Oleg Batishchev

Minihelicon Plasma Thruster

A compact high-power-density helicon plasma source operating on abundant medium atomic weight gases is under examination for a potential electric propulsion application. A high specific impulse, which is easily varied in the range of ~1-2 ks for Ar and ~2-4 ks for N2 propellant, was achieved. A well- collimated plasma plume with emittance ~10deg is produced in a single-magnet configuration. Analysis of the data obtained using in situ and noninvasive spectral diagnostics indicates almost complete gaseous propellant ionization and thrust efficiency etaT ~ 20% with the lowest net ionization cost of ~(80 V)-1 per ion. Direct weighing has yielded a thrusting force of T ~ 10 mN.

Kinetic effects in tokamak scrape-off layer plasmas

The short mean-free path expansion used in fluid modeling of scrape-off layer plasmas is often violated for typical discharge parameters, especially by the superthermal particles, which carry most of the heat flux. Thus, the tail of the distribution function can strongly depart from Maxwellian due to nonlocal mean-free path effects, which can modify plasma transport, impurity radiation, and plasma–neutral gas interactions. These nonlocal effects become particularly pronounced for detached plasma conditions that are characterized by sharp gradients in the plasma parameters along the magnetic field. These problems are being addressed by developing one spatial dimension and two velocity variables, fully kinetic, collisional, and time-dependent particle-in-cell code, W1 [Contrib. Plasma Phys. 34, 436 (1994)], and its parallel-computer version, PW1 [Contrib. Plasma Phys. 34, 424 (1996)]. Comparisons are made with the Fokker–Planck code ALLA [Phys. Plasmas 3, 1634 (1996)] and with experimental results. Kinetic ...

Kinetic effects on particle and heat fluxes in detached plasmas

A model of high recycling scrape‐off layer plasmas in tokamaks is presented where both ion and electron species are described by nonlinear kinetic equations. Coulomb and charged‐neutral particle collisions are included. The ambipolar electric field and electrostatic sheath potential are evaluated self‐consistently. Two models of fluid neutral transport are used to distinguish the neutral density variation for different tokamak divertor geometries. These models are incorporated into a comprehensive three‐dimensional (1‐D, 2 V) hybrid collisional particle‐in‐cell–Monte Carlo code W1 [Contrib. Plasma Phys. 34, 436 (1994)]. This code is used to investigate the effects of neutrals on divertor plasma detachment phenomena and on parallel heat and particle fluxes in the presence of strong gradients where fluid descriptions break down. Results are given for simulations of detached and attached divertor plasmas, and comparisons are made with solutions from a one‐dimensional fluid model and with experimental observa...

Preliminary Characterization of a Helicon Plasma Source for Space Propulsion

The design and construction of the Mini-Helicon Thruster Experiment (mHTX) is discussed. The magnets were designed to produce a maximum field strength of 0.18 T in a Helmholtz configuration so as to ensure axial uniformity. The half-wavelength antenna was designed to excite the m = +1 mode with a frequency and wavelength of 13.56 MHz and 19.56 cm, respectively corresponding to an excitation energy of 20 eV. The propellant gas is fed to the quartz tube through a flow controller and can be varied between 1 - 100 sccm N2. The experiment has been run using nitrogen, argon, xenon, and gas mixtures including air as propellants; however, there are plans to run using hydrogen, water, and ammonia. Optical measurements were taken in the antenna and plume regions using a spectrometer. This allows for a preliminary measurement of specific atomic and molecular emission lines to determine the relative proportion of ion species.

A kinetic model of transient effects in tokamak edge plasmas

Fluid descriptions are not adequate for edge plasma studies when the high collision frequency assumption fails in the presence of rapid transient processes. The results of analytic and numerical modeling of some of the non‐Maxwellian features of edge plasma transport associated with transient heating and cooling phenomena in the scrape‐off layer are presented. It is found that kinetic effects alter the heat conduction coefficient and lead to interesting critical behavior of the distribution function. The numerics are performed with the recently developed time‐dependent Fokker–Planck code ALLA.

