Adam Thomas Drobot

RF ionization of the lower ionosphere

A comprehensive analysis of the ionization rates of air by RF fields is presented. The analysis relies on a time-dependent code which treats the electron energization with a Fokker-Planck type model and the inelastic energy losses with a multiple time scale technique. Derivation of ionization rates for parameters of interest To D region ionospheric by ground-based RF transmitters with frequency much higher than the electron neutral collision frequency is emphasized. The study provides a physical understanding of the ionization proces and its associated efficiency by combining the computational results with analytic theory. It is shown that for values of quiver energies « I, where I is the ionization potential, the electron production time corresponds to the electron energization time from energies below 2 eV to 20–25 eV. The analytic expressions derived are consistent with the computational results over 6 orders of magnitude in ionization rates and over 2 orders of magnitude in values of Power threshold definitions are clarified, and the pitfalls of using fluid descriptions or effective electric field notions are discussed. The paper concludes with an assessment the power requirements for ionization at 70-km ionospheric altitude with RF in the 100–900 MHz frequency range.

Hybrid simulations of whistler waves generation and current closure by a pulsed tether in the ionosphere

The dynamic response of a magnetized collisionless plasma to an externally driven, finite size, sudden switch-on current source across the magnetic field has been studied using a two dimensional hybrid code. It was found that the predominant plasma response was the excitation of whistler waves and the formation of current closure by induced currents in the plasma. The results show that the current closure path consists of: (a) two antiparallel field-aligned current channels at the end of the imposed current sheet; and (b) a cross-field current region connecting these channels. The formation of the current closure and path occurred in the whistler timescale much shorter than that of MHD and the closure region expanded continuously in time. The current closure process was accompanied by significant energy loss due to whistler radiation. 13 refs., 5 figs.

Megahertz electron modulations during TSS 1R

Experiments were conducted during the Tethered Satellite System Reflight in which a 1 keV, 100 mA electron beam was emitted from the shuttle at pitch angles near 90°. Rapid plasma responses measured by the Shuttle Potential and Return Electron Experiment (SPREE) show time-modulated electron fluxes within the beam flux tube. Megahertz modulations fall into two classes: (1) narrow-bands close to harmonics of the electron gyrofrequency fce and (2) broad-bands at interharmonic frequencies in which electrons of different energies had different modulation frequencies. When SPREE intercepted beam electrons after a single gyroturn they, too, were modulated at similar frequencies. Data suggest that beam electrons were modulated by strong plasma interactions near the emission aperture, generating time-varying electric fields that modulated other electrons near the beam cylinder. This is analogous to the electron cyclotron maser responsible for auroral kilometric radiation.

Cylindrical Brillouin Flow In Relativistic Smooth-Bore Magnetrons

A macroscopic cold-fluid model is used to determine the influence of cylindrical effects on the operating range and properties of the electron flow in relativistic smooth-bore magnetrons. Assuming operation at Brillouin flow, it is found that cylindrical effects (such as the centrifugal force on an electron fluid element) can significantly modify several features of the equilibrium flow and diode operating range relative to the case of planar flow.

An analysis and optimization of the pseudo-current method

Abstract The pseudo-current method proposed by B. Marder for eliminating charge conservation errors in electromagnetic particle-in-cell codes has been analyzed and extended. The new method has been shown to be effective and efficient in removing high frequency, short wavelength errors caused by the choice of charge deposition algorithms. To maintain the physical properties of the electromagnetic field the choice of the free parameter in the originally proposed method has been restricted. It is found that the parameter should be homogeneous spatially and that an error minimization technique can be used to determine its value. A comparison is made between this adaptive pseudo-current method and the effects of spatial smoothing on the transverse and longitudinal components of the electromagnetic field.

