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Working Group Specified - Poster |
Thomas
Marshall : WAKE FIELDS EXCITED IN A SEMI-INFINITE
RECTANGULAR DIELECTRIC WAVEGUIDE BY A TRAIN OF ELECTRON
BUNCHES |
A wake field set
up in a planar dielectric having tall but finite
height by the passage of a train of relativistic
electron bunches (Vavilov-Cherenkov radiation) is
considered here. We study peculiarities of excitation
at the entrance boundary, namely the appearance
of a “quenching wave” and transition
radiation. The former cancels Vavilov-Cherenkov
wake field radiation in the region between the entrance
and a front moving with the group velocity, while
the latter distorts the wake field. Exact expressions
for transition and Vavilov-Cherenkov radiation are
obtained, and the spatial profile of the field excited
by an electron bunch of finite size is computed
numerically. Wake field excitation by a train of
equally-spaced fsec bunches is investigated. |
Chunguang
Jing: Mode
Analysis in Multilayer Dielectric-Loaded Accelerating
Structure |
In this paper, multilayer
dielectric loaded structure is adopted to increase
the Quality Factor Q and shunt impedance R for dielectric
based accelerator. Comparing with the conventional
single layer dielectric-loaded accelerator structure,
this multilayer structure is capable of reducing
the RF transmission attenuation introduced by the
wall loss on copper jacket and improving the shunt
impedance at the same time. A general analysis of
this multilayered structure which covers the mode
solutions for both TM (acceleration) modes and HEM
(hybrid transverse mode related to the beam instabilities)
is presented along with their characteristic parameters
such as Q and R. A numerical example for X-band
(11.424 GHz) structure which shows that attenuation
of accelerating field can be significantly reduced
by using mere 2 – 4 layers is also given.
Significant improvement of overall accelerating
efficiency is achieved while maintaining other accelerating
parameters comparable with higher gradient accelerator.
An easy to implement damping method for the hybrid
modes is also presented and discussed. |
Matthew
Thompson: Plasma Density Transition Trapping
of Electrons in Plasma Wake Field Accelerators |
Plasma density transition
trapping is a self-injection scheme for plasma wake-field
accelerators. This technique uses a sharp, downward
plasma density transition to trap and accelerate
background plasma electrons in a plasma wake-field.
The performance of density transition trapping is
competitive with laser based plasma injector schemes
when the techniques are used at similar densities.
Unlike most laser plasma injector schemes, however,
density transition trapping is not dependent on
the ability to achieve sub-ps timing requirements.
A proof-of-principle plasma density transition trapping
experiment is underway at the Fermilab/NICADD Photoinjector
Laboratory (FNPL). The goal of experiment is to
capture a 100 pC beam with 4% rms energy spread
out of a 2 x 10^13 cm^-3 peak density plasma using
a ~ 6nC, 14 MeV drive beam. Progress and status
of the experiment will be reported. |
Stanislav
Zhilkov: Light
beam transformation and the energy exchange with
2-D particle cluster |
Transformation of
optical wave (OB) by loaded dielectric structure
and the proper directing of electron beam (EB) commonly
provide the conditions for the resonant energy’s
transferring OB-EB. Alternatively, OB, which transformed
by SLM (spatial light modulator), can also transfer
the energy effectively. SLM separates the laser
pulse in a set of partial waves. If in the interaction
region the partial waves appear one after another
having time delay as half-period and neighboring
waves have suitable phase difference, then cumulative
field will act so that the particle experiences
stable-directed accelerating. Such solution explains
non-relativistic effects that detected in coherent
control experiments. The approach promises to be
productive for relativistic study as well. Inverse
problem (energy transfer for light modulation by
driven EB) is researched recent years. The today’s
technology makes it available to achieve the modulation
rate of order 5% of frequency of OB (i.e., theoretical
limit). In general, we observe that the dualistic
idea, according with which the regrouping of charged
particles’ assembly and shaping of OB enable
to exchange an energy effectively during the phase-dependant
cross correlation, is now realized for different
practical needs due to the technology’s readiness. |
Amit
Kesar: Experimental Status of The 17 GHz
RF Gun at MIT |
High
frequency photocathode injectors (RF Guns) are important
as sourcesfor ultra-relativistic colliders due to
their high brightness electronbunch output. A 3
cell 17 GHz RF Gun designed and built at HaimsonResearch
Corporation (HRC) is being tested at MIT Plasma
Science andFusion Center. A 1 ps, 10 micro-joule
laser pulse is injected into theback of the first
cell in order to emit the electron bunch. The drivingpower
into the RF Gun is supplied by an HRC 17 GHz relativistic
klystronand is coupled into the middle cell by dual
RF ports. This cell has aracetrack geometry design
in order to achieve a highly symmetrized TM01pi-mode
operation which is necessary for reducing the emittance.
