1 - Nuclear Sciences and Applications - IAEA

6 MAISONNIER
The principal key to such progress has been the increase in gross machine
parameters, and it is the relative simplicity of the tokamak concept which has allowed
us to build the large devices we have today. An example of the virtue of size, and
of course I do not mean only geometrical size, is given by JET, which is the largest
tokamak in the world and which is, I think, second to none in scientific output.
Progress has also been made using smaller devices. For instance, substantial
insight into the complexity of cross-field diffusion in reactor relevant plasmas was
brought about by the discovery of the H-mode in ASDEX with a divertor magnetic
separatrix, pointing to the strong influence of edge localized phenomena on global
confinement. Enhanced confinement regimes have also been achieved, with intense
neutral beam heating and pellet injection, on tokamaks without divertors. But I cannot
resist quoting the slightly provocative statement of Dr. Furth that' 'in this context of
programmatic success, it is sobering to reflect that identification of the specific physical
phenomena responsible for anomalous transport in toroidal configurations has not
progressed decisively since 1956."
Other reactor relevant critical physics issues are under intense investigation
both in theory and in experiments; they concern limits to stable tokamak operation
such as the elongation dependent plasma beta limit, the density limit and the control
of disruptions, and also the so-called innovative ideas, such as current drive (demonstrated
at the megampere scale in JT-60) and various impurity control schemes.
The last untouched physics problem, alpha particle heating, should become
accessible to experimentation about three years from now, when TFTR, JET and TSP
(which is the new name of T-14) will enter into their phase of tritium operation.
So we can say that, while mysteries still remain in some or most of these issues,
the conclusion of the scientific phase of fusion is now in sight. It is therefore timely
to plan for the Next Step device on the road towards a prototype reactor, a Next Step
which should complete the demonstration of the scientific feasibility of fusion and
confront the problems linked with its technological feasibility.
The Next Step
The pace of the major fusion programmes, as they are geared at present, is such
that in some three years from now we will, we hope, have a sufficient scientific and
technical basis to propose to our authorities the launching of the detailed design of
one or more of the various possible incarnations of a Next Step device, national or
international, with a view to its construction.
Next Step devices are at present in the predesign phase both within the large
fusion programmes (for example NET in the European Community, which is the one
I am most familiar with) and in the quadripartite venture, ITER. ITER, whose definition
phase will be formally concluded in November 1988, is conceived as a tokamak
device which, to quote from its terms of reference, "will provide the database in
physics and technology necessary for the design and construction of a demonstration