Progress towards steady state on NSTX

Abstract

In order to reduce recirculating power fraction to acceptable levels, the spherical torus concept relies on the simultaneous achievement of high toroidal beta and high bootstrap fraction in steady state. In the last year, as a result of plasma control system improvements, the achievable plasma elongation on NSTX has been raised from kappa similar to 2.1 to kappa similar to 2.6-approximately a 25% increase. This increase in elongation has led to a substantial increase in the toroidal beta for long pulse discharges. The increase in beta associated with an increase in plasma current at nearly fixed poloidal beta, which enables higher beta(1), with nearly constant bootstrap fraction. As a result, for the first time in a spherical torus, a discharge with a plasma current of I MA has been sustained for 1 s (0.8 s current flat-top). Data are presented from NSTX correlating the increase in performance with increased plasma shaping capability. In addition to improved shaping, H-modes induced during the current ramp phase of the plasma discharge have been used to reduce flux consumption and to delay the onset of MHD instabilities. Based on these results, a modelled integrated scenario, which has 100% non-inductive current drive with very high toroidal, will also be discussed. The NSTX poloidal field coils are currently being modified to produce the plasma shape which is required for this scenario, which requires high triangularity (delta similar to 0.8) at elevated elongation (kappa similar to 2.5). The other main requirement of steady state on NSTX is the ability to drive a fraction of the total plasma current with RF waves. The results of high harmonic fast wave heating and current drive studies as well as electron Bernstein wave emission studies will be presented.