Thediagnostic setup consists of a slit array, YAG
screen and a CCD camerafor beam imaging and emittance
measurement, and deflecting coils and aFaraday Cup
for energy and bunch charge measurement, respectively.
TheRF Gun currently produces 10 pC bunches having
energy of 0.5 MeV. Ourgoal is to increase the input
power in order to obtain a 1 MeV energygain and
to measure the resulting beam emittance. An update
on theexperimental results will be presented. |
Philippe
Piot: Laser-acceleration
of electrons at Fermilab/NICADD photoinjector laboratory |
The possibility of
using a laser to accelerate electrons in a waveguide
structure with dimension much larger than the laser
wavelength was first proposed by Pantell [NIM A
393 pg 1-5 (1997)] and investigated analytically
by M. Xie [reports LBNL-40558 (1997) and LBNL-42055
(1998) available from LBNL Berkeley]. In the present
paper we present the status of our experimental
plan to demonstrate the laser interaction on an
electron beam with initial momentum of 40-50 MeV/c.
A laser (lambda=1.06 micron) operating on the TM*01
mode has been developed. The large wavenumber (k~6x10^6)
together with the initial low electron momentum
pose a serious problem for efficient acceleration.
In the present report we especially discuss the
various options regarding phase matching of the
laser with the electron beam along with our choice
for the coupling structure. |
Levi
Schachter: Roughness
Effect on a Microbunch |
In this study we
investigate the properties of the electromagnetic
wake-field generated by an electron bunch moving
in the vicinity of an optical structure of finite
roughness. The model employed consists of a metallic
cylindrical waveguide to which grooves of random
width, height and location are attached. Based on
this model analytic expressions have been developed
for the average energy emitted per groove and for
its standard-deviation. As expected, both quantities
are virtually independent of the momentum in a highly
relativistic regime and the average energy emitted
per groove is proportional to the roughness parameter.
Moreover, it has been found that the standard-deviation
of the energy emitted per groove is proportional
to the standard-deviation of the roughness parameter
to the power of 1/4. The cumulative effect of surface
roughness was studied resorting to both periodic
and quasi-periodic structures – significant
differences in the spectrum have been observed only
for low frequencies. |
Changbiao
Wang: Rectangular
Dielectric-lined Accelerator Structure |
Results are reported
from analysis of a rectangular X-band dielectric-lined
accelerator structure operating in the symmetric
LSM-11 mode that has a number of favorable attributes,
by comparison with dielectric-lined cylindrical
structures. These attributes include use of two
planar one-piece precision-ground TiN coated dielectric
slabs free of joints, open slots along two opposing
metallic faces to suppress all anti-symmetric higher-order
modes and to facilitate high-speed pumping, and
significant reduction of wall losses by use of evanescent
vacuum gaps beyond the dielectric slabs. It is shown
that a structure operating at 11.424 GHz can be
built with a shunt impedance >60 Mohm/m using
low-loss alumina as the dielectric. |
Sergey
Shchelkunov: A
New Nondestructive Method for Measuring the RMS
Length of Charge Bunches Using Their Wake Field
Radiation Spectrum* |
We report progress
in the development of a nondestructive technique
to measure bunch rms-length in the psec range and
below, and eventually in the fsec range, by measuring
the high- frequency spectrum of wake field radiation
which is caused by the passage of a relativistic
electron bunch through a channel surrounded by a
dielectric. We demonstrate both experimentally and
numerically that the generated spectrum is determined
by the bunch rms-length, while the choice of the
axial and longitudinal charge distribution is not
important. Measurement of the millimeter-wave spectrum
will determine the bunch rms-length in the psec
range. This has been done using a series of calibrated
mesh filters and the charge bunches produced by
the 50MeV rf linac system at ATF, Brookhaven. We
have developed the analysis of the factors crucial
for achieving good accuracy in this measurement,
and find the experimental data are fully understood
by the theory. We also discuss the application of
this technique for measuring fsec bunch lengths,
using a prepared microstructure.
