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

• develop measurement techniques and actuators compatible with the requirements for a
tokamak or st demo.
Disruptions in stellarators
disruptions in stellarators have only been observed during fast current ramps in discharges with
ohmically driven current. With the advent of high-power radiofrequency and neutral beam injection
(nbi) plasma heating systems, most stellarators have operated with little to no ohmic current
during the last several decades. in early experiments in low-shear stellarators, disruptions
did not occur if sufficient vacuum rotational transform was applied (i ≥ 0.14). as in tokamaks, disruptions
are avoided by keeping q > 2 at the boundary, or avoiding fast current ramps. in stellarators,
they are also passively avoided by limiting the plasma current and proper design of the vacuum
rotational profile. Therefore, rotational transform profile control, which is used to improve
confinement in stellarators, is likely to be a desirable element of any research thrust targeted to
avoiding disruptions in “hybrid” and finite-bootstrap fraction stellarators.
at present, there is no observation of disruptions in stellarators with some of the rotational transform
provided by bootstrap current, even at high b. Further research is needed to:
• determine the pressure limits in stellarators under fusion conditions.
• extend stellarator research to higher performance plasmas exploring integrated scenarios
to ensure that finite-bootstrap current stellarators continue to passively avoid disruptions.
• determine the level of vacuum transform, or 3-d shaping, necessary to avoid density
and pressure-driven disruptions in tokamaks to understand the boundaries of robust
disruption-free operation of toroidal plasma devices.
Stellarator discharge termination by radiative collapse
abnormal losses of stellarator discharges can take place through radiative collapse due to rapid cooling
of the plasma. such a collapse typically occurs if the density exceeds an operating limit, substantial
impurity accumulation takes place, or a sufficiently large metallic flake is suddenly injected into
the plasma from the divertor during long-pulse operation. The electron temperature typically collapses
on a time scale commensurate with the confinement time. in some instances, the enhanced
radiation leads to a termination of the discharge while in others, discharges are observed to partially
or fully recover. in tokamak discharges a radiative collapse, for example due to the injection of impurities,
typically triggers a disruption and sometimes generates runaway electrons.
The research requirements in this area are to:
• model and compare with experiment the consequences of a thermal quench of a fusion
plasma discharge via radiative collapse.
• in concert with Themes 3 and 5, understand and improve steady-state divertor power
handling to avoid the uncontrolled injection of microscopic flakes, and develop methods
to counter unpredictable radiative collapses after their onset.
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