Aditi Ray

Developing a finite difference time domain parallel code for nuclear electromagnetic field Simulation

A two dimensional finite-difference time-domain code has been developed for complete simulation of nuclear electromagnetic pulse (NEMP) originating from an atmospheric nuclear detonation. The modules of NEMP simulation code describing various physics aspects are discussed. Typical results of the serial code for Compton current estimated from detailed neutron-gamma transport, induced air conductivity and electromagnetic fields are presented. The need for parallelizing such code has been explained. The parallel implementation using domain decomposition technique of message passing interface paradigm is described. The efficiency of the parallel code has been studied with increasing number of processors. The limitations of speed-up due to communication times are discussed.

Quasi-isentropic compression using functionally graded materials in gas gun and explosive driven systems

Results of hydrodynamic simulations of dynamic compression experiments performed by impact loading of materials are reported. The simulations pertain to a new approach for generating quasi-isentropic compression using functionally graded materials (FGM). First of all, we focus on quasi-isentropic compression waves generated by a constant velocity impactor (similar to that from a gas gun). Quasi-isentropic compression is characterized from the temporal profiles of pressure at target surface and fluid velocity at target-window interface generated from different functional forms of density variation along the FGM flyer. It is shown that quadratic FGM is the best option for increasing rise time of pressure pulse. Secondly, FGM induced quasi-isentropic compressions are studied by accelerating the impactor with high explosive (HE) driven shocks in both the cases when impactor is in contact with the target (contact geometry) and separated from target by air gap (flyer geometry). The study reveals that nearly ise...

Hydrodynamic simulation and thermodynamic characterization of functionally graded material induced isentropic compression: Towards optimum density profile

Hydrodynamic simulations of dynamic compression experiments reveal that heating as well as entropy production in the target are much lower along quasi-isentropes, generated using impactors employing functionally graded material (FGM), than in shock compression. The possibility of achieving quasi-isentropic compression using FGM, in both gas gun and explosive driven systems, was explored in a recent paper. Qualitative analyses of temporal profiles of pressure pulse generated in the target, obtained with various density distributions within FGM impactors, showed that quadratic density variation is most suitable. This paper attempts to re-establish this finding by identifying the signatures of quasi-isentropic compression from basic thermodynamic aspects. It is shown that quadratic density variation is most suitable candidate as it leads to least entropy increase for a specific peak pressure. Further, the optimum density profile, found by genetic algorithm based optimization tool, with density of individual ...

A Z-scan study of nonlinear refraction in sodium vapour

Abstract Resonant enhancement of nonlinear susceptibility in sodium vapour resulting in an intensity dependent nonlinear refractive index was investigated using the Z -scan technique. Estimates for the dipole moment obtained from the values of the near-resonant optical nonlinearity measured by us, in the vicinity of the D 1 and D 2 sodium resonance lines, were found to be in good quantitative agreement with the reported values. The functional dependence of the observed nonlinear susceptibility on the extent of detuning of the laser frequency with respect to the resonant atomic transition was in agreement with the adiabatic following model.

Effects of quantized motion of a trapped atom on non-linear optical processes

It is shown that the second order response of a single two-level atom in a harmonic trap interacting with a classical electromagnetic field is non-zero and that, in that order, the system generates a four-wave mixing signal. The resonances in that signal are analysed by assuming the atom to interact resonantly with one of the sidebands. The non-zero second order response of an atom in a harmonic trap is in contrast with the zero even-order response of an atom to the applied fields in free space.

GENERATION OF SQUEEZED ATOMIC STATES IN CAVITY QED

A squeezed atomic state is that state of a system of two-level atoms for which the intrinsic quantum noise in a process of measurement is less than the minimum noise obtained by using a spin coherent state. It is shown that such a state is generated in certain time intervals when a non-squeezed atomic state evolves on interaction with a single mode coherent field inside a lossless cavity. The atoms are assumed to undergo one-photon or two-photon transitions between the given two levels. The maximum atomic squeezing is found as a function of the number of atoms and the field strength. The effect of the field-dependent Stark shift is investigated in the case of the atoms undergoing two-photon transitions.

Bound-free transitions and the dissociation limit

Following a sequential two-photon excitation, fluorescence is observed from several selectively excited single rotational-vibrational energy levels of theE3π0g+ state of molecular iodine. The re-emittedE →B fluorescence spectrum from each of the populated rovibrational level of theE state consists of a series of sharp lines terminating on the various discrete ro-vibrational levels of theB state and a few broad lines due to transitions taking place on to the continuum of theB state. The point of transition from sharp lines to broad features in the fluorescence spectrum has been utilized to determine theB state dissociation limit. This method of obtaining the dissociation limit of the molecular electronic states appears to be quite simple and straightforward.

Special features of the safety and control systems of theDhruva reactor

The prime requirement of reactor safety combined with the need for high availability of nuclear plants have, in recent years, led to considerable research and development efforts at the Bhabha Atomic Research Centre in the field of reactor safety and control engineering. The areas of special interest have been the development of a fast acting emergency shutdown system, on-line fault detection facility for the reactor protection circuits, enhanced instrumentation capability for measurement of critical plant parameters and computerised systems for plant protection, control, performance evaluation, disturbance analysis, and data acquisition and display with particular attention to the problem of manmachine interface. Some of these recent concepts have been incorporated in safety and control systems of theDhruva reactor which is at present undergoing commissioning trials at Trombay. The special features of these systems are highlighted in the paper. The safety strategy adopted for the reactor and the consequent development of special safety systems are described in detail. The choice of the reactor control scheme and the methodology followed in the design of the automatic power control system are indicated. Campbell instrumentation for measurement of neutron flux or in other words reactor power, extensive use of microprocessors in safety related instrumentation and an improved man-machine interface through suitable design of control room have helped in achieving a high degree of reactor safety. The salient features of these systems are also included.