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BASIC PLASMA EXPERIMENTS 136 Experiments have been conducted to study the propagation of a high- power laser beam through a plasma and the resulting plasma response. These experiments have demonstrated the filamentation of the primary beam into high- intensity beamlets, which trigger secondary plasma-wave instabilities and create associated beams of fast electrons. Magnetic Field Line Reconnection. The first magnetic field line reconnection experiments were done more than a decade ago in plasma pinch devices. Recently, a new generation of precise and well-controlled laboratory experiments has been carried out in which the ions are effectively unmagnetized but the electrons are magnetized. The magnetic field topology was mapped in three dimensions, and its dependence on plasma parameters was investigated. Observations include Alfvénic ion flow from the neutral sheet (i.e., a plane in the plasma at which the local magnetic field vanishes) and the formation of a neutral sheet on time scales less than the Alfvén transit time across the sheet. In the case where the current sheet was much narrower than its length, the breakup of the current sheet into a filamentary structure was observed. Other important nonlinear and three-dimensional phenomena were observed and studied, including the spontaneous generation of whistler-wave turbulence, the local formation of double layers, the generation of magnetic helicity, and the observation of highly non-Maxwellian particle distribution functions. Recent experiments have also studied magnetic reconnection in the merging process that occurs when two spheromak plasmas are brought together. (See Figure 8.1.) These plasmas are isolated structures, spheroidal in shape, that are self-sustained by a combination of currents and magnetic fields. In this case, there are local current sheets with magnetized ions. These experiments indicate that the merging process depends qualitatively on the initial helicities (i.e., the ''twists") of the magnetic fields of the plasmas involved in the merger process. Plasma Reorganization. Several experiments have been done in the past five years on the merging of plasma currents and the propagation of currents across magnetic fields. (See Figure 8.2.) One common feature of these experiments is that the current flows are fully three-dimensional. For example, merging currents in a high-beta plasma (i.e., a plasma in which the plasma pressure is comparable to that provided by the confining magnetic field) were observed to spiral about each other as they coalesced. The currents evolved to become nearly parallel to the local magnetic field and hence force free. An elegant experiment in which an electron current was made to propagate across a magnetic field showed that whistler waves played a key role in the evolution of the current channel. The experiments relied on highly reproducible, repetitive, plasma sources and on probes capable of studying the three-dimensional nature of the plasma behavior. These experiments are relevant to space plasma physics (such as the
