|poster.pdf||2006-03-08 17:43:23||William Hamp|
Machine enhancements to the HIT-SI Spheromak
Author: William T Hamp
Submitted: 2005-12-21 12:34:29
Co-authors: T. R. Jarboe, R. G. O’Neill, A. J. Redd, P. E. Sieck, R. J. Smith, G. L. Sutphin, J. S. Wrobel, University of Washington, B. A. Nelson, NSX Limited
University of Washington
AERB Room 120 Box 352250
Seattle, Washington 98177
Present operating objectives for HIT-SI call for reduction of impurities and increased power into the plasma. To meet these objectives, the HIT-SI power supply has been modified to eliminate voltage spikes on the injector primary, power supply control system improvements are being pursued, and wall conditioning enhancements have been made to reduce the vacuum base pressure.
The tank circuit between the Switching Power Amplifiers (SPA) and the primary voltage coil on the injectors has been changed from a series capacitor design to a parallel capacitor design to prevent large voltage spikes which can arc through the insulating layer to the copper flux conserver. Improvements to the SPA controllers are being actively pursued in collaboration with an SBIR project (NSX Limited). Digital-based controllers are being developed to replace the present analog-based controllers. This will greatly improve the flexibility and fidelity of the HIT-SI feedback control system.
The entire vacuum vessel is baked to 110 C followed by RF glow and pulse discharge cleaning (PDC) in hydrogen and, finally, in helium. Hydrogen discharge cleaning removes residual carbon and helium discharge cleaning removes hydrogen from the walls.
To date (Dec,’05), the base pressure has been reduced to 1X10-8 Torr, and operations with the parallel circuit show the voltage spikes are reduced. As a result, an n=0 object has been produced in the confinement region with 9 kA of toroidal current, driven by 300V loop voltage in the injector. Further, pre-ionization is no longer required to initiate discharges, simplifying operation. A new turbopump is planned to increase the pumping speed from 450 L/sec to 1100 L/sec to further reduce the base pressure.