Laser-Plasma
Acceleration: Plasma Guiding Sub-Group |
J.T.
Mendonca: Plasma waveguides for laser accelerators:
Simulations and experiments |
We present the latest
results of the GoLP/IST group on the production
of plasma waveguides using laser-triggered discharges.
Pic code simulations show that bunches of nearly
10 pC and mono-energetic electrons can attain 1
GeV in 1 cm., in a self-injected configuration.
Our experimental results show that plasma waveguides
compatible with the simulation parameters can be
achieved with the 10 TW laser and 200 fs laser beam
existing in our laboratory. |
Igor
Pogorelsky: ATF experiments on laser and
e-beam transport in a capillary discharge plasma |
A capillary discharge
scheme developed at Hebrew University (Jerusalem)
has been adopted at the BNL ATF as a multi-task
plasma source for proof-of-principle and user’s
experiments. Installed into the 60-MeV electron
linac beamline the capillary has been utilized for
plasma wake production and characterization. This
study evolved later into a pursuit for the resonance
wave excitation using an e-beam microbunched with
a CO2 laser. In a separate experiment, a high-power
CO2 laser has been channeled through a plasma waveguide.
Future plans include exploration of the laser wakefield
electron acceleration with a guided CO2 laser and
Thomson scattering on laser and electron beams counter-propagating
in the plasma channel. The experiments are supported
by hydro-dynamic and PIC simulations that reveal
dynamics of the plasma channel, wake field generation,
laser channeling and ponderomotive focusing, and
monoenergetic electron acceleration. |
Shouyuan
Chen: Evidence of Ionization Blue Shift
Seeding of Forward Raman Scattering |
We report the results
of spectroscopic experiments that wereconducted
by focusing an intense ultr-short laser pulse into
a heliumgas target. The scattered light from the
interaction region wasmeasured spectrally and spatially
from various directions as afunction of laser intensity
and plasma density and correlated withthe accelerated
electron beam. The experiment data showed thatforward
Stimulated Raman Scattering (SRS) was sensitive
to thefocus position of laser relative to the nozzle.
Together with theplasma channel imaged by a CCD
camera, the measurements indicatethat SRS is seeded
by the ionization blue shifted light. The cross-phase
modulation between the SRS and laser beam was also
observedin the experiment.
|
Dimitre
Dimitrov: Particle-in-cell simulations of
intense laser pulses coupling into plasma channels |
The guiding of intense
laser pulses in plasma channels is necessary tomaximize
the energy of electrons accelerated in a laser wakefield
accelerator (LWFA). In general, a significant fraction
of theenergy in the laser pulse can be scattered
when coupling from vacuum into a channel. We present
initial 2D particle-in-cell (PIC)simulations of
this problem, using the VORPAL code. We will discuss
new power spectral diagnostics, based on simultaneous
FFTsin space and time of the PIC data, which enable
quantitative measurements of the scattered pulse
energy. Future work will be directed toward VORPAL
parameter studies designed to optimize the amount
of laser energy that couples into a channel. |
Daniel
Gordon: Detailed
Modeling of Channel Guided Laser Wakefield Accelerators
based on Capillary Discharges |
Capillary discharges
are simple devices that can be used to generate
a plasma channel and guide the drive pulse in laser
wakefield accelerators. However, concerns have often
been raised regarding the effects of magnetic fields
produced by the discharge current, photoionization
of carbon ions, and nonideal channel geometries.
Another difficulty often cited is that a suitable
source of injected electrons must be found. Particle-in-cell
simulations will be presented that include these
processes and show that they do not preclude the
successful use of capillary discharges as accelerating
structures. The optical injection scheme considered
is a high density limit of laser ionization and
ponderomotive acceleration (LIPA). The output from
the LIPA simulations is used as an input into the
model for the acceleration stage, which includes
capillary generated magnetic fields, photoionization
effects, dispersion and plasma nonlinearities, and
the possibility of discrete density tapering. The
simulations of the injector will be compared with
experimental results, and end-to-end simulations
of a joint U.S. Naval Research Laboratory-Icarus
Research experiment to generate near GeV electrons
will be presented. |
Simon
Hooker: Application of the gas-filled capillary
discharge waveguide to laser-plasma acceleration |
One limit to the
acceleration that can be achieved in a laser-plasma
accelerator is the length over which the intensity
of the driving laser can be maintained close to
the focal intensity. In the absence of any method
to guide the laser radiation, diffraction restricts
this length to the order of the Rayleigh range,
which for the laser parameters of interest is typically
a few millimetres. If the target plasma is not fully
ionized, additional ionization of the plasma leads
to refractive defocusing by the induced transverse
electron density profile, which further restricts
the laser-plasma interaction length achieved.One
solution to this problem is the plasma waveguide
in which a plasma is formed, prior to the arrival
of the driving laser pulse, with an electron density
which increases with radial distance from the beam
axis. A profile of this form is able to counteract
refraction and diffraction and so guide intense
laser pulses over long lengths. Several effects
have been utilized to form plasma channels of this
type including: hydrodynamic expansion of a laser-produced
plasma column, the pinch effect in a fast capillary
discharge, and thermal effects in discharge-ablated
or gas-filled capillaries.In this talk we present
the results of our work on one type of plasma waveguide,
the gas-filled capillary discharge waveguide. We
describe the mechanisms by which the channel is
formed, compare the results of magnetohydrodynamic
simulations with interferometric measurements, and
present the results of experiments to guide high-intensity
laser pulses. The potential for using the gas-filled
capillary discharge waveguide in laser-plasma accelerators
will be discussed. |
Min
Hur: Magneto-plasma
waveguide |
A new method of making
a channel to guide an electromagnetic wave in amagnetized
plasma is presented.The basic idea is to use the
external magnetic field to createa transverse variation
in the refractive index. This is possible since
therefractive index of the magnetized plasma is
dependent on the externalmagnetic field as well
as the plasma density. Particle in cellsimulations
with XOOPIC show that the idea works as expected.In
the simulations, a right-hand-polarized pulse was
launched into auniform density plasma with an axial
applied magnetic field whoseintensity varied transversly.
