54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012;
Providence, Rhode Island
Session UI2: High Intensity Interactions and Advanced ICF Ignition
2:00 PM–5:00 PM,
Thursday, November 1, 2012
Room: Ballroom DE
Chair: Natalia Krasheninnikova, Los Alamos National Laboratory
Abstract ID: BAPS.2012.DPP.UI2.4
Abstract: UI2.00004 : Spectra of laser generated relativistic electrons using cone-wire targets
3:30 PM–4:00 PM
Preview Abstract
Abstract
Author:
Hiroshi Sawada
(University of California San Diego)
We report on the characterization of the in situ energy spectrum of fast
electrons generated by ultra-intense (I$\sim $10$^{19}$ W cm$^{-2})$ short
pulse ($\tau \sim $0.7 and 10 ps) laser-plasma interactions using the
TITAN and OMEGA EP lasers. That in situ spectrum is a key component of
ignition efficiency for the Fast Ignition (FI) Inertial Confinement Fusion
(ICF) concept. It is challenging to model and, until now, has resisted
direct experimental characterization; other techniques have very large error
bars or measure the modified spectrum of escaped electrons. This technique
also gives an indication of the forward coupling efficiency of the laser to
fast electrons. This information is derived from the measurement of Cu
K$\alpha $ x-rays emitted from a 1.5 mm long Cu wire attached to the tip of
Au or Al cone targets. Fast electrons, generated in the cone, transport
through the cone tip with a fraction of coupling to the wire. Electrons in
the wire excite fluorescence measured by a monochromatic imager and an
absolutely calibrated HOPG spectrometer. An implicit hybrid-PIC code, LSP,
is applied to deduce electron parameters from the K$\alpha $ measurements.
Experiments on the TITAN laser with Au cones attached to wires show an
increase in pre-pulse energy from 17 to 1000 mJ, decreases the fast electron
fraction entering the wire from 8.4{\%} to 2.5{\%}. On OMEGA EP with Al
cones attached to wires, total K$\alpha $ yield, normalized to laser energy,
drops $\sim $30{\%} for laser pulse length increasing from 1 to 10 ps,
indicative of a saturation mechanism. For Au cones, K$\alpha $ yields were
50{\%} of that measured for Al cones indicating a strong material
dependence. In all cases, the spatial distribution can only be fit with a
two-temperature electron energy distribution, the relative fractions
depending on prepulse level. These results are being used to develop an
optimum cone design for integrated FI experiments.
This work was performed under the auspices of the USDOE by LLNL under
Contract DE-AC52-07NA27344 and DE-FG-02-05ER54834.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2012.DPP.UI2.4