Sergey Averkin

An unstructured direct simulation Monte Carlo methodology with Kinetic-Moment inflow and outflow boundary conditions

The mathematical and computational aspects of the direct simulation Monte Carlo on unstructured tetrahedral grids (U3DSMC) with a Kinetic-Moment (KM) boundary conditions method are presented. The algorithms for particle injection, particle loading, particle motion, and particle tracking are presented. The KM method applicable to a subsonic or supersonic inflow/outflow boundary, couples kinetic (particle) U3DSMC properties with fluid (moment) properties. The KM method obtains the number density, temperature and mean velocity needed to define the equilibrium, drifting Maxwellian distribution at a boundary. The moment component of KM is based on the local one dimensional inviscid (LODI) boundary conditions method consistent with the 5-moment compressible Euler equations. The kinetic component of KM is based on U3DSMC for interior properties and the equilibrium drifting Maxwellian at the boundary. The KM method is supplemented with a time-averaging procedure, allows for choices in sampling-cell procedures, minimizes fluctuations and accelerates the convergence in subsonic flows. Collision sampling in U3DSMC implements the no-time-counter method and includes elastic and inelastic collisions. The U3DSMC with KM boundary conditions is validated and verified extensively with simulations of subsonic nitrogen flows in a cylindrical tube with imposed inlet pressure and density and imposed outlet pressure. The simulations cover the regime from slip to free-molecular with inlet Knudsen numbers between 0.183 and 18.27 and resulting inlet Mach numbers between 0.037 and 0.027. The pressure and velocity profiles from U3DSMC-KM simulations are compared with analytical solutions obtained from first-order and second-order slip boundary conditions. Mass flow rates from U3DSMC-KM are compared with validated analytical solutions for the entire Knudsen number regime considered. Error and sensitivity analysis is performed and numerical fractional errors are in agreement with theoretical errors. The KM method is shown to be a robust technique allowing efficient computation of subsonic flows. Additional verification of U3DSMC is achieved with simulations of the heat transfer process in a stationary argon gas between two flat plates. The numerical heat flux is in agreement with analytical results that cover the transitional to free molecular regimes. Additional validation of U3DSMC is achieved with simulations of hypersonic rarefied flows of nitrogen over a finite thickness flat pate at 0 and 10 degrees angle of attack. Numerical predictions of the pressure and heat flux to the plate are in agreement with experiments.

A Global Enhanced Vibrational Kinetic Model for High-Pressure Hydrogen RF Discharges

A global enhanced vibrational kinetic model (GEVKM) is developed for multitemperature, chemically reacting hydrogen plasmas in inductively coupled cylindrical discharges for lowto high-pressure regimes. The species in a GEVKM are ground-state hydrogen atoms H and molecules H2, 14 vibrationally excited hydrogen molecules H2(v), v = 1 - 14, electronically excited hydrogen atoms H(2) and H(3), groundstate positive ions H+, H2+, and H3+, ground-state negative ions H-, and electrons e. The GEVKM involves volume-averaged steady-state continuity equations for the plasma species, an electron energy equation, a total energy equation, a heat transfer equation to the chamber walls, and a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. The GEVKM is verified and validated by comparisons with previous numerical simulations and experimental measurements of a negative hydrogen ion source in the low-pressure (20-100 mtorr), low-absorbed-power-density (0.053-0.32 W/cm3) regime and of a microwave plasma reactor in the intermediate to high-pressure (1-100 torr), high-absorbed-power-density (8.26-22 W/cm3) regime. The GEVKM is applied to the simulation of a high-current negative hydrogen ion source (HCNHIS). The HCNHIS consists of a high-pressure (20-65 torr) radio-frequency discharge chamber in which the main production of high-lying vibrational states of the hydrogen molecules occurs, a bypass system, and a low-pressure (0.1-0.4 torr) negative hydrogen ion production region where negative ions are generated by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The discharge pressure and negative hydrogen ion current predicted by the GEVKM compare well with the measurements in the HCNHIS.

