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

Develop the means for and demonstrate robust active stabilization of instabilities and fluctuations:
How close to or how far beyond stability limits can ATs operate robustly with maximum efficiency
and negligible probability of control loss using active control?
While the requirement for active control of instabilities to enable increased fusion performance
beyond the passive limits assumed in conventional scenarios greatly increases complexity, the potential
performance gains can be large. a precedent exists in modern aerospace designs, which
have similarly evolved from passively stable early aircraft with limited maneuverability to modern
high-performance fighters that operate in an extended but unstable dynamic range, maintained
by a complex control system.
Specific Challenges:
operation in passively unstable regions requires a robust control system; loss of control can quickly
have serious consequences. While this is already routine in the case of axisymmetric mhd stability,
with the passive plasma elongation limits exceeded in most tokamaks but maintained by
an active feedback system, operation beyond the passive beta limits is still an ongoing research
area. Reliable detectors and actuators that can survive in a burning plasma are needed for the major
beta limiting instabilities: resistive wall modes (RWms), neoclassical tearing modes (ntms),
edge localized modes (elms), core “sawteeth,” and fast ion driven alfvén eigenmodes (aes). The
key actuators are envisaged to be radiofrequency waves and active non-axisymmetric field coils.
The coils and especially the magnetic sensors need to be close to the plasma, and thus must be
protected from the hostile, high-fluence nuclear environment. Radiofrequency systems for locally
maintaining stability require precise localization. For ech, tracking systems and mirrors are required
and need to be shielded. While the at scenarios envisaged generally require suppression
of RWms, ntms, and aes, sawteeth and edge instabilities may play a positive role in eliminating
impurities, including alpha ash, and regulating the profiles in the core and edge respectively. This
must be balanced with effects on core performance and power handling. Thrust 2 will develop solutions
for elms on iteR, but extrapolation to reactor conditions will require further research.
sawteeth and elms also have strong nonlinear couplings with RWms, ntms and aes, so their
regulation must be integrated into the full active feedback control system.
Research Plan:
Short-term: Continue development and optimization of radiofrequency actuators and sensors for
detecting early onset and control of fluctuations resulting from instabilities in existing experiments
and initiate efforts into new methods that can scale to a reactor. For example, develop high-power highefficiency
gyrotrons and remote steering required for MHD stabilization, and test other methods for
MHD stability control.
Medium-term: Test and optimize integrated control system options for controlling all the key instabilities
in D-D plasmas for long pulses, progressively raising beta above the passive limits. This should be done
in collaboration with new Asian tokamaks where appropriate.
Long-term: Test integrated control system options for controlling all the key instabilities in D-T plasmas
for long pulses above the passive beta limits in ITER. Extend to alpha-dominated D-T plasmas above the
passive beta limits. Tests in long pulses and high-fluence environment would be ideal.
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