*This work was supported by Department of Energy
High Energy Physics Division
|
Evgenya
Smirnova: Progress on the MIT Photonic Band
Gap accelerator experiment |
We report the progress
on the design and cold test of metal PBG resonatorsand
accelerator structures. First, an 11 GHz photonic
band gap (PBG) cell,which can be utilized as an
accelerator cavity with reduced long-rangewakefields,
was constructed and tested. The higher-order-modes
suppressionwas proved in the cold test. A Q-factor
of 5000 was achieved for a brazedresonator. Next,
the 11 GHz design was scaled to 17.137 GHz to construct
a6-cell 17.137 GHz PBG accelerator structure with
reduced long-rangewakefields. The accelerator design
was performed with HFSS and a bolted withscrews
structure was built and cold-tested. Engineering
efforts are underwayin order to build a high Q 6-cell
17.137 GHz PBG accelerator. A PBGaccelerator will
be tested with high power at Massachusetts Institute
ofTechnology. |
Noboru
Yugami: Radiation
from Interaction between Short Pulse and Ultra High
Power Laser and Magnetized Plasma |
The radiation from
the interaction between the ultra-short andultra-high
power laser(0.5 TW, 100 fs)and the weakly magnetized
plasmais studied. We observed the short pulse radiation
(200 ps) in themicrowave region(~100 GHz). Two kind
of radiations whose frequenciesare much lower than
the plasma frequency, are observed. One isin the
higer frequency region and polarized in parallel
to theexternal B field(up to 6 kG). The frequency
doesnt depend on the thestrength of the B field.
On the other hand, other componets of theemitted
radiation is vertically polarized and its frequency
depends onthe strength of the B field. The experimental
results suggest that theformer radiation is generated
by the electron oscillation along the Bfield and
the latter is by the electron Bernstein mode. |
Kazuhisa
Nakajima: Head-on
injection and acceleration of a high quality femtosecond
electron bunch in a plasma |
High quality intense
relativistic beams are generated by the interaction
of two head-on colliding laser pulses to inject
plasma electrons into a wakefield excited by one
of laser pulses. The mechanism of injection is analyzed
theoretically and generation of a high quality electron
beam is verified by the numerical simulations. An
electron beam has a small energy spread of 1%, ultrashort
pulse duration less than 10fs and normalized transverse
emittance less than 1 pi mm mrad. |
Scott
Anderson: Sub-picosecond
Compton scattering x-ray generation using a velocity
bunched electron beam at PLEIADES |
The velocity bunching
technique has been employed to compress a high brightness
photo-electron beam to 300 femtosecond duration.
The longitudinal and transverse beam dynamics were
studied experimentally and compared to theory and
simulation. The compressed electron beam has been
utilized in the PLEIADES Compton scattering x-ray
source at LLNL to produce sub-picosecond, 78 keV
x-ray pulses. We present measurements of the compression
achieved in this implementation of velocity bunching
as well as aspects of the use of a velocity-bunched
beam directly in Compton scattering experiments. |
Suzhi
Deng: wake generation and beam stabilization
in self-ionized plasma wakefield accelerators |
A two-stage wake
generation mechanism is found in self-ionized plasma
accelerators. This two-stage mechanism offers the
possibility to eliminate traditional electron hosing,
while introduces a new hosing instability—ionization-induced
hosing. The growth rate of the ionization hosing
is studied and found to be less than that of the
traditional electron hosing. Analytical and 3-D
PIC simulation results are given. |
Adnan
Doyuran: Nonlinear Harmonic Measurements
of the Transverse Thomson Scattering Experiment
at UCLA |
We propose a Thomson
scattering experiment, which will investigate nonlinear
properties of the scattering utilizing the MARS
terawatt CO2 laser in Neptune Laboratory at UCLA.
When the normalized amplitude of the vector potential
a0 is larger than unity the scattering occurs in
the nonlinear region therefore higher harmonics
are also produced. We discuss the experimental setup
and procedure to measure the angular and spectral
distribution of the higher harmonics. We also present
a calculation tool for the Double Differential Spectrum
(DDS) distribution and total number of photon produced
for both head-on and 90o scattering. We decided
to do the experiment at 90o to avoid complications
due to strong diffraction of the incoming laser.