Spectroscopic and Electrostatic Investigation of the Diverging Cusped-Field Thruster

A magnetized plasma thruster variant with a divergent chamber and strong magnetic cusps has been been prototyped and preliminarily characterized at the MIT Space Propulsion Laboratory. This paper expands upon the previous work with a non-invasive, optical examination of the near-field plume and discharge chamber phenomena. Calibrated broadband spectra reveal changes in Xe I and II level populations with operational mode, while high resolution narrow bandwidth scans are used to examine trends in multiply charged ion fractions and Doppler shifted profiles. An internal electrostatic probe apparatus is also decribed which has been installed to observe ion flux to diverging channel surfaces in order to determine the efficacy with which the magnetic topology is used to reduce ion bombardment energies.

Fully Kinetic Modeling of Divergent Cusped-Field Thrusters

A fully kinetic, particle-in-cell plasma simulation tool has been incrementally developed by members of the Massachusetts Institute of Technology Space Propulsion Laboratory. Adapting this model to simulate the performance and plasma dynamics of a divergent cusped-field thruster is discussed. Strong magnetic fields in the cusps (B≈0.5 T) necessitate using a time step on the order of a picosecond in order to resolve electron cyclotron trajectories. As a result, successfully completing a divergent cusped-field thruster simulation with the full magnetic field strength has yet to be accomplished. As an intermediate step, simulation results of a divergent cusped-field thruster with the magnetic field at 1/5 the actual value are presented. Further modifications that may lead to successful simulations of divergent cusped-field thrusters at full magnetic field strength are also outlined.

Adaptively Meshed Fully -Kinetic PIC -Vlasov Model For Near Vacuum Hall Thrusters

Predictive modeling of Hall thrusters requires a new level of simulation capabilities with integrated multi-scaling and accurate anomalous transport models. A PIC-Vlasov hybrid model is undertaken utilizing adaptive meshing techniques to enable automatically scalable resolution of fine structures during the simulation. The code retains the versatility of a PIC simulation while intermittently recalculating and smoothing particle distribution functions within individual cells to ensure full velocity space coverage and to reduce numerical noise. Background theory and algorithmic details are briefly outlined, and numerous benchmarking results are used to demonstrate various portions of the simulation. Initial results of the application of this model to a relatively new and poorly theoretically understood concept in Hall thruster design, a so-called Near Vacuum Hall Thruster (NVHT), are presented and discussed. The importance and practical possibility of incorporating a self-consistent quench model for anomalous electron cross-field transport in 2D3V fully kinetic approximation is also explored.

Predicted effects of parallel temperature gradients on the overestimation of TCV divertor target Langmuir probe Te measurements

During high recycling conditions and divertor detachment in the TCV tokamak, divertor target Langmuir probes never measure electron temperatures much below similar to5 eV. Code simulations using B2-EIRENE of these plasmas indicate that Te approximate to 1 eV. This inconsistency has also been seen in other tokamaks. In this paper, the influence on a theoretical Langmuir probe VI-characteristic of a strong T-e gradient parallel to the magnetic field is examined by using B2-EIRENE simulated T-e and n(e) profiles and adopting a simple numerical approach to compute divertor target electron energy distribution functions. Standard fitting of these theoretically generated H-characteristics results in a temperature very close to the predicted value at the highest densities corresponding to divertor detachment in TCV. At intermediate densities, the computed temperature at the inner target is found to be very sensitive to assumptions regarding the magnitude of the assumed electron collision mean-free-path

Mini-Helicon Plasma Thruster Characterization

A compact mini-Helicon Thruster (mHT) is designed to study possible space applications. High beam and gas utilization efficiencies are of major importance, as well as the compact design and system integration. The concept does not require a cathode for beam neutralization or any auxiliary gas discharge for plasma initiation. We target gases with intermediate weight, such as molecular nitrogen, atomic argon, and mixtures like air, operating at low 50% beam efficiency is achievable. Experimentally demonstrated efficiency is ~20%, which is primarily due to high energy loses in the RFtransition lines that has been recently quantified. Measured heat loads onto inner wall of the discharge chamber are moderate, a few watts per square centimeter. However, power density in the plasma beam is exceptionally high, several mega-watts per square meter, allowing much smaller cross-section of a system in comparison to other EP concepts.