Spheromak Buildup in SSPX using a Modular Capacitor Bank

Abstract

The Sustained Spheromak Physics Experiment (SSPX) [1] was designed to address both magnetic field generation and confinement. The SSPX produces 1.5-3.5msec, spheromak plasmas with a 0.33m major radius and a minor radius of {approx}0.23m. DC coaxial helicity injection is used to build and sustain the spheromak plasma within the flux conserver. Optimal operation is obtained by flattening the profile of {lambda} = {mu}{sub 0}j/B, consistent with reducing the drive for tearing and other MHD modes, and matching of edge current and bias flux to minimize |{delta}B/B|{sub rms}. With these optimizations, spheromak plasmas with central T{sub e} >350eV and {beta}{sub e} {approx} 5% with toroidal fields of 0.6T [3] have been obtained. If a favorable balance between current drive efficiency and energy confinement can be shown, the spheromak has the potential to yield an attractive magnetic fusion concept [4]. The original SSPX power system consists of two lumped-circuit capacitor banks with fixed circuit parameters. This power system is used to produce an initial fast formation current pulse (10kV, 0.5MJ formation bank), followed by a lower current, 3.5ms flattop sustainment pulse (5kV, 1.5MJ sustainment bank). Experimental results indicate that a variety of injected current pulses, such as a longer sustainment flattop [5], higher and longer fast formation [6], and multiple current pulses [7], might further our understanding of magnetic field generation. Although the formation bank can be split into two independent banks capable of producing other injected current waveforms, the variety of current waveforms produced by this power system is limited. Thus, to extend the operating range of the SSPX, a new pulsed-power system has been designed and partially constructed. In this paper, we discuss the design of the programmable bank and present first results from using the bank to increase the magnetic field in SSPX.