BALLISTIC CROSS-FIELD ION BEAM PROPAGATION IN A MAGNETOPLASMA

Long range cross‐field ion beam propagation in a magnetoplasma has been studied in the high kinetic beta regime by using computer simulations and analytic techniques. A new regime of long range ballistic beam propagation has been discovered for narrow high‐density beams. Ion beams with transverse size Δ≪Rb, where Rb is the ion beam gyroradius and mass density nbMb≫npMp, where nb(Mb),np(Mp) are the particle density (mass) of the beam and the ambient ions can propagate ballistically across the ambient magnetic field over distances varying from a minimum of (nb/np)Rb to over an order of magnitude longer depending on the extent of initial interpenetration of the beam and the background plasma. The propagation mode is characterized by the formation of a front at the head of the beam, which forces the plasma to move sideways and prevents beam‐plasma interpenetration. The system momentum is locally balanced by a corresponding displacement of the head of the beam in the opposite direction to the plasma, which is ...

Simulations of rf-driven sheath formation in two dimensions

The results from two‐dimensional particle simulations of sheath formation around periodic metal arrays placed inside magnetized plasmas and driven by oscillating voltages are reported. The main goal is the modeling of the plasma interaction with the Faraday bars surrounding the antennas during ion cyclotron tokamak heating. The study of the time‐averaged potentials shows that the two‐dimensional sheath structure depends on both the sheath length‐to‐thickness ratio and the inclination of the magnetic lines. The equipotential surfaces form closed, nested cells between adjacent bars. When the magnetic lines are nearly perpendicular to the potential gradients, the ion motion is dominated by the E×B drift, and ion streamlines form vortices around the equipotentials. At larger inclinations of the magnetic lines, the flow decouples from the equipotentials and ion transport is mainly along the potential gradients. The critical angle for the transition from vortex circulation to field aligned flow is computed. The...

The flight of the tethered satellite system

The first Tethered Satellite System (TSS-1) Electrodynamics Mission is scheduled for launch aboard the space shuttle ST-46 on July 31, 1992, as a joint mission between the United States and Italy. A 500-kg, 1.6-m-diameter satellite, attached to the shuttle by a thin (.24 cm), conducting, insulated wire (tether), will be reeled upwards from the orbiter payload to a distance of 20 km when the shuttle is at a projected altitude of 300 km. TSS-1 is an extremely ambitious mission with high-risk payoff potential. This is the type of pioneering mission NASA and the United States should be encouraging, with the risk in the achievement of the mission objectives rather than in safety. The mission has been likened to the maiden flight of a new airplane. We expect surprises and hope to set the stage for the next mission, the TSS-reflight.

Studies on longitudinal beam compression in induction accelerator drivers

Longitudinal beam compression is an integral part of the U.S. induction accelerator development effort for heavy ion fusion. It occurs before final focus and fusion chamber beam transport and is a key process determining initial conditions for final focus hardware. Determining the limits for maximal performance of key accelerator components is an essential element of the effort to reduce driver costs. We outline here studies directed towards defining the limits of final beam compression including considerations such as: maximal available compression, effects of longitudinal dispersion and beam emittance, combining pulse‐shaping with beam compression to reduce the totial number of beam manipulations, etc. We summarize inital results on these limits gleaned from a survey including already more than 30 runs of 20–30 minutes each on our Cray computers.In addition, we illustrate one of several possible techniques for utilizing the beam compression process to provide the pulse shapes required by a number of tar...

Solution of Euler's equations on adaptive grids using a fast unstructured grid second order Godunov solver (FUGGS)

We describe a new technique for solving Eulers gasdynamic equations on unstructured triangular grids with arbitrary connectivity. The formulation is based on the second order Godunov method. The use of data structure with only one level of indirectness leads to an easily vectorized and parallelized code with a low level of overhead in memory requirement and high computational efficiency. The performance and accuracy of the algorithm has been tested for a very wide range of Mach numbers starting from very low subsonic to high hypersonic flows, without the need to adjust any code parameters. The algorithm was implemented in a vertex based and triangle based scheme. The computational results produced by the triangle based version showed an extremely low level of artificial viscosity.