In a series of experiments using two-, four-, and eight-beam 10.6-jum-laser irradiation of a variety of target geometries, a significant amount of energy was found to be deposited in regions remote from the focal spots. The deposition patterns can be predicted with a self-generated magnetic field model. PACS numbers: 52.50.Jm Lateral transport of energy away from laser focal spots can play an important role in redistributing energy deposition in laser-fusion targets. Work has been reported investigating the qualitative 1 and quantitative 2 nature of this transport. Recently, using a plasma simulation, Forslund and Brackbill 3 have identified convective transport of electrons in self-generated magnetic fields as an important mechanism for surface transport in laser-irradiated foils. In one simulation with a laser intensity of 5 x 10 13 W/cm 2 in a 60-jum spot and a hot-electron temperature of 20 keV, peak fields of the order of 1 MG were calculated. The calculation has not been performed at higher intensities comparable to those used in experiment (~10 16 W/cm 2 ) because the code does not handle the relativistic effects of the high-energy electrons generated at these intensities. In general, the ratio of electron to magnetic field pressure is of order 1 in a magnetized sheath whose thickness is large compared with the electron gyroradius. This Letter presents experimental evidence for the nonuniformity of energy deposition predicted by the magnetic field model in a variety of target geometries progressing from flats to cylinders to spheres. From the simulations, a simple qualitative moddel has been developed. Briefly, the model describes lateral energy transport by electrons in magnetic fields generated at the periphery of the laser spot by lateral temperature gradients in the corona. These gradients are maintained by electrons confined and drifting in the magnetic field, resulting in the convective transport of energy from the beam spot to the edge of the magnetized region. The interaction of the magnetic field and electrons produces a thermal magnetic wave 4 which propagates across the surface until disrupted by fringing fields at the target edge or by destructive interference with the wave propagating from an adjacent beam.

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Experimental results are presented on a method for extracting well-focused ion beams from plasma sources with time-varying properties. An electrostatic grid was used to stop the flow of plasma electrons so that only ions entered the extraction gap. In this case, ion flow in the gap was controlled by space-charge effects as it would be with a thermionic ion source. Constant extracted current was observed even with large variations of source flux. An insulator spark source and a metal-vapor vacuum arc were used to generate pulsed ion beams. With a hydrocarbon spark, current densities of 44 mA/cm2 were achieved at 20-kV extractor voltage for an 8-μs pulse. With an aluminum-vapor arc, a current density of 15 mA/cm2 (0.3 A total) was measured for a 50-μs pulse.

A 250−J CO2 laser has been used to produce plasma columns 12−20 cm in length, 2−4 mm in diameter, from initially uniform neutral hydrogen in a steady solenoidal field up to 100 kG. Interferometric and spectroscopic measurements indicate densities and temperatures on the order of 100 eV and 5×1017 cm−3, respectively. The backward−going laser−driven wave normally present in laser breakdown has been eliminated by pulsed filling of the tube.