High
Energy Density Physics and Exotic Acceleration
Schemes Working Group |
kunioki
mima: PIC simulation and experimental researches
on high energy ion generation |
High energy ion generation
with ultra-intense lasers has been interested because
of possibilities of applications to high energy
density plasma production, nuclear science opened
up by short lifetime particle acceleration, plasma
diagnostics, medical applications like positron
emission tomography and cancer therapy, and so on.
Multi-10MeV ion generation with ultra-intense laser
has been explored at the Institute of Laser Engineering
of Osaka University by using 50 TW and 500TW /0.5ps
lasers and with large scale PIC simulations. One
of the specific issues investigated by PIC simulations
is a multi stage acceleration of ion by using multi
foil target. It was found that relatively monochromatic
high energy ions can be generated by synchronizing
ion beams with successively generating sheath fields
on rear surfaces of multi foils. In the simulation,
we found that protons are accelerated to 16MeV with
1019 W/cm2 laser intensity, where the 1% of incident
laser energy is converted to the ion energy within
5% band width. The ion energy produced by this scheme
can be higher than 100MeV with 1020 W/cm2 laser
intensity. As for experiments, high current and
high energy ions beams generated by peta watt laser
at Osaka University with hemisphere target are focused
on a solid target to heat dense plasma instantaneously.
This may contribute to high energy density physics
and the fast ignition core plasma heating. The other
geometry like cone target for ion acceleration has
been explored by experiments and simulations. In
this talk, we present the status of the above researches
at ILE, Osaka University on high energy ion acceleration
and related plasma phenomena.
|
Levi
Schachter: Active Media Accelerators |
In either circular
or linear accelerators, electrons are accelerated
by electromagnetic energy stored in macroscopic
cavities or coupled cavities (disk-loaded structure).
In the case of active medium acceleration electrons
may be accelerated by electromagnetic energy (photons)
stored in microscopic cavities. At the level of
a single particle, the effect can be conceived as
the inverse Frank-Hertz effect in the sense that
an electron moving in the vicinity of an excited
atom may gain energy from the latter while the bounded
electron jumps to a lower energy state. Active medium
may accelerate a pulse of electrons pre-bunched
at the resonance of the medium or may actively enhance
the quality factor of a macroscopic cavity that
in turn accelerates electrons. It is the purpose
of this talk to present a variety of phenomena occurring
when electrons move inside or in the vicinity of
a resonant medium. |
Benjamin
Bowes: Ultrafast 2-D radiative transport
in a micron-scale aluminum plasma excited at relativistic
intensity |
Intense, high contrast
femtosecond (fs) laser pulses deposit energy into
the electrons of a solid faster than it escapes
from the initially-excited volume and much faster
than the target surface expands hydrodynamically.
When initial electron temperature kT_e exceeds several
hundred eV radiative heat transport begins to dominate
over collisional transport. The physics of this
regime underlies energy transport in stars, as well
as ultrashort pulse x-ray generation. Past experiments
in this regime used loosely focused, ~1J, ~1ps pump
pulses and probed the target transversely in transmission,
and thus were restricted to observing late stages
of 1D radiative transport in an optically transparent
material on a time scale of tens of picoseconds.
We present new measurements using 1 mJ, 24 fs pump
pulses focused to a diffraction-limited lambda^2-size
spot (1.5µm diameter) to excite a metal target
surface at relativistic intensity (up to 1.8*10^18W/cm^2).
We probe the target in reflection through microscope
optics. This geometry enables us to observe the
earliest stages of radiative transport in 2D on
any target material on a sub-picosecond time scale.
New features of radiative transport are expected
on this space-time scale. For example, simple transport
models in the diffusive limit (i.e. Rosseland radiation
mean free path lambda_Ross << heated spot
size lambda) predict very different temporal evolution
of the thermal/ionization front in 2D vs. 1D. Moreover,
since our focal spot size is on the order of lambda_Ross,
our experiment may open up fs studies of nonlocal
radiative transport. As such, we experimentally
observe a thermal/ionization front expand radially
at ~10^8cm/s from a lambda^2-size spot of an aluminum
target excited at >10^18W/cm^2. |
Kazuhisa
Nakajima: Laser Ponderomotive e+e- Collider
and Proton Acceleration up to the range of Ultra-High
Energy Cosmic Rays |
Relativistic ultrahigh
laser fields can produce plasmas through quantum
mechanical tunneling ionization mechanism, and accelerate
produced electrons and ions to generate a relativistic
electron beam and energetic ions in plasmas. This
process will be followed by creation of electron-positron
pairs through interaction of relativistic electrons
with a Coulomb field of a nucleus in plasma ions
or a strong laser field.
In a relativistic strong laser field, the longitudinal
accelerating force exerted on an electron is proportional
to the square of the electric field, whereas the
transverse quivering force is just linearly proportional
to it. This is essence of the relativistic ponderomotive
acceleration that dominantly produces energetic
particles in interaction of ultraintese laser
fields with particle beams and plasma. Therefore
a tightly focused laser field can accelerate an
electron bunch longitudinally up to a remarkable
energy and at the same time confines it transversely
in the superposed ponderomotive potential of an
intense ultrashort laser pulse. Here we propose
acceleration and focusing of the pair beam by
the ponderomotive acceleration scheme to compose
a high energy electron-positron collider with
very high luminosity.
