Summary of Findings on Spherical Tokamak Research

April 1996

1. The Panel notes that it would be imprudent now to recommend the proper scope and funding level for spherical tokamak research without completing the review of all alternative concepts. That recommendation will be contained in our report in July. In that context, we expect that spherical tokamak research will be one part of a multi-faceted alternative concept research program.

2. The Panel finds that the spherical tokamak concept is scientifically ready to move to a "proof-of-principle" stage program. This conclusion is based on:

(a) The growing data base from "concept-exploration" experiments such as START which shows that confinement in spherical tokamaks is "tokamak-like."

(b) The concept-exploration research has not identified any physics "show-stoppers" to proceeding to the next stage of research.

(c) A large body of tokamak theory and experimental data which can be extrapolated to lower aspect ratio providing a sufficient basis for proceeding to a proof-of-principle stage program.

3. The Panel finds that research in spherical tokamaks can make an important contribution to fusion plasma physics and fusion energy development. The spherical tokamak research can help resolve key issues of tokamaks because the spherical tokamak concept pushes the tokamak physics to the limit of extreme toroidicity. In this context, the spherical tokamak research fits well with the emphasis of the U.S. tokamak program on advanced tokamaks.

Preliminary analysis indicates that spherical tokamaks with small size may be possible for fusion energy development and power plants. However, integration of plasma physics and technological issues such as MHD stability and current drive, design of the center-post, edge physics and divertor heat removal, and wall loading limitations set the optimum parameters of spherical tokamaks. These integration issues should also be addressed in a proof-of-principle spherical tokamak program.

4. Spherical tokamak research is moving into a proof-of-principle stage internationally and several proposals for proof-of-principle experiments are pending. One of these, the MAST experiment, is approved for construction in the United Kingdom. The panel notes that fusion research historically has shown there is great benefit in having more than one proof-of-principle-class experiment. Thus, from a scientific perspective, the construction of a proof-of-principle-class device outside the U.S. should not preclude construction of proof-of-principle-class experiments in the U.S. A programmatic decision to construct a U.S. proof-of-principle-class experiment should be based on the benefits anticipated from such an experiment for the U.S. fusion program.

5. The panel finds that new concept-exploration-class spherical tokamak experiments can provide significant cost effective contributions to key spherical tokamak physics issues. Such experiments may be required for a healthy proof-of-principle spherical tokamak program.