R. Sigel

Plasma Production by Laser

Abstract A review of high density, high temperature plasma production with pulse lasers is presented. In view of possible applications to thermonuclear fusion we limit our discussion to plasma generation from solid targets of low atomic number Z , with particular emphasis on solid deuterium.

Conversion of laser light into soft x rays. Part I: Dimensional analysis

The conversion of laser light into soft x rays by irradiation of a high‐Z material is theoretically investigated for open, planar geometry. The material may be subdivided into a hot, low‐density conversion layer, optically thin for the x rays, and a dense, optically thick reemission zone. The two layers are coupled through radiation only. Dimensional analysis yields asymptotic expressions for the x‐ray conversion efficiency and the reemission coefficient and hence for the total converted flux from the target.

Threshold of the 2ωpe instability in a laser produced plasma

Abstract 3 2 ωL emission from various plane targets irradiated by a 20 J, 5 ns neodymium laser pulse has been investigated. The onset of 3 2 ωL emission and an increase in intensity by five orders of magnitude is observed at a laser intensity of ≌2×1013W cm-2, the threshold intensity predicted for excitation of the 2ωpe instability.

X-ray emission from laser-irradiated plane solid targets

Plane solid targets (D2, C, and Cu) were irradiated with Nd laser pulses of 5‐ns duration with energies of up to 50 J giving peak intensities up to 1015 W/cm2 in the focal spot. The x‐ray emission of the plasma has been studied by using scintillators together with photomultipliers, x‐ray photodiodes, and an x‐ray pinhole camera. Detailed results are presented on the emitted x‐ray spectra up to 20 keV, on their angular dependence, on the conversion of laser energy into x‐ray energy, and on the spatial and temporal behavior of the electron density and temperature. The results are discussed with respect to the hydrodynamics, the absorption of laser light, and the energy transport occurring in the plasma. Two important features are heating at densities larger than critical yielding thermal pressures up to about 1 Mbar, and the development of a large cold plasma by strong lateral heat conduction. The hot electrons observed via the high‐energy tail of the x‐ray spectrum, were important for transporting the absorbed laser energy.

Conversion of laser light into soft x rays. Part II: Numerical results

The conversion of laser light into soft x rays during interaction of intense laser light with a planar gold target was investigated numerically with the help of the multi code [Comput. Phys. Commun. 49, 475 (1988)]. It solves one‐dimensional hydrodynamics including flux‐limited electron heat conduction, multigroup radiation diffusion, and steady‐state nonlocal thermodynamic equilibrium radiation physics. The influence of various parameters such as the laser intensity, wavelength, and pulse duration on the conversion efficiency of laser light into x rays was studied. Particular emphasis was placed on comparing the numerical results with the model presented in the preceding paper (Part I) [Phys. Fluids B 2, 199 (1990)]. According to this model the radiating plasma can be divided into a conversion layer and a reemission zone. Its essential features are confirmed by the numerical results.

Self‐calibration of a thinned, backside illuminated charge coupled devices in the soft x‐ray region

A semiempirical method of calibrating a thinned, backside illuminated charge coupled device (CCD) chip in the soft x‐ray region is presented. It is based on determining the thickness of the dead layer self‐consistently using the continuum emission from laser produced plasmas. The CCD camera system was coupled to a transmission grating spectrometer and recorded the spectrally resolved continuum emission from laser irradiated tungsten targets. The thickness of the dead layer was then determined by comparing the experimental spectra with the calculated quantum efficiency for a thinned CCD using a simplified model. In this way the CCD chip was semiempirically calibrated. The accuracy of the calibration in the soft x‐ray range was assessed by comparing the CCD recorded spectra with those recorded by a spectrometer using the absolutely calibrated Kodak 101 photographic plates and a similar transmission grating. Based on this calibration, the CCD sensitivity is deduced to be about two orders of magnitude higher than that of the Kodak plates in this wavelength range.

Comparative study of laser acceleration of thin foils at wavelengths 0.44 μm and 1.3μm

Abstract The acceleration of thin plastic foils irradiated with frequency-triplet ( λ = 0.44 μm ) and fundamental ( λ = 1.3 μm ) iodine laser light was investigated. Rear-side velocities and two-dimensional behaviour of the foils were recorded with a novel 6-frame photographic diagnostic. Core velocities were inferred from calorimetry and X-ray backlighting. Unlike at 1.3 μm, the rear-side and core velocities were found to be quite different at λ = 0.44 μm . An explanation is offered in terms of lack of lateral smoothing at 0.44 μm. The ablation pressure scales as P abl ≈ I 0.6 abs λ -0.4 .

Laser produced plasma from solid hydrogen foils

Abstract A giant pulse ruby laser was focused onto solid hydrogen foils. The laser light was found to be absorber in a layer which penetrates the solid with a velocity of 3 × 10 6 cm/sec.

Plasma Production with a Nd Laser and Non-Thermal Effects

With the improvement of high power lasers it has become possible to produce plasmas of very high density and temperatures which possibly are of interest for thermonuclear research. Recently substantial neutron emission from such plasmas has been reported1–6. In order to investigate the future possibilities in this field of plasma physics detailed understanding of the properties of these plasmas is necessary. In this paper we report results obtained by using a variety of diagnostic techniques applied to plasmas produced within a wider range of laser power and show the evidence of non-thermal effects occurring in laser produced plasmas at moderate energies.

Spectral Investigation of Light Reflected from a Laser‐Produced Deuterium Plasma

Plasma is created from solid deuterium targets by neodymium laser irradiation. The reflected laser light is shifted by 2.5 A towards longer wavelengths. This is interpreted as a Doppler shift of the laser light reflected at a layer moving in the direction of the incident radiation with a velocity of 3.5×106 cm sec−1.