Confinement studies in high temperature spheromak plasmas

Recent results from the SSPX spheromak experiment demonstrate the potential for obtaining good energy confinement (Te > 350 eV and radial electron thermal diffusivity comparable to tokamak L-mode values) in a self-organized toroidal plasma. In this paper we discuss energy confinement and transport in ohmically-heated SSPX discharges and compare data against several transport models applicable to self-organized systems maintained by DC helicity injection. A strong decrease in thermal conductivity with temperature is observed and at the highest temperatures, transport is well below that expected from the Rechester-Rosenbluth model. Recent improvements to performance (raising Te from 200 eV) result from increasing both gun flux and current to increase the magnetic field while keeping a relatively flat current profile to minimize magnetic fluctuations. In the near term, a new capacitor bank is expected to produce higher magnetic fields and longer pulses, allowing operation with temperatures as high as 0.5 keV. At temperatures above 300 eV, it becomes possible to use modest (1.8 MW) amounts of neutral beam injection (NBI) auxiliary heating to significantly change the power balance in the core plasma, making it an effective tool for improving transport analysis. We are now developing detailed designs for adding NBI to SSPX and have developed a new module for the CORSICA transport code to compute the correct fast-ion orbits in SSPX. This module, coupled to a deposition code (NFREYA), is used to calculate the particle, current and power deposition from Neutral Beam injection. Initial CORSICA results show that a substantial fraction of the injected bean, of order 70%, is confined as fast ions, which is sufficient to raise the electron temperature and total plasma pressure in the core by a factor of two. (author)