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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:

Advanced ignition concepts, such as fast ignition and shock ignition, are being investigated at the Omega Laser Facility. Integrated fast-ignition experiments with room-temperature re-entrant cone targets have begun, using 18 kJ of 351 nm drive energy to implode empty 40 νm thick CD shells, followed by 1.0 kJ of 1053 nm wavelength, short-pulse energy. Short pulses of 10 ps width have irradiated the inside of a hollow gold re-entrant cone at the time of peak compression. A threefold increase in the time-integrated, 2 to 7 keV x-ray emission was observed with x-ray pinhole cameras, indicating that energy is coupled from the short-pulse laser into the core by fast electrons. In shock-ignition experiments, spherical plastic-shell targets were compressed to high areal densities on a low adiabat, and a strong shock wave was sent into the converging, compressed capsule. In one experiment, 60 beams were used with an intensity spike at the end of the laser pulse, and the implosion performance was studied through neutron-yield and areal-density measurements. In a second experiment, the 60 OMEGA beams were split into a 40+20 configuration, with 40 low-intensity beams used for fuel assembly and 20 delayed beams with a short, high-intensity pulse shape (up to 1 × 1016 W cm-2) for shock generation. © 2009 IOP Publishing Ltd.

Abstract:

The energy transport in cone-guided low- Z targets has been studied for laser intensities on target of 2.5× 1020 W cm-2. Extreme ultraviolet (XUV) imaging and transverse optical shadowgraphy of the rear surfaces of slab and cone-slab targets show that the cone geometry strongly influences the observed transport patterns. The XUV intensity showed an average spot size of 65±10 μm for slab targets. The cone slabs showed a reduced spot size of 44±10 μm. The shadowgraphy for the aforementioned shots demonstrate the same behavior. The transverse size of the expansion pattern was 357±32 μm for the slabs and reduced to 210±30 μm. A transport model was constructed which showed that the change in transport pattern is due to suppression of refluxing electrons in the material surrounding the cone. © 2009 The American Physical Society.

Abstract:

Anomalous resistivity for hot electrons passing through a dense core plasma is studied for fast ignition laser fusion. The hot electrons generated via the ultra-intense laser pulse and guiding cone interactions are measured after they pass through a dense plasma with a density of 50-100 g cm-3 in a radius of 15-25 m. When significant neutron enhancements are achieved by the ultraintense laser pulse injection, the energy reduction of fast electrons is observed. Also, a reduction in the number of electrons with energy up to 15 MeV can be seen. We offer a new physical mechanism for the stopping of electrons, involving electron magnetohydrodynamic shock formation in the inhomogeneous plasma density region. The dissipation in the shock region can explain electron stopping with energies of the order of 15 MeV. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.