Edgar Y. Choueiri

Plasma oscillations in Hall thrusters

The nature of oscillations in the 1 kHz–60 MHz frequency range that have been observed during operation of Hall thrusters is quantitatively discussed. Contours of various plasma parameters measured inside the accelerating channel of a typical Hall thruster are used to evaluate the various stability criteria and dispersion relations of oscillations that are suspected to occur. A band by band up-to-date overview of the oscillations is carried out with a description of their observed behavior and a discussion of their nature and dependencies through comparison of the calculated contours to reported observations. The discussion encompasses the excitation of low frequency azimuthal drift waves that can form a rotating spoke, axially propagating “transit-time” oscillations, azimuthal drift waves, ionization instability-type waves, and wave emission peculiar to weakly ionized inhomogeneous plasmas in crossed electric and magnetic fields.

Fundamental difference between the two Hall thruster variants

The fundamental difference between the two variants of Hall thrusters, the stationary plasma thruster (SPT) and the thruster with anode layer (TAL), is illustrated quantitatively using an analytical model that accounts for the effects of secondary electron emission (SEE) from the walls. The model includes a prescription for the quenching of the temperature of the electrons on their way to the anode which results from the enhanced electron energy losses to the wall that occur at, and upstream of, an axial location where the wall potential reverses from electron attracting to electron repellent. For the higher SEE coefficient of an insulator wall (compared to that of a metallic one) this sign reversal occurs at a lower electron temperature and is shown to lead to a more extended acceleration region due to the resulting relaxation of the potential gradient required to balance the electron pressure gradient. By replacing the boron nitride walls (SPT) of an idealized Hall thruster with stainless steel (TAL), the acceleration zone is shown analytically to collapse to a region near the anode having an extent that is about eight times smaller than that for the SPT. The results are used to construct a detailed phenomenological picture of the fundamental difference between the two Hall thruster variants.

A Critical History of Electric Propulsion: The First 50 Years (1906-1956)

∗Chair of AIAA’s Electric Propulsion Technical Committee, 2002-2004. Associate Fellow AIAA. Chief Scientist at Princeton University’s Electric Propulsion and Plasma Dynamics Laboratory (EPPDyL). Associate Professor, Applied Physics Group, MAE Department. e-mail: choueiri@princeton.edu. †Presented at the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, Ft. Lauderdale, FL. Copyright c

A Survey of Propulsion Options for Cargo and Piloted Missions to Mars

Abstract: In this paper, high‐power electric propulsion options are surveyed in the context of cargo and piloted missions to Mars. A low‐thrust trajectory optimization program (raptor) is utilized to analyze this mission. Candidate thrusters are chosen based upon demonstrated performance in the laboratory. Hall, self‐field magnetoplasmadynamic (MPDT), self‐field lithium Lorentz force accelerator (LiLFA), arcjet, and applied‐field LiLFA systems are considered for this mission. In this first phase of the study, all thrusters are assumed to operate at a single power level (regardless of the efficiency‐power curve), and the thruster specific mass and power plant specific mass are taken to be the same for all systems. Under these assumptions, for a 7.5 MW, 60 mT payload, piloted mission, the self‐field LiLFA results in the shortest trip time (340 days) with a reasonable propellant mass fraction of 57% (129 mT). For a 150 kW, 9 mT payload, cargo mission, both the applied‐field LiLFA and the Hall thruster seem reasonable choices with propellant mass fractions of 42 to 45%(7 to 8 mT). The Hall thrusters provide better trip times (530‐570 days) compared to the applied‐field LiLFA (710 days) for the relatively less demanding mission.

Faraday Acceleration with Radio-Frequency Assisted Discharge

A new electrodeless accelerator concept that relies on an rf-assisted discharge, an applied magnetic field, and electromagnetic acceleration using an inductive coil is presented. The presence of a preionized plasma allows for current sheet formation at lower discharge voltages and energies than those found in other pulsed inductive accelerator concepts. A proof-of-concept experiment, supported by optical and probe diagnostics, has been constructed and used to demonstrate low-voltage, low-energy current sheet formation and acceleration. Magnetic field data indicate that the peak sheet velocity in this unoptimized configuration operating at a pulse energy of 78.5 J is 12 km/s. Visual observations indicate that plasma follows the applied magnetic field from the rf discharge to the face of the planar acceleration coil, while magnetic field probing and visualization using a fast-framing camera show the formation and acceleration of the current sheet.