We discuss the electron and laser beam parameters
for the experiment. |
Bonggu
Shim: Time-resolved harmonic generation
from exploding noble-gas clusters |
Title of proposed
report: "Time-resolved harmonic generation
from exploding noble-gas clusters"Summary:
Clustered gases are an attractive medium for plasma
channels and accelerators, because they absorb intense
laser light efficiently and can develop large 3rd-order
optical nonlinearities. I propose to report recent,
unpublished experiments performed in M. Downers
laboratory (U. Texas) in which we measure the third-order
susceptibility of clustered plasma as a function
of time after absorption of a femtosecond pump pulse.
Argon clusters were initially heated by an intense
400nm pump laser and then allowed to freely expand.
An 800nm probe beam was then used to probe the micro-plasma
and generate harmonics. By varying the time delay
between the pump and probe beams using collinear
and non-collinear pump-probe schemes, we have found
evidence of a resonance condition in the third harmonic
production that is different from linear absorption
cases. The third harmonic resonance comes at an
earlier time delay than the linear absorption resonance
case and it has much shorter time duration and we
model it using PIC simulations. Results will be
discussed in the light of a model of the nonlinear
response that involves collective modes of a cold
electron core confined within a positively charged
ion background. Implications for plasma channeling
and acceleration will be discussed. Preliminary
versions of the work were presented at Frontiers
in Optics (Oct.5-9, Tucson), the APS Division of
Plasma Physics meeting (Oct. 27-31, Albuquerque)
and SILAP (Nov. 16-19) in 2003. We will present
latest results at the upcoming IQEC `04 May 16-21
in San Francisco.
|
Rafal
Zgadzaj: Channeling Experiments and Wakefield
Diagnostics |
Channel Experiments:
We report on recent ultrafast pump-probe experiments
in He plasma waveguides using 800 nm, 80fs pump
pulses of 0.2x10^18 W/cm^2 peak guided intensity,
and single orthogonally-polarized 800nm probe pulses
with ~0.1% of pump intensity. The main results,
which are being published in J. Opt. Soc. Am. B
(2004), are: (1) We observe frequency-domain interference
between the probe and a weak, depolarized component
of the pump that differs substantially in mode shape
from the injected pump pulse; (2) we observe spectral
blue-shifts in the transmitted probe that are not
evident in the transmitted pump. The evidence indicates
that pump depolarization and probe blue-shifts both
originate near the channel entrance. I will also
report on current experiments in which we inject
pulses of fully relativistic intensity into plasma
channels generated in a differentially-pumped cell.
Wakefield Diagnostics: We report on recent progress
on the development of an optimized real-time single-shot
diagnostic (Optimized Frequency Domain Holography,
OFDH) for measuring spatio-temporal structure
of wakefields and other ultrafast phenomena. Using
a short (70fs) and a long (1 ps) second harmonic
probe pulse in a colinear geometry we have demonstrated
the feasibility of the technique by measuring
temporally and spatially resolved cross-phase
modulation of the long probe due to a short pump
in a glass coverslip. Real-time measurement and
analysis capability enables us to fine-tune the
laser and probe parameters during the experiment
to optimize generation and measurement of the
wakefield. Our current work is focused on developing
methods for tailoring the OFDH technique for optimal
sensitivity to wakefield structures, and I will
present numerical results demonstrating the effectiveness
of our optimization procedure. I will also report
on our current experiments to measure the wakefields.
|
David
Yu: A POLARIZED ELECTRON PWT PHOTOINJECTOR
FOR LINEAR COLLIDERS |
A new
S-band Plane-Wave-Transformer (PWT) photoinjector
with capabilities to achieve high vacuum is under
development at DULY Research Inc. The PWT is equipped
with NEG pumps and a cathode load lock which is
designed to handle semiconductor photocathodes such
as GaAs. Polarized electrons with high charge, low
emittance and high rep rate can be produced from
the PWT, suitable for future linear colliders and
other high brightness beam applications. Ultra short
bunches can also be produced from the PWT for advanced
accelerators and light sources.
