Hot electron characteristics were studied using
ultra-intense laser systems which can deliver up to 500 J in
energy and a few times 10^19 W/cm^2 in intensity
within a 1 psec pulse width. In fast ignition model
experiments, a plastic (CD) shell with a Au cone was
imploded with 2.5 kJ green laser beams to create a 50 g/c.c.
density core with a 50 micron diameter. 0.5 PW heating laser
pulse was injected onto the core through the Au cone at the
max core density. Neutrons were enhanced by 3 orders of
magnitude with the heating pulse. Hot electron spectra were
measured through compressed cores of CD shells with Au
cones. The reduction of hot electrons was observed when the
electron spectra were compared with and without the cores.
The details of the measurement are reported.
[RO2.002] Petawatt laser heating of uniformly imploded plasmas and thermal neutron enhancement
Yoneyoshi Kitagawa, Shin Akamatsu, Wataru Sakamoto, Kazuo Tanaka, Ryosuke Kodama, HIroaki Nishimura, HIsanori Fujita, Takayoshi Norimatsu, Atsushi Sunahara (Insitute of Laser Engineering, Osaka University), Yasuhiko Sentoku (University of Nevada, Reno), IY02 Series Experiment on GXII Collaboration
Directly illuminating the PW laser onto a CD shell target,
we have enhanced thermal neutrons from 1\times 10^6 to 4
\times 10^6. The target used here is a CD shell sphere of
501 \pm12 \mu m in diameter and 6.9\pm 0.62 \mu m in
thickness with no gas filling. The green GEKKO XII laser of
2341 \pm 452 J in 1.3 ns super Gaussian imploded the core
up to a 100 times the solid density. The PW laser, 1 \mu m
wavelength of 312 \pm67 J in 500 \sim 700 fs, was
focused at the cutoff density layer, which is typically 220
\mu m far from the target center with an off-axial parabola
of F number of 7.6. We have varied the PW laser timing from
the GXII intensity peak ( t = -800 ps) through the first
bounce of the centripetal shock (t = 0 ps) after the
compression. At 80 ps and 180 ps, we have found two strong
enhancement peaks of thermal neutrons. The streaked
intensity of 2-3 keV X-ray XSS from the imploded core plasma
shows similar feature as the thermal neutrons. Hot electrons
were ejected into the core plasma at 10^o cone angle
to the laser axis direction, much narrower than the
preliminary predicted 30^o cone angle. It seems
that the so narrow hot electron emission has effectively
heated the core and enhanced thermal neutrons.
[RO2.003] Cone Dynamics in Fast Ignitor Cone Targets on OMEGA
R.B. Stephens (General Atomics), S.P. Hatchett, R. Turner (Lawrence Livermore National Laboratory), C. Stoeckl, J.A. Delettrez, T.C. Sangster (Laboratory for Laser Energetics, U. Rochester), A. Fujioka, H. Shiraga, K.A. Tanaka (Osaka U.)
A reentrant cone is used in fast ignition targets to provide
a clear path to the core of the compressed fuel for the
ultra-fast ignition pulse, and to efficiently convert that
pulse to a beam of relativistic electrons. It has been
assumed to date that the interior surface of the cone is
free of plasma when the ignition pulse is delivered.
Experiments on OMEGA with both direct and indirect drive
fast ignitor targets show that the outside tip of the
reentrant cone is heated to 10Õs of eV during the
compression. Since an ignitable target requires a cone wall
\sim10-20 \mum thick, and the exterior heating occurs
>10 ns before the ignition pulse, the assumption of a
plasma free cone interior is questionable. The various
heating mechanisms, and approaches to dealing with them will
be discussed.
[RO2.004] Fuel Assembly Experiments with Fast-Ignitor Cone Targets on OMEGA
C. Stoeckl, J.A. Delettrez, T.C. Sangster (Laboratory for Laser Energetics, U. of Rochester), R.B. Stephens (General Atomics), S.P. Hatchett (LLNL), J.A. Frenje, C.K. Li, F.H. Séguin (PSFC, MIT), S. Fujioka, H. Shiraga, K.A. Tanaka (ILE, Osaka U.)