In a plasma where a group velocity of laser pulse
is less than the vacuum speed of light, ponderomotive
potential scatters particles on the front of the
laser pulse above a threshold intensity. In this
regime of ponderomotive acceleration the laser
pulse energy is effectively transferred to particle
energy in a point-like interaction. It is feasible
to conjecture that the over-threshold ponderomotive
acceleration can accelerate protons in a cosmic
relativistic plasma up to extreme high energies
of the order of 10^20eV, known from observations
of ultra-high energy cosmic rays.
|
Paul
Bolton: Ion Production via Optical Field
Ionization of Atoms and Ions: Results from Early
Work |
Optical field ionization
(OFI) of atoms and molecules is ultrafast and distinguished
with high laser intensities of ultrashort duration.
For an initially neutral atom or molecule, field
driven ionization rapidly progresses sequentially
through successively higher charge states. Consequently,
the time-dependent field of a single high intensity
laser pulse can multiply ionize a multielectron
atom during its risetime such that the peak intensity
predictably ‘sees’ ion targets with
the highest possible charge state (as determined
by the peak intensity level). Within a short time
interval (during which ions are relatively immobile)
the spatial distribution of charge states is then
determined by the laser field and target distributions.
Closed form expressions for threshold and saturation
laser intensities (in terms of relevant ionization
potentials) bracket a useful range for ion production.
They can also facilitate the design of ion sources
and laser diagnostics. Results from the early OFI
studies conducted at the Lawrence Livermore National
Laboratory will be presented. |
Eric
Esarey: Beam Manipulation with Laser-Plasma
Wakefields: Beam Conditioning, Emittance Selection,
and Beam Chopping |
The strong electric
fields associated with laser-plasma wakefields are
discussed and how, also, tailoring the transverse
profiles of the laser and plasma density allows
one to control the wakefields. This ability enables
a number of applications to electron beam manipulation.
Three possible applications are considered. The
first is beam conditioning,which enhances free electron
laser (FEL) gain by introducing a correlation between
the amplitude of transverse oscillation and theparticle
energy. The second is transverse emittance selection,
again advantageous for FELs, in which particles
with large amplitudeoscillations are removed. The
third is the generation of short (100 fs) pulses,
accomplished by chopping a beam longitudinally.
This work supported by DoE, DE-AC03-76SF0098.
|
Alex
Kanareykin: New Low-Loss Ferroelectric Materials
for Accelerator Application |
Ferroelectric ceramics
have an electric field-dependent dielectric permittivity
that can be altered by applying a bias voltage.
Ferroelectric ceramics have been widely used recently
in rf communication technologies, radar applications,
etc. Ferroelectrics have unique intrinsic properties
that makes them attractive for high-energy accelerator
applications: very small response time of ~10-11
sec, considerably high breakdown limit of more than
100 kV/cm, good vacuum properties. Because of these
features, bulk ferroelectrics may be used as active
elements of tunable accelerator structures [1],
or in fast, electrically - controlled switches and
phase shifters in pulse compressors or power distribution
circuits of future linear colliders [2]. One of
the most critical requirements for ferroelectric
ceramic in these applications is the dielectric
loss factor. In this paper, the new bulk ferroelectric
ceramic is presented. The new composition shows
a loss tangent of 4-5*10-3 at 11 GHz. The ceramics
have high tunability factor: the bias voltage of
50 kV/cm was enough to reduce the permittivity from
500 to 400. The material chemical compound, features
of the technology process, and mechanical and electrical
properties are discussed. It is shown that there
is no fundamental physical limitation in reducing
the loss tangent down to 1*10-3. [1] A. Kanareykin,
W. Gai, J. Power, E. Sheinman, and A. Altmark, AIP
Conf. Proc. 647, Melville, N.Y., 2002, p. 565.[2]
V.P. Yakovlev, O.A. Nezhevenko, J.L. Hirshfield,
and A.D. Kanareykin, AIP Conf. Proc. 691, Melville,
N.Y., 2003, p.187. |
Teh
Lin: Characteristics of High-Intensity Laser
Produced Proton Beams |
We report on the
dependence of high-intensity laser accelerated proton
beams on material properties of various thin-film
targets. Evidence of star-like filaments and beam
hollowing (predicted from the electrothermal instability
theory) is observed on Radiochromic Film (RCF) and
CR-39 nuclear track detectors. The proton beam profile
also varies with initial target conductivity and
target thickness. For resistive target materials,
these structured profiles are explained by the inhibition
of current due to the lack of a return current.