*Work supported by DOE SBIR grant number DE-FG02-03ER83846
|
Serguei
Kalmykov: Laser wakefield acceleration by
petawatt ultra-short laser pulses |
Focusing an ultra-short
(about 30 fs) petawattlaser pulse in a wide focal
spot (with a radius up to 100 microns) in rarefiedplasma
(electron density of order 10^{17} cm^{-3}) gives
an opportunity toaccelerate electrons up to 1 GeV
by means of laser wake-fieldacceleration scheme
without pulse channelling. In theseconditions, the
laser pulse with an over-critical power forrelativistic
self-focusing propagates in plasmas quite similar
tovacuum. The nonlinear quasi-plane wake plasma
wave, whoseamplitude and phase velocity vary along
the path of laser pulse,effectively traps and accelerates
injected electrons with a widerange of initial energies.
Electrons accelerated within twice theRayleigh length
(about 8 cm) can gain the energy up to 1 GeV. Inparticular,
acceleration of a rather long(about 0.3 ps) non-resonant
electron bunchwith initial energy of particles about
1 MeV forms a bunch of lowemittance electrons with
the energies in the range 900 MeV and energy spread
about 50 MeV.All these conclusions follow from two-dimensional
simulationsperformed in cylindrical geometry by
means of fully relativisticparticle code WAKE. |
David
Yu: NEW DEVELOPMENTS ON PBG RF CAVITIES |
Performance
and design features of metal PBG and rod-loaded
cavities for single-beam and multi-beam rf accelerating/generating
devices are considered. Fundamental differences
of the performance between single-defect and multi-defect
structures are identified. A six-beam cavity design
with external coupler is optimized for a multi-beam
PBG klystron. Preliminary design of a compact, 6-beam,
X-band MBK demonstrates feasibility of generating
high power with high efficiency.
*Work supported by DOE SBIR grant number DE-FG02-03ER83845
|
Mitsuru
Uesaka: X-band RF gun and linac for medical
Compton scattering X-ray source |
Compton
scattering hard X-ray source for 10~80keV are under
construction using the X-band (11.424GHz) electron
linear accelerator and YAG laser at Nuclear Engineering
Research laboratory, University of Tokyo. This work
is a part of the national project on the development
of advanced compact medical accelerators in Japan.
National Institute for Radiological Science is the
host institute and U.Tokyo and KEK are working for
the X-ray source. Main advantage is to produce tunable
monochromatic hard (10-80 keV) X-rays with the intensities
of 10^8-10 photons/s (at several stages) and the
table-top size. Second important aspect is to reduce
noise radiation at the beam dump by adopting the
deceleration of electrons after the Compton scattering.
This realizes one beam line of a 3rd generation
SR source at small facilities without heavy shielding.
The final goal is that the linac and laser are installed
on the moving gantry. We have designed the X-band
(11.424 GHz) traveling-wave-type linac for the purpose.
Numerical consideration by CAIN code and luminosity
calculation are performed to estimate the X-ray
yield. X-band thermionic-cathode RF-gun and RDS(Round
Detuned Structure) type X-band accelerating structure
are applied to generate 50 MeV electron beam with
20 pC microbunches (10^4) for 1 microsecond RF macro-pulse.
The X-ray yield by the electron beam and Q-switch
Nd:YAG laser of 2 J/10 ns is 10^7 photons/RF-pulse
(10^8 photons/sec in 10 pps). We design to adopt
a technique of laser circulation to increase the
X-ray yield up to 10^9 photons/pulse (10^10 photons/s).
50 MW X-band Klystron and compact modulator have
been constructed and now under tuning. The construction
of the whole system starts. X-ray generation and
medical application will be performed in the early
next year. |
Winthrop
Brown: Development of a Higher Brightness,
Ultra-Fast Thomson Scattering X-ray Source for Dynamic
Diffraction on Atomic Time Scales |
Thomson
scattering of a short, intense laser pulse by a
relativistic electron bunch is a promising method
for producing high brightness, hard x-ray pulses
capable of probing the structural dynamics of high-Z
materials at ultra-fast (femtosecond) time scales.
