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

netic field, so one of these must be varied. a thrust element addressing this issue would be to develop
variable frequency gyrotrons. Thus, instead of slowly ramping the magnetic field strength
to move the interaction region, which is the technique used on alcator c-mod, the frequency of
the ecdc could be varied at fixed magnetic field (which iteR requires). additional control can be
implemented by application of a variable poloidal field. a possible ancillary benefit could be its use
in freeing tritium from the chamber walls, which would be useful in controlling the tritium inventory
in the vessel. another benefit would be to resume research and development in the Us on
high-power, high-frequency, steady-state gyrotrons, an important enabling technology with additional
uses in fusion experiments.
transport during transient phases. simulations of iteR scenarios have revealed the need for
careful initiation and termination of plasma discharges. For the current ramp-up and ramp-down
phases, simulations have been developed to model the evolution of the current profile, which is
important for assessing magnetohydrodynamic (mhd) stability, but the results depend on electron
thermal transport during these transient states, which is poorly understood. The issue is:
What is the energy and particle transport during ramp-up and ramp-down phases, and how does it depend
on the evolving current profile? Uncertainty concerning transport during current ramp-up and
ramp-down can be reduced by coordinated research on this poorly studied topic. to be most relevant
to iteR, it is desirable to use dominant electron heating, likely from radiofrequency sources.
sawtooth mixing and temperature recovery between sawtooth crashes constitute a transient condition
that may be of more importance for iteR than present-day devices. to study transport
within the sawtooth mixing region, high time resolution is required for measurements of magnetic
pitch angle, ion temperature, and flow speed. experiments need to vary the heating mechanism
between thermal (e.g., electron cyclotron resonance heating) and fast ion thermalization to
validate models of the effect of suprathermal sawtooth stabilization. like sawteeth, elms will effectively
produce significant radial transport in the periphery, and the influence of elms on the
plasma boundary needs to be quantified for iteR.
H-mode access and dependence on ion species. access to the h-mode is essential for iteR
to fulfill any part of its experimental mission. an example of a low- to high-confinement (l-h)
transition, including the observation of elms during the h-mode phase, is shown in Figure 1. a
high-priority research activity is determining the heating power required for attaining various
regimes: (1) the transition threshold from l-mode to h-mode and from h-mode to l-mode, (2)
steady h-mode plasmas with (small, rapid) type iii elms, (3) steady h-mode plasmas with “good”
confinement, as determined by standard scaling relations, and (4) h-mode access and back transition
during the plasma current ramp-up and ramp-down phases. These studies need to be done in
iteR-relevant plasma conditions. The isotope mass and species scaling (i.e., hydrogen and helium
plasmas) for the heating power required to achieve the regimes listed are also of high value when
considering the nonnuclear phase of iteR.
259