges present an obvious challenge since the channel cannot normally
be observed from the outside. Zigler (Hebrew U.) reported a new autocorrelation
technique that deduces the average plasma density in the capillary from the time delay
arising from the difference in the pulse group velocity from c. Jones (NRL) reported
ablative wall discharge experiments in transparent, rectangular capillaries that
permitted direct observation of the plasma inside the capillary. He also described a
temporally-resolved Raman backscatter diagnostic that has been used on both
discharge and laser-ablated capillaries.
Diagnostic improvements have also been made in axicon-generated channel
experiments. Downer (U. Texas) reported pump-probe experiments where the probe
frequency is near that of the pump to avoid slippage or "walk-off" associated with the
difference in the group velocities of the two pulses. Alexeev (U. Marland) reported a
direct measurement of the apparent "superluminal" velocity of the ionization front in
an axicon-focusing experiment. Kim (U. Maryland) described a new single shot
supercontinuum spectral interferometry technique, which offers a direct measurement
of the phase shift AO in a plasma or gas.
Channel guiding and focusing models
Hafizi (Icarus/NRL) described a generalized envelope equation model that includes
relativistic focusing, ponderomotive channeling, and tapered plasma channel guiding.
The model agrees well with simulation results from LEM and WAKE.
Hubbard (NRL) described how short plasma channels may be used as lenses to
control intense laser pulses and compared simulation results with an analytical model
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based on the envelope model described above. He showed simulations of a standard
LWFA using a series of plasma channel lenses as periodic focusing elements
embedded in a uniform plasma. This is a potential solution to the difficulties
associated with making long, low density plasma channels.
Energy gain in channel-guided LWFA
There has been a significant effort to predict the performance of future channelguided LWFA experiments using phased electron injection. Sprangle (NRL) and
Penano (NRL) reported simulations of multi-GeV energy gain from a standard LWFA
propagating in a tapered plasma channel. With optimal density tapering in the
channel, the phase of the accelerating electrons can be held relatively constant for
distances well beyond the classical dephasing length. Energy gains of 5GeV in a
single stage appear possible, with "moderate" laser intensity (a ~ 0.5) and little pulse
distortion. Much higher energies are possible if the LWFA can be staged with proper
phasing at each stage.
Penano (NRL) also reported simulations of forward Raman scattering and selfmodulation in longer pulse LWFAs. The phase slippage rate is much faster than in the
standard LWFA regime, so channel density tapering is probably essential. This
channel-guided SM-LWFA offers an interesting regime for near term experiments
because the channels are easier to make than those required for a standard LWFA.
Phased injection is generally believed to be a requirement for a high quality LWFA
electron beam. However, two simulation studies reported remarkable high quality
beams without phased injection. TurboWAVE simulations reported by Gordon
(Icarus/NRL) showed that low emittance and energy spread could be obtained in a
GeV channel-guided LWFA with 1 MeV external injection even though particles were
injected over all phases. Those particles near the optimum injection phase were
selected for acceleration, while others were left behind. Narang (UCLA) reported
both TurboWAVE and OSIRIS simulations of channel-guided LWFAs. Of particular
interest was a case with high intensity (a ~ 2) that exhibited self-trapping of plasma
electrons and acceleration of a single bunch to several hundred MeV. This may be the
first simulation that exhibited self-trapping in the standard LWFA regime and opens
up an interesting regime for future experiments.
NOVEL ACCELERATORS AND RADIATION SOURCES
There continues to be an active effort to develop new acceleration techniques or
introduce major refinements on existing approaches. Some of this work has
applications to advanced radiation sources as well as acceleration.
Shvets (IIT), Hur (UC-Berkeley), and Puhkov (U. Dusseldorf) have examined an
RF-driven inverse Cherenkov accelerator concept that produces large amplitude
plasma waves and substantial pulse compression. For a 300 GHz RF source, a plasma
density of 1015 cm"3, and a 1 T transverse magnetic field, the predicted gradient is 1
GeV/m. Shvets, Hur, and Wurtele (UC-Berkeley) have also proposed a novel
accelerator based on electromagnetically induced transparency (EIT) in a plasma. The
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BIT dispersion relation predicts perfect matching with vacuum (ck = co), strong pulse
compression, and a predominantly longitudinal electric field, which are good
properties for particle acceleration.
