Phase-resolved optical emission spectroscopy of a neutralizer-free gridded ion thruster

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Transient propagation dynamics of flowing plasmas accelerated by radio-frequency electric fields

International audienceFlowing plasmas are of significant interest due to their role in astrophysical phenomena and potential applications in magnetic-confined fusion and spacecraft propulsion. The acceleration of a charge-neutral plasma beam using the radio-frequency self-bias concept could be particularly useful for the development of neutralizer-free propulsion sources. However, the mechanisms that lead to space-charge compensation of the exhaust beam are unclear. Here, we spatially and temporally resolve the propagation of electrons in an accelerated plasma beam that is generated using the self-bias concept with phase-resolved optical emission spectroscopy. When combined with measurements of the extraction-grid voltage, ion and electron currents, and plasma potential, the pulsed-periodic propagation of electrons during the interval of sheath collapse at the grids is found to enable the compensation of space charge

Control of electron, ion and neutral heating in a radio-frequency electrothermal microthruster via dual-frequency voltage waveforms

The development of low power micro-propulsion sources is of recent interest for application on miniature satellite platforms. Radio-frequency. (rf) plasma electrothermal microthrusters can operate without a space-charge neutralizer and provide increased control of spatiotemporal power deposition. Further understanding of how the phase-resolved rf plasma heating mechanisms affect the phase-averaged bulk plasma properties, e.g. neutral gas temperature, could allow for in-flight tailoring of plasma thrusters. In this work, experimentally validated two-dimensional fluid-kinetic simulations were employed to study the spatially resolved electron and ion power deposition and neutral gas heating in a dual-frequency rf electrothermal microthruster operating at 1.5. Torr plenum pressure in argon. Experimental validation was performed through a comparison of the measured and simulated phase resolved Ar(2p(1)) excitation rates, showing close agreement. Two types of dual-frequency voltage waveforms were investigated, and comprise the combination of a 13.56 MHz voltage waveform with 27.12 MHz and 40.68 MHz waveforms, respectively. Varying the phase offset of the higher harmonic relative to the fundamental 13.56. MHz voltage waveform was found to modulate the dc self-bias voltage by 11% and 3% of the maximum applied peak-to-peak voltage, respectively. The 13.56. MHz, 27.12. MHz dual-frequency voltage waveform provided the highest degree of control, where the fraction of total rf power deposited into Ar+ ions was found to vary from 57% to 77%, modulating the on-axis neutral gas temperature by 35%. This control is attributed to the variation in the fraction of the rf phase cycle for which the sheath is collapsed, altering the phase-averaged electric field strength adjacent to the radial wall. The application of dual-frequency waveforms provides the ability to optimize the particle heating mechanisms with application to electrothermal propulsion.The work presented herein was funded by the Engineering and Physical Sciences Research Council (EPSRC, EP/M508196/1)

Observations of a mode transition in a hydrogen hollow cathode discharge using phase resolved optical emission spectroscopy

Two distinct operational modes are observed in a radio frequency (rf) low pressure hydrogen hollow cathode discharge. The mode transition is characterised by a change in total light emission and differing expansion structures. An intensified CCD camera is used to make phase resolved images of Balmer α emission from the discharge. The low emission mode is consistent with a typical γ discharge, and appears to be driven by secondary electrons ejected from the cathode surface. The bright mode displays characteristics common to an inductive discharge, including increased optical emission, power factor, and temperature of the H2 gas. The bright mode precipitates the formation of a stationary shock in the expansion, observed as a dark region adjacent to the source-chamber interface.This research was partially funded by the Australian Research Council Discovery Project (DP1096653)

The formation of atomic oxygen and hydrogen in atmospheric pressure plasmas containing humidity : picosecond two-photon absorption laser induced fluorescence and numerical simulations

Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio

Inducing locally structured ion energy distributions in intermediate-pressure plasmas

Ion energy distribution functions (IEDFs) incident upon material surfaces in radio frequency (rf) capacitively coupled plasmas are coupled to spatial and temporal sheath dynamics. Tailoring the ion energy distribution function within intermediate-pressure plasmas (133 Pa, 1 Torr), which find application in surface modification and aerospace industries, is challenging due to the collisional conditions. In this work, experimentally benchmarked 2D fluid/Monte-Carlo simulations are employed to demonstrate the production of structured IEDFs in a collisional (200 Pa 1.5 Torr argon) rf hollow cathode discharge. The formation of structures within the IEDFs is explained by an increase in the Arþ ion-neutral mean-free-path and a simultaneous decrease in the phase-averaged sheath extension as the rf voltage frequency increases over 13.56–108.48 MHz for a constant rf voltage amplitude (increasing plasma power) and gas flow rate. Two distinct transitions in the shape of the IEDF are observed at 450 V, corresponding to the formation of “mid-energy” (60–180 eV) structures between 40.68 and 54.24 MHz and additional “high energy” (180 eV) structures between 81.36 and 94.92 MHz, with the structures within each region displaying a distinct sensitivity to the applied voltage amplitude. Transitions between these energy ranges occurred at lower applied voltages for increased applied voltage frequencies, providing increased control of the mean and modal ion energy over a wider voltage range. The capabitlity to extend the range of access to an operational regime, where the structured IEDFs are observed, is desirable for applications that require control of the ion-bombardment energy under collisional plasma conditionsThe work presented herein was funded by the Engineering and Physical Sciences Research Council (EPSRC), Grant No.: EP/ m508196/1