Solenoid-Free Startup Experiments in DIII-D

Full text: A series of DIII-D experiments was performed to investigate the potential for initiating plasma current using poloidal field (PF) coils located outside the central solenoid region of the DIII-D tokamak, i.e. 'solenoid-less startup' (SS). Inductive plasma current startup was achieved using only divertor and outer PF coils and strong electron cyclotron (EC) heating. Synergistic experiments were carried out using a standard solenoid generated plasma that transitions to non-inductive current drive (CD) using: EC, neutral beam (NB), and fast wave (FW). Conditions obtained in these latter experiments allowed a wide range of parameters to be studied than were obtained in SS experiments, and were designed to quantify the present DIII-D CD capabilities in low current, wall-limited plasmas. The key goal is to transition from the SS scenario to a fully noninductive CD scenario and to establish the feasibility of plasma initiation in a solenoidless tokamak, a requirement for viability of the low aspect ratio (LAR) path to fusion energy. Plasma current to 170 kA was achieved in the SS configuration limited by coil and power supply constraints in the DIII-D system. This was achieved utilizing ∼ 2 MW of EC pre-ionization and heating. Flux conversion to plasma current was only slightly degraded relative to standard DIII-D startup. In the SS scenario, ECCD levels were small and prior to gaining closed loop radial position control, NBCD efficiency was decreased by plasma radial motion. Experiments using our standard solenoid initiation followed by noninductive CD provided valuable insight into the present DIII-D capabilities in plasmas typical of the SS configuration. While full noninductive current drive has been achieved in low current (340 kA), diverted, H-mode helium plasmas [1], the present DIII-D capability in a limited, L-mode plasmas has not been previously evaluated. Preliminary CD results indicate substantial noninductive current can be driven in a limited, L-mode plasma; however, self-sustaining levels with our present 6 co-injection system will require further optimization and possibly H-mode operation. (author)