Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session DI02: LTP/MFE: Divertors and Low-Temperature PlasmasInvited Live
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Chair: Alexandros Gerakis, Luxembourg Inst of Science and Technology Room: Ballroom C |
Monday, November 8, 2021 3:00PM - 3:30PM |
DI02.00001: Quantification of ELM and inter-ELM heat and particle flux to a secondary divertor and its consequences for future fusion machines Invited Speaker: Renato Perillo We show that the ELM-driven heat flux to the secondary divertor of DIII-D can be up to 1/3 of the total heat flux, which could reach several tens of MW/m2 in ITER. Resolving the mechanisms that deliver heat and particle fluxes to the divertor target is crucial because ITER is designed to operate in lower-single-null (LSN) but with a secondary X-point (XPT) inside or near the tiles coated with beryllium, possessing a lower heat flux tolerance (2 MW/m2) than tungsten. DIII-D type-I ELMing discharges with a secondary XPT inside the vessel were used for a quantitative analysis with infra-red thermography and fast reciprocating probe measurements of the ELM and inter-ELM power distribution between the primary and secondary divertors by varying the up-down magnetic balance (dRsep) from -5 (LSN) to +16mm (upper-single-null, USN). We experimentally demonstrate for the first time that, with dRsep < 10mm, ELM plasma is transported to the secondary inner target, which is magnetically isolated from the outer leg, leading to ELM peak heat values that are comparable to those at the secondary outer strike point. Both the integrated and the ELM peak heat flux to the secondary divertor decay below ~50% of the maximum as dRsep is varied from -5 to +6mm, but the integrated heat flux decay asymptotes. Values of dRsep above ~25mm are needed to reduce the ELM heat flux to the secondary divertor below 10% of heat flux deposited to a well-defined SN. These findings imply that the up-down magnetic balance will have to be accurately controlled (+/- 5mm) in order to preserve the upper wall. This poses a concern for any future tokamak that will operate in quasi-DN configuration and where the secondary inner target is not yet designed to withstand significant heat loads. |
Monday, November 8, 2021 3:30PM - 4:00PM |
DI02.00002: Weakly ionized plasmas as tunable elements for radio-frequency systems Invited Speaker: Sergey Macheret Weakly ionized electric-discharge plasmas have unique properties making them promising as tunable elements in radio-frequency (RF) electronics, especially in high-power applications. Plasma discharges can combine resistive, capacitive, and inductive properties and all three can be tuned over very wide ranges. In recent breakthrough experimental studies, capacitively coupled radio-frequency (CCRF) discharges were driven by variable-frequency and/or variable-power sources so that the discharge impedance experienced by a much higher frequency weak probing signal was tuned widely, including change from capacitive to inductive behavior. Tunable plasma antennas are also of great interest, but their gain was, until recently, expected to be poor compared with that of conventional metallic antennas. However, recent experiments demonstrated that plasma antennas can have a gain comparable with that of metallic antennas despite the fact that the electrical conductivity of the plasma is orders of magnitude lower than that of metals. |
Monday, November 8, 2021 4:00PM - 4:30PM |
DI02.00003: Pulsed Streamer Discharge Development in Inhomogeneous Media and External Magnetic Fields Invited Speaker: Andrey Starikovskiy A numerical modeling is used to simulate the properties of positive and negative streamers emerging from a high-voltage electrode in a long air gaps with density gradients and external magnetic fields. It is shown that photoionization in front of the streamer head is important not only for the development of strong positive discharges, but for the development of strong negative discharges as well. An increase in the photoionization rate increases the propagation velocity of the positive streamer and retards the propagation of the negative streamer. A completely new mechanism was proposed to explain the generation of runaway electrons in atmospheric discharges. Conditions were found when the electric field at the streamer head exceeds the breakdown threshold by a factor of 30–50 and significantly exceeds the critical values for electron runaway. As a result, the majority of electrons could transform into the runaway regime. It was shown that positive streamer deceleration in the undisturbed atmosphere by itself can generate extremely high electric fields exceeding the runaway threshold. In such fields, intensive relativistic electron beams, as well as electron-induced x-ray radiation, could be formed. The numerical simulation of the development of a streamer discharge in a gap with an external longitudinal magnetic field was used to demonstrate the self-focusing of such discharges. Self-focusing is caused by a sharp deceleration of the radial ionization wave due to a change in the electron energy distribution function, a decrease in the average electron energy, the rate of gas ionization and the electron mobility in crossed electric and magnetic fields as compared to the case of the discharge development without a magnetic field. The self-focusing effect of a streamer discharge in an external longitudinal magnetic field is observed for both polarities. |
Monday, November 8, 2021 4:30PM - 5:00PM |
DI02.00004: Molecular Dynamics simulations of low temperature plasma processing Invited Speaker: Pascal Brault As plasma processes are atomic and molecular by nature, simulations at the molecular level will be relevant for providing us with insights into core and interface plasma chemistry basic phenomena. Moreover, statistical averaging allows to provide/predict macroscopic data as reaction rates, diffusion coefficients, ... directly comparable with experiment outputs. Among all the available molecular simulation tools, (reactive) molecular dynamics (MD) simulation technique is a good compromise between quantum mechanical and kinetic Monte-Carlo methods, especially due to the availability of robust and accurate reactive force fields [1]. Moreover, such reactive forcefields are also calculating partial chages at any relevant times using charge equilibration algorithm. In the context of plasma chemistry this will allow to adress charge transfer mechanisms. Using this forcefields, MD simulations are able to calculate the trajectories of a set of particles, by solving the appropriate set of Newton equations of motion. For realistic comparisons with experiments, inputs of MD simulations have to be selected for matching experimental conditions. |
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