Hariswaran Sitaraman

Magneto-hydrodynamics simulation study of deflagration mode in co-axial plasma accelerators

Experimental studies by Poehlmann et al. [Phys. Plasmas 17(12), 123508 (2010)] on a coaxial electrode magnetohydrodynamic (MHD) plasma accelerator have revealed two modes of operation. A deflagration or stationary mode is observed for lower power settings, while higher input power leads to a detonation or snowplow mode. A numerical modeling study of a coaxial plasma accelerator using the non-ideal MHD equations is presented. The effect of plasma conductivity on the axial distribution of radial current is studied and found to agree well with experiments. Lower conductivities lead to the formation of a high current density, stationary region close to the inlet/breech, which is a characteristic of the deflagration mode, while a propagating current sheet like feature is observed at higher conductivities, similar to the detonation mode. Results confirm that plasma resistivity, which determines magnetic field diffusion effects, is fundamentally responsible for the two modes.

Advances in Microcavity Discharge Thruster Technology

Recent advances in the effort to demonstrate the propulsion capabilities of a microcavity discharge (MCD) thruster are discussed. The MCD thruster is being developed primarily through an experimental effort with support from computational modeling, and predicts an ultimate performance of 1 mN of thrust per cavity, a thrust efficiency exceeding 60%, and an argon specific impulse of 150 seconds. Because the MCD thruster has low specific mass and is scalable over a large number of cavities, successful implementation would ultimately result in an advanced propulsion system useful for primary (orbit transfer, maneuvering) and secondary (attitude, position and acceleration control) applications for a wide range of satellites. Experimental measurements are taken with MCD thrusters designed and fabricated according to parameters suggested by computational simulations. These measurements determine the stagnation temperature, from which specific impulse, thrust efficiency, heat loss and micronozzle efficiencies can be derived. Since the extremely small dimensions of the MCD thruster and fast oscillatory phenomena associated with AC excitation make experimental diagnostics of the device very difficult, a detailed first-principles computational model provides time-accurate solutions of the multi-species, multi-temperature, self-consistent plasma governing equations for discharge physics, coupled to the compressible Navier-Stokes equations for the bulk fluid flow through the MCD thruster. The computation employs a hybrid unstructured computational mesh for the MCD thruster geometry that represents both the solid dielectric structure that confines the flow and a separate mesh for the plasma and flow fields.

Simulation Studies of Alternating-Current Microdischarges for Microthruster Applications

The Microcavity Discharge Thruster (MCDT) concept is built on the idea of a microdischarge operated using alternating current (AC). The thruster geometry comprises of a constant area section and a divergent micronozzle with two ring electrodes placed close to the throat. Power is deposited into a flow of argon gas which increases the specific impulse of the thruster. A suitable dielectric material is used to separate the electrodes from the bulk plasma. This prevents the sputtering and erosive action of energetic particles thus increasing the life of the microthruster. A detailed flow model is used to study the effect of inlet stagnation conditions on wall heat losses which is crucial in the optimization of power deposition in the thruster. Operation at higher pressures close to 500torr has been found to be effective due to lower wall heat losses. At this inlet total pressure and total temperature of 300K, a cold gas specific impulse of 50 seconds is seen. Subsequent heat addition of 113mW at these conditions in the thruster gives a specific impulse of 80 seconds and a thrust efficiency of 50%. The use of a dielectric-barrier to cover the electrodes requires an alternating current to drive the discharge. A detailed plasma dynamics model is used to study the operation of microdischarges as a DBD. Dielectric Barrier Discharges (DBD) operate at near atmospheric pressures (760torr) using alternating currents to give stable glow discharges. The study compares the effect of plasma joule heating and collision source terms on gas temperature in millimeter and micrometer length scale DBDs. It has been found that the gas temperature in a micrometer scale DBD reaches a value of 800K compared to smaller changes in a millimeter length scale DBD. The large power densities observed in the micro DBD gives rise to higher gradients in temperature with lower length scales adding to the effect. This considerably increases heat transfer to the walls. Therefore operation at higher Reynolds number is crucial to optimize the efficiency of the thruster.

Magneto-hydrodynamics simulation study of high-density plasmas in electromagnetic guns

Electromagnetic plasma guns use Lorentz forces to accelerate high density plasmas to velocities ~ km/s. This concept has been used widely in space propulsion systems and in thermonuclear fusion devices. One of the important factors that influence the performance of these devices is the interaction of the high density plasma with the bounding solid surfaces. We perform a numerical modeling study of the plasma in an electromagnetic gun to understand the discharge physics and in particular study the plasma-surface interactions. We use the resistive Magneto hydrodynamics (MHD) equations which include the mass, momentum and energy equations for a conducting fluid along with the Maxwells equations to study the plasma phenomenon in these devices. The equations are solved on an unstructured mesh using a cell-centered finite volume formulation. Simulations are performed on the operation of a generic plasma accelerator in the plasma detonation mode with current inputs ~ 400-1200 kA/m. Results obtained reveal the formation of a current sheet that propagates from the breech to the muzzle. It is also seen that the viscous shear stresses and thermal fluxes at the electrodes are dominant in the region of the current sheet. The time averaged viscous drag acting on the plasma is seen to increase rapidly with the current input.