Scaling of Thrust in Self-Field Magnetoplasmadynamic Thrusters

The magnetoplasmadynamic thruster (MPDT) has recently passed milestones in performance and lifetime that have prompted renewed interest in its unique advantages for energetic space missions. Mission and system studies, as well as ongoing performance characterization, require the use of simple relations for the scaling of performance parameters. The Maecker formula has long played such a role for the thrust of the self-e eld MPDT. The formula is shown to be too simplistic to account for the trends in measured thrust data that exhibit departures from the model, particularly at low current. We show that at high currents, the departures can be explained by the evolution of the current densities over the electrode surfaces that ine uence the spatial distribution of the volumetric Lorentz force densities. At low current levels the departures are attributed to the scaling of gasdynamic pressure distributions induced by the pinching components of the volumetric electromagnetic forces. The insight was used to formulate a more accurate empirically based model for the scaling of the thrust of an MPDT.

Anomalous resistivity and heating in current-driven plasma thrusters *

A theory is presented of anomalous resistivity and particle heating in current-driven plasma accelerators such as the magnetoplasmadynamic thruster (MPDT). An electromagnetic dielectric tensor is used for a current-carrying, collisional and finite-beta plasma and it is found that an instability akin to the generalized lower hybrid drift instability (GLHDI) exists for electromagnetic modes (i.e., with finite polarization). Weak turbulence theory is then used to develop a second-order description of the heating rates of particles by the waves and the electron-wave momentum exchange rate that controls the anomalous resistivity effect. It is found that the electron Hall parameter strongly scales the level of anomalous dissipation for the case of the MPDT plasma. This scaling has recently been confirmed experimentally [Phys. Plasmas 5, 3581 (1997)]. Polynomial expressions of the relevant transport coefficients cast solely in terms of macroscopic parameters are also obtained for including microturbulence effect...

Performance optimization criteria for pulsed inductive plasma acceleration

A model of pulsed inductive plasma thrusters consisting of a set of coupled circuit equations and a one-dimensional momentum equation has been nondimensionalized leading to the identification of several scaling parameters. Contour plots representing thruster performance (exhaust velocity and efficiency) were generated numerically as a function of the scaling parameters. The analysis revealed the benefits of underdamped current waveforms and led to an efficiency maximization criterion that requires the circuits natural period to be matched to the acceleration timescale. It is also shown that the performance increases as a greater fraction of the propellant is loaded nearer to the inductive acceleration coil

Scaling laws for electromagnetic pulsed plasma thrusters

The scaling laws of pulsed plasma thrusters operating in the predominantly electromagnetic acceleration mode (EM-PPT) are investigated theoretically and experimentally using gas-fed pulsed plasma thrusters. A fundamental characteristic velocity that depends on the inductance per unit length and the square root of the capacitance to the initial inductance ratio is identified. An analytical model of the discharge current predicts scaling laws in which the propulsive efficiency is proportional to the EM-PPT performance scaling number, defined here as the ratio of the exhaust velocity to the EM-PPT characteristic velocity. The importance of the effective plasma resistance in improving the propulsive performance is shown. To test the validity of the predicted scaling relations, the performance of two gas-fed pulsed plasma thruster designs (one with coaxial electrodes and the other with parallel-plate electrodes), was measured under 70 different operating conditions using an argon plasma. The measurements demonstrate that the impulse bit scales linearly with the integral of the square of the discharge current as expected for an electromagnetic accelerator. The measured performance scaling is shown to be in good agreement with the theoretically predicted scaling. Normalizing the exhaust velocity and the impulse-to-energy ratio by the EM-PPT characteristic velocity collapses almost all the measured data onto single curves that uphold the general validity of these scaling laws. [12pt]This paper is dedicated to the memory of Dr Daniel Birx

Characterization of Oscillations in Closed Drift Thrusters

The nature of oscillations in the 1 kHz-60 MHz frequency range that have been observed during operation of closed drift thrusters (CDT), such as the stationary plasma thruster (SPT) and the anode layer thruster (ALT), is quantitatively discussed. We use contours of various plasma parameters measured inside the accelerating channel of an SPT[1, 2] as the starting point of our investigation and calculate the magnitude and spatial distribution of various associated natural and collision frequencies, characteristic lengths and velocities. This leads to a thorough characterization of the plasma in the channel under typical operation. This detailed picture is then used to evaluate the stability criteria and dispersion relations of oscillations that are suspected to occur. Using various dispersion relations, we present a band by band overview of the oscillations with a description of their observed behavior and discuss their nature and dependencies through comparison of the calculated contours to reported observations. In particular we discuss the excitation of low frequency azimuthal drift waves that can form a rotating spoke at lower values of B∗ r , axially propagating “transit-time” oscillations, high frequency azimuthal drift waves, ionization instability-type waves, and wave emission peculiar to weakly ionized inhomogeneous plasma in crossed electric and magnetic fields.