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

esearch requirements
installation of neutral beams at larger energies, currents, and time duration are required to study
the effects of significant populations of super-alfvénic particles. The energy content and momentum
of the beams must be comparable to or higher than that of the background plasma. The beam
duration must be longer than both the plasma confinement time and the fast ion confinement
time. Theoretical and numerical tools, such as tRansP, nova, and oRbit, require adaptation
for the RFP configuration.
DEtERMining bEta LiMiting MECHaniSMS
The required value ~20% of poloidal beta (ratio of plasma pressure to poloidal magnetic field pressure)
for an attractive RFP reactor has already been experimentally achieved, but complete theoretical
understanding is lacking. it is therefore not yet possible to project beta limits in a burning
plasma. optimization and control of the plasma pressure profile, possibly as a function of plasma
cross section and aspect ratio, may become important to avoid instability and maintain the high
beta necessary to achieve the iteR-era goals.
research requirements
by upgrading the existing facilities, especially the neutral beam injection system in terms of power
and fueling rate, it is possible that the beta limit could be identified at moderate S for both axisymmetric
and helical RFP equilibrium. however, the plasma parameters (energy confinement
time, fast ion slowing down time, and plasma duration) make fast ion thermalization difficult,
most likely limited to higher density operation provided by pellet injection. dependence of beta
limits on cross section and aspect ratio cannot be studied on the existing facilities, requiring new
facilities to explore geometric optimization.
testing beta limits at the larger S available in an advanced, long-pulse proof-of-principle facility
and a subsequent performance extension facility with full heating and fueling capabilities is essential
in achieving the RFP goal.
comprehensive theoretical analyses are required beyond the standard suydam criteria (critical
pressure gradient), which have been surpassed by experimentally achieved values. extended capability
for computational tools at larger S and with two-fluid physics is also required, in conjunction
with theory, to confirm and quantitatively predict the beta limits.
aCtiVE COntROL OF MHD inStabiLitiES
both resonant and nonresonant mhd instabilities play an important role in the RFP. The control
of these instabilities and their consequences by means of non-axisymmetric coils (both saddle
and flux loops) is an important area of investigation, since this may be essential in any major
next-step RFP device and a future reactor. The t2R and RFX-mod facilities in europe have significant
active control experimental capability used to completely stabilize resistive wall modes. This
establishes the critical physics requirement that RFP plasmas be maintained beyond the time
scale of passive stabilization by an electrically conducting wall. in addition, saddle loops located
at cuts or breaks in the conducting vessels surrounding the plasma have been used to mitigate lo-
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