Physics
Basis and Extrapolation to ITER
A great deal of past and present
world fusion program effort has gone into understanding the physics
basis for energy confinement in tokamaks and similar magnetic confinement
systems and in applying this understanding to the design and optimization
of tokamak burning plasma experiment and reactor prototype designs. Empirical
scaling and nondimensional similarity
scaling extrapolation methods and firstprinciple
microturbulencebased numerical simulation methods have been applied,
and the ITERassociated 'physics expert group' process has been responsible
for the development of a series of increasinglyrefined empirical 'Hmode'
tokamak energy confinement scalings that have been used as the basis
for
selection of the current ITER design parameters. Application of nondimensional
similarity and firstprinciple modelling methods supports — with
certain important caveats, particularly about the key role that the plasma
boundary temperature plays in determining overall plasma confinement — the present ITER parameter selection and the likelihood that ITER
will be able to achieve its Q = 10 design objective and sustain Q > 5
over a credible range of plasma conditions. These achievements will open unique opportunities to conduct
'reactorregime' confinement science experiments without need for
plasma parameter extrapolation or equivocation about the simultaneity
of the plasma conditions.
Advanced
Confinement
Positivism about the
likelihood of ITER being able to meet or exceed its fusion gain and 'reactorregime'
objectives derives in part from studies in present tokamaks that identify
a number of ITERapplicable regimes of 'advanced' plasma operation, wherein
local and global energy transport is reduced — owing to reduction
in microturbulence — relative to the standard 'ELMy Hmode' design
basis assumption. The fusion performance enhancement so obtainable can
either be exploited directly, to obtain Q > 10 or likely even ignition (fullyselfheated sustained fusion
burn), or can be used as a basis for 'advanced tokamak' steadystate
operation modes wherein nominal performance (Q = 510) is obtained at
a reduced plasma current that is sustainable without the need for continuing
inductive current drive. In this regard, ITER
will be a unique research facility — with steadystatequalified
superconducting coils and power and particle exhaust — for extending
'advanced tokamak' science and development to the reactor
regime and for explicitly demonstrating what many fusion researchers
believe is the ultimate inherently steadystate embodiment of a tokamak
fusion power reactor.

Recent
developments in microtubulence modelling now allow explicit calculation
of transport in ITERlike plasma geometry
ITER
will allow extension of present
'advanced tokamak' science studies to
a true steadystate burning plasma regime.
Success can yield a full demonstration of
the reactor potential of the tokamak.
