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Using both 0D and 1D fluid models, we revisit the formation of the breathing mode in Hall thrusters and show that it is an ionization instability associated with nonlinearity in the electron power absorption. As the plasma density increases, the axial electric field profile changes and the magnitude of the electric field is enhanced in the ionization zone. This causes a nonlinear increase in the power absorbed by electrons and an increase in the electron temperature and ionization rate factor that is able to partially compensate for the decreasing neutral density to keep the ionization rate high. This sets up a positive feedback mechanism where the electric field continues to be enhanced as the plasma density increases and, consequently, the neutral density needs to decrease even further before plasma growth can be halted. At this point, the neutral density is so low that the plasma can no longer be "sustained," and time is needed for neutrals to refill the thruster channel before "reignition" can occur and the process repeated. By treating the breathing mode as an AC excitation, a carefully designed external circuit can be used to counteract the change in the axial electric field by appropriately varying the anode voltage to stabilize the discharge. [ABSTRACT FROM AUTHOR]

A radio-frequency (RF) inductively coupled electrothermal plasma thruster operating with argon is experimentally characterized for different mass flow rates, RF powers, and propellant injection configurations. Depending on the propellant mass flow rate, significant neutral gas heating is observed with effective stagnation temperatures around 2000 K (giving a maximum estimated thrust and specific impulse of about 100 mN and 125 s, respectively) for absorbed powers between 300 and 500 W. A self-consistent theoretical discharge model is developed and used to study the basic physics and operation of RF electrothermal thrusters, and predictions of the gas temperature are in good agreement with experimental measurements. The model identifies primary power inefficiencies as electron-neutral excitation losses and neutral gas heat losses to the thruster walls. Both experimental and theoretical results indicate that a relatively high stagnation pressure (of the order of 100 Torr or higher) is critical for high performance. For pressures significantly below this the electron-neutral collisional power transfer is too low to effectively heat the neutral gas. [ABSTRACT FROM AUTHOR]

Ion-induced secondary electron emission (SEE) is a fundamental surface interaction that strongly influences many plasma discharges. Recently, interest in iodine plasmas is growing due to new material processing and space propulsion applications, but data for SEE yields due to iodine ion bombardment remain scarce. Additionally, due to the formation of multiple ion species in typical iodine plasmas and surface chemical reactions leading to iodide layer formation, the effective SEE yield is expected to differ from that for individual ion species on clean surfaces. In this work, we measure the SEE yield of multi-species iodine ion beams bombarding different target materials (Mo, W, Al, Ti, Cu, carbon-carbon, and steel) in the energy range of 0.6–1.4 keV. An ion beam is produced by extracting and accelerating ions from a gridded ion source based on an inductively coupled plasma (ICP). SEE yields of downstream targets are measured using a conventional electrostatic probe technique, and the ion beam composition is determined using time-of-flight spectrometry. The beam is composed predominately of atomic ( I + ) and molecular ( I 2 + ) ions whose ratio changes depending on the ICP power. Yields depend strongly on the target material and beam composition and vary between 0.05 and 0.4 depending on whether potential or kinetic emission processes dominate. [ABSTRACT FROM AUTHOR]

We report investigations of capacitively coupled carbon tetrafluoride (CF4) plasmas excited with tailored voltage waveforms containing up to five harmonics of a base frequency of 5.5 MHz. The impact of both the slope asymmetry, and the amplitude asymmetry, of these waveforms on the discharge is examined by combining experiments with particle-in-cell simulations. For all conditions studied herein, the discharge is shown to operate in the drift-ambipolar mode, where a comparatively large electric field in the plasma bulk (outside the sheaths) is the main mechanism for electron power absorption leading to ionization. We show that both types of waveform asymmetries strongly influence the ion energy at the electrodes, with the particularity of having the highest ion flux on the electrode where the lowest ion energy is observed. Even at the comparatively high pressure (600?mTorr) and low fundamental frequency of 5.5?MHz used here, tailoring the voltage waveforms is shown to efficiently create an asymmetry of both the ion energy and the ion flux in geometrically symmetric reactors. [ABSTRACT FROM AUTHOR]