Because of the limitation in the magneticfield strength
which can be made by current technology, the guided
wave is mostlyin the microwave regime. We investigate
the possiblity of extending thefrequency range to
the laser regime. |
Vinod
Kumarappan: Guiding of intense pulses in
efficient end-pumped plasma channels generated by
self-focusing in Ar and H_2 clusters |
We demonstrate that
self-guiding of intense short pulses in clustered
gases can be utilized to generate long plasma channels,
which upon expansion form waveguides suitable for
propagation of laser pulses at high intensity. This
scheme has several advantages over waveguide-generation
in non-clustered gases. The absorption of energy
by the target depends on the size of the clusters
and not on the average density of the gas, which
allows greater control of the density encountered
by the guided pulse. In particular, electron densities
less than 10^18 per cm^3 are feasible. Moreover,
since clusters absorb sub-picosecond pulses very
efficiently, channel generation by an auxiliary
long-pulse laser is no longer necessary and a considerably
simpler setup suffices. The problem of taper at
the channel entrance, an old bugbear of side-pumped
waveguides in gases, is shown to be significantly
reduced.Evidence will be presented of waveguide
generation in gases of argon and hydrogen clusters,
using different cryogenic sources. A slit source
is used for argon, and waveguides < 1 cm could
be formed, in which laser pulses with intensity
> 10^17 W/cm^2 were guided. The results of a
propagation code suggest that even longer channels
are well within experimental reach. Argon, however,
has the disadvantage that a super-intense pulse
would likely produce further ionization, and hence
suffer ionization induced defocusing. Hydrogen clusters,
which can easily be fully ionized, were formed using
a more efficient conical nozzle cooled to 90 K,
limiting maximum waveguide lengths to < 3 mm.
Though these channels are short, there is no obvious
reason why a longer target wont allow longer waveguides
to be generated, and the experiments demonstrate
the utility of this novel scheme.
|
Arie
Zigler: Guiding of Ultrahigh Laser Intensities
in Ablative Capillary Discharge Plasma Channel with
Laser Ignition |
We are reporting
on the guiding of laser intensities above 5 x 10^18
W/cm^2 over distances of 1.5 cm by ablative capillary
discharges. Generation of plasma channels with narrow
and deep axial minimum in electron density profiles
will be reviewed. Channels with temperatures around
2-4 eV and electron densities of 6 x 10^17 cm^-3
– 2.5 x 10^18 cm^-3 were produced by low-current
discharge with laser ignition in polyethylene capillaries
of 0.3 mm diameter and 15 mm length. The diameter
of the resultant hollow plasma channel at half-minimum
electron density was approximately 25% of the diameter
of the capillary, and the relative profile depth
was as much as 70% of the axial electron density.
The ignition of the capillary discharge was obtained
using an auxiliary 10 mJ, 10 nsec Nd:YAG laser.
This triggering is characterized by both low delay
and jitter times. The guiding was achieved using
both CH2 and BN capillaries. It was demonstrated
that the BN capillaries are capable of withstanding
over ~1000 shots. |
Chieh
Sung: Guiding of 10 μm laser pulses
by use of hollow waveguides |
Guiding of intense
10 μm laser light over many Rayleigh ranges
is important in many advanced accelerator schemes.
We have carried out such guiding experiments using
both evacuated and gas-filled tubes of different
materials. The CO2 laser pulse length was typically
200ps (FWHM) and the CO2 laser peak power was up
to 100GW. A maximum transmission of 15% was obtained
when a 2 cm long, 1mm diameter, evacuated Stainless-steel
waveguide was used. This dropped to about 8% when
180 mTorr of hydrogen was added to the waveguide.
At the peak intensity of 5x10^14 W/cm^2 at the waveguide
entrance, we expect the formation of a fully ionized
plasma throughout the waveguide. The implications
of the study to acceleration experiments will be
discussed. |