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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

Plasma was generated by 35 ps 1.06 mu m laser pulses, focused on to plane Perspex slabs in vacuum. Reflectivity and electron temperature were measured as functions of incident laser intensity between 2*1011 and 2*1014 W cm-2. The measured scaling laws were compared with a one-dimensional code. In this short pulse regime, the coronal electron temperature is determined by the balance between inverse bremsstrahlung plasma absorption and enthalpy flow. The electron temperature measured from X-ray emission is interpreted as that of the cooler overdense plasma. For the interpretation of experimental measurements, the importance of the ratio between density scale length and focal spot diameter is stressed.

tation are not yet complete, they clearly show spectrally shifted peaks away from the ruby-laser wavelength. The fact that the scattered radiation is shifted, in addition to the large enhanced signal levels detected, precludes Rayleigh scattering from excited hydrogen atoms as being responsible. Observed signals are at least two orders of magnitude greater than near-resonant Rayleigh scattering from hydrogen could ever provide for our experimental conditions. Significantly, the scattering was sufficiently enhanced that it was necessary to use 10 3 attenuation in detection to obtain counts within the dynamic range of the analyzer (800 counts). As further evidence of the highly nonthermal nature, a search for the electron feature (several hundred angstroms in width) made by removing the 10 3 attenuation yielded no scattered signal. Since for thermal scattering the peak ion to electron ratio is ~ 10, the detection sensitivity sets a lower bound of ~ 10 4 enhancement over thermal.In fact, a direct calibration using Rayleigh scattering showed typical enhancements of 10 4 -10 5 with occasional shots up to 10 6 . Now for the probed k = 2ir/\ = 9X10 4 cm" l , thermal fluctuations would imply density fluctuations (6w/n) th ~ (wX 3 )" 172 ~ 10" 3 . Thus since the Thomson-scattered signal ~ Ibn k I 2 , an enhancement of 10 4 implies 6n k /n~ 0.1 with correspondingly higher values for greater enhanced scattering. Clearly, very large levels of low-frequency ion (and electron) fluctuations have been induced by C0 2 -laser heating of the gas-target plasma.The presence of such fluctuations levels forThe investigation of "fast ions" emitted from a laser-generated plasma is an active research topic 1 " 7 of particular importance for laser fusion. The energy carried by these ions (greater than &A D~0 .5 andlc-i-k 0 of the C0 2 laser, as well as the experimentally observed hn/n ^0.1 for k\ D ~ 0.065 with 5 = 2E 0 , clearly shows significant ion turbulence over a broad geometry and spectral range. Potentially efficient absorption of laser radiation can result from these fluctuations.Since we have directly measured the fluctuation level for k\ D~ 0.5, it is interesting to calculate the anomalous heating rate ^* for our plasma assuming 6n k /n~ 0.1 to be true for a broad spectrum of k as indicated from our experimental results. Taking < cos 2 0,) *4 and < Ime/ I el 2 ) ^ 1 where brackets denote averages over k, we find i>*~ i>ci/3. Evidently fluctuation levels of 20% would make v* £ v clo Thus for short periods during the relatively long-pulse C0 2 -laser heating of the underdense plasma, anomalous absorption may very well be contributing to heating.We wish to thank M. Cervenan, B. Hadley, and P. Haswell for assistance in this experiment. -Nee, Rev. Sci. Instrum. 45, 1400.the thermal expansion energy of the plasma) is generally considered to be the result of ion acceleration by fast electrons. Examples of possible fast-electron sources are resonantly driv-Plasma was generated by focusing a 35-psec, 1.06-jum laser onto Perspex. Ion expa...

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Plasma was generated by focusing a 1.06-μm 35-psec laser pulse obliquely onto a solid surface at the optimum resonance absorption angle. The scaling of nonthermal ion emission energy with laser intensity was determined. A computer model for resonance absorption showed nonthermal ion emission to be due to electron acceleration by a resonantly driven electrostatic field.

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