In the ``cone-in-shell'' concept for fast-ignition (FI)
targets, a re-entrant cone is used to keep the path of the
ultrafast ignition pulse clear of plasma. A dense core of
fuel needs to be assembled close to the tip of the cone,
which can be effectively heated by the hot electrons
produced by the fast-ignition pulse. Gas-tight, direct-drive
FI targets have been used on OMEGA to study the implosion
dynamics and fuel assembly. A step in the cone surface and
Al on the shell was required to make the assembly gas-tight.
The targets were filled with up to 10 atm of either D_2
or D^3He. Time-resolved and time-integrated x-ray
imaging was used with backlighting and in self-emission to
study the core assembly. Nuclear diagnostics provided
important information about the core conditions, both the
primary yield of DD-filled capsules, which is very sensitive
to the core temperature, and the downshift of primary
D^3He protons, which can be used to infer the total
areal density of the core assembly. This work was supported
by the U.S. Department of Energy Office of Inertial
Confinement Fusion under Cooperative Agreement No.
DE-FC03-92SF19460.
[RO2.005] First Simulations of Sub-Ignition Implosions of ``Cone-in-Shell'' CH Targets with DRACO
J.A. Delettrez, C. Stoeckl, P.B. Radha (Laboratory for Laser Energetics, U. of Rochester)
The ``cone-in-shell'' concept has been proposed for the
generation of a relativistic electron beam near a
high-density core in the fast-ignition scheme.(P.A.
Norreys, Phys. Plasmas 7), 3721 (2000). Sub-ignition
experiments are being carried out on the OMEGA laser system
to study the effect of a gold cone on the core uniformity of
gas-filled CH shell implosions. To simulate these
experiments, the capability to handle the presence of a cone
has been added to the two-dimensional hydrocode DRACO.
The effect of the cone on the shell motion and the core
symmetry at stagnation and that of the shock expansion from
the core on the cone apex are presented for different
positions of the cone apex with respect to the core. The
issue of the illumination pattern near the cone is also
discussed. This work was supported by the U.S. Department of
Energy Office of Inertial Confinement Fusion under
Cooperative Agreement No. DE-FC03-92SF19460.
[RO2.006] Intense Electron-Beam Transport in Dense, Cryogenic, DT, Fast-Ignition Fusion Targets
J. Myatt, A.V. Maximov, R.W. Short, J.A. Delettrez, C. Stoeckl (Laboratory for Laser Energetics, U. of Rochester)
One of the central issues surrounding the fast-ignitor (FI)
approach to inertial fusion is the possibility of
propagating an intense relativistic electron beam over
distances in excess of 100 \mum from its origin near the
critical surface to the dense compressed core. Motivated by
future integrated FI experiments with the OMEGA EP laser
system, we have studied the penetration of an intense
electron beam into highly overdense plasma with the hybrid
electromagnetic particle-in-cell code LSP(D.R.
Welch et al.), Nucl. Instrum. Methods Phys. Res. A
464, 134 (2001). in both two and three dimensions. We have
assumed prescribed parameters for the electron beam at the
critical surface and a plasma profile consistent with OMEGA
implosions. We have determined the relative roles of
resistive electric fields,(M.E. Glinsky, Phys.
Plasmas 2), 2796 (1995); A.R. Bell et al., Plasma
Phys. Control. Fusion 39, 653 (1997). self-generated
magnetic fields, and collisional stopping in determining the
penetration and collimation of the hot electrons over a
parameter space that will likely be encountered in future
fast-ignition studies. This work was supported by the U.S.
Department of Energy Office of Inertial Confinement Fusion
under Cooperative Agreement No. DE-FC03-92SF19460.
