The role of the longitudinal ambipolar electric field, present inside a pre-formed plasma, in electron heating and beam generation is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and “V” shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field a0, to normalized longitudinal electric field Ez, exceeds unity, i.e., a0/Ez≳1. For a more realistic potential well of finite depth, present inside the pre-formed plasma, the condition for stochastic heating of electrons gets modified to 1≲a0/Ez≲L, where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales ∼a02/Ez, which can exceed the laser ponderomotive energy (∼a0) in the stochastic regime.
Fast electron generation in the presence of coronal plasma in front of a solid target (typically referred to as preformed plasma) in laser-matter interaction in the intensity range of 10(19)-10(21) W/cm(2) is studied in a one-dimensional slab approximation with particle-in-cell (PIC) simulations. Three different preformed plasma density scale lengths of 1, 5, and 15 μm are considered. We report an increase in both mean and maximum energy of generated fast electrons with an increase in the preformed plasma scale length (in the range 1-15 μm). The heating of plasma electrons is predominantly due to their stochastic motion in counterpropagating electromagnetic (EM) waves (incident and reflected waves) and the presence of a longitudinal electric field produced self-consistently inside the preformed plasma. The synergetic effects of this longitudinal electric field and EM waves responsible for the efficient preformed plasma electrons heating are discussed.
The effect of preplasma on fast electron generation and transport has been studied using an intense-laser pulse ͑I =2ϫ 10 18 W / cm 2 ͒ at the Osaka University. An external long pulse laser beam ͑E Ͻ 1.5 J͒ was used to create various levels of preplasmas in front of a planar target for a systematic study. K␣ x-ray emission from a fluorescence layer ͑copper͒ was absolutely counted and its spatial distribution was monitored. Experimental data show K␣ x-ray signal reduction ͑up to 60%͒ with an increase in the preplasma level. In addition, a ring structure of K␣ x rays was observed with a large preplasma. The underlying physics of the ring structure production was studied by integrating the modeling using a radiation hydrodynamics code and a hybrid particle-in-cell code. Modeling shows that the ring structure is due to the thermoelectric magnetic field excited by the long pulse laser irradiation and an electrostatic field due to the fast electrons in the preplasma.
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