FPo5.45 MAGNETIC CONFINEMENT OF AN EXPANDING LASER-PRODUCED PLASMA M. S. Tillack, S. S. Harilal, F. Najmabadi and J. O’Shay UC San Diego, Center for Energy Research 9500 Gilnan Drive, mail stop 0438 La Jolla, CA 92093-0438 mtillack@ucsd.edu High energy ions from IFE target explosions threaten the survival and lifetime of dry chamber walls. Magnetic fields have been proposed as a possible means to divert or extract energy from the expanding plasma. We have performed experimental studies to characterize the expansion dynamics of laser ablation plumes into several magnetic field configurations, including fields aligned with or transverse to the expansion direction, and a curved field with the axis aligned with the expan- sion direction. Plasma was produced using pulses from a Q-switched Nd:YAG laser supplying power density in the range of 5–50 GW/cm 2 . Significant changes were observed in the plume dynamics, including enhanced emission, confinement of the plasma, and guiding of the expansion along field lines. I. INTRODUCTION The idea of using magnetic fields to confine or divert high-energy ions emanating from IFE target explo- sions has been considered from the early days of IFE power plant research 1 . It has been postulated 2 that a cloud of laser-produced plasma will be stopped by a magnetic field B in a distance R~B -2/3 . In the diamag- netic limit, applying such simple estimates indicates that a 200 MJ fusion reaction could be confined to a b =1 bubble of radius just under 5 m using a magnetic field of 1 T. In addition to confinement, the presence of a mag- netic field may lead to ion acceleration, enhanced emis- sion intensity, and various kinds of instability. Further research is needed in order to assess the feasibility of this concept and to better understand the various options and issues for magnetic confinement of IFE target emissions. Considerable work has been performed previously on the interaction of an expanding plasma cloud with a magnetic field. Dimonte and Wiley 3 investigated the expansion of plasma across a transverse magnetic field (B=0.35 T) and found that the plasma is contained within the magnetic cavity up to the point of peak diamagnet- ism. Later, the magnetic field was observed to diffuse into the plasma anomalously fast compared to classical diffusion rates. This is believed to be caused by insta- bility as the plasma is decelerated, and is evidenced by observations of flute structures. Mostovych, et. al. 4 investigated the expansion of laser-produced plasma in 0.5-1 T transverse magnetic fields, and reported flows that were collimated into two dimensional jets that became focused and driven un- stable by the field. Even though the magnetic pressure P B =B 2 /8p exceeded both the plasma ram pressure P r =nMV 2 /2 and the thermal pressure P t =nkT, the jet’s tip velocity was not reduced. The profile of the plasma jet in the plane normal to the magnetic field became wedge-shaped and exhibited an asymptotically narrow- er and denser tip. Ripin et. al. 5,6, studied sub-Alfvenic plasma expan- sion in the limit of large ion Larmor radius and reported that the magnetic confinement radius R b followed the expected B -2/3 dependency within ±20% error at inter- mediate magnetic field values. Before the plasma reached R b , the leading edge developed distinct flute structures or spikes that projected out from the main plasma body into the magnetic field. The onset of the instability curiously occurred at about the same distance and time regardless of field strength, and the wave- lengths appeared to be insensitive to the field strength as well. The experimental linear growth rate of this instability is consistent with that of the large-Larmor- radius instability theory developed by Huba et. al. 7 , and is nominally 6 times faster than the conventional MHD Rayleigh-Taylor growth rate. II. EXPERIMENTAL SET-UP Details of our experimental set-up are given in a recent publication 8 . 1.06 m m pulses from a Q-switched Nd:YAG laser (8-ns pulse length) were used to create an aluminum plasma in a stainless steel vacuum chamber. The chamber was pumped to a base pressure ~10 -8 Torr. The laser beam was focused onto the target