Injection and sustainment of plasma in a preexisting toroidal field using a coaxial helicity source

Abstract

The spheromak device SPHEX has been modified by adding a current-carrying rod along the geometric axis, providing a preexisting toroidal field. We show that plasma can be successfully injected into such a field from a helicity source; the field assists plasma ejection from the gun and improves the coupling between gun and plasma, so that T^., Ti, and the toroidal current all increase with rod current. The mechanism of plasma sustainment appears to be the same as that of the spheromak. These results represent a step towards the achievement of steady-state tokamak operation.PACS numbers: 52.55.Hc SPHEX is a gun-injected spheromak device similar to the Compact Toroid Experiment (CTX) [1] at the Los Alamos National Laboratory; it is described in the preceding Letter [2], which presents results suggesting the outline of a relaxation mechanism involving a largescale coherent mode of oscillation which, we believe, drives the toroidal current in the plasma.In this paper we describe the modification of SPHEX in which the plasma is injected into a pre-existing toroidal field. This is generated by a current-carrying rod placed along the geometric axis of both the gun and the flux conserver (see Fig. 1 of Ref. [2]). This configuration was suggested by results from the Heidelberg HSE experiment [3]; a similar scheme has been proposed [4] to sustain a tokamak discharge by helicity injection. Our results (presented briefly in Ref.[5]) show for the first time that a coaxial helicity source can form and sustain a plasma in an externally generated toroidal field; although it is not clear that our configuration can properly be described as a tokamak, our results suggest that sustainment in a tokamak regime may be possible.In an ideal spheromak configuration the toroidal field vanishes at the wall but the safety factor q does not [6]; in fact, in a closed spheromak it varies by < 20% over the radius. We expected that because of the tight aspect ratio and strong toroidicity, the addition of toroidal field might lead to a significant shear even at modest rod currents. We have therefore studied numerically the effects of toroidal field on force-free equilibria described by VxB=/iB, both for constant p (the relaxed state [7]) and for p =='p^,+c\i//y/o, where if/ is the poloidal flux coordinate, y/G is the gun solenoid flux, and p^ is the value of p at the wall. This form has been used to describe spheromak equilibria in CTX [8]; with j/^ = 0 at the wall, a driven spheromak has <: < 0, the relaxed state r=0, and a decaying plasma c > 0 [8]. Solutions are obtained [9] by the SOR (successive over-relaxation) method applied to the corresponding Grad-Shafranov equation (linearized when p is not a constant). We have so far studied the driven or relaxed cases, c :< 0.We first consider spheromaklike solutions with IR =0. Figure 1 (a) shows a typical set of flux surfaces. Since this system has flux entering and leaving through the electrodes, it includes a separatrix dividing the field into regions of *'short open flux,'' *'long open flux,'' and...

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