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

Research should focus on developing a physical basis for extrapolating h-mode power thresholds
reliably and accurately and should develop strategies for minimizing the power requirements. examples
of the latter include pellet injection, plasma shape modification, inboard gas puffing, and
varying the plasma current ramp-rate. Progress will require improvements in edge diagnostics,
particularly those that measure profiles of relevant quantities such as density, temperature, and
flows. characterization of edge fluctuations leading up to l-h transitions is also important. ample
heating power should be available in multiple forms (neutral beams, radiofrequency waves,
etc.). efforts should be made to resolve differences in power threshold results that may arise
among different heating schemes.
H-mode sustainment. Plasma conditions in iteR — dominant electron heating, electron-ion
temperature equilibration, low neoclassical collisionality and low torque injection — will differ
from those in most present-day tokamaks. The issue is: What is the physical basis for extrapolating
H-mode confinement to this reactor-relevant condition? The definition of reactor-relevant conditions
should include those processes necessary to reduce the heat flux on plasma facing surfaces, such
as the suppression of edge localized modes and radiative divertor operation.
since plasmas are complex physical systems, determining the scaling of transport phenomena
with dimensionless parameters is a valuable tool. a transport extrapolation to smaller relative gyroradius
(r*), and perhaps smaller collisionality (n*), while keeping the other dimensionless variables
fixed is the preferred scaling path. a high priority for future transport experiments is to use
dominant radiofrequency heating to better simulate the burning plasma regime with strong electron
heating, equal ion and electron temperatures, and low torque injection.
Rotation and velocity shear have a beneficial effect on confinement and play a role in the l-h
transition and the stability of various mhd modes. While extrapolation of intrinsic rotation from
an inter-machine database appears favorable, it is important to determine the origin of spontaneous
rotation and validate its size scaling. Recent observations of flow drive from mode conversion
of radiofrequency waves look promising, and electron cyclotron waves and lower hybrid waves
should be pursued as potential profile control tools. experiments also need to validate models of
the effects of resonant and nonresonant drag from non-axisymmetric magnetic fields. The counter-current
offset to rotation from neoclassical toroidal viscosity could potentially cancel out externally
driven rotation in the co-current direction and needs to be documented for extrapolatation
to future devices.
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Figure 1. Example of transition (dashed line)
from low to high-confinement mode on NSTX:
(a) plasma current, (b) neutral beam injection
power, (c) edge D a recycling light with ELMs indicated
by spikes, and (d) energy confinement
time. (Figure courtesy of Stan Kaye.)