| EM
Structure-Based Accelerators Working Group |
| Vitaly
Yakimenko: In Vacuum Laser Acceleration
of Electrons at the Brookhaven ATF |
| Phase velocity of
the electric field for accelerated electrons is
larger than the speed of light when the electrons
travel over long distance larger than 3 times laser
Rayleigh length. This present major difficulty in
using laser fields in the vacuum to accelerate the
electrons. Expected acceleration is small and one
of the challenges of such experiment is to detect
acceleration. We use Inverse Cherenkov based Accelerator
as a starting point and gradually demonstrate change
in the acceleration gradient as a function of gas
pressure. This way we can predict much more precisely
acceleration at the vacuum level and judge if it
is sufficiently high when compared to the energy
spectrometer resolution. Recent experimental results
will be presented. |
| Feng
Zhou: Proof-of-principle experimental test
for the novel vacuum electron-laser acceleration
at the BNL-ATF |
| A recently developed
theoretical model [Y.K.Ho, et al., Phys. Rev. E,
66, 066501 (2002)] demonstrated the injection electrons
with low energy (5-20 MeV) and small incident angle
(q~0.1) relative to the laser propagation direction
are captured and significantly accelerated in a
strong laser field (a0³4), and the accelerating
gradient can reach GeV/cm with a0>30. To verify
this mechanics, we propose to use the BNL-ATF upgrading
Tera-Watters CO2-laser and high-brightness electron
beam to carry out a proof-of-principle experiment.
Complete simulation results with ATF parameters
(including both electron and laser beams) are presented.
Layout for this experiment including the laser injection
optics and electrons extraction system is described.
Diagnostics to measure angular distribution and
energy spectrum of output electrons are discussed. |
| Thomas
Marshall: GeV/m WAKE FIELDS GENERATED BY A TRAIN
OF pC, FEMTOSECOND BUNCHES IN A PLANAR DIELECTRIC
MICROSTRUCTURE |
| A tall, dielectric-lined
rectangular wake field microstructure is analyzed
as a possible element of an advanced linear wake
field accelerator. This accelerator would be driven
by a train of 3fs 500MeV electron microbunches that
would be chopped out of a longer bunch using e.g.
a powerful CO2 laser, and then formed into a train
of rectangular-profile bunches using a quadrupole.
The bunches set up a periodic wake field in the
microstructure that can be built up to 1100 MV/m,
for example, using a train of ten 2pC 3-fs bunches
spaced by the period of the wake fields in the structure.
Stability is examined for drive and accelerated
bunches using computations of test particle orbits
in the longitudinal and transverse wake fields excited
by the drive bunches. It is found that nearly all
test electrons in the drive bunches are confined
within the structure for a travel distance of 30
cm or more, while test electrons located in an accelerated
bunch can have stable motion over at least 100cm
without passing through the structure walls, thereby
gaining >1GeV. |
| Changbiao
Wang:GeV/m WAKE FIELDS GENERATED BY A TRAIN OF pC,
FEMTOSECOND BUNCHES IN A PLANAR DIELECTRIC MICROSTRUCTURE |
| A tall, dielectric-lined
rectangular wake field microstructure is analyzed
as a possible element of an advanced linear wake
field accelerator. This accelerator would be driven
by a train of 3fs 500MeV electron microbunches that
would be chopped out of a longer bunch using e.g.
a powerful CO2 laser, and then formed into a train
of rectangular-profile bunches using a quadrupole.
The bunches set up a periodic wake field in the
microstructure that can be built up to 1100 MV/m,
for example, using a train of ten 2pC 3-fs bunches
spaced by the period of the wake fields in the structure.
Stability is examined for drive and accelerated
bunches using computations of test particle orbits
in the longitudinal and transverse wake fields excited
by the drive bunches. It is found that nearly all
test electrons in the drive bunches are confined
within the structure for a travel distance of 30
cm or more, while test electrons located in an accelerated
bunch can have stable motion over at least 100cm
without passing through the structure walls, thereby
gaining >1GeV |
| Benjamin
Cowan: Photonic Crystal Laser-Driven Accelerator
Structures |
| We discuss simulated
photonic crystal structure designs, including two-
and three-dimensional planar structures and fibers.
