Spheromak

This paper summarizes several studies of fusion reactors based on the compact torus (CT). A wide variety of reactor configurations can be projected within present understanding of the possible types of CT and their macroscopic stability and confinement properties. Three types of CT are considered here, the field-reversed-configuration having B oroidal 0, the Spheromak with 0, and CT s formed with particle rings. 

In the following discussion, various reactor designs will be considered in the broad categories FRC, Spheromak, and particle-ring CT based reactors. 

The Spheromak reactor22 based on ideal and resistive MHD stability of a CT having an internal toroidal field. Stability considerations indicate that the overall shape must be sufficiently oblate and with a surrounding conducting shell, and that the overall configuration must be nearly force-free. The calculated maximum local 0 limit is 2-4% using the... 

Table 9. Representative Parameters for a Large Spheromak Ignition Reactor With Resistively Decaying...

Representative parameters of a small Spheromak TCT reactor are given in Table 10. Here it is argued by the authors of Ref. 22 that, for a single ion component, the Q for resistivity losses,... 

An interesting possibility for continuous Spheromak operation is to increase T in Qj above by providing the current with a small fraction of energetic electrons (as discussed later). If reasonable can be achieved in this case, high Q, small, continuous, Spheromak reactors may be possible. 

Table 10. Representative Parameters for a Small Spheromak TCT Reactor With Beam-Driven Currents22...

We discuss (1) the theoretical analysis of the spheromak, (recent theoretical studies are summarized and a comparison is made with other approaches), and (2) a spheromak plasma formation scheme. One of the difficult aspects of spheromak research is to produce a toroidal plasma in the absence of coils down the axis of rotational symmetry. A novel plasma formation scheme has been developed at PPPL to create a toroidal plasma by using a flux generating core technique. This formation scheme has been optimized with the assistance of resistive MHD computer codes. The parameters of the S-1 device, now under construction, were determined with various theoretical aids. 

Recently a email device (1/6 scale of S-1) has successfully produced a spheromak plasma using this scheme. 

Historically, the configuration was first studied theoretically [7,8,9] in an astronomical context in the 1950 s. Later it was reported that the spheromak configuration was formed in a laboratory experiment [10]. In the late 1960 s some theoretical calculations were carried out from the view point of plasma confinement [11]. 

Many theoretical and experimental physics issues concerning the spheromak such as MHD stability, transport phenomena, microinstabilities, and plasma formation schemes had remained virtually unstudied. The reexamination of this configuration from the reactor viewpoint recently became of great interest [5] and the study of these issues has been accelerated, since... 

Advanced computational physics based on up-to-date theoretical understanding can provide substantial guidance for experimental procedure. Although an experimental arrangement need not follow theoretical predictions, it is very beneficial to evaluate experimental procedure with theoretical and computational assistance as has been amply demonstrated by the resistive MHD code modeling of the PPL spheromak formation method to be discussed later. 

The term spheromak is widely used to distinguish the family of equilibria with zero external toroidal field from other compact tori. For example, the "classical" spheromak shown in Fig 1 is the "aspect-ratio-unity limit," and the stabilized diffuse pinch [14] (operation of diffuse pinch with zero external toroidal field) corresponds to the large aspect-ratio limit. Before we discuss the spheromak in detail, it may be instructive to briefly survey the relation between the spheromak and other approaches. A comparison is made in table 1. The field is the poloidal field strength at the plasma edge, is the toroidal field strength at the magnetic... 

The reversed field 0 pinch with no toroidal field inside the plasmoid is a more advantageous reactor-oriented device than the spheromak, in some respects, since the plasma formation is... 

Fig. 2 Comparison of classical spheromak configuration (solid line) with Hill s vortex configuration (dotted line).

Since no minimum B exists in the spheromak configuration, the pressure gradient driven modes must be stabilized by magnetic shear. The beta-limit due to local modes has been investigated by using the Mercier criterion [21],... 

We obtain the "classical spheromak with 5=0 and ellipticity K=b/d=1.0, and the stabilized diffuse pinch in the large aspect-ratio limit as delta approaches one. The "classical" spheromak has q(>J ) = 0.82 and q(Yg)=0.72 at the plasma surface due to the finite... 

Global MHD modes in the spheromak configuration can be categorized into two types (1) the toroidal number n = 1 modes related to the motion of the axis, such as tilting/sliding modes,which are relevant in the small aspect ratio limit, and (2) n > 2 modes similar to those in toroidal devices. Tilting/sliding modes will be discussed in the section 3-3. 

The choice of the low-beta plasma approach (plasma = 2% ) with R/a =2 is based on the theoretical predictions given in the section-3.1. This low beta approach contrasted with the high beta approach in general diffuse pinch devices, is probably favorable in the low aspect ratio spheromak, since bad curvature could enhance macro/micro instabilities. Low-beta further minimizes the risk of the deterioration of the transport properties since there are no definite theoretical predictions about resistive g modes, which are thought to be dangerous instabilities in tokamaks. 

The immediate aim of the spheromak experiment is to study the qualitative behavior and plasma transport properties of a high temperature spheromak plasma. In order to carry out this goal/ it is necessary to increase the electron temperature high enough to minimize the ambiguity related to atomic/molecular processes and impurity radiation losses (T > 100 -200 eV). The choice of the metallic liner and equi-psi surface approach should also reduce the impurity migration from the core surface. 

A schematic diagram of the S 1 spheromak is given in Fig. The vacuum vessel has a major radius of 1.5m and an oblate... 

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