Electron heating and ion acceleration in sheaths from ultra-high intensity laser-solid interactions

The behaviour of high power laser-plasma interaction from solid targets, and the resultant ion generation, at the extreme intensities available at state-of-the-art laser facilities is an important topic for realising potential applications. We will present experimental data investigating electron heating and proton acceleration in a sheath field using the ultra-high intensity, high contrast J- KAREN-P laser. Using a 10 J, 40 fs pulse focused to an intensity ~5x1021 Wcm-2 resulted in generation of protons up to 40 MeV at 0.1 Hz from a 5 μm steel tape target. The high repetition rate of the tape target allowed large statistically relevant investigations into the scaling of the electron and proton beam with laser energy, pulse length and spot size.We demonstrate that the laser accelerated electron temperature depends not only on laser intensity but also on focal-spot size, in which the restriction of the transverse acceleration distance causes saturation of the electron temperature at increasingly small foci. However, the accelerated electron beam profile becomes more collimated and asymmetric with small focal spots. Measurements of the proton beam show only limited benefit to using increasingly small focal spot sizes, and the best scaling for achieving higher maximum proton energies from sheath acceleration is achieved with increasing the pulse energy, rather than reducing the spot size or pulse length.Imperial College London Plasma Physics Group Semina