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The Influence of Neutral Flow Rate in the Operation of Hall Thrusters.

This article presents a review of work performed over the past ten years in France, centered on the utilization of laser-induced fluorescence (LIF) spectroscopy to diagnose the low-pressure magnetized dc discharge of a Hall thruster (HT). The latter is a gridless electric propulsion device in a crossed electric and magnetic field configuration, which is used onboard satellites and space probes for various types of maneuvers. Although the design of a HT is relatively simple, the physical mechanisms that govern thrust generation and efficiency are not yet fully understood. Characterization of the ion and atom velocity distribution function (VDF) appears to be a powerful way to obtain insights into the underlying physics. The VDF of xenon and krypton—the most common propellants—is therefore locally interrogated by means of LIF on excited levels. In this review emphasis is placed on time-averaged and time-resolved continuous-wave LIF measurements, associated quantities and recent outcomes. Results will be presented concerning a variety of phenomena: velocity vector field structuring, ion population interaction, electric field generation, ion magnetic drift, apparent atom acceleration, interaction of the plasma plume with background gas and low-frequency electric field oscillations, to name only a few.

https://iopscience.iop.org/article/10.1088/0963-0252/22/1/013001/pdf

Laser-induced fluorescence diagnostics of the cross-field discharge of Hall thrusters

This paper reviews several novel uses of lasers in atomic spectroscopy. A tutorial discussion is given of the basic processes involving the interaction of laser radiation with atoms and the measurement approaches. Laser microprobes, especially laser induced breakdown spectroscopy and laser ablation-inductively coupled plasma-optical emission or mass spectrometry are reviewed thoroughly with respect to principles, instrumentation and applications. Laser excited atomic fluorescence and atomic absorption spectrometry with diode lasers are considered primarily with respect to recent publications. Laser-enhanced ionization, resonance ionization and resonance ionization imaging are also thoroughly reviewed with respect to recent publications. Diagnostical measurements of plasmas and atom reservoirs are discussed. The principles of six laser based atomic absorption methods are given and the methods are compared with respect to detection limits. Finally, future uses of lasers in atomic spectroscopy and a comparison of the characteristics of various atomic methods for trace elements are given.

Novel uses of lasers in atomic spectroscopy. Plenary Lecture

This work presents the near exit plane velocity field of a 200-W laboratory xenon Hall thruster at a single operating condition with a 250-V anode potential. The ionized propellant velocities were measured using laser-induced fluorescence of the 5d[4] 7/2 -6p[3] 5/2 excited state xenon ionic transition at 834.72 nm. Ion velocities were interrogated from the acceleration channel exit plane to a distance 107 mm from the exit plane (3.3 exit plane diameters). Both axial and radial velocities were measured. A nearly uniform axial velocity profile of approximately 13,800 ± 500 m/s (130 ± 10 eV) was measured at the thruster exit plane. The maximum axial velocity, measured 107 mm from the exit plane, was 16,800 m/s (192 eV). The ion flow exiting the thruster acceleration channel mixes downstream due to both the coaxial thruster geometry and a possible ion-acoustic shock. This behavior appears in regions where multiple, or broadly distributed, radial and axial velocity components occur. These regions also exhibit broadened fluorescence line shapes, likely indicative of collisions between the various velocity populations as well as possible ionization of background neutral, and correspond to the brighter, more visible portions of the plume. This region has been previously identified as a possible ion-acoustic shock. This hypothesis appears consistent with the low radial velocity ion populations measured in this more luminous portion of the plume. In addition, a limited study at five off-nominal conditions near a region of high insulator erosion indicates that the impinging ion energy on the protruding center pole boron nitride insulator is predictably changed by flow rates and anode potentials; however, it also appears to vary significantly with applied magnetic field strength.

Near Exit Plane Velocity Field of a 200-Watt Hall Thruster

A particle-based Monte Carlo numerical method is developed for computation of flow in a low-thrust hydrogen arcjet. The method employs the direct simulation Monte Carlo technique to compute the nonequilibrium fluid mechanics and thermochemical relaxation. Simulation of plasma effects is included at a simple level that ensures charge neutrality. A new model is developed to include Ohmic heating in the simulation. Results are presented for a flow condition that has been studied experimentally with a number of different diagnostic techniques. A strong degree of thermal nonequilibrium is found in the flow. It is demonstrated that Ohmic heating is important, and that the Monte Carlo model captures the important physics very well. The predicted results are in very good agreement with most of the experimental data.

Monte Carlo simulation of nonequilibrium flow in a low-power hydrogen arcjet

0→6p[3/2]2(3P2-1D2) transition at 823.2 nm and the xenon-ion 5d[3]7/2→6p[2]5/2
0(4D7/2-4P5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar+-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s-1 to 400 m s-1 and ion velocities as high as 12 km s-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm-1.

Laser-induced fluorescence study of a xenon Hall thruster

Laser-induced fluorescence of the H(α) transition of atomic hydrogen has previously been performed in the plume of a hydrogen arcjet thruster. Measurements of plasma velocity and temperature, based on the Doppler shift and broadening of the H(α) line shape, were previously published [Appl. Opt. 32, 6117 (1993)]. In that paper the Stark broadening of the H(α) transition was estimated from static-ion calculations performed in the early 1970's and found to be negligible in comparison with the Doppler broadening. However, more recent dynamic-ion calculations have shown the Stark broadening to be considerably larger than was previously assumed, resulting in inaccurate temperature measurements. We present a reanalysis of the fluorescence data, taking into account the improved Stark broadening calculations. The correct atomic hydrogen translation temperature and electron number density are obtained from the Doppler and Stark broadening components of the measured line shape. The results indicate a substantial drop in temperature from those previously reported.

Stark broadening corrections to laser-induced fluorescence temperature measurements in a hydrogen arcjet plume

The first use of laser-induced fluorescence for spatially resolved plasma velocity measurements within an arcjet nozzle are described.The arcjet was operated on a propellant of pure hydrogen.The Balmer-α transition of atomic hydrogen was scanned by the excitation laser source while simultaneously detecting the spectrally integrated resonance fluorescence.Velocity was determined from the Doppler shift in the peak of the laser excitation spectrum relative to a stationary reference.The arcjet operating conditions were chosen to facilitate a comparison between these measurements and arcjet modeling results of Butler et al.

Fluorescence velocity measurements in the interior of a hydrogen arcjet nozzle

We review past experimental measurements of internal flow properties of arcjet thrusters. These measurements are generally classified as either intrusive, requiring design changes to prototype thrusters, or nonintrusive, and include measurements of cathode temperature, as well as static pressure, flow temperatures (vibrational, rotational, electronic, translational), electron density, and velocity throughout the interior region extending to the exit plane. Comparisons are made to available model predictions. These measurements, performed on a wide range of thrusters, and operating on variety of propellants, indicate that the nozzle plasma flow may be removed from local thermodynamic equilibrium.

P. Storm

Papers

Laser-induced fluorescence study of a xenon Hall thruster

Laser-induced fluorescence study of a xenon Hall thruster

Stark broadening corrections to laser-induced fluorescence temperature measurements in a hydrogen arcjet plume

Fluorescence velocity measurements in the interior of a hydrogen arcjet nozzle

Interior Plasma Diagnostics of Arcjet Thrusters