P. D. Gupta

Spectroscopic determination of mass ablation rate in laser produced plasmas

The measurements of mass ablation rate and ablation thickness in laser produced plasmas from layered targets using a luminosity time of flight spectrometer are reported. The results show a steady state ablation in the laser intensity range of 1011–5×1012 W/cm2 of a Nd:glass laser. A mass ablation rate of 2×104 g/cm2/sec is observed for aluminum plasma at the laser intensity of 5×1011 W/cm2 which is consistent with the estimate obtained using a self‐regulating model for plasma heating. This technique offers a simple and direct method for measuring the ablation parameters.

V‐I characteristics of a plane disc probe in collisionless streaming plasmas

Expansion of high‐temperature high‐density plasmas produced by focussing high‐power lasers on solid targets gives rise to collisionless streaming plasmas with flow velocity U such that Ci ≪U≲Ce, where Ce and Ci are electron and ion thermal velocities, respectively. Dimensionless V‐I characteristics of a plane disc probe in such streaming plasmas are obtained. The characteristics show a nonsaturation behavior of the probe current in the electron attracting region of the probe operation. It is observed that in the electron repelling region, the slope of the V‐I characteristics increases with the decrease in the probe potential.

Study of thermally induced active birefringence in Nd:glass laser rods

A study of active birefringence arising due to the thermal stresses in Nd:glass laser rods under different experimental conditions of pumping is reported. The extent of birefringence was measured in terms of depolarization of a pulsed probe beam from a Q-switched Nd:YAG (yttrium aluminum garnet) laser. The maximum depolarization for a 38-mm-diam rod pumped by 12 xenon flash lamps in a circular diffuse reflector configuration was determined to be 2.3%. This value for the depolarization as well as its radial profiles agree well with those determined from a cylindrically symmetric gain profile. In cases of deviation from the cylindrically symmetric pumping, the observed birefringence was found to be more for a clover leaf reflector as compared to that for a circular diffused reflector.

Effect of recombination on ion expansion velocity in laser‐produced plasmas

A study of the ion expansion characteristics in plasmas created from carbon, copper, and gold targets using a 1‐GW, 5‐ns neodymium glass laser is presented. The ion expansion velocity is found to vary with the distance of the probe from the target and the variation is observed to be different for low and high atomic number (Z) targets. Analysis shows that the strikingly different variation for low and high Z targets is a manifestation of their different recombination characteristics during plasma expansion.

Dependence of x‐ray intensity scaling on target atomic number in laser produced plasmas

We have measured the x‐ray emission intensity from plasmas created from a number of targets with atomic number (Z) in the range 6≤Z≤73 using a 1‐GW, 5‐ns neodymium glass laser. The variation of x‐ray intensity with laser intensity (Φ) was observed to depend on the target atomic number and showed a scaling of Φα where α varies from 1.9 to 2.8 for different elements. The observed scalings were found to be consistent with a self‐regulating model for plasma heating.

Estimation of ablation thickness by ion expansion analysis in laser‐produced plasmas

Ion expansion analysis of the plasma produced from targets consisting of thin layers of aluminum on carbon substrates is used to estimate the thickness of the material ablated from the target. Ablation depth obtained in this manner is consistent with a steady‐state self‐regulatory model for plasma heating.

Isotopic enhancement in laser‐produced plasmas

This paper reports the investigations carried out on isotopic enrichment in boron oxide plasma produced by a Nd : glass laser giving 20 MW in 30 nsec (FWHM). Energy distributions of various ions in the plasma expanding in vacuum were obtained with an electrostatic ion analyzer. Energies corresponding to peaks of distribution for B+10 and B+11 were found to be different by the ratio of their masses, consistent with hydrodynamic expansion of plasma. An anomalously high ratio of B+10 to B+11 ions as compared to their natural abundance was observed.

X‐ray luminosity time‐of‐flight spectrometer for laser‐produced plasmas

A simple and compact x‐ray luminosity time‐of‐flight spectrometer for measurements of the velocity distribution of highly charged ions in laser‐produced plasmas is presented. The spectrometer is based on making time‐ and space‐resolved measurements of x‐ray line emission from these ions in the expansion zone. Design, operation, and performance of this system are described. The spectrometer is useful in studying processes of interaction between highly charged ions and buffer gases, and nonthermal behavior of laser‐produced plasma expansion.

Effect of lateral energy transport on the momentum transfer to targets in laser produced plasmas

We have studied the ablation driven momentum transfer to single and layered planar targets, irradiated by a 1‐GW, 5‐nsec Nd:glass laser in the intensity range 1011–1013 W/cm2. The results show an effect of lateral energy transport on the scaling of momentum with laser intensity. Corroborative evidence for this is presented from experiments on thin foil targets.

Fluorescence emission from Nd:yttrium aluminum garnet laser amplifier

Study of on axis emission from a Nd:YAG (yttrium aluminum garnet) laser amplifier is reported. At high input energies, the fluorescence pulse shows a sharp peak and its intensity keeps on increasing with the pump energy. This is attributed to superposition of parasitic oscillations from the laser amplifier in the axial direction on the amplified fluorescence intensity.