543 



66 



III. Background 



In parallel with the impressive progress in tokamak development, fusion 

 researchers have continued to explore other magnetic confinement 

 geometries and to improve the tokamak itself. This research has two 

 motivations: (1) to explore geometries that might lead to fusion 

 reactors having qualitatively different features; and (2) to explore new 

 plasma phenomena that contribute to improved understanding of plasma 

 confinement as a whole. Indeed, it is the interplay between these two 

 motivations that has often brought forth our most creative science and 

 invention. 



The importance attached to reactor concept improvements is evidenced 

 by their strong support in the national programs. Also, there is 

 already a considerable degree of international cooperation in these 

 areas. The Appendices discuss each concept in more detail. 



A. Concepts, Reactor Features 



The Concepts covered by the Subpanel are compared to the tokamak in 

 Table I. 



The line labelled Reactor Features lists the ways in which these 

 concepts might eventually improve on the present tokamak, which appears 

 to lead to large reactors in the range of 1000 MWe and reactors that 

 are pulsed in time, since the plasma current essential for heat 

 confinement in tokamaks is inductively driven. Thus, two main themes 

 of improvement are: (1) smaller, cheaper reactors (lower power 

 output, smaller dimensions), mainly by increasing the parameter 

 "beta",* and (2) D.C. operation and other engineering simplifications. 



Briefly, in Table I, Tokamak Improvements refers to those aspects of 

 tokamak research that address the goals of smaller unit size and D.C. 

 operation; for example, by means of a highly non-circular cross section 

 to increase the beta (more elongated than in the present large 

 experiments) and rf current drive to replace the pulsed inductive 

 current drive in present tokamak experiments. In the Open Systems 

 category, the tandem mirror is a D.C, high-beta system with a linear 

 geometry that simplifies blanket design and may eventually lead to 

 smaller units of lower output power, depending on the length required 

 for confinement. Finally, the category labelled Alternative and 

 Supporting Systems refers to a diverse group of concepts with the common 

 goal of retaining the good heat confinement expected in "closed", or 

 toroidal, systems (and exhibited in tokamaks) while improving the system 

 in other ways. Three examples listed in the table are: (1) 

 stellarators/heliotron, which resemble tokamaks in concept and good 

 plasma confinement performance but do not require current drive or power 

 input to the plasma in order to operate D.C; (2) the Reversed Field 

 pinch (RFP), for which high beta and low field may permit the use of 



*Beta is defined as the ratio of plasma pressure to magnetic energy density. 

 Higher beta implies either higher power density or weaker magnetic fields. 



