Sambaran Pahari

Achieving ultra high vacuum conditions in SMARTEX-C: control of instabilities and improved confinement

SMall Aspect Ratio Toroidal Experiments in a C-shaped trap (SMARTEX-C) attempts to confine electrons in a toroidal trap using static electric and magnetic field. Confinement of these electrons is largely limited by charge losses due to the presence of background neutrals and by short pulsed magnetic field. Ions formed due to electron impact ionization of background neutrals lead to instabilities that are ultimately observed to limit the confinement. Lowering the amount of neutrals in the trap is therefore of paramount importance. In SMARTEX-C, prior to electron injection, the trap is pumped down to base pressures of the order of 2 ± 1 × 10−8 mbar. However, as one turns on the electron source (tungsten filament emitting thermionically) pressure in the system increases to ~ 1 × 10−7mbar. Partial pressure analysis of the vacuum system indicates predominant presence of H2 during filament operation. Replacing the copper current-leads for electron source with stainless steel-304 led to reduction of H2 outgassing. Additionally, viton seals have been replaced with Aluminum wire-seals that allow baking at elevated temperatures and further reduce out-gassing. All these measures have led to an improvement in the base pressure to 7±1 × 10−9 mbar even during filament operation. Confinement of electron plasmas in the trap has thus significantly improved.

Development of an Electromagnetic Acceleration Facility for Impact and Fracture Studies at High Strain Rates

Experimental studies of strain time history and fracture & penetration resulting from the high velocity impact of solid projectiles on solid targets have been initiated. Design, fabrication, testing and commissioning of an electromagnetic impact facility driven by a capacitor bank have been carried out in this regard. The facility presently has an induction coil gun driving a cylindrical hollow/solid projectile on to a target. 3–7 kJ capacitor banks have been used to drive the launchers. The parameters of the coil gun are in consonance with a computer code developed in‐house for the validation and optimization of the coil dimension and bank parameters. Systematic studies have been carried out for validation of code and understanding and benchmarking coil performance. Reproducible velocities of the order of 100 m/s have been successfully achieved with projectiles of masses 20 gm. Preliminary impact studies carried out on Alumnium target plates have given the strain time history.

Significance of Armature Resistivity and Deformation in Modeling Coilgun Performance

We have performed 2-D axisymmetric magnetohydrodynamic simulations for a single stage induction coilgun system and validated the results against in-house experiments. The simulations, besides computing the currents and electromagnetic forces, also capture the associated hydrodynamic phenomena. This paper highlights the importance of appropriate material strength for handling hydrodynamic phenomena and temperature-dependent electrical resistivity model for self-consistent electromagnetic calculations. These are very essential for accurate prediction of armature dynamics and velocity. Inappropriate resistivity model, even in case of no deformations, produced velocity deviation of up to 50% as compared with experiments. Inclusion of appropriate models shows average deviation when compared with experiments for known armature material (Al6061-T6). For higher energies, predicted armature deformations appear to be in a good agreement with experiments. If material deformation is ignored, velocities get over-estimated. Finally, subsequent to our validation at low energies, an optimized armature profile and dimensions are reported that have allowed our armatures to achieve velocity with single stage operation and 300 m/s with two-stage operations of the coilgun. This paper shows that accurate modeling of Joule heating and armature deformation, along with self-consistent evolution of currents, is essential for coilgun modeling. Although the system is not presently optimized for efficient energy conversion ( ~ 1.5%), the performance of induction accelerator at moderate velocity of armature (50-300 m/s) is not only useful for validating our code, but can act as an effective catapult for impact testing of materials.

Numerical Simulations of an Electromagnetic Coilgun for Impact and Fracture Studies

CAD is setting up an electromagnetic acceleration and impact facility for studies of material fracture at high strain rates. The objective is to electromagnetically accelerate metallic projectiles to velocities in the range 100–500 m/ for impact studies. The velocity of the projectile depends upon various design parameters of the coilgun, the projectile, the driving capacitor bank and the initial system configuration. We have developed a computational model that can predict projectile velocities as a function of time, taking into account magnetic field diffusion, joule heating and electromagnetic forces and the resulting deformation of the coilgun and projectile. Initial results indicate a reasonable match with experimental results, although there are some regimes where large deviations arise. This paper focuses on issues related to the development of the computational model.

Buckling and longitudinal cracks in electromagnetically accelerated hollow cylinders

Hollow cylindrical Al-6061 T6 projectiles, driven in a coilgun system, suffer radial compression and buckle into quadrilateral or pentagonal cross sections. In some cases, the projectiles fail by developing longitudinal cracks in the compressed region. Simulations of the coilgun-projectile system, using Johnson–Cook and Bao–Wierzbicki failure models, reproduce buckling and formation of longitudinal cracks via localization of plastic strain and high temperatures around the bends of the buckled geometry. Failed specimens were micro-graphically investigated and the cause of failure attributed to synergistic effect of buckled geometry and localized high temperature zones.

Confinement time of electron plasma approaching magnetic pumping transport limit in small aspect ratio C-shaped torus

A long confinement time of electron plasma, approaching magnetic pumping transport limit, has been observed in SMARTEX-C (a small aspect ratio partial torus with Ro/a∼1.59). Investigations of the growth rate reveal that they are governed by instabilities like resistive wall destabilization, ion driven instabilities, and electron-neutral collisions. Successful confinement of electron plasmas exceeding >1×105 poloidal E→×B→ rotations lasting for nearly 2.1±0.1 s is achieved by suppressing these instabilities. The confinement time has been estimated in two ways: (a) from the frequency scaling of the linear diocotron mode launched from sections of the wall that are also used as capacitive probes and (b) by dumping the plasma onto a charge collector at different hold times.

Investigation of diocotron modes in toroidally trapped electron plasmas using non-destructive method

Experiments with trapped electron plasmas in a SMall Aspect Ratio Toroidal device (SMARTEX-C) have demonstrated a flute-like mode represented by oscillations on capacitive (wall) probes. Although analogous to diocotron mode observed in linear electron traps, the mode evolution in toroids can have interesting consequences due to the presence of in-homogeneous magnetic field. In SMARTEX-C, the probe signals are observed to undergo transition from small, near-sinusoidal oscillations to large amplitude, non-linear “double-peaked” oscillations. To interpret the wall probe signal and bring forth the dynamics, an expression for the induced current on the probe for an oscillating charge is derived, utilizing Greens Reciprocation Theorem. Equilibrium position, poloidal velocity of the charge cloud, and charge content of the cloud, required to compute the induced current, are estimated from the experiments. Signal through capacitive probes is thereby computed numerically for possible charge cloud trajectories. In ...