Test of Point-Source Helicity Injection for Non-Solenoidal Startup in NSTX
This grant supported research activity focused on the development of Local Helicity Injection (LHI) as a technique for initiation and growth of spherical tokamak plasmas without the use of a traditional ohmic solenoid for magnetic induction. The LHI technique consists of injection of high-density current streams into a vacuum magnetic field consisting of toroidal and vertical fields. Under appropriate conditions, these streams become unstable and support magnetic reconnection to allow the system to reconfigure itself into a tokamak-like plasma magnetic configuration. This research program focused on three aspects of this plasma startup technique: 1) development and tests of appropriate electron current sources that provide sufficiently high injection currents into the near-plasma edge without strongly contaminating the resultant tokamak plasma with impurities; 2) development and validation of a 0-D power balance model to describe the evolution of the plasma current Ip(t) during this startup process; and 3) initial estimates of Ip levels achievable with this technique in NSTX-U scale plasmas. The 0-D model development included extremely low aspect ratio corrections to analytical formulae for the plasma inductances that produce voltages due to rapid changes in plasma size and location. Applications of this model to sample plasmas in the Pegasus ultra-low aspect ratio tokamak confirms the expectation that Ip(t) is dominated by these induced voltages when injection occurs from the outer low-field side of the plasma. Conversly, Ip(t) is dominated by an effective toroidal loop voltage arising from helicity conservation when injectors are placed in the inner high-field side divertor region. The activities pursued here, combined with more general research on LHI followed by the Pegasus experiment group, provide the tools needed for initial consideration of a MA-level plasma startup capability for the NSTX-U experiment. Simplified modeling for LHI on NSTX-U indicates that no showstoppers exist, and ~ 0.8MA appears accessible using a moderate current injection facility. However, many issues remain to be resolved to address extrapolating this technique to the larger sizes and magnetic field strengths of NSTX-U and larger fusion-grade tokamaks. These include, among others: Ip scaling with helicity drive input; extending current injector technology; understanding electron confinement and the resulting helicity dissipation processes that determine the achievable Ip; the role of plasma stability in controlling the current evolution; and controlling plasma-material interactions and impurity content. These open issues will be further explored in a future experiment dedicated to integrated studies of non-solenoidal startup at higher magnetic fields and plasma currents.
- Research Organization:
- Univ. of Wisconsin, Madison, WI (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Fusion Energy Sciences (FES)
- DOE Contract Number:
- SC0006928
- OSTI ID:
- 1488607
- Report Number(s):
- DOE-UWisconsin-0006928
- Country of Publication:
- United States
- Language:
- English
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