A global model of high current negative hydrogen ion source

Summary form only given. Dissociative electron attachment to rovibrationally excited hydrogen molecules is one of the key mechanisms of volume negative hydrogen ion formation. Usually production of high-lying vibrational states of H2 molecules is attributed to collisions with energetic electrons1 (> 20eV). At the same time these electrons are effective to destroy negative ions. Therefore, the volume sources are mostly based on space separation of vibrationally excited molecules formation and negative hydrogen ions generation regions. A new concept for negative ion production is investigated. The production of vibrationally excited molecules is accomplished in a high pressure discharge followed by the generation of negative hydrogen ions in a second chamber connected with a nozzle. This concept has an advantage over existing negative ion sources, by keeping the electron temperature low thus eliminating the need of magnetic filter. A global model of the high pressure discharge chamber is presented. The chemical composition is assumed to contain ground state hydrogen molecules and atoms, 14 vibrationally excited hydrogen molecules, three positive hydrogen ions (H+, H2+, H3+), two negative species (H- and electrons). The volume-averaged continuity equations with assumed space profiles are solved in conjunction with electron and neutral energy equations assuming drift diffusion approximation for particle fluxes. Compared to conventional global models ours does not assume a neutral temperature but obtains it through the energy equation. The number densities of all species, electron and neutral temperatures are obtained as a function of absorbed power and volume flow rate to the discharge chamber. This new global model was verified, validated and used in a parametric study in order to obtain optimum operational parameters of the high pressure discharge.

Global model of a negative hydrogen ion source with caesiated plasma grid

A global model is applied to investigate the complex chemistry in a negative hydrogen ion source with caesiated plasma grid. This global model includes electrons, neutral hydrogen molecules with all vibrational states (H2(v)), hydrogen atoms in the first 3 electronic states (H(n)), and ground state ions (H+, H2+, H3+ and H−). It uses a comprehensive set of surface and volume chemical reactions including a model for negative hydrogen ion production from caesiated surfaces. In the global model, steady state species continuity equations, electron energy and total energy equations, heat transfer to walls, and quasineutrality are solved simultaneously in order to calculate number densities and temperatures of plasma components in the discharge over a wide range of pressures and absorbed powers. We present preliminary global model results for a plasma composition and species temperatures in a caesiated plasma grid extraction chamber of a negative hydrogen ion source. These results may be used to extract the mos...

Investigation of the Radio-Frequency Discharge in a High Current Negative Hydrogen Ion Source With a Global Enhanced Vibrational Kinetic Model

A numerical investigation of the radio-frequency hydrogen discharge in the high current negative hydrogen ion source (HCNHIS) is presented using a global enhanced vibrational kinetic model (GEVKM). The HCNHIS consists of a high-pressure (2–65 torr) radio-frequency discharge chamber where the main production of high-lying vibrational states of the hydrogen molecules occurs. The hydrogen plasma flow in the discharge chamber is reduced by a series of bypass tubes and enters through a nozzle into a low-pressure (1–15 mtorr) negative hydrogen ion production chamber where H− are generated mainly by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is applied to the HCNHIS discharge and involves volume-averaged equations for 21 hydrogen species (atoms, ions, and molecules in excited states) and electrons. The GEVKM is supplemented with outlet boundary conditions for the nozzle and bypass tubes of the HCNHIS and accounts for compressibility, viscous, and rarefaction effects. GEVKM simulations of the RF discharge are performed with inlet flow rates of 5–1000 sccm and absorbed powers of 200–1000 W using the HCNHIS-2 design which is configured with an extractor grid attached to a short negative ion production region. These simulations investigate the effects of the absorbed power and the inlet flow rate on the chemical composition, electron and heavy particles temperature, wall temperature, the maximum extractable H− current in the discharge chamber, as well as optimum operational parameters of HCNHIS-2. GEVKM simulations of the HCNHIS-2 discharge are used to obtain estimates of the H− current and compared with Faraday cup measurements taken at the extraction grid.

Simulation of Cold Nitrogen Flows in Nanonozzles with Atmospheric Inlet Pressures

A computational investigation of flow phenomena in conical nanonozzles is carried out with an unstructured 3D Direct Simulation Monte Carlo with Kinetic-Moment boundary conditions (U3DSMC-KM). Nanonozzles considered have inlet diameter of 150 nm, exit diameter from 250-1000 nm, throat radius of curvature of 125 nm, length of 1065 nm, throat diameter of 100 nm resulting in the conical exit half angle  from 4° to 25° and operate at stagnation pressure from 0.1 atm to 2 atm, inlet temperature of 300 K and pressure ratios from 0-0.33. The nanonozzle walls are modeled as diffusive with full thermal accommodation at 300 K. Results show that these nanoflows are characterized by relatively low Reynolds numbers and high Knudsen numbers covering the slip to free molecular flows. Results show that the flows remain subsonic at the throat. It is also found that the flows are in translational non-equilibrium. Performance characteristics such as mass flow rate, thrust and specific impulse are evaluated.

Electrodynamic interactions of nanodroplets in plasmas

The interactions of nanosize droplet jets with a plasma beam are important in electrosprays and has similarities with dusty plasmas.