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

most relevant to iteR. Recent observations of flow drive from mode conversion of radiofrequency
waves look promising, although verification on a variety of devices, along with a more stringent
comparison with theory, should be undertaken. Further work with heating from electron cyclotron
waves and lower hybrid waves should be pursued as potential tools for profile control.
Particle transport: The transport of particles (electrons, fuel ions, and impurity ions) has been
experimentally shown to combine both turbulence-driven and collisional effects. The turbulencedriven
transport is suspected to depend on particle charge and mass, whereas collisional effects
are known to be strongly dependent on the charge of the particle. These differences in transport
properties among particle species could be leveraged to isolate control of a particular species.
to make firm predictions about density peaking and impurity accumulation, research should be
targeted on micro-instabilities that are relevant to iteR. Future experiments should attempt to
verify the pinch effects predicted by turbulence simulations and should include measurements
of both majority and impurity ion particle transport as well as plasma fluctuations. a diagnostic
for measuring electric potential fluctuations in the core of the plasma, such as a heavy ion beam
probe, is especially desirable. Use of current drive to remove the effect of the Ware pinch (an inward
drift of toroidally confined trapped particles) would make these studies cleaner.
core fueling efficiency in iteR by gas puffing and pellets is critically dependent on both “normal”
particle transport and transport of the pellet-sourced ions; in fact, pellet modification of the plasma
profiles may affect the particle transport. experiments need to be done to determine the effect
of pellet injection angle and launch location and of the particle pinch on the core fueling efficiency.
This includes experiments on high-field-side injection, simulations with realistic particle
pinch, and loss due to pellet-induced edge localized modes. it is highly desirable to find some way
to explore post-pellet transport in low-collisionality plasmas because the observed and predicted
pinch is absent at high collisionality; perhaps frequent, very small pellets would be appropriate.
much of this work is already included on the lists of high-priority research for the international
tokamak Physics activity (itPa).
Pedestal Structure: Projections for iteR from models of turbulence-driven transport have
shown that the achievable confinement is sensitive to the assumed temperature just inside the
edge transport barrier in h-mode plasmas (a region known as the pedestal). Recent modeling has
been successful at reproducing the height of the h-mode pressure pedestal by combining theoretical
predictions of stability limits with an empirical scaling for the width of the pedestal region.
however, further tests are required. in addition, many other pedestal structure models are now
emerging, which should be tested against experimental data from a range of devices. in particular,
models are needed that dynamically capture the evolution of the pedestal when 3-d fields are
applied or when transients occur. once validated, these models should be coupled with models for
heat and momentum transport in the core region to produce a comprehensive, predictive confinement
model.
other outstanding issues include the effects on the quality of the h-mode pedestal from (1) helium
or hydrogen operation, (2) the near-unity ratio of input power to h-mode threshold power,
(3) the relatively small separation between the primary and secondary separatrices, and (4) high
opacity to edge fueling.
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