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

cal maximum operating frequency for high-power transmitters is presently ~ 120-140 mhz, corresponding
to the fourth harmonic of the deuterium cyclotron frequency at a magnetic field of 2
telsa. optimal wave coupler design needs to be determined. excitation of surface waves and radiofrequency
sheath effects must be evaluated during the plasma initiation period.
non-central current drive (including ebW, ech/eccd, and mch/mccd) have many possible
uses to control the current profile, but no such technique has been demonstrated in an st. steering
of eccd may not be so straightforward in sts with high magnetic field. it is not presently
known how to steer ebW, and mch/mccd is not steerable. coupler design is an issue for all
techniques. For mch/mccd, couplers in the icRF range are large, and must be designed to tolerate
large neutron fluxes when fusion reaction rates are high. The icRF coupler design for iteR
provides a starting point. For ebW, it is not presently clear that couplers can be effectively designed
to function over a wide enough range of plasma edge density gradient and magnetic field
line angle to be efficient throughout the plasma initiation and ramp-up period. efficient coupling
during constant plasma conditions is probably closer to realization. st coupler design for ech/
eccd can be heavily leveraged using conventional tokamak research. simulations have indicated
that ebWcd (ohkawa current drive) may be quite efficient in the plasma edge region. lhcd is a
candidate edge current drive technique. however, lhcd has never been seriously investigated in
the st. modeling is needed at magnetic fields representative of an st-ctF or st-demo. other research
needs include modeling and higher-field st experiments to validate theory.
ST AREA 2: PLASMA-MATERIAL INTERFACE
While the majority of plasma-material interface issues related to plasma-wall interactions, plasma
facing components, and internal device components are common to all magnetic configurations,
the taP report identified several st-specific knowledge gaps. The main goal for the st research
is the development of normal and off-normal heat and particle flux handling strategies
compatible with plasma core and extreme edge plasma operating conditions, especially long plasma
duration divertor and first wall power handling, particle control at low normalized plasma
densities, and integration of these with high normalized plasma pressure and high plasma confinement
operation. at present, two experimental facilities with graphite tiles facing the plasma
(nstX and mast) contribute to the experimental understanding of these issues.
research requirements
1) divertor and first wall power handling techniques: These need to be developed to reduce steadystate
peak steady-state heat fluxes from the projected 20-60 mW/m2 to 10 mW/m2 or lower, and
transient loads to 0.5 mJ/m2 or lower in future st-based devices with input power of tens of
megawatts. This goal includes experimental and theoretical understanding of extreme edge plasma
(scrape-off layer — sol) and divertor plasma electron and ion heat transport to develop predictive
modeling and scalings for peak flux and scrape-off layer flux width for projections to future
devices. The roles of conduction, convective (turbulent) transport, radiation transport, particle
flow to the divertor and first wall at low plasma collisionality need to be clarified. at low sol
density, kinetic effects will play a larger role in heat transport. new numerical analysis with gyro-fluid
and gyrokinetic transport and turbulence models will be required. to help validate the
models, new diagnostic measurements to elucidate the edge electron and ion energy distribution
189