Shashank Chaturvedi

High velocity impact of metal sphere on thin metallic plates: A comparative smooth particle hydrodynamics study

Four different shock-capturing schemes used in smooth particle hydrodynamics are compared as applied to moderately high-velocity impacts (at 3km/s) and hypervelocity impacts (at ~6km/s) of metallic projectiles on thin metal plates. The target and projectile may be of same metal or different. The simulations are compared with previously published experimental data and simulations. The schemes differ in how artificial viscosity (AV) is treated and include (i) standard SPH AV, (ii) Balsaras AV (BAL), (iii) Morris and Monaghans AV (MON) and (iv) Riemann-based contact algorithm (CON) of Parshikov et al. At moderately high impact velocity, CON performed best overall, in particular, in being free from numerical fracture and formation of clumps, problems that plague SAV and its modifications, BAL and MON. For the hypervelocity impact, CON does not produce correct debris cloud. The BAL and MON while reproducing the debris cloud more closely, overestimate the crater diameter significantly. An excessive AV, by enhancing transverse momentum flow, may be the source of the overestimation of crater diameter. Experimental agreement is generally worse when the target and projectile are formed of distinct metals.

Three-dimensional computation of reduction in Radar cross section using plasma shielding

We have performed three-dimensional (3-D) finite-difference time-domain (FDTD) simulations for calculating microwave scattering from metallic objects shielded by a plasma shroud. Such simulations are of interest for plasma-based stealth technology. The simulations yield a reasonable match with experimental measurements. A physical interpretation has also been provided for these results, in terms of the flow of electromagnetic power. Such an analysis is only possible using the detailed spatio-temporal evolution of electromagnetic fields that is provided by the FDTD method. We find that apart from absorption, the bending of waves toward regions of lower plasma density plays an important role in determining the extent of backscatter. This has major implications for plasma stealth applications, which have heretofore assumed that plasma absorption is the main mechanism. Also, bending could actually enhance radar scattering in directions oblique to the incident direction. We have also identified situations where 3-D simulations become necessary, and other situations where a composite one-dimensional simulation may be enough. This has practical relevance since it could help reduce the demand for computational resources while modeling large objects like aircraft.

Study and Optimization of Plasma-Based Radar Cross Section Reduction Using Three-Dimensional Computations

The radar cross section (RCS) of a flat plate covered with a cold collisional inhomogeneous plasma has been studied using a 3-D finite-difference time-domain (FDTD) method for electromagnetics. Two problems have been considered. In problem 1, using experimentally reported plasma density profiles, we have observed some interesting features in the bistatic RCS and provided simple physical interpretations for some of these features. The simulations confirm that a plasma shroud can successfully be used for reducing the RCS of a flat plate at almost all scattering angles, although the RCS could increase at some other angles. This is a novel extension of the FDTD method for the calculation of the bistatic RCS of an object shielded by a nonuniform collisional plasma. Problem 2 involves an optimization study for the input power required to achieve a desired RCS reduction (RCSR), examining a variety of plasma density levels and spatial profiles. For this optimization study, we have considered a helium plasma produced by a high-energy electron beam. We find that the maximum achievable reduction increases monotonically with power up to an optimum point, beyond which the RCSR decreases, finally showing some tendency to saturate. This is of practical importance and indicates the usefulness of FDTD simulations in identifying the optimal point. Furthermore, at a given power level, there can be a considerable scatter in the RCSR achievable. This is because various combinations of the plasma parameters, differing considerably in their RCSR abilities, could require the same power to sustain them. Simulations would be of great use in helping to identify the best profiles to be used for a given input power level.

Modal analysis for a bounded-wave EMP Simulator - part I: effect of test object

We have analyzed the electromagnetic-mode structure inside a bounded-wave electromagnetic pulse (EMP) simulator. This has been done by the application of the singular value decomposition method to time-domain data generated by self-consistent, three-dimensional finite-difference time-domain simulations. This combination of two powerful techniques yields a wealth of information about the internal mode structure which cannot be otherwise obtained. To our knowledge, this is the first time such a comprehensive study has been done. In the absence of a test object, the transverse electromagnetic (TEM) mode is dominant throughout the simulator length. TM/sub 1/ dominates over other transverse electromagnetic (TM) modes over most of the length. Close to the termination, the TEM mode weakens marginally, while higher order TM modes become stronger. The enhancement of TM/sub 2/, and the weakening of TEM near the termination, have been explained in physical terms. Placement of a perfectly conducting test object in the parallel-plate section increases the strength of higher order TM modes, at the cost of TEM. Hence, the object is subjected to electromagnetic fields that deviate significantly from the desired TEM form. A physical interpretation has been provided for this phenomenon. The enhancement of electromagnetic fields near the top and bottom faces of the object are explained in terms of the Poynting flux distribution.

