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

The barrier gradient is generally limited by mhd stability, and its transient collapses, referred to
as elms, will not be tolerable in future devices as the associated burst of energy and particles expelled
from the plasma severely limit the lifetime of material surfaces. similarly, other large transients
can result in loss of the plasma configuration, and cannot be tolerated in future devices. it
is clear that the mhd issues for a high fusion power core can interact strongly, requiring a consistent
demonstration in a burning plasma.
Heating and current drive
What heating and current drive sources will be the most effective in creating and sustaining
the desired plasma transport and MHD stability properties? As the fraction of plasma
current and heating power to the plasma diminishes, which heating and current sources
provide new and more effective control tools?
neutral beam (nb) injection has dominated most of the tokamak experiments around the world,
as a reliable and effective plasma heating, current drive, and rotation drive tool, with particle
energies less than 120 kev. to provide central deposition in burning plasmas, the particle energy
must increase, requiring a transition from the positive to the negative ion acceleration methods,
going to 1 mev particle energy for iteR, and over 2 mev particle energy for a power plant.
The nbs at these energies may drive fast particle instabilities. as one approaches demo, these
sources must develop solutions to acceleration, cryopump regeneration, and the neutron streaming
problem with the large apertures required for nbs.
although wave propagation and absorption are theoretically well developed in all frequency regimes,
to apply radiofrequency power as a tool for control of the pressure and current profiles requires understanding
how radiofrequency waves interact in a burning plasma environment. The interaction of
icRF waves and lhRF waves with energetic particles is not well understood, and the coupling of icRF
and lhRF waves from the launchers to the core plasma, over long distances (10-15 cm), must also be
quantified, accounting for the exposure of the launchers to the harsh nuclear environment. The interaction
of radiofrequency launchers with the edge plasma and ultimately material surfaces must
be minimized, to guarantee that power enters the plasma and impurities are not generated. The primary
challenge for application of power in the electron cyclotron range of frequencies (ecRF) is technological
in nature and related to the need for gyrotron sources at higher frequencies (> 200 Ghz),
as the magnetic field and plasma density in the plasma increase. an important integration problem
for ecRF will be to develop the understanding of the stabilization of sawteeth and neoclassical tearing
modes. This requires understanding the coupling of the ecRF to the time-dependent evolution of
resistive mhd modes. all radiofrequency sources and nb for driving current noninductively will be
challenged by the higher densities typical of burning plasmas, and these sources must be optimized
to simultaneously provide current drive, heating, and possibly flow drive, as efficiently as possible, in
a plasma that is strongly driven by its internal physics.
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