Filip (UCLA) analyzed and compared nonresonant heatwave excitation with the
traditional PWFA heatwave approach. TurboWAVE simulations predict peak
longitudinal electric fields of up to 100 MV/m. Guang (UCLA) reported on
experiments using the Neptune laser to drive Cherenkov wakefields in magnetized
plasma. This is expected to be a strong source of THz radiation.
Pukhov (U. Dusseldorf) is using the 3-D simulation code VLPL to maodel wave
breaking and bubble or cavity formation at very high intensities, electrons are trapped
inside the bubble and accelerated to energies up to 500 MeV. The 33 fs, 12 J laser
pulse is compressed down to a single optical cycle.
Gordon (Icarus/NRL) modeled a two-stage SM-LWFA concept in the regime where
the pulse is weakly-guided by relativistic self-focusing. He also proposed controlling
Raman forward scattering (RFS) and limiting the injection phase of trapped electrons
by seeding the laser with a second ultra-short pulse. Seeded RFS concepts were also
discussed by Downer (U. Texas) and Steinhower (STI-Optronics), who considered
both electron beams and lasers as possible sources for the seed pulse.
Lindberg (UC-Berkeley) reported on scaling laws XOOPIC simulations of a
colliding beam accelerator concept proposed by Shvets. The 2-D simulations included
a plasma channel to guide the laser pulse.
Hartemann (LLNL) described the new Pleiades facility, which includes a 150 Mev
linac and the 500 ml, 50 fs FALCON laser. The primary goal is to produce a bright
Compton x-ray source with femtosecond pulse length.
Lee (KERI) described simulations and possible experiments on photon acceleration.
Propagation in a downward density gradient produces a blue shift. A conceptual
design for a ring recursive photon accelerator was also presented.
MODELING OF CURRENT EXPERIMENTS AND LASERPLASMA INTERACTIONS
Energy gain in SM-LWFA experiments
Although SM-LWFA acceleration experiments have been successfully carried out
at a number of laboratories, the details of the pulse propagation and electron
acceleration processes are still not well understood. There are a number of
complicated competing processes that occur, and much of the past theoretical and
numerical work has relied on 2-D models that use either unrealistic slab geometries or
that make other assumptions that may become invalid as the pulse propagates. Fully
explicit 3-D simulations are just now becoming available.
Both Mori (UCLA) and Gordon (Icarus/NRL) reported full 2-D and 3-D
simulations of the recent 200 MeV acceleration experiment at LOA. Both models
produce energy spectra that are in reasonable agreement with the experiment. Strong
relativistic focusing is observed, with highly nonideal wakes. Gordon's results suggest
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that self-phase modulation and self-focusing dominate RFS in this regime.
Diagnostics indicate that axial wakefields dominate over the transverse fields that are
the source of the direct laser acceleration (DLA) mechanism.
Faure (LBNL) carried out WAKE simulations of SM-LWFA to determine the
relative importance of laser (DLA) and plasma acceleration mechanisms. He
concluded that DLA was important but not dominant in the LOA experiments.
Plasma-driven electron acceleration may not necessarily be confined to a well-defined
wake bucket, so that stochastic heating becomes an important contribution as well.
Stochastic heating and acceleration processes were also discussed by Saleh (U.
Michigan).
lonization and Instability Processes
lonization is often ignored in laser plasma accelerator modeling since the intense
laser pulse produces very rapid ionization near the front of the laser pulse. However,
the ionization details are important in some cases.
An example is in modeling the blue shifting that has long been observed in the SMLWFA regime. The effect is believed to be due to tunneling ionization at the head of
the beam. Dimitrov (Tech-X) has carried out detailed OOPIC simulations of 1'OASIS
blue-shifting experiments at LBNL. The simulations include a new ionization model
and produce blue shifts that agree well with the experiments.
Hafizi (Icarus/NRL) presented results on modeling of the energy distribution of
ionized electrons. In the tunneling limit, the spectrum is monotonic and peaked
towards lower energies, while in the multi-photon regime, electrons are generated with
nonzero energies.