The success of such sources depends heavily on the
ability to produce high quality electron beams capable
of meeting the femtosecond pulse duration, kilo-amp
peak current, and micro-meter spot size requirements
needed to push the performance of Thomson x-ray
sources to the next level. In this talk, the dynamics
of electron beam production and final focus are
discussed, along with the practical limitations
on the electron beam parameters. In addition, the
dynamics of Thomson scattering is reviewed and scaling
laws relating x-ray source flux and brightness to
the electron beam emittance, pulse duration, and
spot size are introduced. The PLEIADES facility
has produced 5 x 10^6 photons per pulse in the 40-80
keV range, with an estimated peak x-ray brightness
of 10^16 photons/s/mm^2mrad^2/0.1%b.w., by colliding
a 50-60 MeV, 10 picosecond, 0.25 nC electron beam
with an 800 nm, 50 fs laser pulse. Details of the
current experiment will be discussed, and upgrade
paths for enhancing the x-ray source performance
will be examined, including the use of ultra-high
gradient permanent magnet quadrupole magnets for
minimizing the electron beam spot size, and longitudinal
electron bunch compression for minimizing the x-ray
pulse duration. In addition, the production of chirped
x-ray pulses for subsequent slicing and/or compression
will be discussed in detail. It is shown that sufficient
temporal correlation in the scattered x-ray spectrum
to achieve sub-100 fs resolution can be produced
from state of the art, high brightness electron
beams. |
David
Yu: BEAM DYNAMICS AND PERFORMANCE OF A 15.6
GHZ CERAMIC HIGH POWER RF GENERATOR |
Beam
dynamics calculations of a 15.6 GHz ceramic rf power
generator have been performed for an experiment
at the upgraded AWA facility at ANL. The expected
peak power to be generated is 100-150 MW. Analytical
and numerical calculations address the following
issues: 50A+ heavy beam loading in both linac and
slow-wave structures, beam breakup and dipole mode
suppression, end-to-end beam dynamics and transport,
generated rf waveform and spectrum. Comparison is
made with an earlier DULY/CERN/ANL 21GHz experiment.
*Work supported by DOE SBIR grant number DE-FG03-01ER83232
|
Michael
LaPointe: A Status Report on Ka-band Transmission
Line Component Development* |
The
development of the 34 GHz magnicon in the Omega-P,
/ Yale Universtiy Beam Physics Lab requires various
transmission line components to transport the mm-wave
power from the magnicon to other devices to be tested
(accelerator section, surface fatigue experiment,
etc.). Components include dual directional couplers
in WR28, phase shifters, calorimetric loads, mode
converters from rectagular to circular waveguide,
13mm -63mm tapers, low loss miter bends with mode
converters, windows, pumping ports for 13mm annd
63.5mm circular waveguide, a resonant ring for high
power testing. Current staus and future plans will
be presented.
*Research sponsored by U.S. Department of Energy,
Division of High Energy Physics.
|
Richard
Hubbard: Prospects for Integrated Injection
and Acceleration Experiments with Capillary Discharge
Guiding |
Several laboratories
are planning integrated injection and acceleration
experiments to demonstrate the basic principles
of a channel guided laser wakefield accelerator
(LWFA). This paper discusses several recent advances
in experimental techniques and theoretical/simulation
modeling that have improved the prospects for successful
demonstration experiments in the near future. (1)
Simulation and Hamiltonian models have demonstrated
that a channel-guided LWFA may produce high quality,
ultrashort accelerated electron bunches without
precisely-phased, monoenergetic injection of an
ultrashort bunch. (2) Recently demonstrated all-optical
injectors that produce injected bunches with large
energy spreads are potentially capable of producing
high quality accelerated electron beams in a LWFA.
(3) New laser-triggered ablative wall capillaries
have demonstrated deeper plasma channels and guiding
at substantially higher intensities, thus reducing
laser power requirements. (4) The required minimum
energy for injection may be substantially lower
in a channel than in a uniform plasma because of
a favorable shift in the size and phase of the portion
of the wake that is both accelerating and focusing.
(5) Finally, this phase shift can also result in
a significant increase in the dephasing length and
thus the overall final energy of the accelerated
bunch. This combination of factors should significantly
relax the requirements for successful near-term
demonstration experiments. *Supported by the Department
of Energy and Office of Naval Research |
Kei
Nakamura: Laser triggered injection of electrons
in a laser wakefield accelerator with colliding
pulse method. |
Laser
driven acceleration in plasmas has succeeded in
producing electron beams containing multi-nCs of
charge, with some fraction of the electrons having
energies in excess of 10s of MeVs but 100 % energy
spread. One of the current challenges is to produce
electron beams with much reduced energy spread.
