DYNAMICS OF A LASER CREATED PLASMA EXPANDING IN A MAGNETIC FIELD

Measurements have been made on a plasma created in a vacuum magnetic field by the focused beam from a Q-spoiled neodymium laser. The target material was either a 20 mu quartz or pyrex fiber or a paraffin (CNH 2N+2) sphere 20 mu dia. suspended by a 1 mu quartz fiber. The laser power was approximately 150 MW with 20 nsec pulse width. Plasma parameters were measured by means of diamagnetic loops and probes, Langmuir probes and a biased ion electron collector. High speed framing photographs show an initial spherical expansion. In the presence of the magnetic mirror field of 1200 G the expansion is stopped at a radius of approximately 1.5 cm. In agreement with theoretical predictions a slight 'bounce' is observed after which the emission light configuration becomes elliptical in shape with a dark non-luminous central region. The diamagnetic probes show beta >or=1 at this 1.5 cm radius, and the signals show a 'bounce' for r>or=1.5 cm. By scanning a photodiode across the laser beam at the exit window of the vacuum system, the time dependence of the absorbed energy by the target as a function of radius was determined.