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

Disruption avoidance
most disruptions are, in principle, avoidable. many causes of disruptions are well understood, including
loss of axisymmetric stability (vde), radiative collapse (density limit), and ideal and resistive
mhd instabilities (beta limit, current limit, locked modes). Their onset can be predicted on
an empirical or a theoretical basis. by avoiding such conditions, present tokamaks routinely operate
at pulse durations of tens of seconds without disruptions. avoidance of disruptions in iteR
will require reliable maintenance of the desired plasma configuration, detection of stability limits,
corrective action when approaching stability limits, and active suppression of instabilities. The
associated research needs include:
• diagnostics and actuators for control of the pressure and current profile, particularly in
conditions in which these profiles are largely determined by internal processes (e.g., self
heating, large bootstrap current).
• diagnostics to detect stability limits (see disruption prediction in Figure 11).
• actuators for direct control of instabilities, including localized current drive, localized
rotation drive, and non-axisymmetric magnetic coils.
• “intelligent” control algorithms that can stably maintain the plasma state away from stability
limits and recover from changes in the plasma state caused by non-disruptive instabilities and
other unplanned occurrences. if an instability and subsequent disruption cannot be avoided,
the control system must be able to attempt a controlled “soft shutdown” of the discharge.
much of the development of diagnostics, actuators, and control can be carried out in existing shortpulse
facilities. it will be important for the new generation of superconducting tokamaks (east,
kstaR, sst-1, and Jt-60sa) to demonstrate that “disruption-free” long-pulse or steady-state discharges
with high performance, as needed for iteR and demo, can be realized. however, a full
test of plasma control and disruption avoidance in a self-heated plasma situation will only become
possible in iteR. disruption avoidance with high reliability must be developed and demonstrated
in iteR before proceeding to demo, where the requirements are even greater than for iteR.
Figure 11. ITER will provide an important test of the ability to reduce the frequency of allowable disruptions
(“disruptivity”) in high-performance discharges, compared to that in current facilities. SSTR is a conceptual design
for a steady-state tokamak reactor. (Figure reproduced from J. Wesley, “Disruptions: A Personal View,” Research
Needs Workshop white paper #18, http://burningplasma.org/web/renew_whitepapers_theme1.html.)
53