Rostislav Spektor

Evolution of the parallel and perpendicular ion velocity distribution functions in pulsed helicon plasma sources obtained by time resolved laser induced fluorescence

The temporal evolution of parallel and perpendicular ion velocity distribution functions (ivdf ) in a pulsed, helicon-generated, expanding, argon plasma is presented. The ivdf’s temporal evolution during the pulse was determined with time resolved (1 ms resolution), laser induced fluorescence. The parallel ivdf measurements indicate that, in the expansion region of the plasma and for certain operational parameters, two ion populations exist: a population moving at supersonic speeds (1.1 Mach) resulting from acceleration in an electric double layer (EDL) and a slow moving population (0.7 Mach) generated by local ionization. After 100 ms, although present, the EDL is not fully developed and has not reached a steady-state. Measurements of the perpendicular ivdf indicate constant radial expansion, with ion speeds of ≈ 400 ms −1 , in the expansion region.

Laser induced fluorescence in a pulsed argon plasma

A time-resolved laser induced fluorescence (LIF) technique for pulsed argon plasmas is described. A low power, tunable diode laser pumps a three level Ar II transition sequence at a wavelength of 668.6138 nm. With a standard LIF system designed for steady-state plasmas (e.g., 4 kHz optical chopper, 20 kHz band-width detector, and a lock-in amplifier), we demonstrate that the evolution of the ion velocity distribution can be resolved with a time resolution of 1 ms through a combination of time-series averaging and post-acquisition digital signal processing.

Laser induced fluorescence measurements of the cylindrical Hall thruster plume

An investigation of a fully cylindrical Hall thruster was performed using laser induced fluorescence (LIF) to measure ion velocity profiles in the plume. The measurements confirm a previously reported 9% increase in the exhaust energy when the cathode keeper draws an excess current (overrun mode). Furthermore, the velocity directions in the plume remain relatively unchanged for the cusped and direct magnetic field configuration in both overrun and nonoverrun modes. Previously reported plume narrowing in the overrun mode was confirmed and found to be due to the shift of the acceleration and ionization regions toward the anode. The electric field inferred from the LIF measurements allowed calculation of the electron E×B drift. Close to the centerline of the thruster, electrons drift azimuthally with velocity decreasing away from the centerline, thus creating shear. This shear can be a source of plasma instabilities and influence electron transport. Further away from the centerline, electrons drift in the op...

Coherent Ion Acceleration using Two Electrostatic Waves

We analyze a recently discovered coherent ion acceleration mechanism that relies on the nonlinear interaction of a magnetized ion with multiple electrostatic waves, at least two of which differ in frequency by an integer multiple of the cyclotron frequency. The mechanism does not require the ion to be in resonance and can accelerate an ion with an arbitrarily low initial energy, hence its basic importance to practical applications. We illustrate the fundamental features of the mechanism through a parametric numerical study of the ion’s nonlinear interaction with two electrostatic waves. Compared to the wave frequencies, the wave amplitudes were found to have a weak effect on setting the energy bound of the coherent portion of the acceleration but have a strong impact on the nature of the acceleration as they control the connectivity between the coherent and stochastic domains of phase space. The lack of requirements on the initial ion energy and other fundamental properties of the mechanism point to its promise as an ion energization method for plasma propulsion.

Computation of two-dimensional electric field from the ion laser induced fluorescence measurements

This paper presents a method of computing two-dimensional electric field from ion laser induced fluorescence (LIF) measurements in a plasma flow. The expression for the field is derived by taking velocity moments of the Boltzmann equation for ions. It was found that the pressure tensor, related to the width of the ion velocity distribution, plays a critical role in the computation of the electric field. Even with the assumption of cold ion flow, the pressure tensor contribution may be significant when velocity spread is caused by other forces. Such a situation occurs in the flow of a Hall thruster, where velocity spread is caused by the ions born at different potentials. LIF measurements of the cylindrical hall thruster plume were used to demonstrate practical application of the derived method. Whenever the pressure tensor components are small as compared to the mean ion drift velocity, the electric field calculations reduce to a simple expression given in terms of mean ion drift velocity and its divergence.

