No abstract

The interaction of an intense electromagnetic field with the critical-density layer in a nonuniform plasma is a problem of topical interest in laser-heating and -compression experiments. Microwave experiments in a tenuous plasma 1 have demonstrated resonant enhancement of electric fields at this layer, and a density cavity, or caviton, was observed for times of order 10 5 (cu />e )"* 1 . Relativistic computer simulations of such interactions have been undertaken 2 * 3 and the importance of resonance absorption, 4 its relation to self-generated magnetic fields 5 and secondharmonic generation, 6 its enhancement at sharp gradients near the critical surface, self-consistent modifications to the density profile, 2 and the influence of density scale length on parametric instability thresholds 7 have been widely discussed. The relationship between these instabilities, selfmodulation and filamentation instabilities, 8 and European to be published), Paper No 0 A3.7. soliton formation 9 " 12 have been active areas of related research. This paper describes the first observation of a density caviton in a laser-produced plasma; a preliminary description of the technique has been presented elsewhere. 13 Measurements were made on plasma generated by an unpolarized 1.5-GW C0 2 laser pulse focused onto plane carbon targets at an intensity of 9x 10 12 W cm" 2 , with a 50-nsec (full width at halfmaximum) pulse duration and an energy of 75 J. 13 A holographic interferometer 14 was used to measure !n e dl, using a ruby oscillator which generated 100-MW, 10-nsec pulses to probe the plasma at 90° to the C0 2 -laser axis. The interferograms were recorded on Agfa 10E 75 plates 25 nsec after initiation of the C0 2 -laser pulse; the object resolution was 40 jum. Line densities deduced from fringe shifts were converted to radial density profiles by Abel inversion (Fig. 1). Mea-Measurements of electron density and x-ray emission have been made on a C vn plasma, generated by a 9 xio 12 -W-cm~2 C0 2 laser beam. A density cavity and x-ray filamentation are observed. The relevance of these results to theories of resonance absorption, soliton formation, self-modulation, and filamentation of laser light is noted. 467

Existing theories of the cusp containment of quiescent plasma are reviewed. Some technical and economic requirements of any fusion reactor are then summarized and used to deduce the probable dimensions of open-ended high-β reactors. It is shown that such reactors must be long, pulsed, and of very high output power. Crucial physical problems are the microstability of the high-β plasma boundary and, to a lesser extent, problems associated with energy transport by hot electrons. Assuming that these physical and technological problems are capable of solution, it is shown that mirror-symmetric versions of the hybrid-cusp discussed previously by Bickerton may have significant advantages over the simpler linear θ-pinch geometry.

The penetration time, crater top radius, and melted zone for aluminum plates drilled with a CO2 laser beam have been measured and analyzed on the basis of various assumptions concerning absorptivity of the liquid metal, attenuation by the metal vapor and heat exchange with the vapor. By comparison with the experimental data it is shown that each of the interaction parameters must lie within a predetermined range in order to correlate all the experimental measurements. This ascertains the reliability of the theoretical approach and yields estimates of the a priori unknown parameters for the specific exposure conditions.