Typical evolution of an uncontrolled runaway electron beam in DIII-D. Argon pellet is launched at 2.0 s. The dashed line in (c) indicates an exponential fit to the initial L/R decay of the thermal plasma, extrapolated into the runaway plateau.
Compression of runaway beam boundary occurring during the current quench. The light “target boundary” indicates the target plasma shape prior to the arrival of the argon pellet. The black “RE beam boundary” is the runaway beam boundary after the current quench.
Effects of excessive radial compression during and after the current quench. Solid line indicates a highly compressed, short lived runaway beam. Dotted line indicates a long-lived runaway beam for comparison.
Effects of control scenario on early runaway beam survival. Solid line indicates an uncontrolled case and dashed line indicates a case with enhanced controls implemented.
Example of well controlled runaway beam position and current, sustained to the flux limit of the ohmic solenoid. Current quench (obscured) occurs at 2.0 s.
Vertical controllability of an elongated runaway electron beam. (a) 2D image of visible synchrotron emission from runaway beam superimposed upon an outline of the DIII-D limiter boundary and the EFIT flux contours (plasma boundary indicated by thick line). (b) Vertical growth rate calculated using rigid plasma model. (c) Calculation of minimum controllable DZmax required. Marginal control area (light shading below dashed horizontal line) indicates when vertical control begins to saturate. Dark shaded area below solid horizontal line indicates where complete loss of vertical control occurs. (d) Runaway beam vertical position.
Appearance of enhanced wall interaction during a slow radial compression. The dashed vertical line indicates the time at which the threshold is crossed. EFIT fails at that point, so no further minor radius data is available in (b).
Correspondence of synchrotron core impacting inner wall and onset of enhanced wall interaction. The images depict the EFIT boundary (solid line) and a 2D image of runaway beam's visible synchrotron emission superimposed over an outline of DIII-D limiter structure (black background). The shaded zones in plots (a) and (b) indicate when the enhanced wall interaction is occurring.
Runaway beam trajectories in and minor radius from current quench (solid triangles) until inflection (solid circles).
Example of long-distance runaway beam interaction with outer limiter.
“Safe Zone” for radial positioning of runaway beam with minimal wall interaction. The solid circles indicate the mean of each dataset, and the vertical lines the full radial extent. Shaded regions indicate “danger zones” for positioning the runaway beam that result in an enhanced wall interaction threshold being crossed.
Example of controlled runaway beam ramp-down. Dashed line in (a) indicates control target. EFIT minor radius data is not available in (d) after the enhanced wall interaction threshold is crossed at 2.6 s.
Example of full runaway beam ramp-down. Large negative loop voltage (a) produces a rapid ramp-down in (b), and corresponding reduction in minor radius (c). Left dashed vertical line indicates where a = 0.3 m, which is the time slice shown in (f). Right dashed vertical line indicates the clear onset of enhanced wall interaction at a = 0.24 m. (d) Photo-neutron signal and (e) EFIT . Arrows in (f) indicate the direction in which the coils are pushing the runaway beam.
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