|raman_icc2006.pdf||2006-03-22 08:45:26||Roger Raman|
Solenoid-free Plasma Start-up in HIT-II and NSTX using Transient CHI
Author: Roger Raman
Submitted: 2005-12-20 17:34:43
Co-authors: T.R.Jarboe, B.A.Nelson, M.G.Bell, D.Mueller
University of Washington
Seattle, WA 98195
Solenoid-free Plasma Start-up in HIT-II and NSTX using Transient CHI*
R.Raman 1, T.R.Jarboe 1, B.A.Nelson 1, M.G.Bell 2, D.Mueller 2
and the HIT-II and NSTX Research Teams
1 University of Washington, Seattle, WA, USA
2 Princeton Plasma Physics Laboratory, Princeton, NJ, USA
Elimination of the central solenoid is a consideration for the design of toroidal confinement devices which will then require alternative methods for initiating the plasma current. A new method of non-inductive startup, referred to as transient coaxial helicity injection (CHI), has been successfully developed on the HIT-II experiment. In this method, a plasma current is rapidly produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields. The initial poloidal field configuration is chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes, with the toroidal plasma current forming closed flux surfaces. By self-consistently increasing both the injector flux and the externally produced toroidal flux, the useful CHI-produced current on closed flux surfaces has been increased to 100 kA. This initial currrent can be retained during a subsequent inductive ramp. On HIT-II, CHI-started plasmas outperform discharges initiated by induction alone and consume less volt-seconds.
The method has now been successfully used on NSTX for an unambiguous proof-of-principle demonstration of closed-flux current generation without the use of the central solenoid. Significant improvements over the HIT-II results are (a) demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor, (b) a remarkable multiplication factor of 60 between the injected current and the achieved toroidal currrent, compared to six in previous experiments, and (c) significantly more detailed experimental measurements, including, for the first time, fast time-scale visible imaging of the entire process that shows discharge formation, disconnection from the injector and the reconnection of magnetic field lines to form closed flux. In some discharges the generated current persists for an unprecedented 400ms, which is an unanticipated result. These significant results, which were obtained on a machine designed with mainly conventional components and systems, indicate favourable scaling with machine size. Experimental results from these experiments in HIT-II and NSTX will be presented.
*Work supported by US DOE contracts DE-FG03-9ER54519 and DE-AC02-76CH03073.
Workshop topics: A.1, C, D