Forward Raman scattering plays a central role in many experiments. Schroeder
(LBNL) has carried out detailed analytical and simulation modeling of the effects of
frequency chirp and compared the results with experiments. These results show that a
positive chirp enhances instability growth, but the effect of frequency chirp is
generally weaker than the effect of the pulse shape. Penano (NRL) showed that
density tapering can detune and thus suppress FRS while enhancing self-modulation.
FUTURE EXPERIMENTS AND APPLICATIONS
Most designs for future laser-driven accelerators require phased injection of
electrons, the generation of an appropriate stable accelerating electric field, and optical
guiding to overcome diffraction. Previous experiments have generally looked at these
components individually or have operated in the unstable SM-LWFA regime. Future
experiments are moving towards integrated systems that have injection, acceleration,
and guiding simultaneously and can in principle produce high quality electrons beams
with substantial energy gain. Most of the major facilities are upgrading primary lasers
and infrastructure to carry out these complicated integrated experiments.
The major facility in Japan is at JAERI/APR. They have ambitious plans for future
integrated LWFA experiments that include a 150 MeV photocathode microtron or an
all-optical plasma cathode as the injector, a 100 TW, 20 fs laser as the primary driver,
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and a fast z-pinch capillary discharge for optical guiding. With channel guiding in a
10 cm long capillary, their point design has a predicted energy gain of 4.5 GeV.
LBNL also has a large facility with plans for future integrated LWFA experiments.
They are planning to use a SM-LWFA injector and a laser-generated gas jet plasma
channel for guiding. With the 1'OASIS upgrade, the 40 TW, 80 fs laser and 12.5 cm
long channel at 2.5xl017 cm"3 density may produce accelerated electrons with energies
up to 1 GeV. Colliding pulse injectors will also be investigated.
UCLA experiments are centered on the plasma heatwave accelerator using the 1
TW Neptune CC>2 laser and an 11 MeV electron injector. Future experiments will
likely use the IFEL to provide phase locking and microbunching of the injected beam.
Acceleration to 100 MeV with good beam quality remains the primary goal.
U. Texas is currently upgrading their laser to 4 TW at 50 fs. A differentiallypumped H2 gas cell will be used for future channel-guiding and acceleration
experiments. Possible future experiments on a Raman-seeded SM-LWFA and a
channeled colliding beam accelerator (CBA) were also described.
U. Michigan has a new laser (HERCULES) that can be separated into two pulses.
It is in the process of being upgraded from 20 TW to 100 TW.
NRL has completed a major upgrade to its T3 laser and plans a series of integrated
experiments involving optical injection and primary acceleration stages both with and
without optical guiding. The initial experiments will use an SM-LWFA injector with
energy selection followed by a lower plasma density acceleration stage. Later
experiments will use either seeded SM-LWFA or LIPA for phased injection and a
capillary discharge for guiding.
BNL has a 60 MeV electron beam, ablative wall discharges for plasma channels,
and a CC>2 laser that will be upgraded to 2 TW at 2 ps pulse length. A variety of future
experiments are being considered. Experiments on x-ray generation from the
Thomson scattering of the laser and counter-propagating electron beam in a plasma
channel will continue. A possible LWFA experiment with the upgraded laser and a 10
cm long channel could produce gradients in excess of 1 GeV/m and energies gains
above 100 MeV. Later experiments could involve staged LWFA as a natural
extension of the STELLA program.
A major new facility is being constructed at U. Strathclyde in Glasgow, Scotland.
The primary goal is to use the LWFA as a short pulse driver for an x-ray source. The
facility will have a 10 MeV injector, a femtosecond laser driver, a capillary discharge
for guiding, and a SASE PEL for generating the x-rays.
SUMMARY
There continues to be substantial progress in laser-plasma accelerators and
considerable future promise. Current experiments are mostly confined to the SMLWFA regime or to targeted studies on injection and optical guiding. Modeling and
simulation studies support these experiments and address issues that will be
encountered in future experiments. These experiments will involve the standard
LWFA, the PBWA, and possible new acceleration concepts, and many will integrate
injection, acceleration, and guiding.
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