Plasma Dynamics Modeling Laboratory (PDML) focuses on development of computational and theoretical models to investigate weakly-ionized gas, collisionless plasmas, and plasma discharges.
Hall Thruster Plasmas
A Hall thruster, or a Hall-effect thruster, is one type of electric propulsion (EP) devices that are used in space missions such as satellite, deep space missions, and asteroid retrieval. Despite the use of Hall thrusters in actual missions, the physics of Hall thruster has not been fully understood yet. We have investigated the mode transition of discharge oscillations in Hall thrusters using a hybrid-direct kinetic (DK) simulation code and ionization oscillation theory. We showed that the stability and excitation of such low-frequency oscillations is strongly dependent on the electron transport and electron heating/cooling mechanisms.
One of the most challenging problems in the community is related to electron transport, which determines the efficiency, operating mode (oscillatory/stable), and thrust level. It is known that the electron mobility obtained from experimental results does not agree with classical theory, which is attributed to "anomalous" electron transport. There are signatures that the anomalous electron transport is attributed to high frequency oscillations, turbulence, plasma waves, and etc. In PDML, we are developing both kinetic and fluid codes to investigate the electron transport mechanism and its effect on the thruster performance.
Direct Kinetic (DK) Simulation
In gas kinetics, the velocity distribution functions (VDFs) of the gas species play an important role in the overall gas dynamics and chemical reactions. When the flow is collisional, the VDFs relax to Maxwellian distribution, for which fluid description (conservation laws) is valid. On the other hand, when the flow is rarefied or collisionless, the VDFs can be any non-Maxwellian distribution and kinetic approach must be used. The gas dynamics is further coupled with the electromagnetic forces, which further make the system more nonlinear and complex. The most commonly used kinetic simulation technique is particle based methods, such as particle-in-cell (PIC) and direct simulation Monte Carlo (DSMC). Such particle-based kinetic methods can model non-Maxwellian nature of the gas and plasma flows, but the statistical noise due to the use of such discrete particles may lead to unphysical oscillations and instabilities. We have been developing a new kinetic approach, which we call the direct kinetic (DK) method, in which the kinetic equations such as the Boltzmann and Vlasov equations are solved directly on discrete phase space. As the kinetic equations are hyperbolic partial differential equations (advection type), the numerical methods and algorithms developed in the computational fluid dynamics (CFD) community can be employed. The advantage of using a DK method is that the statistical noise is essentially absent, making the method applicable to investigate oscillations and instabilities.
Plasma Instabilities and Oscillations
Due to the nonlinearity of the system, plasmas can experience a variety of instabilities and oscillations. We have used both particle and grid based kinetic methods to investigate the instabilities and oscillations. Due to the discrete velocity space, DK method has been used for nonlinear problems in thermal plasmas, including trapped particle instabilities and ladder climbing of plasma waves using external chirped field. On the other hand, PIC simulations are useful for beam-plasma interactions such as the instabilities induced by neutralized ion beam. Additionally, ionization oscillations in Hall thruster discharge plasmas are investigated using a hybrid-DK method. Utilization of different numerical modeling techniques (fluid and kinetic) is important to gain understanding of such oscillation phenomena.
For any plasma applications, plasma-material interaction plays an important role in controling the plasma. Typically, electrons are much faster than ions due to the mass difference (more than 1800 times smaller). This results in the plasma-immersed materials to be negatively biased and formation of a potential drop near the material, which is called the plasma sheath. The potential drop accelerates the ions, which may lead to enhanced sputtering of the material, and decrease the electron heat flux to the wall. In the presense of plasmas, it is known that electrons can be emitted from the materials due to the ion and/or electron bombardment. The emitted electrons, either by thermionic or secondary electron emission, reduces the potential drop, which can in turn affect how the plasma is confined. As the distribution functions are non-Maxwellian and the plasma is non-neutral in the sheath region, DK simulation is useful to model the plasma sheath.
Facility Effects on EP Plasma Discharges