(a) Magnetic surfaces and profiles in the RFP, (b) a scale drawing of the 25 kV, 1 MW tangential NBI on MST, (c) a top view of MST indicating the relative location of the NBI and neutral particle analyzer.
Example of the measured fast ion dynamics during NBI. (a) The time trace of a typical 300 kA plasma current discharge with timing of the neutral beam injection. (b) The neutron flux is an indicator of the volume-averaged fast particle content (of the small fraction of deuterium beam fuel), while panel (c) is a color contour depiction of the measured core-localized parallel fast hydrogen distribution, by normalizing the NPA signals by background neutral density strength versus time. Panels (d) and (e) are typical radial electron density and temperature profiles as measured at 26 ms into the discharge.
Expanded view of fast hydrogen behavior following beam turn-off. With zero source, the fast hydrogen particles slow classically as the vertical dashed lines are the expected times of peak signals in each of the next four lower energy channels for classical slowing at .
TRANSP prediction of fast ion distribution in MST. In plot (a), the fast ion density in MST is core-peaked and ramps up for the 20 ms injection. Plot (b) shows the predicted distribution midway through the beam injection period; the ions are predominantly high pitch and peaked near the beam injection energy.
NBI-induced suppression of the core-most (n = 5 in this case) tearing mode. Panel (a) is the plasma current and NB injection versus time, (b) is the line-averaged electron density and neutron flux versus time, and panel (c) is the measure of the magnetic fluctuation amplitude of the core-most tearing mode with and without NBI. Also defined (d) is the mode suppression factor, used as a proxy of core fast ion content.
For two distinct equilibria, the NBI has a stabilizing influence on the core-most tearing mode while the secondary modes are unaffected. In these two examples, q profiles plotted in (a) are separated by a strong variation in the q(a) boundary condition. Reduction of the core-most resonant modes (b) is observed at different toroidal mode numbers, n in each case; modes resonant at larger radius are unaffected.
q profiles of NBI (red) and non-NBI (black) discharges at 300 kA and q(a) = 0. The island width for each case is included (offset minimally from 0.2 in each curve for clarity). Also plotted is a typical fast ion gyro-orbit (blue) for a 25 keV fast hydrogen ion, indicating the gyro motion is of the same length scale as the island width.
Two ensembles of discharges with full beam power (1 MW red) and reduced beam power, (0.6 MW green). In panel (a), the two different beam injection conditions are plotted versus time. The high frequency magnetic activity associated with beam-driven instabilities are plotted for the 1MW injection case (b), and the reduced power case (c). Panel (d) is the fusion neutron flux (a proxy for the volume averaged fast deuterium content) versus time for the two cases; while panel (e) is the ANPA measure of core-localized, high pitch fast hydrogen particles. A measure of the fast-ion induced suppression of the core-most tearing mode is plotted for each case in panel (f).
The n = 4 beam driven instability, measured by edge Mirnov coils, exhibits a long-lived distinct chirping feature, where the frequency can drop by more than a factor of two over the course of about 1 ms.
Article metrics loading...
Full text loading...