Research Needs for Magnetic Fusion Energy Sciences - US Burning ...

of recent accomplishments that have advanced research toward the goal of “predictable, high-performance,
steady-state plasmas.” many more can be found in the scientific literature, including
excellent review papers, and in recent community reports. some suggestions for further reading
are given at the end of this chapter.
accurate measurements of hot magnetically-confined plasmas have had a profound effect upon
our ability to understand and manipulate those plasmas. This in turn has enabled the development
of attractive operational regimes. measurement of the local magnetic field inside a tokamak
plasma, which was not possible until the 1990s, provides information on the current density
profile. This is critical to understanding plasma confinement; current-profile control is one of
the cornerstones on which advanced tokamak scenarios are based. measurements of fluctuations
and fluctuation-induced fluxes, associated with various instabilities as well as turbulence and turbulent
transport, have provided direct quantitative evaluation of some of the sources of anomalous
transport in plasmas. a powerful suite of diagnostic techniques has been developed over
the years, including active spectroscopy, reflectometry, various kinds of imaging, and magnetics.
even more capability in this area is desired to measure other fluctuating quantities over shorter
wavelengths. For steady-state or very long-pulse plasmas, the interactions between the plasma
and the plasma facing wall, and the characteristics of the wall, become crucial. This is due to the
long “wall-equilibration” time scale for such processes as particle retention and deposition, as well
as erosion and ablation of material. our understanding of various wall materials and of these interactions
has increased enormously over the past 20 years, primarily as a result of the measurement
capability that has been developed.
impressive advances in plasma control, including avoiding transient events, have also been
achieved, in part due to the improvements in measurements. These accelerated substantially in
the 1990s with the advent of control-intensive advanced tokamak and related regimes in highly
shaped, diverted, high-b plasmas operating routinely near or beyond magnetohydrodynamic
(mhd) stability limits. some key achievements worldwide over the last decade include:
• complex model-based multivariable axisymmetric shape and stability control, giving
routine actively stabilized operation of highly-elongated tokamak plasmas within 2% of
the ideal limit for the axisymmetric mhd instability.
• suppression of non-axisymmetric resistive mhd modes (including resistive wall modes,
by applying non-axisymmetric fields, and neoclassical tearing modes, using electron
cyclotron current drive). enabling operation significantly beyond no-wall stability limits.
• suppression of edge localized modes by the application of non-axisymmetric fields.
• multi-point regulation of current profiles using multiple combined actuators.
• control of formation and subsequent regulation of internal transport barriers.
• simultaneous control of divertor heat flux and total energy confinement by varying
fueling, pumping, and impurity injection.
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