Parshotam Dass Gupta

Interferometric determination of thickness of free-standing submicron Formvar films

A method for measuring the thickness of free-standing films of Formvar in the range of 2000 A to 5 μm is presented. The optical path difference of the film with reference to air is measured using a modified Jamin interferometer, and the refractive index is determined by a thin wedge technique. The method provides thickness measurements with an accuracy of ±300 A. The results are found to be in good agreement with those obtained from a profilometer. This simple method can be very useful for thickness measurement of free-standing thin plastic films for extreme ultraviolet (XUV) soft x-ray research applications.

Pressure dependence of enhanced x-ray line emission from laser-produced plasmas expanding in an ambient gas

An experimental study of space-resolved x-ray emission from a laser produced magnesium plasma expanding in a helium gas background has been performed at different gas pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s2 1So - 1s2p 1P1 at 9.17Å) and intercombination (1s2 1So - 1s2p 3P1 at 9.23Å) line emission is observed in the blast wave region extending upto ~6 mm from the target. Results of measurements of pressure dependence of peak x ray line intensity, onset distance, size of the emission region, and intensity ratio of the two lines are described and the role of background gas to explain the observed behavior is discussed. At optimum background pressure of about 40 mbar there is an order of magnitude increase in the x-ray line intensity compared to that for a gas pressure of few mbar. Charge exchange and three body recombination rates have also been analyzed for their relative contribution towards this intensity enhancement.

Features of electrical current in the x-ray source based on the vacuum diode with the laser-plasma cathode

Instabilities of the discharge, as well as temporal and energetic characteristics of x-ray radiation a source based on the vacuum diode with laser-plasma cathode in a wide range of energies, power densities, duration of plasma forming laser pulse, have been experimentally studied. It has been observed that the characteristics of laser radiation significantly affect the dynamics of vacuum discharge and allow to control radiation processes at the early stages of its development. The minimum recorded duration of x-ray pulse (with photon number 1011 per pulse) in titanium K-shells (~4.5 keV) was 10 ns in the case of generation of plasma cathode using laser radiation with the duration of 27 ps and intensity ~1013 W/cm2. It has been established that in the regime of instability suppression and at the application of accelerating voltage (about 3 - 4 times the excitation threshold), the maximum value of contrast of characteristic radiation over Bremsstrahlung is achieved.

X-ray line emission from H-like and He-like ions in magnesium plasma produced by multi-picosecond laser pulses

We report a spectroscopic study of x-ray line emission from a magnesium plasma produced by Nd:glass laser pulses of 30 ps (FWHM) duration at a laser intensity of ~ 3 x 1014 Wcm-2. High-resolution x-ray line spectrum was recorded using a crystal spectrograph. Measurements of continuum radiation intensity were also performed for different x-ray filter cutoff energies. A peak electron temperature of ~ 2.4 keV derived from these measurements is in agreement with that calculated from an analytical model describing laser plasma heating in the short pulse regime. The intensity ratio of MgXII 1s-2p, λ = 8.42 Å and MgXI 1s2-1s2p, λ = 9.17 Å is observed to be much smaller than that calculated from the spectroscopic code RATION. Analytical estimates of ionization equilibrium time for different ionization states in the heated plasma as well as during its expansion using a simple hydrodynamic model can explain this behavior.

A novel geometry for uniform intensity line focus of Gaussian laser beams

Gain measurements in x-ray lasers involves monitoring the intensity of lasing transition as a function of line focus length. Having a line focus of variable length with uniform intensity is important in these measurements. In addition, uniform line focusing of Gaussian laser beams has many other applications in material processing, pumping of dye lasers etc. In this paper, we describe a simple technique using a wedge to displace and overlap two halves of a Gaussian laser beam to obtain a uniform intensity line focus. Variation of the line length at a fixed intensity is facilitated by use of appropriate masks. X-ray emission pictures of a line focused laser produced plasma show the effectiveness of this geometry.

Dependence of soft x-ray conversion on atomic composition in laser-produced plasma of copper-gold mix-Z targets

In the indirect scheme of ICF, a high-Z ablator is used as the inside wall material of the hohlraum cavity. This choice is made because a high-Z plasma is an efficient x-ray convertor, and it offers a high opacity to radiation conduction into the cavity wall. Recently it had been shown that a mixture of two or more high-Z elements can provide a higher opacity than that of any single materials used, and optimum composition was predicted from computer simulations. It is expected that such a wall will provide maximum conversion of absorbed laser energy into soft x-ray radiation. In this paper, we present a comparative experimental study of soft x-ray emission from copper-gold mix-Z planar targets of different atomic compositions irradiated at a laser intensity of ~1013 W/cm2 (λ = 0.532μm). Spectral features of the soft x-ray radiation were recorded using a transmission grating spectrograph. Maximum enhancement in radiation intensity in the spectral region ~ 40 - 120 Å was observed for a mix-Z target of atomic composition of Cu 0.57 - Au 0.43. This is in broad agreement with theoretical results of opacity calculations performed for different atomic compositions using a screened hydrogenic average atom model.

Spectral, spatial, and polarization characteristics of harmonics generated at interaction of intense laser radiation with aluminum foils

Investigations on harmonic generation in laser-matter interaction of Nd:glass laser radiation with solid surfaces have been performed. Detailed measurements were made on the intensity of second, third and fourth harmonics, their polarization properties and spectral distribution, as well as dependencies of these characteristics on intensity and polarization of the incident laser radiation. Intensity dependence of second, third and fourth harmonics for p- polarized laser pump shoed a power law scaling of 1.5, 1.8 and 3.8 respectively. Maximal conversion efficiencies for second, third and fourth harmonic generation were observed in the range of 10-8 to 10-12. For p- polarized laser radiation, the generation efficiency was more than ten and hundred times higher in comparison to that for the s-polarized radiation for second and third harmonics respectively. Among other features, rotation of second harmonic polarization as a function of laser intensity showed a quite different behavior for p- and s-polarized laser radiation, and the second harmonic observed in the specular reflection direction was red-shifted for laser intensity exceeding 5 by 1014 W cm-2.

Sub-angstrom study of plasma x-ray emission by transmission grating spectrometer

A simple XUV transmission grating spectrography operating in the wavelength range of 3-90 angstrom with sub-angstrom resolution without using any XUV imaging optics is described. This is based on a free-standing gold microstructure grating of 2000 angstrom period in normal incidence geometry. A spectral resolution of 0.6 angstrom is obtained by optimizing the slit aperture, the source to grating and the grating to detector distances. Electron temperature deduced form analysis of the spectrum of a laser produced magnesium plasma using a plasma spectroscopic code are consistent with theoretical considerations.