Bulletin of the American Physical Society
73rd Annual Gaseous Electronics Virtual Conference
Volume 65, Number 10
Monday–Friday, October 5–9, 2020; Time Zone: Central Daylight Time, USA.
Session JT2: Magnetized PlasmasLive
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Chair: Peter Hartmann, Wigner Research Center, Hungary |
Tuesday, October 6, 2020 1:00PM - 1:30PM Live |
JT2.00001: Electron heating and the Magnetic Asymmetry Effect in magnetized capacitive radio-frequency plasmas Invited Speaker: Julian Schulze Capacitively coupled radio frequency (RF) magnetrons are frequently used for sputter deposition of ceramic layers. However, fundamentals of their operation such as the effects of the magnetic field on the electron power absorption dynamics and the formation of process relevant flux-energy distribution functions are not understood. In order to address these issues, we characterize such a discharge operated in argon with oxygen admixture at low pressure by a synergistic combination of different experimental diagnostics [current/voltage measurements, retarding field energy analyzer, multipole resonance probe, phase resolved optical emission spectroscopy (PROES), magnetic field measurements]. These experimental results are compared to Particle in Cell (PIC) simulation and model results. We find that the magnetron magnetic field induces a discharge asymmetry. This Magnetic Asymmetry Effect affects the DC self bias and ion flux-energy distribution functions at boundary surfaces, which can be controlled by adjusting the magnetic field [1]. Tuning the magnetic field also allows to magnetically control the self-excitation of plasma series resonance oscillations of the RF current and, thus, Non- Linear Electron Resonance Heating (NERH) [2]. PROES and PIC simulations reveal space and time resolved insights into the dynamics of the electron power absorption in the presence of the magnetic field. [1] M. Oberberg et al. 2018 Plasma Sourc. Sci. Technol. 27 105018 [2] M. Oberberg et al. 2019 Plasma Sourc. Sci. Technol. 28 115021 [Preview Abstract] |
Tuesday, October 6, 2020 1:30PM - 1:45PM Live |
JT2.00002: On the relation between deposition rate and ionized flux fraction in high power impulse magnetron sputtering Jon Tomas Gudmundsson, Hamidreza Hajihoseini, Martin Rudolph, Nils Brenning, Michael A Raadu, Daniel Lundin The deposition rate in high power impulse magnetron sputtering (HiPIMS) is known to be lower than for dc magnetron sputtering (dcMS) operated at the same average power. Back-attraction of ions of the sputtered species to the cathode target is believed to be the main cause for this, while some other mechanisms have also been suggested. We discuss how the magnetic field strength $|{\bf B}|$ and geometry (degree of balancing) influences the deposition rate and ionized flux fraction $F_{\rm flux}$ in dcMS and HiPIMS operation both axially [1] and radially [2]. We relate the deposition rate and the ionized flux fraction to the ionization probability $\alpha_{\rm t}$ and the back attraction probability of the sputtered species $\beta_{\rm t}$. A significant transport of the film forming material is found to travel radially or parallel to the target surface for both sputter techniques. and a significantly higher number of ions traveling radially in the HiPIMS discharge. We discuss the tradeoff between a high ionized flux fraction of the sputtered species and a high deposition rate referred to as the HiPIMS compromise, and approaches to optimize the sputter process. [1] Hajihoseini et al., Plasma {\bf 2} (2019) 201, [2] Hajihoseini et al., J. Vac. Sci. Technol. A {\bf 38} (2020) 033009 [Preview Abstract] |
Tuesday, October 6, 2020 1:45PM - 2:00PM Live |
JT2.00003: Characterisation and control of an ion-acoustic plasma instability downstream of a diverging magnetic nozzle Scott Doyle, Alex Bennet, Dimitrios Tsifakis, James Dedrick, Rod Boswell, Christine Charles In this investigation, carried out at the Space Plasma, Power and Propulsion laboratory (SP3), electrostatic probes were employed to measure a 4~-~20~kHz instability in the ion saturation current downstream of an electric double layer (DL) in the \textit{`Chi Kung'} expanding helicon plasma source. The amplitude and frequency of the instability were found to vary in inverse proportion to the operating argon gas pressure (0.2~-~0.6~mTorr) and in direct proportion to the applied rf power (100~-~600~W) and applied solenoid current (3~-~8~A). A spatially resolved characterisation of the maximum instability amplitude downstream of the DL determined two radial maxima, corresponding to the locations of most positive radial ion density gradient. Through the application of 2~-~12~kHz voltage amplitude modulations of the 13.56~MHz radio-frequency driving voltage, the instability was reduced by up to 65\%; exhibiting a greater reduction at higher applied modulation frequencies. This effect is ascribed to a reduction in the radial ion density gradient via asymmetrically attenuated ion acoustic density perturbations, induced by the applied voltage amplitude modulation. This work demonstrates a novel potential control mechanism for density gradient driven instabilities in magnetised plasmas. [Preview Abstract] |