The conductors, however, can support large propagating
currents due to the substantial cold return current
which is composed of free charge carriers in the
conduction band. A plot that shows the relation
between the maximum proton energy and the target
thickness also supports the return current and target
normal sheath acceleration (TNSA) theory. We have
also observed filamentary structures in the proton
beam like those expected from the Weibel instability
in the electron beam. The maximum proton energy
and the spectrum of protons energy are obtained
by the time-of-flight method. The electron distributions
produced from the interaction between the high-intensity
laser and solid targets are also investigated in
this report.This work is supported by National Science
Fundation. |
Peter
Messmer: Ion acceleration and terahertz
wave generation by ultrashort laser-foil interaction |
The interaction of
a strong fs laser pulse with a thin over-dense foil
target is investigated via particle-in-cell (PIC)
simulations. The interaction leads to the formation
of an electrostatic shock wave propagating through
the foil and accelerating bulk ions to keV - MeV
energies. These ions leave the foil as a second
population of high-energy ions, in addition to the
sheath accelerated ions from the foil backside.
Terahertz perturbations are seen in the post-foil
region. This will be discussed in more detail. |
Atsushi
Ogata: Energy Enhancement of Produced Protons
by Prepulses and Back Focusing in Interaction between
T-cube Lasers and Thin Foils |
The maximum proton
energy is enhanced by factor of five in the interaction
between thin organic foils and TW laser pulses with
prepulses, when the waist is focused behind the
target foil. The maximum energy obtained by our
50mJ 50fs 800nm laser is 900keV. This finding gives
possibility for small TW-class lasers to obtain
MeV protons. A model is proposed in which prepulses
first ablates the target to produce neutral gases.
The Kerr effect of the gas then guides the main
pulse and tighten its focusing on the target. |
Tomas
Plettner: New experimental approaches for
the LEAP experiment |
The
availability of commercial high peak power compact
near infrared lasers in the past decade has drawn
interest to explore laser driven particle acceleration
in vacuum as a feasible scheme for achieving high
acceleration gradients. The LEAP experiment is a
proof of principle experiment for laser driven particle
acceleration in vacuum caused by the longitudinal
electric field produced by two crossed Gaussian
laser beams.
The LEAP experiment
is located at the SCA-FEL facility at Stanford
University. During its first 6 years the instrumentation,
software and diagnostics for the experiment were
developed. Also, a fused silica accelerator cell
having a 1mm long laser-electron interaction space
for the acceleration and with 10 microns wide
entrance and exit apertures was implemented.
Single electron
bunches were successfully transported through
the entire system and characterized, however the
very narrow entrance and exit apertures of the
accelerator cell caused significant loss of beam
and made the electron beam tuning a very difficult
task. Furthermore the damage threshold of the
cell limited the laser power and thus the acceleration
from the cell to 20 keV, which was impossible
to detect with the existing noise characteristics
at the SCA-FEL facility. Finally, our old timing
resolution between the laser and the electron
beam was about 100 psec, forcing us to very long
time scans of the laser.
Due
to these shortcomings key elements of the experiment
have been drastically modified. To circumvent
the laser damage threshold problem a “disposable”
cell consisting of a very thin reflective boundary
in the form of a tape has been designed. The reflective
boundary stops only the laser while allowing the
electrons to traverse. The need for slits and
the transmission problem through the cell has
disappeared. Furthermore we have added an IFEL
upstream of the tape as a precise timing diagnostic
between the laser and the electron beam to within
1 psec.
|
Carl
Schroeder: Electron
Beam Conditioning by Thomson Scattering |
A novel method is
proposed for conditioning electron beams via Thomson
scattering. The conditioning provides a quadratic
correlation between the electron energy deviation
and the betatron amplitude of the electrons, thereby
circumventing emittance limitations, resulting in
enhanced gain in free-electron lasers. Quantum effects
imply conditioning must occur at high laser fluence
and moderate electron energy. Conditioning of x-ray
free-electron lasers (e.g., LCLS) should be achievable
with present laser technology, leading to significant
size and cost reductions of these large-scale facilities.
This work supported by DoE,
DE-AC03-76SF0098.
|
Jonathan
Wurtele: Beam Conditioning for FELs: Consequences
and Methods |
The consequences
of beam conditioning in four example cases of soft
x-ray to hard x-ray Free electron lasers (FELs)
are examined. In parameter regimes of interest for
x-ray FELs the emittance is a major limitation to
FEL performance. It is shown that in these cases
proper conditioning allows operation with larger
emittance, stronger focusing in the undulator, and
decreasing gain lengths by up to a factor of two,
while increasing the saturated power output. General
analytic work is presented showing it is possible
to design beam lines that both produce conditioning
and, simultaneously, provide exact matching into
an FEL. Various conditioners, employing conventional
components, are considered, and expressions derived
for the amount of conditioning provided. The possibility
of using laser and plasma systems for conditioning
is explored in a companion paper [Esarey, Sessler
and Wurtele, AAC04]. |
Donald
Umstadter: Acceleration of an Electron Beam
in Vacuum by a High-Intensity Laser Pulse |
The interaction of
a laser produced electron beam with an ultra-intense
laser pulse in free space is studied. We show that
a transversely propagating optical pulse imparts
longitudinal momentum to the electron beam, causing
it to deflect. The results are found to be in good
agreement with the theoretical model that takes
into account the longitudinal fields present when
a Gaussian pulse is tightly focused. It is also
demonstrated that this technique can be used to
temporally characterize a sub-picosecond electron
bunch. |