RSX experimental configuration. (a) Side view of RSX with (1) plasma guns, (2) vacuum vessel, (3) external anode, (4) external magnet coils, schematic of two flux ropes with their magnetic fields, and the coordinate system. Each flux rope current flows in the positive direction. (b) Expanded view of the miniaturized plasma injector with main circuit elements (not to scale). (c) Time history of gun bias currents during the formation of two flux ropes.
Time history of plasma parameters during the interaction of two flux ropes. (a) Bias current driven at each gun (top gun, dashed line; bottom gun, solid line). (b) Distance between flux rope centers . (c) Peak current density driven in the current sheet. (d) Thickness of the current sheet .
(Color online) Structures of the transverse magnetic field during the interaction of twin flux ropes at the different times during the current ramp up. Magnetic field lines corresponding to lines of constant vector potential are also shown as continuous lines.
Axial current density at evaluated along the line that connects the centers of the two flux ropes. Different times (same as in Fig. 3 ) are shown during the current ramp up. Positive current is driven in each flux rope, which induces a negative current sheet during the coalescence process. We define the thickness of the current sheet as the distance between points where , e.g., points and in frame (d).
(Color online) Contours of electron pressure at four times that encompass the creation and merging of two flux ropes. Data were obtained by a movable triple electrostatic probe (100 RSX discharges) in the plane perpendicular to the axis of the device 0.5 m from the two plasma guns, background .
Current sheet width and current density at different values of axial magnetic field, evaluated at the time of peak layer compression . (a) Current sheet thickness (circles, left-hand axis) together with the estimated ion gyroradius (squares, right-hand axis). (b) Reversed peak current density in the center of the current sheet.
Comparison of experimental aspect ratio with predictions from Eq. (4) including (circles) and neglecting (squares) the contribution from . The case in Eq. (4) corresponds to the original Sweet-Parker model. The gray box indicates our estimated experimental error in determining the current sheet aspect ratio. The inset shows the Sweet-Parker geometry.
(Color online) False color images of two magnetic flux ropes in hydrogen plasma reveal helically twisted structures (b,c). Times are given from the start of the applied current drive . The silhouetted plasma guns in the foreground are outlined for early times (a). The viewing perspective of the fast camera (represented by an eye) is schematically shown by a continuous line in the side and top views of the RSX device. For clarity, the viewing perspective is artifically exaggerated in the figure. In the actual setup, , .
Electron heating during a 300 W pulse (small fraction of this was actually coupled to the plasma) of rf power. The afterglow lasts over , which allows a long rf free time interval with elevated and Lundquist number S.
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