H.W.Kugel,1 J.Boedo,2 M.Chance,1 R.Goldston,1 S.Jardin,1 J.Kesner,4 S.Luckhardt,2 M.Manickam,1 K.McGuire,1 M.Okabayashi,1 J.Schmidt,1 L.Schmitz,2,3 L. Zakharov1
1Princeton Plasma Physics Laboratory, Princeton, NJ 08543
2University of California, San Diego, La Jolla, CA 92093
3University of California, Los Angeles, CA 90024
4Massachusetts Institute of Technology, Cambridge, MA 02139
Active control of plasma boundaries using edge currents has been demonstrated in numerous experiments to produce edge conditions favorable to MHD stability, non-inductive current drive, fueling control, impurity and helium exhaust from the core plasma, reduced divertor heat loading, and access to enhanced performance regimes. There has been, however, no routine operational use of feedback stabilization to achieve and maintain favorable edge conditions for high performance plasmas. This is due in part to the complexity of controlling simultaneously edge density, edge profiles of density and pressure, edge radial transport, edge potential and radial electric field, and scrape-off layer particle and heat flow, independent of intrinsic core plasma parameters. This paper investigates the experimental facilities required to evaluate five innovative methods for edge control and feedback stabilization.
Stabilization of the vertical instability by means of halo currents involves utilizing biased electrodes in the vacuum vessel. The electrodes drive a force-free current in the plasma halo, and this current creates a field which acts to stabilize the plasma. Edge plasma feedback control by electrostatic biasing utilizes a segmented passive stabilizer shell for feedback control of ballooning and kink modes by poloidal edge current drive. Edge stabilization by toroidally segmented divertor biasing and current injection makes use of non-axisymmetric, time varying SOL currents to suppress the growth of resistive-wall modes, through the generation of controllable time and space varying magnetic pressure. Stabilization of divertor heat loads by plasma edge ergodization utilizes segmented divertor biasing to produce a controlled turbulence at the plasma edge so as to distribute the outwardly transported plasma power over a large area.
It has been found that all of these proposed methods can be implemented using the available PBX-M electrically isolated and biasable passive shell and floating divertors as electrodes for applying edge biasing, under feedback control in concert with several available core stabilization techniques. The stabilization of the plasma edge boundary is a neglected and next-step need that can be addressed in the near term.
*Work supported by US DOE Contract No. DE-AC02-76-CHO3073.