High power electron beam for plasma heating

Analysis of C-2W electrode arcing experimental data
James Sweeney, Erik Granstedt, Peter Yushmanov, Manjit Kaur, Daniel Sheftman, Deepak Gupta, and the TAE Team
Data Analysis of Arcing
The state of arcing at any time point within a shot is determined from the number and intensity of peaks in a given electrode region averaged over a rolling time
window. This arcing data is combined with data from several other diagnostics in C-2W to enable analysis of arcing. Diagnostics utilized in this study include measurements of the biasing power supply, neutral pressure measurements from cold cathode gauges, Thomson scattering measurements of electron density and temperature, and Mirnov probe magnetic field measurements which infer the plasma radius and total energy.
Arcing I-V Characteristics
Measurements of the power supply voltage and current were combined with
arcing data and demonstrate the electrical characteristics of arcs in each electrode region. It is clear that arcs in the outer region result in the power supply being shorted to ground, the outer region arcs are thus defined as bipolar arcs (plate to plate). The arcs in the center and inner regions do not necessarily drop the impedance of the circuit, and are considered to be unipolar arcs (plate to plasma).
Arcing and Neutral Pressure
Cold cathode gauges measure the neutral gas pressure in the divertor vessel, these are combined with arcing data and presented with respect to bias current. This data shows that bipolar arcs commonly coincide with high neutral pressure in the divertor. Within a single shot, it can be observed that arcing often precipitates a rise in neutral pressure. An explanation for this rise is that the arc discharge releases neutrals from the electrode surface.
Gettering and Arcing
Arcing and Plasma Parameters
In C-2W, electron temperature scales with bias voltage by a factor of ~10. The mechanism for this
relationship between Te and bias voltage is still under study. Bipolar arcing results in low voltage on the electrode and correspondingly low electron temperature. Unipolar arcing from the inner region is more common at higher bias voltage and higher Te than unipolar arcing from the center region.
Electron density generally trends with bias current, which is understandable in part because a component of the bias current is comprised of losses from the confined plasma. Bipolar arcing results in high bias current but does not lead to accumulated density. The highest density shots in C-2W commonly include high current and unipolar arcing from the inner electrode region, though most of the increase in density can occur with just under 1 kA of bias current.
Electron pressure and total plasma energy both increase with bias power to a limit, but exhibit distinct trends in increase indicative of different kinds of stored energy. For either plasma
parameter, the highest values occurred with high bias power and inner region unipolar arcing. It seems that while bipolar arcing is always detrimental, unipolar arcing does not necessarily hinder plasma performance and may have positive effects. It should be noted from this data that most of the increase in either plasma parameter can be achieved with relatively low bias power (<3 MW with uncertain coupling efficiency).
ABSTRACT
The TAE Technologies C-2W experimental device (also called "Norman")1 produces advanced beam-driven field reversed configuration (FRC) plasmas. Norman incorporates electrode
biasing for improved plasma stability and heating. Extending the operational boundaries of the electrodes enhances plasma performance, but arc discharges can occur on the electrodes under extreme conditions. Arcing can have detrimental consequences, which require the phenomenon to be understood operationally. An analysis tool was developed to detect and categorize electrode arcs through image processing of high-speed camera data. The information about the electrode arcing state is combined with measurements from other diagnostics to enable statistical analysis of arcing. Correlations of plasma and experimental parameters and their relationship to electrode arcing are presented.
1H. Gota et al, Nucl. Fusion 59, 112009 (2019)
C-2W and Electrode Biasing
+ The C-2W experimental device generates FRC plasmas - sustained with neutral beam injection and electrode biasing. Concentric electrodes produce a radial electric field and drive radial currents which induce plasma ExB rotation. The resulting electric circuit increases FRC energy and contributes to plasma
cold cathode gauge
Mirnov probe
Thomson beamline
bipolar arc initiation
heating.
The C-2W Electrode
C-2W utilizes concentric ring plate electrodes. Typically the inner plates are negatively biased and the outer plates grounded to produce a radial electric field. At the center of the electrode is an orifice for the plasma gun, a discharge producing device which initiates plasma for biasing.
Electrode Arcing
Electrode biasing can result in arc discharges on the surface of the electrode. They can be observed in fast camera video using narrow light filters for hydrogen and metal species. Typically the discharge is observed as multiple concentrated regions of light on the electrode surface. The occurrence of arcing can be either beneficial or detrimental to the plasma, and one way to examine the difference is through the location of the arc in relation to the biased and grounded electrodes.
Automatic Arc Detection
C-2W electrode
Inner region unipolar arcing
pressure rise
Arcing and Electron Heating
A number of shots were identified in which Te does not scale with bias voltage. A commonality among these shots was found to be the absence of unipolar arcing on the electrode. Within a single shot, the transition from low to high Te state can be observed to coincide with the initiation of unipolar arcing. The precise mechanism of electron heating remains uncertain.
Summary
unipolar arcing
In an effort to understand electrode arcing, a tool was built to automatically detect arcs in fast camera video from C-2W experiments. The tool makes use of a maximum filter to identify peaks of emission, then categorizes them based on intensity and location.
Three electrode regions of interest are chosen: a center region encompassing the plasma gun, an inner region consisting of the negatively biased electrodes, and an outer region including the grounded electrodes and the edge of the biased electrodes.
outer
center inner
Gettering of the electrode surface and divertor vessel is one mitigation strategy employed on C-2W to reduce arcing. Gettering information was extracted from experimental logs and related to arcing data and cold cathode gauge neutral pressure measurements. It was found that gettering recency coincides with higher pumping of neutrals in the divertor, and somewhat diminished arcing. Gettering appears to have a modest effect on arcing, but is an imperfect tool for controlling arcs.
• C-2W includes electrode biasing for stability and heating, but arc discharge can occur
• Arcing can be detected visually and combined with diagnostic data for analysis
• Arcing is either unipolar or bipolar, distinguished by electrical impedance
• Arcing releases neutral gas, gettering pumps neutral gas and somewhat diminishes arcing
• Bipolar arcing is detrimental, unipolar arcing correlates with higher plasma parameters