We report on experimental progress in the laser
triggered injection of electrons in a laser wakefield
accelerator using the colliding pulse method [1],
[2]. The experiments use the lOASIS multi-beam 10
Hz high power Ti:Al2O3 laser system [3]. In the
present experiments, two counter propagating beams
30˚ angle are focused onto a high density
(~1019/cm3) gas jet. Preliminary results indicate
that electron beam properties are affected by the
second beam. Details of the experiments will be
shown as well as comparisons with simulations. This
work supported by DoE under contract No DE-AC-03-76SF0098.
C. G. R. Geddes acknowledges support from the Hertz
Foundation.
[1] E. Esarey et al., Phys. Rev. Lett. 79, 2682
(1997).[2] C. B. Schroeder et al., Phys. Rev.
E 59, 6037 (1999).[3] W.P. Leemans et al., Phys.
Rev. Lett. 89, 174802 (2002).
|
Michail
Tzoufras: Electron acceleration from 650fsec
and 50fsec ultraintense laser pulses |
In order
to obtain a better understanding of the mechanisms
related to theelectron acceleration observed in
recent laser-plasma interaction experimentswe have
performed PIC simulations with the code OSIRIS.
Two regimes have beeninvestigated. For the first
regime, which corresponds to a 650fsec, 1PW laser
pulse travelling through underdense deuterium plasma,
we haveperformed 2D3V simulations. Even though the
pulse shows no sigificant selfmodulation, electrons
up to 350 MeV are observed. The main acceleration
mechanismis believed to be stochastic direct acceleration
by the laser field.
The second acceleration regime is
generated by the interaction of 50fsec (5-25)TW
laser pulseswith underdense plasma. Clear wakefield
structure is observed that accelerates particles
up to 100 MeV. Full 3D (and 2D3V) simulations
have been performed for this regime for a variety
of plasma densities.
Ionization effects will be addressed.
|
Chieh
Sung: Study of a THz IFEL prebuncher for
laser-plasma accelerators |
For
monoenergetic acceleration of electrons, the injected
particles need to be bunched with the same periodicity
as the accelerating structure. In the Plasma Beat
Wave Acceleration experiment in the Neptune laboratory,
the accelerating structure has a periodicity of
340 μm. The plasma wave is phase-locked
to the CO2 beat-wave used to drive it. We are proposing
to use the same beat-wave to generate a high power
340 μm EM radiation via difference frequency
mixing in GaAs. This radiation would have the same
phase relationship as the plasma wave and therefore
can be used to prebunch an existing nominally 10
MeV electron beam based on an IFEL concept. We will
present the design of such a prebuncher which uses
a 50 cm long undulator. The injected 5ps long electron
beam is expected to form a series of 45 μm
long microbunches containing over 40% of the injected
current after 1.6 m drift space following the undulator. |
Vladimir
Gorgadze: Beam Physics Studies in Nonneutral
Plasmas: Injection, Virtual Cathodes, and Autoresonant
Control |
Nonneutral
plasmas have been put forward over the last five
years as a novel system with which to conduct intense
beam transport studies. Other aspects of beams physics
can be investigated with nonneutral plasma systems.
The surprisingly rich injection physics in nonneutral
plasmas has been studied both theoretically and
experimentally [1]. Simulations using a PIC code
have confirmed experiment results and offer insights
into the phase space structure beyond what is observed
experimentally. This includes two-stream instabilities,
virtual cathodes, and self-field induced changes
to the transverse emission profile. Theoretical
estimates are consistent with the simulations. Autoresonance
has been intensively studied by Friedland and co-workers
in a wide range of physical systems. Basically,
the idea is to control a driven nonlinear oscillator
through a frequency chirped drive. Recently, we
considered using an unpowered external circuit with
a frequency chirp to extract power from a diocotron
oscillation. The connection of these ideas to beam
physics will be presented. [1] G. Gorgadze, T. Pasquini,
J. Fajans, and J. Wurtele, in preparation. Co-Authors
Joel Fajans, Jonathan Wurtele, UC Berkeley/LBNL
Lazar Friedland, Hebrew University |