Demonstration of Tokamak Ohmic Flux Saving by Transient Coaxial Helicity Injection in the National Spherical Torus Experiment

Transient coaxial helicity injection (CHI) started discharges in the National Spherical Torus Experiment (NSTX) have attained peak currents up to 300 kA and when coupled to induction, it has produced up to 200 kA additional current over inductive-only operation. CHI in NSTX has shown to be energetically quite efficient, producing a plasma current of about 10 A/J of capacitor bank energy. In addition, for the first time, the CHI-produced toroidal current that couples to induction continues to increase with the energy supplied by the CHI power supply at otherwise similar values of the injector flux, indicating the potential for substantial current generation capability by CHI in NSTX and in future toroidal devices.

Figure 1

Schematic of the NSTX machine components including the location of the insulating gaps between the divertor plates, the lower divertor coils used for generating the CHI injector flux and the absorber poloidal field coils.

Figure 2

Shown are (a) the plasma current and preprogrammed loop voltage for two discharges. Discharge 135622 is a reference discharge in which the absorber PF coils were not energized. In discharge 135616 the absorber PF coils provided a buffer flux to the evolving CHI discharge. Also shown are (b) the injector current, signal from a central chord bolometer, the upper and lower divertor O-II signals. (c) Fish-eye camera images just prior to, during, and after the absorber arc for both discharges.

Figure 3

(a) Shown are plasma current and radiated power traces from a scan in which the size of the CHI capacitor bank power supply was increased from 5 to 20 mF. Discharge 135621 is a reference inductive-only discharge. (b) CHI injector current and spectroscopic traces for the four discharges.

Figure 4

Electron temperature, density, and pressure profile from a Thomson scattering diagnostic for a CHI-started discharge 135614 and a reference inductive-only discharge.