Two Dimensional Simulations of Micro Cavity Discharges for Micro-propulsion applications

The Micro Cavity Discharge (MCD) thruster concept is built on the idea of a microdischarge with dielectric covered electrodes operated using an alternating current (AC) excitation. The thruster geometry comprises of a constant area section and a divergent micro-nozzle with two ring electrodes placed close to the throat. Power is deposited into a flow of argon gas which increases the specific impulse of the thruster. A suitable refractory wear-resistant dielectric material is used to separate the electrodes from the bulk plasma. This minimizes erosion of the plasma-facing material components due to heating and sputter mechanisms, thereby increasing the life of the microthruster. A detailed plasma dynamics model coupled with the compressible Navier-Stokes equations is used to study the flow and plasma phenomenon in the thruster. Results indicate a highly pulsed microdischarge with plasma densities of ~10 19 m -3 and current densities ~ 700 mA/cm 2 for an alternating current excitation in the radio frequency (RF) regime. It is also seen that the dominant heating mechanism in these discharges is through ion Joule heating. Higher electron densities and spatially dominant thermal source terms are observed at 20 MHz excitation compared to the 10 MHz case. A peak gas temperature rise ~ 150 K is seen for a cycle-integrated power deposition of 68 mW.

Magneto-hydrodynamics simulation study of high speed flow control using the Rail Plasma Actuator (RailPAc)

The RailPAc (Rail Plasma Actuator) is a novel flow control device that uses the magnetic Lorentz forces for fluid flow actuation at atmospheric pressures. The device consists of two parallel flush mounted rail electrodes along which a plasma current sheet is accelerated by virtue of ~ J × ~ B Lorentz forces. Experimental studies reveal actuation ∼ 10-100 m/s can be achieved with this device which is much larger than conventional electro-hydrodynamic (EHD) force based plasma actuators. A magneto-hydrodynamics simulation study of this device is presented. A one dimensional model to study the arc motion in the RailPAc is first formulated. The snowplow model, typically used for studying parallel plate pulsed plasma thrusters is used to predict the arc velocities, which agree well with experimental measurements. Two dimensional simulations are performed using a finite volume based resistive magneto-hydrodynamics model. The model is further developed to include the effect of applied electric and magnetic fields seen in this device. The effect of Lorentz forcing and heating effects on fluid flow actuation is studied by using two electrical conductivity models an equilibrium air plasma conductivity as a function of temperature and an assumed conductivity accounting for the moving non-equilibrium plasma. The equilibrium conductivity model is unable to self consistently capture the motion of the current sheet thereby confirming the importance of its non-equilibrium nature. Simulations are performed with a moving region of high conductivity with arc velocities predicted by the snowplow model. Actuation on the order of 100 m/s is attained at the head of the current sheet due to the effect of Lorentz forcing alone. The inclusion of heating effects led to isotropic blast wave like actuation which is detrimental to the performance of RailPAc. This study also revealed the deficiencies of a single fluid model and a more accurate multi-fluid approach is proposed for future work.

Simulation studies of micrometer scale dielectric barrier discharges for microthruster applications

Summary form only given. The recently designed Microplasma thruster employs the use of direct-current (DC) microdischarge to cause heating of argon gas during the expansion process through a nozzle. This helps in enhancing the cold gas thrust. However, one of the deficiencies of direct-current microdischarges is the need for exposed electrodes that incur ion bombardment with large thermal fluxes and consequently subject to wear. A different approach is to use an alternating current (AC) with electrodes covered by a suitable dielectric to prevent electrode erosion. The discharge will then be similar to a micrometer scale Dielectric Barrier Discharge (micro-DBD). It is thus important to investigate the power densities that are attained by this system and assess its effectiveness in gas heating. Classical large-scale (mm gap) Dielectric Barrier Discharges (DBD) is used in excimer lamps, ozone generation and in materials processing and are not subject to any significant gas heating. In this study we compare the gas heating effects in a micro-DBD and a classical large-scale DBD over a range of frequencies.A detailed first-principles computational model is used to provide time-accurate solutions of multi-species, multitemperature, self consistent plasma governing equations for the discharge physics. We use a finite rate argon chemistry which has been validated through simulations on micro hollow cathode discharges. The paper will present details of important discharge parameters, power densities and gas temperature in a micro DBD and a millimeter scale classic DBD.