[RO2.007] Simulations of Laser Coupling to a Fast Ignitor Cone for Long Ignitor Pulses
Y. Sentoku (U. Nevada), R.B. Stephens (General Atomics)
The high intensity laser pulse needed to ignite compressed
fuel in the fast ignitor (FI) concept is estimated to
require an intensity 10^19\, W/cm^2 and a duration of
10s of ps. During this long, intense pulse the interior cone
surface will heat up and develop a plasma gradient, which
will substantially change the coupling of the laser energy
into the FI target. We have investigated the coupling under
this condition with a two dimensional particle-in-cell
simulation of 10~n_c cone whose inner surface is coated
with a 10~\mum thick pre-plasma, and whose outside is
surrounded by the same plasma density (to simulate the blow
off from the nearby compressed fuel core) interacting with a
Gaussian focus, 300 fs, 110^19\, W/cm^2 laser pulse. We
find that the inside plasma gradient (for 10~n_c < 1)
quickly evolves to a steady state condition, where the laser
photon pressure balances the plasma pressure. This
pre-plasma enhances laser absorption at the tip of the cone.
The consequences of these changes on FI target design will
be discussed.
[RO2.008] Subignition fusion yields generated by fast heating of compressed deterium-tritium
S.A. Slutz, R.A. Vesey, I.M. Shoemaker, T.A. Mehlhorn, K. Cochrane (Sandia National Laboratories, Albuquerque, NM 87185-1186)
A simple model is used to calculate the fusion yields from
the fast heating of compressed deuterium-tritium fuel. This
model is in good agreement with detailed numerical
simulations when the self heating of the fuel can be
neglected and can thus be used to estimate the yields for
near term fast ignition experiments where the deposited
laser energies will be insufficient to cause ignition and
propagating burn. The model is used to calculate the fusion
yields as a function of the important parameters such as the
fuel density, deposition energy, etc. It is found that the
fusion yield approaches the deposited energy when more than
2 kJ is deposited into fuel with densities above 300 g/cc.
[RO2.009] PIC Simulations for the First Picosecond in the Fast Ignition Scheme
C. Ren, M. Tzoufras, F.S. Tsung, W.B. Mori (UCLA), S. Amorini, R.A. Fonseca, L.O. Silva (IST), J.C. Adam, A. Heron (Ecole Polytechnique)
We study the fast ignition scheme using two-dimensional
(2-1/2 D) particle-in-cell (PIC) simulations with the code
OSIRIS. The simulations are performed in a 100 \mu m\times
100\mu m box with a target size of 50 \mu m to reduce the
influence of the box boundary conditions. The target
consists of fully-ionized plasma with a bulk density of 40
n_c (critical density). The laser intensity used is about
10^20 W/cm^2 and the pulse length is about 1 ps. Both s-
and p-polarization are used to infer any three-dimensional
effect. Various target geometries have been studied,
including square, circular and circular with a cone, and
with a sharp boundary or ramped-up plasma density. We find
that the fast electron filaments are largely determined by
the laser filaments and the laser-plasma interface
instability, not by the Weibel instability. The current
filaments do not coalesce into one even after 1ps. The fast
electron spectrum is found to obey a power law. Laser
absorption rate and extrapolated efficiency of the energy
transported to the core will also be presented.
[RO2.010] Isochoric heating into the warm dense matter regime by laser-solid produced K_\alpha x-rays
G. Dyer, T. Ditmire (University of Texas at Austin), J. Kuba, P. Patel, D. Price, R. Shepherd, A. Wootton, R.W. Lee (Lawrence Livermore National Laboratory), E. Fill (Max-Planck-Institut für Quantenoptik)
The study of matter at near solid density and at
temperatures of 1-10 eVr is a great challenge to both
experimentalists and theorists, because such matter exhibits
internal energy density which is very high but insufficient
to overpower the inter-atomic potentials. This form of
matter, intermediate to condensed matter and plasmas, exists
in many astrophysical systems. In this paper, we describe an
experimental program to study solid-density matter heated to
temperatures near 1 eV per atom with ultrafast pulses of
x-rays. An intense, ultra-short laser pulse incident upon a
thin foil produces a burst of K_\alpha x-rays, which are
used to flash heat an adjacent bulk sample. Optical
interferometric probing of the sample with sub-ps time
resolution allows us to measure its expansion into vacuum
upon heating. K_\alpha source target properties are
optimized for irradiation of the adjacent sample. Initial
results on K_\alpha yields and heating of Al foils will be
discussed.
