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

• determine minimum heating power required to obtain and maintain (1) h-mode during
ramp-up and ramp-down, (2) steady h-mode with (small, rapid) type iii edge localized
modes (elms) and (3) steady h-mode with good confinement (compared to scaling
relations).
• elucidate the physics and minimize interactions of ion cyclotron resonance heating
antennas with edge plasmas.
• develop models for particle transport and for gas and pellet fueling applicable to iteR.
• test models of h-mode pedestal structure against experiment, determine the effect of
pellet fueling and low flow/torque, and pursue coupling to core models.
• Understand the physics of the hybrid scenario to make reliable predictions for iteR,
develop predictive understanding of steady-state modes of operation, and define
requirements for implementation in iteR.
success in iteR is predicated on the development of operational scenarios that achieve the necessary
levels of plasma performance. accessing high-performance states in iteR requires successfully
forming, heating, controlling, and safely shutting down a high-temperature plasma in
a predictable and reproducible manner. achieving this requires greater understanding, gained
through an integrated experimental campaign under plasma conditions as similar as possible to
those expected on iteR, e.g., h-mode plasmas with low input torque, equal ion and electron temperatures,
and low collisionality. Uncertainties in the projected performance of ITER arise from many
considerations, and the issues deemed most critical, and approaches for resolving or mitigating them, are
described in the following sections. With upgrades to the tools for heating and current drive, particle
control, and heat flux mitigation on existing tokamaks, and a possible new tokamak facility,
the Us fusion community would be well-positioned to address these burning plasma issues along
with others that may arise prior to or during iteR operation.
Wall preparation and cleaning. it is well known that the chemical composition of the plasma
facing surfaces has a strong impact on the quality of tokamak discharges. Questions in this area
are: What cleaning methods can be used for ITER? Will wall-conditioning methods applied during and
between discharges be effective? a related question is: Can techniques be developed to remove tritium
from the walls, thereby extending the number of discharges before the operational limit on in-vessel tritium
is reached?
The number of wall preparation techniques available for iteR is limited due to its large steadystate
magnetic field. one approach would be to use high-power radiofrequency waves near the
ion cyclotron frequency. This technique has been used on a number of existing tokamaks, including
the superconducting devices tore supra and east. a possibly more attractive approach would
be to use electron cyclotron discharge cleaning (ecdc). in this method, millimeter waves are injected
into the chamber vessel at or near the electron cyclotron frequency corresponding to the
static magnetic field. in both cases, the resulting low-temperature plasma is effective at removing
lightly bound elements from the chamber walls. The ecdc plasma tends to be most intense and
effective at cleaning over only a relatively small region of the vessel at fixed frequency and mag-
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