Excimer laser induced electron beams on an Al target: Plasma effect in a ‘‘nonplasma’’ regime

Abstract

Experiments on electron-beam generation from an Al target illuminated by XeCl and KrCl excimer lasers were carried out under plasma ignition threshold intensities. Direct experimental evidence of such a plasma formation under ignition threshold is given for the first time. By Langmuir-like probe measurements, positive potentials as large as 0.36 and 0.75 V at distances down to about 2 μm from the grounded target over the laser spot area were reproducibly observed during the XeCl and KrCl laser action, respectively. From this result and in absence of cathode voltage, an accelerating electric field on the target of about 375 kV/m was estimated. The highest positive charge density near the cathode that resulted was about 3.3 mC/m3. The strong electric field caused by the plasma formation modified the acceleration conditions for a space-charge dominated electron beam and it should be the main cause of the discrepancies between the experimental current values and the calculated ones.