Background gas pressure dependence and spatial variation of spontaneously generated magnetic fields in laser−produced plasmas

A spontaneous aximuthal magnetic field generated by electron currents which flow during the interval that a high−brightness 4−nsec full width at half−maximum intensity ruby laser was incident on a metallic target has been detected, its spatial characteristics evaluated, and its dependency upon background gas pressure investigated both experimentally and theoretically. The propagation velocity of the early component of this magnetic field is greater than 108 cm/sec, more than an order of magnitude larger than the plasma convective velocity, and indicates that the electron currents responsible for this fast field must flow through the background photoionized gas. The early component is first detected at a pressure of 5×10−6 Torr, and for a fixed spatial location, reaches a peak intensity at a pressure of 2×10−3 Torr. Its intensity is typically on the order of 1−10 G, a factor 10−100 times smaller than the spontaneously generated magnetic fields which are driven by pressure gradient effects in the expanding laser plasma. The pressure dependence of the pressure gradient magnetic field has also been evaluated and compared with previous experimental results, obtained at laser pulse lengths of greater than 20 nsec.