Christopher V. Young

Ion velocity and plasma potential measurements of a cylindrical cusped field thruster

Measurements of the most probable time-averaged axial ion velocities and plasma potential within the acceleration channel and in the plume of a straight-channeled cylindrical cusped field thruster operating on xenon are presented. Ion velocities for the thruster are derived from laser-induced fluorescence measurements of the 5d[4]7/2-6p[3]5/2 xenon ion excited state transition centered at λ=834.72nm. Plasma potential measurements are made using a floating emissive probe with a thoriated-tungsten filament. The thruster is operated in a power matched condition with 300 V applied anode potential for comparison to previous krypton plasma potential measurements, and a low power condition with 150 V applied anode potential. Correlations are seen between the plasma potential drop outside of the thruster and kinetic energy contours of the accelerating ions.

Self-organization in planar magnetron microdischarge plasmas

Evidence is presented of rotating azimuthal wave structures in a planar magnetron microdischarge operating at 150 mTorr in argon. Plasma emission captured using a high frame rate camera reveals waves of azimuthal modes m = 3–5 propagating in the −E→×B→ direction. The dominant stable mode structure depends on discharge voltage. The negative drift direction is attributed to a local field reversal arising from strong density gradients that drive excess ions towards the anode. The transition between modes is shown to be consistent with models of gradient drift-wave dispersion in the presence of such a field reversal when the fluid representation includes ambipolar diffusion along the direction parallel to the magnetic field.

Ion Velocimetry Measurements and Particle-In-Cell Simulation of a Cylindrical Cusped Plasma Accelerator

The Stanford Cylindrical Cusped Field Thruster (CCFT) has been experimentally and numerically investigated with particular focus on the exit plane acceleration region near the top magnetic cusp. Time-averaged xenon ion laserinduced fluorescence measurements using the 5d[4]7/2 -6p[3]5/2 (λ = 834.72-nm air) Xe II transition have mapped the total ion velocity vectors in this region. The thruster is also simulated using the fully kinetic 3-D particle-in-cell code F3MPIC. The consistent experimental and numerical results give physical insight into the mechanisms of ion acceleration and the role of the magnetic field topology in determining ion trajectories and plume divergence. The electrons are strongly magnetized and follow the magnetic field structure, grouping near the cusps. A steep potential drop over a few millimeters near the exit plane follows the magnetic separatrix of the top cusp, and is consistent with measured ion velocity vectors. A characteristic conical region of high ion density, peak ion velocity, and visible emission is observed in the experimental and simulated plume, with an estimated divergence half-angle of 30°.

Ion dynamics in an E × B Hall plasma accelerator

We show the time evolution of the ion velocity distribution function in a Hall plasma accelerator during a 20 kHz natural, quasi-periodic plasma oscillation. We apply a time-synchronized laser induced fluorescence technique at different locations along the channel midline, obtaining time- and spatially resolved ion velocity measurements. Strong velocity and density fluctuations and multiple ion populations are observed throughout the so-called “breathing mode” ionization instability, opening an experimental window into the detailed ion dynamics and physical processes at the heart of such devices.

A zero-equation turbulence model for two-dimensional hybrid Hall thruster simulations

We present a model for electron transport across the magnetic field of a Hall thruster and integrate this model into 2-D hybrid particle-in-cell simulations. The model is based on a simple scaling of the turbulent electron energy dissipation rate and the assumption that this dissipation results in Ohmic heating. Implementing the model into 2-D hybrid simulations is straightforward and leverages the existing framework for solving the electron fluid equations. The model recovers the axial variation in the mobility seen in experiments, predicting the generation of a transport barrier which anchors the region of plasma acceleration. The predicted xenon neutral and ion velocities are found to be in good agreement with laser-induced fluorescence measurements.

A finite mass based method for Vlasov-Poisson simulations

A method for the numerical simulation of plasma dynamics using discrete particles is introduced. The shape function kinetics (SFK) method is based on decomposing the mass into discrete particles using shape functions of compact support. The particle positions and shape evolve in response to internal velocity spread and external forces. Remapping is necessary in order to maintain accuracy and two strategies for remapping the particles are discussed. Numerical simulations of standard test problems illustrate the advantages of the method which include very low noise compared to the standard particle-in-cell technique, inherent positivity, large dynamic range, and ease of implementation.

Time-resolved laser-induced fluorescence measurement of ion and neutral dynamics in a Hall thruster during ionization oscillations

The paper presents spatially and temporally resolved laser-induced fluorescence (LIF) measurements of the xenon ion and neutral velocity distribution functions in a 400 W Hall thruster during natural ionization oscillations at 23 kHz, the so-called “breathing mode.” Strong fluctuations in measured axial ion velocity throughout the discharge current cycle are observed at five spatial locations and the velocity maxima appear in the low current interval. The spatio-temporal evolution of the ion velocity distribution function suggests a propagating acceleration front undergoing periodic motion between the thruster exit plane and ∼1 cm downstream into the plume. The ion LIF signal intensity oscillates almost in phase with the discharge current, while the neutral fluorescence signal appears out of phase, indicating alternating intervals of strong and weak ionization.

Excited state population dynamics of a xenon ac discharge

We measure the time evolution of the – (834.68 nm, air) excited neutral xenon transition lineshape in a xenon 60 Hz oscillatory discharge by applying time-synchronized laser induced fluorescence (LIF) spectroscopy. Two different time-synchronized LIF techniques are demonstrated, yielding consistent results and revealing distinct features: a reduction of peak fluorescence intensity (representative of the state density) is observed at high values of the discharge current, the maximum fluorescence intensity occurs at low values of the discharge current, and the excited state populations quench as the alternating current passes through zero. This behavior is reproduced and explained by collisional-radiative modeling, which highlights the role of collisional and radiative mixing between excited energy states throughout the current cycle.