The discussion of 2D structures demonstrates guiding
of a speed-of-light accelerating mode by a defect
in a photonic crystal lattice and reveals design
considerations and trade-offs. With a three-dimensional
lattice, we introduce a candidate geometry, discuss
beam dynamics in that structure, and present possible
manufacturing techniques. In addition we discuss
W-band scale tests of a fiber structure. The computational
methods are also discussed. |
| John
Power: Observation of Multipactor in an
Alumina-based Dielectric-Loaded Accelerating Structure |
| We report a new regime
of single-surface multipactor that was observed
during high-power testing of an 11.424-GHz alumina-based
dielectric-loaded accelerating structure. Previous
experimental observations of single-surface multipactor
on a dielectric occurred in cases for which the
rf electric field was tangential and the rf power
flow was normal to the dielectric surface (such
as on rf windows), and found that the fraction of
power absorbed at saturation is ~1%, independent
of the incident power. In this new regime, in which
strong normal and tangential rf electric fields
are present and the power flow is parallel to the
surface, the fraction of power absorbed at saturation
is an increasing function of the incident power,
and more than half of the incident power can be
absorbed. A simple model is presented to explain
the experimental results. |
| Wayne
Kimura: Detailed Model Comparisons with
STELLA Experimental Results |
| High-trapping efficiency
and narrow energy spread in a staged laser acceleration
system was demonstrated during the STELLA experiment
[1]. The experiment used inverse free electron lasers
(IFELs) driven by the BNL ATF CO2 laser. The 1st
IFEL modulated the electron beam energy. A subsequent
chicane created a train of ~3 fs-long microbunches
separated by 10.6 microns. These microbunches are
trapped and accelerated in a 2nd IFEL where up to
80% trapping efficiencies and energy spreads down
to 0.36% (1-sigma) were measured. This paper presents
the detailed model comparisons with the data including
results as a function of the phase delay time between
the microbunches entering the 2nd IFEL and the laser
field. The model also provides information on the
energy-phase space and longitudinal density distribution
of a microbunch. The energy-phase plots are particularly
useful for understanding the observed energy spectra,
and the processes occurring to achieve high-trapping
efficiency and narrow energy spread.
[1] W. D. Kimura, et al., Phys. Rev. Lett. 92,
054801 (2004).
|
| Rodney
Yoder: DESIGN FOR A SLAB-SYMMETRIC DIELECTRIC-BASED
RESONANT STRUCTURE FOR LASER-POWERED ACCELERATION |
| We present a design
for a slab-symmetric optical accelerating structure
which is to be resonantly excited at a wavelength
of 340 micron but can be scaled to other frequency
ranges. The device consists of a vacuum gap between
dielectric-lined conducting walls and partakes of
the well-known advantages of slab symmetry (including
suppression of transverse beam wakefields and low
power density). Accelerating fields on the order
of 100 MeV/m are predicted when the structure is
powered by a high-power (~100 MW) FIR radiation
source now in development at UCLA. Optimal strategies
for power coupling into the structure will be discussed.
