Dhruba J. Biswas

Effect of longitudinal flow velocity on the emission spectrum of CO2 lasers

By operating a low‐pressure cw CO2 laser at very high flow velocities in order to smooth out the nonuniform spatial distribution in the inverted population, experimental evidence has been provided of the important role played by the velocity of flow in the longitudinal direction in determining the rotational emission spectrum of CO2 lasers.

Switching of a TEA CO 2 laser with its own UV emitting parallel spark channels

The efficient operation of a TEA CO(2) laser wherein the parallel spark channel preioniser of the laser itself functioned as a switch is reported. Simultaneous closure of the parallel gaps without an external switch has been achieved by ballasting them with mutually coupled inductances. The repetitive operation capability of such a laser is also discussed.

Study of laser assisted decontamination of commonly used clad surfaces

The decontamination factor, defined as the ratio of initial to residual activity, is studied as a function of laser fluence for three kinds of clad surfaces viz., plain zircaloy, autoclaved zircaloy, and stainless steel with cesium as the simulated contamination. It has been found that the decontamination factor exhibits a maximal behavior with the laser fluence and its maximum value occurs at different laser fluences in the three cases. The maximal behavior is attributed to reduced coupling of energy from the laser beam to the substrate due to the initiation of surface-assisted optical breakdown. The results obtained in the experiment carried out in helium environment qualitatively support this explanation.

Switch-less operation of a TEA CO2 laser.

We report here the operation of a UV pre-ionised mini-TEA CO2 laser where the use of an external switch has been dispensed with. This was rendered possible by making the preioniser spark array play a dual role- that of a switch as well as of a source of UV photons that pre-conditioned the inter-electrode volume.

Multiline oscillation from a hybrid CO2 laser by cavity length tuning

A simple method of obtaining multiline oscillation on the P‐branch transitions of the 10.6‐μm band from a hybrid CO2 laser by precisely controlling the cavity length is described. To explain the mechanism of this scheme, theoretical results on multiline oscillation in a hybrid CO2 laser using the rate equation model are presented.

A highly efficient electron beam controlled switchless multi-joule TEA CO2 laser

Abstract We report multi-joule operation of an electron beam controlled switchless TEA CO2 laser. The system is capable of delivering up to 75 J of energy per pulse at an electro-optical efficiency of ∼26%. More energy could be extracted from the laser, however, at a reduced efficiency. At the maximum operating efficiency, the small signal gain of the laser has been measured to be ∼5% per cm.

A novel method of measuring the delay between pre-ionizing and main discharges in TE gas lasers

A simple and reliable method of measurement of the very short delay (tens of nanoseconds) between the pre-ionizing and main discharge currents in a TEA CO2 laser operated in the spiker sustainer configuration has been demonstrated. In this method light emitted from the pre-ionizing spark and the main discharge are first collected separately using optical fibres and then the delay between the onsets of their emission is measured with the help of simple opto-electronic circuits. This method is noise free, since it does not involve measurement of large pulse currents, and can be useful for the measurement of similar delays in the operation of other TE gas lasers.

Latch proof operation of a switch of a high repetition rate laser with dc resonant charging

We report on the operation of a high voltage high current switch in which command resonant charging capability is built in. Elimination of switch latch up problems do not require any extraneous and complex circuitry. The indigenously developed switch delivers power in excess of 3 kW into a dummy load typical of a TE laser at a pulse repetition rate of 200 Hz. The test on a real TEA CO2 laser load was, however, limited up to 50 Hz repetition rate beyond which the accumulation of dissociation products within the active volume led to arcing.

Laser-Assisted Decontamination of Fuel Pins for Prototype Fast Breeder Reactor

Decontamination of fuel pins is an important process step in nuclear fuel fabrication. Decontamination assumes greater significance with respect to fuels containing plutonium, owing to both plutonium’s high radiotoxicity arising from its long biological half-life and its relatively short radioactive half-life. The advantages of using a laser to decontaminate such radioactive surfaces over conventional cleaning techniques are well recognized. This paper describes detailed process optimization and field implementation of laser-assisted decontamination of fuel pins of the Prototype Fast Breeder Reactor (PFBR). A short-pulsed Nd-YAG laser has been effectively used to decontaminate the fuel pins by exposing their outer surface to laser radiation of an appropriate fluence. The laser parameters were controlled to achieve the required cleaning without causing any clad surface damage. Achievement of such was confirmed by evaluating the laser-cleaned surface using scanning electron microscopy, chemical composition studies by electron probe microanalysis, and Vicker’s microhardness test.

High repetition rate operation of a rotating dielectric spark gap

The reliable operation of a spark gap at 300 Hz pulse repetition frequency by rotating a suitably configured dielectric plate between its electrodes is reported. The triggering of the spark gap has been achieved by the use of an optical sensor.