The detection of -100-keV x radiation and of directly back-scattered light is described for neodymium-glass-laser light pulses focused on a polyethylene target. These observations can be explained in terms of the nonlinear excitation of plasma waves by the laser light.We recently made some measurements of x rays and light reflection from a laser-produced plasma which suggest that plasma instabilities have been produced. Our neodymium laser, which includes a multipass glass-disk system, has been described elsewhere. ' 3 The 1.06pm-wavelength neodymium-laser light was focused by means of an f/7 lens to an irregular-shaped spot with a mean diameter of approximately 80 pm. s The pulse energy ranged from 20 to 7P J; the pulse length ranged from 2 to 5 nsec. The maximum power during the pulse was 10 GW, corresponding to an intensity at the target of = 2x 10'4 W/cm~. The laser pulse was focused on flat targets of "deuterated" and ordinary polyethylene, (CD&)" and (CH2)", respectively. The targets were tilted about 5' to prevent back reflections from causing spontaneous oscillations in the laser amplifier chain. After each shot the target was moved to expose a fresh new surface. It was found necessary to wait 1 h between shots to allow the disk laser to cool. Failure to allow ample cooling resulted in a larger focal spot and a decrease or an absence of neutrons and hard x rays.Diagnostics for the first set of experiments included a large plastic scintillation counter, intended for neutron detection and located in close proximity to the laser target. The detector was initially shielded with 9. 5 mm of aluminum plus 6. 3 mm of lead. Nevertheless, when the laser was fired, scintillation pu1. ses were seen from the (CHz)" target as well as from (CD&)". When the shielding thickness was increased to 9. 5 mm of aluminum plus 12. 7 mm of lead, no pulses were seen when the (CH~)" target was in place, but small pulses corresponding to 103 and 104 neutrons were seen when the (CD&)" target was used. s ' The fact that large pulses were seen from a (CHz)" target with a scintillation detector shielded by 6. 3 mm of lead was interpreted as evidence that considerable quantities of hard x rays were being produced in these experiments.Similar hard x radiation has been reported recently by the I ebedev group in the USSR.To measure the x-ray spectrum more accurately, additional measurements were taken with four scintillation detectors using a target of polyvinyl chloride. One pair of detectors used europium-activated calcium fluoride fluors in conjunction with aluminum absorbers; the other pair used thalliumactivated sodium iodide fluors and nickel absorbers. To minimize background corrections, the nickelabsorber pair was shielded from scattered hard x rays by at least 6 mm of lead on all sides, except for a narrow cone pointed at the focal spot. The aluminum-absorber pair did not need such shielding, because the soft-x-ray signal was much greater than the hard-x-ray background. The absorption ratios for a given absorber pair were normalized...
Two Nd-glass laser systems have been assembled for producing high-temperature plasmas. The smaller laser, which gives subnanosecond pulses of about 5 J, has been used primarily for x-ray studies. The spectrum above 2 keV is being explored using Bragg reflection from a cylindrically curved graphite crystal. Line radiation appears to be present in the spectra measured for laser-produced plasmas of lead and gold. Plasma temperatures implied by x-ray transmission measurements are found to vary with absorber thickness. Temperatures of a few tens of keV, inferred from thick-absorber measurements of low-Z plasmas, suggest the operation of anomalous heating mechanisms. A second glass laser system, involving face-pumped disks 14 cm in diameter, is also in use for plasma-heating experiments and has led to the production of D-D neutrons. This system has been used to produce pulses of several tens of joules in a few nanoseconds for recent experiments. A calibrated plastic scintillator with two photomultipliers is used f r neutron detection. Targets of CD(2) have produced total yields of more than 10(4) neutrons. Effective neutron production appears to require laser pulses longer than 2 nsec and to be accompanied by a large reflected pulse.
The x-ray energy emitted from laser-produced plasmas has been measured under various experimental conditions. Two Nd-glass lasers were used in separate experiments to focus pulsed laser light on planar targets. X-ray fluences were measured with newly developed silicon detector calorimeters. Results for various experimental conditions are reported in terms of the efficiency with which the laser light was converted to x-ray energy by plasma production.
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