Transient-mode multipactor discharge

Multipactor discharge is a resonant condition in which electrons impact material surfaces in phase with an alternating rf or microwave electric field. A growing and/or sustained discharge requires the electrode secondary electron coefficient, δ(Ee), to be greater than unity at the impacting electron energy (Ee). E1, the minimum energy for δ ≫ 1, is highly dependent on electrode surface preparation and conditioning, and contaminated surfaces will generally experience a lower E1 than the same clean or “conditioned” surface. A transient or pulsed mode of multipactor discharge can exist when the rf voltage and the corresponding electron energy distribution occur near E1. Initially, a discharge may form if sufficient number of electrons have impact energies greater than E1. Yet, as surface contaminants are removed with subsequent electron impacts, E1 can quickly increase, causing the electron impact energy to become insufficient for discharge-sustaining secondary emission. As a result, the discharge extinguishes until surface contaminants return. This new TMM discharge has been shown to occur indefinitely for a constant contamination source at a range of rf voltages. Both TMM frequency and intensity have been shown to depend on the contamination rate and the rf voltage relative to E1, and experimental TMM results are shown for copper and aluminum. Experimental results are presented along with supporting Monte Carlo electron-tracking simulation results.

Effect of multipactor conditioning on technical electrode surfaces

Historically, multipactor conditioning has been utilized to remove surface contaminants from rf electrodes by electron‐stimulated gas desorption, and such conditioning has been shown to reduce multipactor susceptibility. Multipactor threshold improvements are due to increasing E1, the minimum energy for the secondary electron coefficient, δ>1, such that resonant electrons are incapable of producing discharge‐sustaining secondary emission. Using an rf amplitude sweep technique, the evolution of the multipactor threshold is measured as a function of multipactor conditioning time for a series of technical electrode surfaces. Results show over +3 dB of threshold improvement in copper and gold electrodes, while the aluminum threshold actually decreases with conditioning exposure. Additionally, these conditioning results indicate the possible voltage region for transient‐mode multipaction (TMM), which can cause significant risk to rf systems such as space satellite components for which in‐situ conditioning is g...

Multipactor Breakdown Threshold Reduction Due to Magnetic Confinement in Parallel Fields

In certain high-power RF devices, such as isolators and circulators, a static magnetic field is present in parallel with the oscillating RF electric field. This parallel magnetic field imposes electron Larmor motion, which restricts the trajectories of electrons accelerated by the RF field. In open geometries, magnetic confinement reduces electron losses in the multipactor region and can reduce the breakdown threshold. In this paper, the effect of magnetic confinement on multipactor breakdown threshold was measured experimentally using a stripline fixture with an externally applied uniform magnetic field. Multipactor threshold power was reduced from the unmagnetized case by as much as 3 dB at magnetic fields of about 0.5 kG. Multipactor gap aspect ratio has a significant impact on the degree of threshold reduction.

Reduction in multipactor breakdown threshold due to a parallel magnetic field

Summary form only given. Multipactor breakdown may occur in an RF component when the electron transit time across a gap satisfies a resonant condition and the secondary electron yield from surfaces is sufficiently high to overcome electron losses from the breakdown region and produce net electron population growth. In devices such as isolators and circulators that have a magnetic field oriented parallel to the RF electric field, transverse mobility of electrons is inhibited. This electron confinement reduces losses and can lower the multipactor threshold power by several decibels compared to the unmagnetized case. Fundamentals of this process have been studied in a simple stripline geometry with an externally applied uniform magnetic field of variable strength. Laboratory experiments are compared with particle tracking simulations as well as an analytical theory that takes into account the distribution of secondary emission energies.

PRINCE: A Software Tool for Characterizing Waves and Instabilities in Plasma Thrusters

An interactive software tool is developed to characterize the waves and instabilities present in plasma thrusters by solving for the complex zeros of plasma dispersion relations. This tool helps researchers explore plasma waves and instabilities in their particular plasma thruster configuration. A user-friendly graphical interface allows versatile data input and parametric control. The zeros of the dispersion relation are located and tracked by root-finding algorithms based on Cauchy’s Argument Principle and Newton-Raphson’s method. Information about the instabilities found is presented through various data visualization options. The software tool is validated by reproducing previous work concerning instabilities arising in Hall thrusters from solutions to a simplified Esipchuk-Tilinin dispersion relation.