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Abstract:

The production of highly energetic beams of both electrons and ions is a major part of the experimental programme at the Central Laser Facility (CLF), STFC Rutherford Appleton Laboratory. Every year sees a significant number of experiments done in both areas. This has been complemented by theoretical studies that have been carried out at the CLF and UK universities. In a recent consultation on plans to build a 10 PW upgrade to the VULCAN facility, laser-driven particle acceleration formed a very significant part of the science case that emerged from this consultation. In this talk, I will review the experimental progress that has been made in particle acceleration, and I will also examine what theoretical investigations suggest the future of this field will be. Experimental studies of laser-driven ion acceleration of the CLF using both the VULCAN and ASTRA systems have looked at a number of aspects including focussing and control of the ion beam, manipulation of the energy spectrum, energy scaling with laser and target parameters, and direct use of the proton beam in both isochoric heating of secondary targets and proton radiography. Recently there has been great interest in a number of theoretical studies which indicate that it should be possible to explore radiation-pressure driven ion acceleration for intensities above 1021 Wcm-2, which will be accessible with the ASTRA-GEMINI system. This very exciting prospect will also be discussed. Electron acceleration in laser wakefields is also a well established part of the CLF programme. Experimental studies of laser-driven electron acceleration using the ASTRA laser have explored electron acceleration in both supersonic gas jets and gas-filled capillaries. This has led to the production of electron bunches with up to 1 GeV energy and a few percent energy spread. The influence of tuneable parameters such as the evolution of the plasma channel inside a capillary or the position of the laser focus with respect to the gas jet is actively being investigated. These efforts are backed up by a matching numerical campaign. Recent experiments have also shown that electron bunches trapped on a downward density ramp can have a very small absolute energy spread, and the potential consequences of these results will also be discussed. © 2011 Optical Society of America.

Abstract:

The study of the basic physical processes underlying the generation of fast electrons during the interaction of high-intensity short laser pulses with solid materials and the transport of these fast electrons through the target material are of great importance for the fast ignition concept for inertial confinement fusion and for the development of ultra-short X-ray sources. We report on the experimental investigation of fast electron transport phenomena by means of the spatial and spectral characterization of the X-ray emission from layered targets using bent crystal spectrometers and a new diagnostic technique based on a pinhole-camera equipped with a CCD detector working in single-photon regime for multi-spectral X-ray imaging [1]. In particular, differences of fast electron transport features depending on the atomic number and/or the resistivity of the target material have been studied. The experiments were carried out at relativistic laser intensities, both in the longer (≃ps) pulse interaction regime relevant for fast ignition studies [2] and in the short-pulse (≃100 fs) interaction conditions related to basic physics studies as well as to the development of ultrashort Kα X-ray sources.

Abstract:

A comprehensive scientific program is being pursued at LLE to explore the physics of fast ignition. The OMEGA EP Laser was completed in April 2008, adjacent to the 60 beam, 30 kJ OMEGA Laser Facility. OMEGA EP consists of four beamlines with a NIF-like architecture, each delivering up to 6.5 kJ of UV laser energy in long pulse (ns) mode into the OMEGA EP target chamber. Two of the beamlines can operate as high-energy petawatt lasers, with up to 2.6 kJ each with 10 ps pulse duration. These beams can either be injected into the OMEGA EP target chamber or combined collinearly into the existing OMEGA target chamber for integrated fast-ignitor experiments. Fuel-assembly experiments on OMEGA have achieved high fuel areal densities, and the effects of a cone on the fuel assembly are being studied. Experiments on short-pulse laser systems in collaboration with other institutions are being pursued to investigate the conversion efficiency from laser energy to fast electrons. A coherent transition radiation diagnostic to study the transport of the electrons in high-density material is being developed. Integrated experiments with room-temperature targets on OMEGA will be performed in 2008. Simulations of these integrated experiments show significant heating of up to 1 keV due to the hot electrons from the short-pulse laser. © 2008 IOP Publishing Ltd.