Scaling questions lead to the investigation of an
entirely nonmetallic design, which has significant
effects on the accelerating modes. |
| Stanislav
Zhilkov: Driven radiation of ribbon electron
beam as a tool for optical modulation |
| The amplitude of
Cherenkov radiation (CR) or Smith-Purcell radiation
(SPR), which produced by electrons that compactly
move along surface, can be controlled by the deflector
of double-metal-plates type. The height of passing
of ribbon electron beam (EB) over surface is regulated
depending on voltage that applied to the deflector’s
plates. If EB produces suitable radiation at the
same time when optical beam (OB) is in the interaction
region, then due to superposition the amplitude
of OB is changed. So as the deflector’s voltage
regulates EB-radiation, the amplitude modulation
of OB occurs. Three schemes – CR dielectric
grating FEL, SPR metal grating FEL and flat-layered
dielectric unidirectional optical amplifier (UOA)
– have been analyzed in accordance with their
effectiveness to provide optical modulation. Theory
predicts that the rate of modulation can be 5% of
frequency of OB (or 1000 Gbps for visible light);
technical means assume that driven voltage is of
order 1V. Feasibility of the concept can be tested
for THz range by modified SP FEL in Dartmouth. |
| Steven
Gold: Development of a 20-MeV Dielectric-Loaded
Accelerator Test Facility* |
| This paper will describe
a joint project by the Naval Research Laboratory
(NRL) and Argonne National Laboratory (ANL), in
collaboration with the Stanford Linear Accelerator
Center (SLAC), to develop a dielectric-loaded accelerator
(DLA) test facility powered by the high-power 11.424-GHz
magnicon that was developed by NRL and Omega-P,
Inc. The magnicon can presently produce 25 MW of
output power in a 250-ns pulse at 10 Hz, and efforts
are in progress to increase this to 50 MW [1]. The
facility will include a 5-MeV electron injector
being developed by the Accelerator Laboratory of
Tsinghua University in Beijing, China. The DLA test
structures are being developed by ANL, and some
have undergone testing at NRL at gradients up to
~8 MV/m [2]. SLAC is developing a means to combine
the two magnicon output arms, and to drive an injector
and accelerator with separate control of the power
ratio and relative phase. RW Bruce Associates, working
with NRL, is developing means to join short ceramic
sections into a continuous accelerator tube by ceramic
brazing using a millimeter-wave beam. The installation
and testing of the first dielectric-loaded test
accelerator, including injector, DLA structure,
and spectrometer, should take place within the next
year. The facility will be used for testing DLA
structures using a variety of materials and configurations,
and also for testing other X-band accelerator concepts.
The initial goal is to produce a compact 20-MeV
dielectric-loaded test accelerator.
* Work supported by DoE and ONR. (a) Plasma Physics
Division, Naval Research Laboratory, Washington,
DC 20375(b) LET Corporation, Washington, DC 20007(c)
High Energy Physics Division, Argonne National
Laboratory, Argonne, IL 60439(d) Stanford Linear
Accelerator Center, Menlo Park, CA 94025(e) Tsinghua
University, Beijing 100084, China(f) RW Bruce
Associates, Arnold, MD 21012[1] O.A. Nezhevenko
et al., Proc. PAC 2003, p. 1128.[2] S.H. Gold
et al., AIP Conf. Proc. 691, p.
|
| Levi
Schachter: Optical Bragg Acceleration Structure |
| We have made extensive
calculations recently regarding wakes in dielectric
structures. In an attempt to optimize the efficiency
a unique structure has been designed. It resembles
an optical fiber except that contrary to the latter,
the dielectric coefficient is unity (vacuum) in
the center due to the need of ensuring electron
propagation. The similarity between the two is the
fact that for field confinement the effective dielectric
coefficient of the surrounding structure is smaller
than in the center – in our case it is smaller
than unity. This is accomplished by a series of
periodic dielectric layers forming a Bragg structure.
In spite of the complexity of the structure it was
possible to establish some analytical estimates
of the interaction impedance, group velocity, maximum
electric field and most importantly of the wake.
A general approach of evaluating wake fields in
dielectric structures has been developed and will
be presented. |
| Evgenya
Smirnova: Photonic Band Gap structures for
accelerator applications |
| A photonic band gap
(PBG) structure is a one-, two- or three-dimensionalperiodic
metallic and/or dielectric system (for example,
of rods), whichacts like a filter, reflecting rf
fields in some frequency range andallowing rf fields
at other frequencies to transmit through. PBG structureshave
many promising applications in active and passive
devices at millimeterwave and higher frequencies.
Metal PBG structures can be employed at X andKu-band
accelerators to suppress wakefields whenever dielectric
PBGstructures are attractive at higher frequency
for construction of low-losslaser-driven accelerators.
For both applications two-dimensional (2D) PBGstructures
are of main interest. In this talk I present a review
oftheoretical studies and computer modeling of 2D
metal and dielectricstructures. Also world-wide
experimental efforts on constructing and testingmetal
and dielectric PBG accelerators and microwave devices
are discussed. |
| Chunguang
Jing: High power rf test for MCT based 11.424GHz
dielectric loaded structures |
| Recently we carried
out two high power RF tests of dielectric-loaded
accelerating structures based on a ceramic material
made of Magnesium Calcium and Titanium (MCT), which
has advantages over alumina-based tubes due to higher
dielectric constant (around 20). In both experiment,
rf breakdown occurred at small gaps in the joints
between ceramic tube sections. The RF power applied
to the tube was limited to 1.2 MW due to permanent
arcing which occurred when the incident power reached
1 MW. Based on numerical simulation, the peak surface
electric field gradient at this power level was
equivalent to greater than 65MV/m because of field
enhancement at the ceramic gap. This suggests that
MCT-based dielectric loaded accelerating structures
may handle 60~~80MV/m electric field without breakdown.
