Editing Plasma stability (section)

=== Internal Structure ===
Control of the internal structure of the plasma allows more active avoidance of MHD instabilities. Maintaining the proper [[current density]] profile, for example, can help to maintain stability to tearing modes. Open-loop optimization of the pressure and current density profiles with external heating and current drive sources is routinely used in many devices. Improved diagnostic measurements along with localized heating and current drive sources, now becoming available, will allow active feedback control of the internal profiles in the near future.
Such work is beginning or planned in most of the large tokamaks ([[Joint European Torus|JET]], [[JT-60|JT&ndash;60U]], [[DIII-D (tokamak)|DIII&ndash;D]], [[Alcator C-Mod|C&ndash;Mod]], and [[ASDEX Upgrade|ASDEX&ndash;U]]) using [[radio frequency|RF]] heating and current drive. Real-time analysis of profile data such as MSE current profile measurements and real-time identification of stability boundaries are essential components of profile control. Strong plasma rotation can stabilize resistive wall modes, as demonstrated in tokamak experiments, and rotational shear is also predicted to stabilize resistive modes. Opportunities to test these predictions are provided by configurations such as the ST, spheromak, and FRC, which have a large natural diamagnetic rotation, as well as tokamaks with rotation driven by neutral beam injection. The [[Electric Tokamak]] experiment is intended to have a very large driven rotation, approaching [[Alfvén wave|Alfvénic]] regimes where ideal stability may also be influenced. Maintaining sufficient plasma rotation, and the possible role of the RWM in damping the rotation, are important issues that can be investigated in these experiments.