Tuesday, October 6, 2020 2:00PM - 2:15PM Live |
JT2.00004: Investigation of Drift-Driven Turbulence on Electron Transport in a Low Power Magnetic Nozzle Shadrach Hepner, Benjamin Jorns This work investigates the presence of instabilities and their effects on electron detachment in a low power magnetic nozzle. We have previously observed a lower hybrid drift instability (LHDI) and an anti-drift instability (ADI) in these devices. This work expands on the previous works with a sweep of operating conditions to determine when the LHDI and ADI are present and significant in inducing electron transport. We employ three diagnostics for this test. First, we implement a Langmuir probe sweep in two dimensions to verify the ADI and LHDI dispersion relations with background plasma parameters. We then supplement these measurements using probe pairs observing frequency and three-dimensional wavevectors of local fluctuations. The spectra read from the probe pairs act as direct measurements of unstable modes. We then take high speed imagery data in the upstream portion, observing the coherent modes non-invasively. The relative impact of each of these waves is evaluated by calculating the effective collision frequency. We determine the dependencies of the presence and impact of these waves on the plasma and thruster conditions and discuss means of altering their significance in electron transport. [Preview Abstract] |
Tuesday, October 6, 2020 2:15PM - 2:30PM Live |
JT2.00005: Neutral Atomic-Hydrogen Measurements in a Mirror/FRC Plasma Device using fs-TALIF Arthur Dogariu, Eugene Evans, Sangeeta Vinoth, Samuel Cohen We report on temporally- and spatially-resolved neutral atomic H density measurements carried out in a mirror/field-reversed configuration plasma device. The facility uses multiple RF-heating techniques with powers exceeding 100 kW to create 5-100ms duration quasi- and fully-steady-state magnetized (50-500G and mirror ratio \textasciitilde 10) plasmas from various gases, achieving densities approaching 10$^{\mathrm{14}}$ cm$^{\mathrm{--3}}$ and electron temperatures in excess of 100 eV. The density of neutral H atoms 50cm off-midplane is imaged and time-resolved via fs two-photon absorption laser-induced fluorescence using a fs laser system at 205nm with 200$\mu $J/pulse at 1kHz rep rate. A fast CMOS camera with a 5ns gated image intensifier records the H-$\alpha $ fluorescence at 656nm for every laser shot. By varying the delay between the RF trigger and laser pulses, the temporal dynamics of the H atoms is measured across multiple discharges to better than 15$\mu $s resolution. For high-power RF, the measured H atom density rises to 10$^{\mathrm{12}}$ cm$^{\mathrm{-3}}$ in 10's of $\mu $s, resolving axial and temporal dynamics of the neutrals near the device's central region. The steady-state ``seed'' plasma generated using low-power (10-500W) RF exhibits unexpectedly slow production and depletion of neutrals. Comparing the dynamics of the H neutrals and plasma-excited H atoms yields a measured lifetime of 160$\mu $s for the neutrals. Calibration is performed using 1mTorr of Kr. [Preview Abstract] |
Tuesday, October 6, 2020 2:30PM - 2:45PM Live |
JT2.00006: Heating in numerical simulations of anomalous transport in E$\times$B discharges Salomon Janhunen, Andrei Smolyakov Instability driven cross-field transport is an important feature affecting the operation and performance of $\mathrm{E}\times \mathrm{B}$ discharges. Significant electron heating is observed in traditional particle-in-cell simulations of the electron cyclotron drift instability (ECDI). Traditionally the heating is mitigated by introducing atomic physics and sources, but we have been able to reduce heating rate by using a different numerical scheme that eliminates some of the numerical error. We present results from particle-in-cell simulations of the electron cyclotron instability using the control variate direct-$\delta{f}$ scheme. [Preview Abstract] |
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