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

stand the relative roles of long-to-intermediate wavelength versus short wavelength turbulence
in causing electron transport. Given the very high plasma energy confinement potentially needed
for the st-ctF application, very low recycling regimes should be tested on present and near-term
experiments to assess predictions of improved confinement at the reduced toroidal magnetic field
and high-b of the st.
increased nbi current drive efficiency and control is needed to test fully noninductive operation
of an st as needed for an st-ctF. Reduced collisionality, increased nbi heating power, and injection
geometries more favorable for nbi current drive and current profile control are needed
to assess the predicted nbi current drive capabilities. These additional capabilities are especially
important for testing ramp-up of the plasma current that is assumed to be achieved using nbi
heating and current drive in an st-ctF. st operating scenarios could have a substantial population
of super-alfvénic ions and associated energetic particle instabilities. since these instabilities
can result in fast ion transport — which is not presently well understood — improved diagnosis
and theory and computation of fast ion redistribution by alfvénic activity is needed for reliably
projecting the performance of future sts. sufficient plasma duration (~10 1 to 10 2 s) is required to
achieve full plasma current profile relaxation following plasma ramp-up and during the sustainment
phase, and to demonstrate profile controllability.
exhaust solutions for high-particle flux and ctF-relevant high-heat flux must also be developed
and assessed for compatibility with a sustained, fully noninductive, integrated high-performance
core plasma. such solutions might include liquid metal plasma facing components and/or high
flux expansion and increased scrape-off layer magnetic field line-length as embodied in the X and
super-X divertor configurations. The above core and edge plasma integration must ultimately be
achieved using the first-wall materials and operating conditions expected in an st-ctF and/or
reactor to ensure that acceptably low tritium retention can be achieved and that the first wall can
tolerate infrequent off-normal events such as elms and disruptions.
Finally, new computational and theoretical capabilities supporting comprehensive time-dependent
integrated modeling of the plasma evolution, using reduced models for core and edge transport
and stability, are needed to develop and understand st integrated scenarios, and to reliably
project to the performance needed for an st-ctF and beyond.
St available Means for Research and actions to address Research needs
Present Us and large international st research facilities include (in order of decreasing plasma
current capability) the nstX (1 ma), PeGasUs, and ltX in the Us, mast (1 ma) in the U.k., and
QUest in Japan. many smaller devices are operating internationally (see Fesac taP report for
greater detail). Proposed upgrades for these devices that have gone through initial review include
nstX-U and mast-U (both 2 ma). These upgrades are critical to pursuing the iteR-era st goal,
but they and further upgrades may not be sufficient to fulfill this goal. defining additional steps
to resolve the st physics issues described above and advance to an st-ctF is evolving through
the ReneW process and beyond. The fusion nuclear science and technology actions for a ctF are
addressed in Thrust 13.
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