Plasma cavity enhanced ion acceleration

Summary form only given. Laser driven ion acceleration is particularly interesting due to its many potential applications, including (isochoric) heating of matter which has been proposed as an attractive method for heating nuclear fuel in fusion reactions. In theory ignition is predicted to be possible with currently achievable proton temperatures, however conversion efficiencies of laser energy to protons must be increased beyond the few percent so far routinely achieved to upwards of ten percent for this to be a feasible concept [Roth, M., et al., 2001]. In laser-plasma interactions a large fraction of the total incident laser energy, up to two thirds, has been measured to be reflected from the ionised target surface [Streeter, M.J.V., et al., 2011]. Here a novel target design is introduced which is engineered to increase conversion efficiency of the laser energy into protons by reusing the large fraction of laser energy reflected from the initial interaction. In this scheme, in addition to the planar foil that the laser primarily interacts with, a second foil is attached forming a half-cavity which collects the reflected laser light and reflects it back towards the initial interaction point. By shaping this structure in the form of a quarter-sphere the reflected laser light is refocused, maintaining a high beam intensity and giving a double pulse interaction [Robinson, A.P.L., et al., 2007; Markey, K., et al., 2010]. We present experimental results showing that ion flux enhancements of up to 90 % are achievable in this scheme and evidence of a low energy proton spectral modification which may lend it itself to applications such as uniform isochoric heating.