Balancing current drive and heating in DIII-D high noninductive current fraction discharges through choice of the toroidal field

In order to maintain stationary values of the stored energy and the plasma current in a tokamak discharge with all of the current driven noninductively, the sum of the α-heating power and the power required to provide externally driven current must be equal to the power required to maintain the pressure against transport losses. In a study of high noninductive current fraction discharges in the DIII-D tokamak, it is shown that in the case of present-day tokamaks with no α-heating, adjustment of the toroidal field strength (BT) is a tool to obtain this balance between the required current drive and heating powers with other easily modifiable discharge parameters (βN, q95, discharge shape, ne) fixed at values chosen to satisfy specific constraints. With all of the external power sources providing both heating and current drive, and βN and q95 fixed, the fraction of externally driven current scales with BT with little change in the bootstrap current fraction, thus allowing the noninductive current fraction to be adjusted. © 2011 IAEA, Vienna.