Experimental demonstration of transition from J x B heating to stochastic heating in ultrashort high intensity laser foil interaction

We experimentally demonstrate the transition of fast electron generation mechanism from J x B heating to stochastic heating by varying preplasma scale length in the interaction of ultrashort (similar to 25 fs) high intensity (similar to 3-4 x 10(19) W/cm(2)) laser with thin foil. At sharp plasma density laser interaction (contrast similar to 2 x 10(-10) at 1 ns, L/lambda << 1), fast electrons were observed along the laser propagation direction demonstrating J x B heating. Interestingly, fast electron temperature in this case was less than ponderomotive scaling. The reasons were identified to be the small excursion length of electron compared to laser wavelength in sharp density interaction along with energy loss while escaping through the rear surface. A simplistic model has been proposed to understand the energy loss mechanism from the rear surface. Next, preplasma was introduced gradually by varying the amplified spontaneous emission contrast and additional picosecond prepulse at different delays. It resulted in an increase in the energy and temperature of fast electrons. Most importantly, at larger scale length (L/lambda >> 1), fast electron temperature beyond the ponderomotive limit was observed. The temperature scales with scale length as T proportional to L0.59 and shows a saturation effect at longer scale length. The results indicate a gradual change in the fast electron generation mechanism to stochastic heating producing superponderomotive energy. Particle-in-cell simulation also very well reproduces our experimental findings. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license