The pressure dependence of spontaneous magnetic fields in laser produced plasmas

Abstract

A plasma has been produced by irradiation of an aluminum slab with a 6.6 joule ,22 nanosecond (10 10 watt/cm 2 ) pulse of 1.06 micron radiation. Magnetic fields were observed to arise spontaneously when the laser-produced plasma was formed. The space and time behavior of the spontaneous magnetic fields and their relationship to the plasma density profile for expansion into various background pressures of H2, He, N2 and Ar was investigated using magnetic and electrostatic (double) probes. The magnitude and direction of the magnetic fields was found to depend on the background gas pressure. The generation of spontaneous magnetic fields at the front of the expanding laser plasma was observed long after laser irradiation ceased. These fields were axially symmetric and in a direction opposite to the initial field direction. Reverse fields were observed only above a "critical" background gas pressure. Magnetic fields were also observed to arise when a laser plasma impinged on a glass plate. The observed increase of the magnitude of the initial spontaneous magnetic fields is interpreted as arising from the momentum interaction between the expanding laser plasma and the pre-ionised ambient background plasma. The reverse field is attributed to the development of an axial electron temperature gradient at the plasma front due to snowplowing of the ambient plasma.