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

Heating and fueling. during the ramp-up and flattop phases in iteR, heating and fueling must
be carefully programmed to reach the high-gain state where alpha particle heating becomes dominant.
iteR will be heated by a combination of neutral beam injection and various radiofrequency
heating methods (ion cyclotron, electron cyclotron and possibly lower hybrid waves). The heating
physics of these methods is well understood, and the full injected power is expected to be effectively
absorbed by the plasma; however, there are plasma interface or technological issues associated
with each of the methods. of particular interest in the Us is the interaction of the ion cyclotron
antenna with the edge plasma. The questions are: What will be the nature of the radiofrequency
sheaths formed by the ion cyclotron antenna? What will be its effect on impurity production and the effect
of impurities on antenna operation? What will be the antenna impedance and will the required antenna
voltage be acceptable?
The specific Thrust element is a program focused on the interaction of ion cyclotron antennas
with the edge plasma, including development, verification, and validation of models for radiofrequency
sheaths and their effects. closely coordinated work using experimental, theoretical, and
computational tools is required. it is expected that this activity will point toward the need for advanced
antenna designs, and these should be implemented and their performance validated in
existing machines. it is also expected that methods of reducing the antenna voltage below breakdown
levels will be developed and validated within the frame of this Thrust element.
control of the density and impurities and, to the extent possible, their spatial profiles is important
for the transient and steady discharge phases. The limits of gas fueling at high neutral opacity
and the role of transport in setting the density profile need to be explored. The fact that the
particle transport differs among species could be leveraged to isolate control of particular fueling
and impurity species. Questions to be resolved in this domain are: What is the particle transport,
including impurity transport, for the various ITER operating scenarios, and what pellet ablation and fuel
deposition models are appropriate? Can diagnostics be developed to determine the core deuterium and tritium
ratio to aid burn control and as an adjunct to determining tritium retention? The specific activity
in this Thrust element involves measuring and characterizing transport in transient phases and
validating models of fueling via (inside launch) pellet injection. existing tokamaks properly outfitted
with inside launch pellet fueling and an appropriate diagnostic set should suffice for carrying
out this mission. in addition, advanced fueling techniques, e.g., compact tori injection, should
be pursued.
H-mode pedestals. The ability of the iteR device to achieve its high fusion gain (Q=10) mission
depends on having an edge pedestal sufficient to maintain high core confinement. typical profiles
for the density and temperature edge pedestals are shown in Figure 2. The questions are: What is
the physics of the edge pressure pedestal in H-mode plasmas, and how does the pedestal interact with core
heat and momentum transport? Recent modeling has reproduced the height of the h-mode pressure
pedestal in several tokamaks by combining a prediction of edge mhd stability limits from
peeling-ballooning mode theory with an empirical scaling for the pedestal width. transport can
play a role when elms are mitigated or suppressed so that the pressure gradient is held below the
threshold for peeling-ballooning instability. models of the h-mode pedestal structure and of the
complete elm cycle need to be further developed and thoroughly tested against experiment. This
includes the effect of pellet fueling and low flow/torque. in addition, models of the h-mode ped-
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