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

will dominate the external sources of heating and current drive, and the plasma will self-organize
to a state consistent with its underlying physics. The power from alpha particles (P alpha ), in
the demo regime, will be larger than the externally injected power (P input ) by a factor of 4-9, i.e.,
the ratio Q=P fus /P input = 20-45. in addition, the ratio of plasma self-driven current (i bootstrap ) to
the total plasma current (i plasma ) is expected to reach values of 0.65-0.90, or perhaps even higher.
although we have concentrated on the plasma transport and current drive couplings, shown schematically
in Figure 3, it is recognized that there are also couplings in the mhd area, among fast
alpha particles, global instabilities, and the pedestal.
transformer source
of poloidal flux
V loop
J ohm
J cd
J bs
magnetic flux
diffusion
B pol
bootstrap
current
conductivity
profile
auxiliary
current drive
α-particle
heating
p, T, n, V
profiles
Figure 3: Some of the many feedback loops important in plasma scenarios with strong self-heating and selfdriven
currents. From P. Politzer et al, Nucl. Fusion 45 (2005) 417-424.
This regime will generate dominant heating of the electron channel, where our understanding is
limited, as opposed to the ion channel, where experimental evidence and theoretical understanding
have made significant progress. operation at high density, near the empirical Greenwald density
limit, which is traditionally avoided, will become the norm as the requirement to generate fusion
power forces the plasma density higher than in deuterium (d-d) experiments. There is limited
understanding of the processes determining this empirical limit. The impact of driven and
intrinsic rotation on transport and stability, which is only now being explored in detail on existing
tokamaks, is difficult to project to future devices, particularly with a dominant alpha heating
source. it is recognized that the plasma rotation present on existing tokamaks has a fundamental
82
auxiliary
heating
Σ
heat, particle, &
momentum fluxes Σ
transport coefficients
turbulent & neoclassical
auxiliary angular
momentum
fueling
& pumping
external
internal
wall sources
and sinks