Inductive scenarios are inherently pulsed, as they are sustained by ramping a central solenoid, inducing a large plasma current. The International Thermonuclear Experimental Reactor (ITER) aims to achieve its mission of producing a fusion reaction efficiency of ten running an inductive scenario, the Iter Baseline Scenario. Dr. Turco explores the coupling of MHD stability and performance to external magnetic fields, plasma shape variation, plasma heating, and current drive schemes, as well as impurity control.
ITER represents a major step toward realizing fusion energy's potential. Though ITER will be a research machine, tokamaks with similar parameter space access and size are candidates for fusion power generation. Time will tell whether these power plants operate inductively or noninductively.
Non-inductive, steady-state scenarios rely on operation at high normalized pressure and utilize less plasma current than inductive scenarios. The plasma current in these non-inductive scenarios is generated by neutral beam injection, electron cyclotron systems, and the self-generated 'bootstrap' current. Dr. Turco's efforts are concentrated on the high βN hybrid scenario, a favorable candidate for high gain reactor operation regularly achieved on the DIII-D tokamak. Other candidate advanced tokamak scenarios include high internal inductance, high βp, high qmin, and super-H mode.
The High βN Hybrid Scenario is remarkably stable and permits very efficient current drive. Current drive efficiency is high enough that hybrid plasmas have been sustained fully noninductively, meaning the central solenoid was turned off during operation, and the plasma was sustained for multiple current relaxation times by noninductive means. This marks recent progress in realizing some of the first reactor relevant fully noninductive plasmas, a necessary proof of concept for steady state reactor scenarios.
Electron cyclotron current drive (ECCD), the most proven reactor compatible means of current drive, is most efficient injected at the very center of a plasma's magnetic axis. Noninductive scenarios often feature broad current density profiles, requiring off-axis and therefore inefficient current deposition. Hybrid plasmas feature a benign core MHD mode that redistributes core plasma current to the midradius, even if the majority of current is driven on axis. This effect coupled with the bootstrap current delivered by a high pressure pedestal accomplishes efficient off-axis current drive. Broadness in current profiles is favorable to steady state operation as this prevents the magnetic field from twisting up too much, which can trigger damaging disruptions.
Due to its self-organization, the hybrid is also resilient to temporary lapses in noninductive current drive, a foreseeable issue in a steady state reactor expected to run for extended periods of time. The central solenoid could be used to start up the plasma, turned off, then reactivated temporarily to sustain or even reenter the hybrid regime. This quality is not common among steady state scenarios, as access to each requires tailored conditions.
Present research includes quantitatively relating Ideal and Resistive MHD stability limits and raising these limits by tuning kinetic profiles. Operation at higher pressure raises the bootstrap current fraction, reducing the recycled power required to drive current, thus increasing fusion gain.
"Negative triangularity" refers to a shape parameter of tokamak plasmas, designating a shape distinct from those preferred in ITER or Hybrid scenarios. Most cutting edge plasmas feature a "D" shaped cross section, as this shape generally enhances performance metrics. Negative triangularity scenarios invert this "D" to face inward toward the central solenoid. This improves confinement and allows tokamaks to operate at high power in the easier to control, lower confinement, 'L-mode' regime. Professor Carlos Paz-Soldan has lead the work on negative triangularity physics at the DIII-D Tokamak. Negative triangularity research has recently enjoyed more attention from the international plasma physics community, and exploratory work at Columbia in this interesting regime is underway.