[RO2.011] Three-dimensional simulations of fast electron heating of solid targets
Laurent Gremillet, Regis Melizzi, Alain Decoster (Commissariat a l'Energie Atomique, DPTA, BP12, 91680 Bruyeres-le-Chatel, France), Guy Bonnaud (Commissariat a l'Energie Atomique, DSE, 31-33 rue de la Federation, 75752 Paris cedex 15, France)
The heating of a high-density target induced by a beam of
intense laser-generated relativistic electrons is critical
for Fast Ignition(M. Tabak \emphet al.), Phys.
Plasmas \textbf9,941 (2002).It is governed by the
inelastic collisions suffered by the beam electrons and the
ohmic dissipation of the return current, which depends on
the local resistivity. We present numerical simulations of
fast electron transport into solid targets using a fully 3-D
upgrade of the hybrid code PÂRIS(L. Gremillet
\emphet al.), Phys. Plasmas \textbf9, 941 (2002) coupled
with the atomic physics package NOHEL (C. Bowen, A.
Decoster \emphet al.), JQSRT \textbf81, 71 (2003).The
short (\sim 100 fs) time-scale of the interaction leads to
a non-equilibrium regime where the target electrons are much
hotter than the ions. This requires a two-temperature model,
both for the equation of state and the resistivity. The
results obtained for a phenomenological model of the latter
drawing upon the works of Lee and More (Y. T. Lee,
R. M. More, Phys. Fluids \textbf27), 1273 (1984) and
solid-state theory are discussed.
[RO2.012] Fast Ignitor using Nonlinear Force driven Plasma Blocks
Heinrich Hora, Frederick Osman (University of Western Sydney, Australia, 1797), EU-Team Warsaw Team
The discovery of the essential difference of maximum ion energy for TW-ps laser plasma interaction compared with the 100 ns laser pulses led to the theory of a skin layer model [1] where the control of prepulses suppressed the usual relativistic self-focusing. The subsequent generation of two nonlinear force driven blocks has been demonstrated in extensive numerical studies where one block moves against the laser light and the other into the irradiated target. These blocks of nearly solid state density DT plasma correspond to ion beam current densities exceeding 10E10 A/cm2 where the ion velocity can be chosen up to highly relativistic values. Using the results of the expected ignition of DT fuel by light ion beams, a fast ignition may be possible even into uncompressed solid DT fuel similar to the Nuckolls-Wood [2] scheme which uses the laser driven relativistic electron beam for producing more than 100 MJ fusion energy from 10 kJ laser pulses of ps duration. Problems of the block ignition scheme are discussed including the swelling by a prepulse arriving 100 ps before the main pulse.
[1] H. Hora, J. Badziak et al Opt. Commun. 207, 333 (2002)
[2] J.L. Nuckolls et al, Preprint UCRL-JC-14860,
www.llnl.gov/tid/Library.html
[RO2.013] Genuine Two-Fluid Computations of PW-ps Laser Interaction with Plasma for the Block Ignitor
Cang Yu, Frederick Osman, Heinrich Hora, Robert Beech (University of Western Sydney, Australia, 1797)
One modification of the fast ignitor is the block ignitor [1] for laser fusion similar to the Nuckolls-Wood scheme [2]where PW-ps laser pulses ignite nearly uncompressed solid DT fuel generating more than 100 MJ fusion energy from 10 kJ laser pulses using relativistic electron beams. Instead of the complexities of the electron beams, the block ignition uses nearly sub-relativistic ions produced by the nonlinear forces in the new skin layer interaction scheme. It is essential that the prepulses are carefully controlled in the skin layer interaction where a ten wavelength thick corona needs a swelling by a factor of about 5. The numerical treatment is using the genuine two-fluid model where simultaneously the stochastic pulsation is suppressed by smoothing.
[1] Heinrich Hora, Hansheng Peng, Weiyan Zhang, Frederick Osman, SPIE Proceedings 4914, 37 (2002); F. Osman et al IAEA Fusion Energy Conf., Lyon Oct. 2002 IAEA-CN-94/IF/P-10 [2] J.L. Nuckolls et al, Preprint UCRL-JC-14860, www.llnl.gov/tid/Library.html