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

3-d shaping, and beta control to significantly reduce disruption probability and variations
in fusion reactivity.
• demonstrate sustained operation at normalized beta values beyond st-ctF, approaching
demo levels, to reduce performance risk, and rapidly achieve neutron fluence goals.
• develop validated computational tools for the experimental research actions.
Links to other thrusts: Thrust 2 (transient/disruption control), Thrust 5 (continuous,
high-performance operations), Thrust 6 (predictive modeling), Thrust 17 (3-D fields for
stability), Thrust 18 (low field)
5. employ energetic particle beams, plasma waves, particle control, and core fueling techniques to
maintain the current, and control the plasma profiles.
The sustainment of high-performance integrated st scenarios requires full noninductive current
drive and sufficient control of the plasma profiles. Future st applications are expected to rely on
a combination of current drive sources, including the neoclassical bootstrap current, nbi current
drive, and plasma wave current drive (fast-wave, electron bernstein, and/or electron cyclotron).
spherical torus operation at high plasma current (4-10 ma) will require access to a new st parameter
regime of sustained low collisionality, high thermal confinement, and high beta to provide
high current drive efficiency and high bootstrap fraction. This regime must be self-consistently
accessible with the heating and current-drive techniques proposed for st applications — including
during plasma current startup and ramp-up. control of the nbi and wave current drive can in
principle be achieved by varying injection parameters to vary the power deposition profile. however,
the bootstrap current profile is largely determined by the plasma thermal and particle transport.
Thus, new control techniques must be developed to optimize and ultimately control the plasma
pressure profile to control the current profile.
actions:
• access one to two orders of magnitude lower collisionality by increasing plasma temperature
with a two to four-fold increase in magnetic field, plasma current, and heating power.
• implement particle pumping to achieve factor of two to four reduction in normalized
density to support access to very low collisionality values. Pumping techniques for access
to very low recycling regimes should also be investigated.
• assess and optimize the impact of reduced collisionality on core and edge transport — in
particular, the impact of transport on the bootstrap current density profiles.
• test the ability of improved pumping and low recycling, combined with the development
of deep core fueling, to enable modification and control of the core transport and pressure
profile (and therefore the bootstrap current profile) for plasma sustainment and optimal
stability.
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