Also, during the process of raising the RF power
to the point of breakdown, the s-parameters behaved
similarly to that observed for the alumina-loaded
tube test, suggesting that MCT undergoes a similar
multipactor process at high levels of RF input power.
However, the MCT structure did not show any visible
light emission at the point where the transmission
coefficient of the tube began to fall, which occurred
at an incident power of 90 kW (equivalent to 1.7MV/m
E-field). We began to observe light emission at
440 kW incident power. We have fabricated additional
MCT taper sections with their small dimension modified
for better RF power transmission efficiency, and
a corresponding copper module is being machined.
We anticipate better performance will be achieved
in the near future. |
| Anatoly
Blanovsky: Conceptual Design of Dielectric
Accelerating Structures for Intense Neutron and
Monochromatic X-ray Sources |
Bright compact photon
sources, which utilizing electron beam interaction
with periodic structures, may benefit a broad range
of applications in the medical, industrial and scientific
fields. A class of dielectric-loaded periodic structures
for hard and soft X-ray production has been proposed
to provide a high accelerating gradient when excited
by an external RF and/or primary electron beam.
It is demonstrated that target-distributed accelerators
(TDA), in which an additional electric field compensates
for lost beam energy in internal targets, provide
all necessary means to drive a high flux subcritical
reactor (HFSR) for nuclear waste transmutation.
In addition to the prime concept, the TDA for Boron
Neutron Capture Therapy, nuclear isomer-based compact
energy sources, monochromatic computer tomography
and X-ray lithography are proposed. At present,
synchrotron radiation from an electron storage ring
is practically the only high-quality source of monochromatic
x-rays with intensities that are adequate for these
applications. However, their high cost, large size
and low x-ray energies are serious limitations.
In the proposed quasi-monochromatic X-ray sources,
the combination of a low atomic number of internal
targets and low prime-accelerator voltage reduces
the intensity of continuous spectrum to the point
at which characteristic radiation assumes a greater
importance.One of the early assumptions of the theory
of dielectric wake-field acceleration was that,
in electrodynamics, the vector potential was proportional
to the scalar potential. The analysis takes into
consideration a wide range of TDA design aspects
including wave model of observed phenomena, layered
compound separated by a Van der Waals gap and a
high energy density source powered by fission electric
cells (FEC) with a multistage collector. The FEC
is essentially a high-voltage power source that
directly converts the kinetic energy of the fission
fragments into electrical potential of about 2MV.
|
| Alex
Kanareykin: Enhanced Transformer Ratio Experiment
at Argonne Wakefield Accelerator
|
| In this talk we present
an experimental program and our group recent results
for the design, development and demonstration of
an Enhanced Transformer Ratio Dielectric Wakefield
Accelerator (ETR-DWA). We present here an experimental
design of a 13.625 GHz dielectric loaded accelerating
structure, a laser multisplitter producing a ramped
bunch train demostration, and simulations of the
bunch train parameters required. Test results of
the accelerating structure bench testing and ramped
pulsed train generation with the laser multisplitter
are shown as well. The principal goal of the project
is to increase the transformer ratio, the parameter
that characterizes the energy transfer efficiency
from the accelerating structure to the accelerator
electron beam. |
| Chris
Sears: IFEL-Chicane
based Microbuncher at 800 nm |
| As a first stage
to net acceleration in a laser based EM structure
RF electron pulses must be microbunched to match
the laser wavelength. We report on the design of
an undulator and chicane for microbunching at 800nm
using an inverse free electron laser (IFEL) interaction.
This includes design considerations for the hardware
itself, the laser IFEL interaction and bunching
performance, and a full 3D particle tracking simulation
to study the focusing effects and possible emittance
growth due to the fringe fields of the magnets.