Modeling of an EMP simulator using a 3-D FDTD code

Performed self-consistent, three-dimensional (3-D), time-domain calculations for a bounded-wave electromagnetic pulse simulator. The simulator consists of a constant-impedance transverse electromagnetic structure driven by a charged capacitor, discharging through a fast closing switch. These simulations yield the detailed 3-D electromagnetic field structure in the vicinity of the simulator. The prepulse seen in these simulations can be explained quantitatively in terms of capacitive coupling across the switch and the known charging waveform across the capacitor. Placement of a test object within the simulator significantly modifies the electric fields within the test volume, in terms of field strength as well as the frequency spectrum. This means that, for a given simulator, larger objects would be subjected to somewhat lower frequencies. The E-field waveform experienced by a small test object is reasonably close to that for free-space illumination, but the mismatch increases with object size. The use of a resistive sheet as a matching termination significantly reduces radiation leakage as compared to two parallel resistive rods. For a given termination, larger test objects marginally reduce leakage. A physical interpretation of these conclusions is also included. This work is a first step toward full-fledged optimization of such simulators using 3-D modeling.

Determination of best-fit potential parameters for a reactive force field using a genetic algorithm

The ReaxFF interatomic potential, used for organic materials, involves more than 600 adjustable parameters, the best-fit values of which must be determined for different materials. A new method of determining the set of best-fit parameters for specific molecules containing carbon, hydrogen, nitrogen and oxygen is presented, based on a parameter reduction technique followed by genetic algorithm (GA) minimization. This work has two novel features. The first is the use of a parameter reduction technique to determine which subset of parameters plays a significant role for the species of interest; this is necessary to reduce the optimization space to manageable levels. The second is the application of the GA technique to a complex potential (ReaxFF) with a very large number of adjustable parameters, which implies a large parameter space for optimization. In this work, GA has been used to optimize the parameter set to determine best-fit parameters that can reproduce molecular properties to within a given accuracy. As a test problem, the use of the algorithm has been demonstrated for nitromethane and its decomposition products.

Particle-in-cell simulations for virtual cathode oscillator including foil ablation effects

We have performed two- and three-dimensional, relativistic, electromagnetic, particle-in-cell simulations of an axially extracted virtual cathode oscillator (vircator). The simulations include, for the first time, self-consistent dynamics of the anode foil under the influence of the intense electron beam. This yields the variation of microwave output power as a function of time, including the role of anode ablation and anode–cathode gap closure. These simulations have been done using locally developed particle-in-cell (PIC) codes. The codes have been validated using two vircator designs available from the literature. The simulations reported in the present paper take account of foil ablation due to the intense electron flux, the resulting plasma expansion and shorting of the anode–cathode gap. The variation in anode transparency due to plasma formation is automatically taken into account. We find that damage is generally higher near the axis. Also, at all radial positions, there is little damage in the ea...

Comparison of wave propagation studies in plasmas using three-dimensional finite-difference time-domain and ray-tracing methods

Power-flow trajectories of electromagnetic waves through a spatially nonuniform plasma have been computed using direct solutions of Maxwell’s equations using the three-dimensional finite-difference time-domain (FDTD) method. This method yields accurate information on refraction as well as absorption effects. The method can be used to compute power-flow trajectories for plasmas with arbitrarily varying density profiles, including effects due to arbitrarily shaped conducting or dielectric surfaces bounding the plasma. Furthermore, since FDTD is computationally expensive, especially for parametric studies, it is desirable to use ray tracing to estimate refraction effects. A quantitative comparison is performed between two different methods of obtaining exact and approximate solutions of Maxwell’s equations in order to assess their relative utility in different situations. In the present work, we limit ourselves to a cold, collisional, unmagnetized plasma, where the response to electromagnetic waves is fully ...

Secondary Virtual-Cathode Formation in a Low-Voltage Vircator: PIC Simulations

We have performed 2-D relativistic electromagnetic axisymmetric particle-in-cell simulations for an axially extracted vircator. During periods of significant microwave emission, we observe the formation of a secondary smaller virtual cathode (VC) at some distance from the main VC. For a given vircator geometry, this appears only during low-voltage operation. The electron density in this secondary structure tends to increase with the microwave emission. We have also provided a qualitative explanation for the creation of this secondary structure and for its appearance during low-voltage operation. We find that electron trajectories in the vircator can be divided into four broad categories, based on their shapes and also on the temporal variation of electron kinetic energy. Three of these trajectories are those reported earlier by Alyokhin, while the fourth appears to be a new one. The fourth type is linked to the appearance of the secondary VC and, hence, becomes significant only at times of significant power emission.

Calculation of Accurate Resistance and Inductance for Complex Magnetic Coils Using the Finite-Difference Time-Domain Technique for Electromagnetics

A finite-difference time-domain (FDTD) method for electromagnetics, which is adapted for magnetic-field diffusion problems, has been applied for accurate calculation of the resistance and inductance of arbitrarily wound helical coils of interest in magnetic flux-compression systems. These simulations have been performed using a locally developed 3-D variable-mesh FDTD code. The resistance calculations automatically take account of skin and proximity effects and are capable of handling arbitrarily complex multimaterial systems. The simulations also yield a detailed 3-D picture of the magnetic-field diffusion through a complex multimaterial coil in the presence of arbitrary time-dependent current waveforms. Hence, they can provide critical insight into coil performance in real-life systems. We report on the critical issues that must be kept in mind for such simulations and the results of test problems with simple coil geometries. To our knowledge, this is the first application of this powerful technique to the flux-compression systems.