Optimize Access Approaches to fNI=1, bN>4, Elevated-qmin Plasma With Off-Axis CD

2023 Research Campaign, Thrust: High qmin & Steady State Scenarios

Purpose of Experiment

Various net-electric, steady-state tokamak fusion pilot plants designs have been published in the literature. These typically rely on high normalized beta (betaN) operation with betaN=3.5-4.5 and safety factor profile q with min(q)>2 and edge q (q95)=5-6.5. This combination of parameters is projected to achieve high enough fusion power and bootstrap current fraction to sustain the plasma current non-inductively. This DIII-D experiment seeks to identify paths and options for achieving such operation, with a focus on control of plasma stability. The experiment has three main components described next.

Experimental Approach

The experiment will restore previous DIII-D plasmas with qmin>2, betaN~3, and q95=6. It will then add additional electron cyclotron current drive, including using a relatively new injection geometry called top launch, and explore limits to sustained high betaN>3. The operating space will be explored by scanning the sustained betaN and q95, an by exploring changes to plasma shaping, fueling, and finally, changes to the pressure-ramp-up phase. The later includes testing options for getting to high betaN earlier in the discharge with more efficient heating and current drive. Interspersed with many of these steps will be special plasma shots taken specifically to compare the current drive efficiency of electron cyclotron current drive using the traditional launch angle and the new top launch angle. Also interspersed with changes to the plasma scenario are dedicated discharges to apply multi-mode MHD spectroscopy. This technique uses small 3D magnetic field coils to perturb the plasma – the measured magnetic plasma response is analyzed to infer growth rates of the least stable plasma kink or tearing modes, thus providing valuable information to further optimize these high betaN steady-state scenarios.

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).