The talk will close with a discussion of laser-electron
beam diagnostics for overlap in the undulator and
for diagnosing microbunching performance. |
| Wei
Gai: 1 GeV
acceleration using electron beam driven wakefields
in structures |
| In this talk, we
first discuss the transformer ratio problem in collinear
wakefield acceleration. Then, we review three wakefield
schemes that have potential to accelerate a witness
beam to 1 GeV: 1) Collinear wakefield acceleration
with non-symmetric drive bunch shape or bunch trains;
2) Collinear wakefield acceleration with multiple
drive beam which uses staging techniques; 3) Two
beam acceleration scheme where the drive and the
witness beam travel in different but RF connected
structures. We will discuss the drive beam and structure
requirements for each scheme that can support >
100 MV/m gradient and sustained acceleration of
> 10 meter length. Particular examples that use
dielectrics as the wakefield structure for each
scheme will be given. Finally, we will discuss possible
future research plans that will achieve the goal
of 1 GeV acceleration. |
| Gerald
Dugan: Advanced
accelerator requirements for synchrotron radiation
facility linacs |
The general electron
beam requirements of few GeV electron linacs for
the production of synchrotron radiation will be
reviewed, with an emphasis on the requirements which
could be satisfied using advanced accelerator systems.
The talk will cover the beam requirements needed
for the generation of the synchrotron radiation
in conventional storage rings, wigglers and undulators
driven by an energy-recovery linac, and SASE free-electron
lasers.
|
| Wayne
Kimura: Conceptual Design for a 1-GeV IFEL
Accelerator |
| A conceptual design
for an inverse free electron laser (IFEL) electron
accelerator with a net energy gain of 1-GeV will
be presented. This design was developed using various
models available at STI Optronics and UCLA. These
models have been anchored with previous successful
IFEL experiments. The electron beam characteristics
while being accelerated will be examined including
trapping efficiency, phase-space distribution, and
preservation of the microbunch length. |
| Yu
Ho: Exploring
Acceleration Channel of Vacuum Laser Beams |
| A new scheme of the
electron laser acceleration in vacuum has been studied
with theoretical investigation and 3D simulations.
In this scheme, relativistic electrons injected
with small incident-angle relative to the laser
propagation direction are not expelled by the laser
beam as predicted by the ponderomotively scattering
model, but are captured and significantly accelerated
in the strong laser field region. We call this new
scheme CAS (capture and acceleration scenario).
It has been found that there exists a lower wave
phase velocity region (less than c) for any focused
laser beam propagating in vacuum. Thus the overlapping
regions of the lower phase velocity region with
that of the axial electric field component form
ideal acceleration channels. Relativistic electrons
injected into this channel can be trapped in the
acceleration phase and remain in phase with the
laser field for sufficient long times, thereby receiving
considerable energy from the field. The crucial
conditions for CAS to work are the laser intensity
should be strong enough, the electron incident angle
is sufficiently small, and the optimum incident
electron energy should be in the range of 5-15 MeV.
The main features of CAS are the principal acceleration
force is the axial electric field component and
the maximum energy gain is linearly proportional
to the field strength. A typical value of the energy
gain is 100 MeV (a = 10), 2 GeV (a = 100). There
is an intensity threshold, which is critically dependent
on the beam width (thereby the phase velocity in
the acceleration channel). The output beam properties
of the CAS scheme have been investigated in quantitatively
detail, such as the energy spectra, angular distributions,
energy-angle correlation, fractions of accelerated
electrons in the total incident electron bunch,
emittances, etc. In CAS, optics and medium placed
near the laser focal region are not necessary, thereby
allowing use of high intensity lasers and large
energy gain. |
| 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 10MeV
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 4ps long electron
beam is expected to bunch in a series of microbunches
each only 45 μm long containing over 40%
of the injected current after 1.5 m drift space
following the undulator. |
| Chieh
Sung: Single stage GeV-class IFEL accelerator |
| A high power 0.8
μm laser, in a diffraction-dominated mode
is used to self-trap and to accelerate electrons.
1D and 3D simulation codes are used to explore the
feasibility of a single-stage, 1GeV IFEL accelerator. |
| 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
|
