Inertial Confinement Fusion with Shock Ignition relies on a very strong shock created by a laser pulse at an intensity of
the order of 1016W/cm2. In this context, an experimental campaign at the Prague Asterix Laser System (PALS) has been
carried out within the frame of the HiPER project. Two beams have been used, the first to create an extended preformed
plasma (scale length of the order of hundreds of micrometers) on a planar target, the second to generate a strong shock
wave. Different diagnostics were used to study both the shock breakout at the rear surface of the target and the laserplasma
coupling and parametric instabilities. This paper is focused on back-scattering analysis to measure the backreflected
energy and to characterize parametric instabilities such as stimulated Brillouin and Raman scattering. Our
experimental data show that parametric instabilities do not play a strong role in the laser plasma coupling. Moreover,
preliminary analysis of the back reflected light from the interaction region shows that less than 5% of the total incident
laser energy was back-reflected, with only a small fraction of that light was originating from parametric instabilities.
This paper presents the goals and some of the results of experiments conducted within the Working Package 10 (Fusion
Experimental Programme) of the HiPER Project. These experiments concern the study of the physics connected to
"Advanced Ignition Schemes", i.e. the Fast Ignition and the Shock Ignition Approaches to Inertial Fusion. Such schemes
are aimed at achieving a higher gain, as compared to the classical approach which is used in NIF, as required for future
reactors, and making fusion possible with smaller facilities.
In particular, a series of experiments related to Fast Ignition were performed at the RAL (UK) and LULI (France)
Laboratories and were addressed to study the propagation of fast electrons (created by a short-pulse ultra-high-intensity
beam) in compressed matter, created either by cylindrical implosions or by compression of planar targets by (planar)
laser-driven shock waves. A more recent experiment was performed at PALS and investigated the laser-plasma coupling
in the 1016 W/cm2 intensity regime of interest for Shock Ignition.
We present the results of an experiment concerning laser-plasma interaction in the regime relevant to shock ignition. The
interaction of high-intensity frequency tripled laser pulse with CH plasma preformed by lower intensity pre-pulse on
fundamental wavelength of the kJ-class iodine laser was investigated in the planar geometry in order to estimate the
coupling of the laser energy to the shock wave or parametric instabilities such as stimulated Raman or Brillouin
scattering, or to the fast electrons. First the complete characterization of the hydrodynamic parameters of preformed
plasma was made using crystal spectrometer to estimate the electron temperature and XUV probe to resolve the electron
density profile close to the critical density region. The other part of the experiment consisted of the shock chronometry,
calorimetry of the back-scattered light and hard X-ray spectrometry to evaluate the coupling to different processes. The
preliminary analysis of the measurements showed rather low energy transfer of the high-intensity pulse to back-scattered
light (< 5%) and no traces of any significant hot electron production were found in the X-ray spectra.
HiPER (High Power laser Energy Research) is the first European plan for international cooperation in
developing inertial fusion energy. ICF activities are ongoing in a number of nations and the first ignition
experiments are underway at the National Ignition Facility (NIF) in the USA. Although HiPER is still in the
preparatory phase, it is appropriate for Europe to commence planning for future inertial fusion activities that
leverage the demonstration of ignition. In this paper we shall detail some of the key points of the laser design
and the way this design is connected to the capsule requirements.
Spatially-engineered "top-hat" laser beams are used in solid-state high-energy lasers in order to increase the energy
extraction efficiency in the amplifiers. To shape the laser beam, an efficient alternative to serrated apertures is to modify
a laser cavity so that it naturally generates this "top-hat" beam, replacing a mirror of the laser cavity by a graded phase
mirror. Its complex shape can be approached by microlithographic techniques based on an iterative mask and etch
technique, but many steps are required to avoid large phase steps. The broad-beam ion-etching technique is well suited to
manufacture such surfaces, with a good precision and a perfectly smooth surface. We shall present the technique we used
for square top-hat beam generation. We shall detail the mask optimisation, combining simultaneous simulation of the ion
etching and the beam build-up in the front-end laser. We shall present the results of the surface testing and the final test
of the component in the laser.
To deviate and focus of the beams of the future Laser Integration Line (LIL) and Megajoule laser (LMJ), CEA has chosen an original setup using two large 420 x 470 mm2 transmission gratings. The first grating is an holographic plano transmission master grating with straight and equispaced ruling, 25 degree(s) incidence angle and working at 1.053 micrometers . The second one is an holographic plano transmission master grating, with curved and non equispaced ruling, 25 degree(s) incidence angle which combines both focusing and deviation properties. Groove profile of both gratings is deep laminar. High damage threshold, improved wavefront quality and high efficiencies are the main issues for those two gratings. Jobin Yvon's was selected by CEA in 1999 to develop, industrialize and manufacture gratings reaching LIL/LMJ specifications. A dedicated plant and facilities were built to manufacture the gratings directly engraved into the fused silica substrates provided by CEA. After process developments, Jobin Yvon manufactured the two first 1(omega) and 3(omega) gratings in mid 2001. After a short summary of the specification of these gratings, we present in this paper the production process and the performances of the 1(omega) and 3(omega) gratings manufactured. Wavefront data, efficiency measurements and damage threshold performances are detailed.
A new calculation mode has been developed inside the Miro software to treat the broad-band effects involved in optical smoothing. Bandwidth effects such as gain narrowing in amplifiers or group velocity difference in frequency converters can be computed. This program has been used to simulate the Megajoules laser with smoothing by spectral dispersion.