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

shaping and plasma profile/RWm/elm control models, actuators, and diagnostics should be extensively
tested for reliable disruption avoidance in present facilities. The frequency of disruptions
should be quantified as a function of proximity to the key stability limits to assess the potential
trade-off between higher fusion performance and reduced disruption probability. additional
design and engineering activities are needed to determine the allowable frequency and
magnitude of disruptions for various st applications based on allowable damage limits to internal
components from electromagnetic and thermal loads and high-energy electrons. it is noted
that 10 6 seconds of continuous plasma operation is the long-term objective for the component
testing mission of a ctF, and thus ctF will likely require disruption occurrence rates of less than
10 -6 s -1 (a demo reactor would likely require one to two orders of magnitude lower disruption frequency).
to make substantial progress toward the disruption avoidance requirements of future
fusion devices, st pulse durations should be extended by one to three orders of magnitude beyond
present capabilities. This implies that the present 1s plasma duration should be extended to
10 1 -10 3 s, and ~10 3 s/year of accumulated operating time should be extended to 10 4 -10 6 s/year with
a substantial fraction of that time achieved free of major disruptions.
eneRGetic PaRticle instabilities
The ReneW st Panel extends the taP knowledge gap statement for this area to be “impact of energetic
particle instabilities on neutral beam current drive, heating, electron transport, and alpha
channeling in the st” to include important energetic particle (eP) instability effects on electron
transport, as well as so-called alpha particle channeling issues.
research requirements
a unique feature of sts is that energetic particles created in the plasma by external heating such
as nbi and icRh, or by fusion alpha particles in an st reactor, have velocities far greater than
the velocity of transverse magnetic waves in the plasma (super-alfvénic particles). This produces
a rich variety of eP instabilities. high velocity (fast) ions can effectively interact with various collective
phenomena in the plasma, notably low frequency (alfvén) modes capable of inducing energetic
particle radial transport. instabilities with high frequencies (below, or near the ion cyclotron
frequency) can change particle energy and be used to transfer the energetic particle energy directly
to plasma thermal ions, thereby avoiding heating of plasma electrons. high-frequency modes
can resonantly interact with plasma thermal electrons, which is a newly discovered phenomenon
that can pose a great challenge for st design.
Energetic particle anomalous transport effects on heating and current drive: experiments
in recent years on conventional tokamaks and sts demonstrated large losses of fast ions
due to the alfvénic instabilities — in some discharges up to 40% of the heating nbi ions, affecting
heating efficiency and the current drive. These losses are poorly understood theoretically. The
following steps are needed to close the knowledge gap:
• develop numerical and theoretical models for eP transport predictions in the presence of
multiple instabilities expected in burning plasmas and iteR: This should include linear
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