Observation of Burning Plasma Dynamics in DIII-D

The key power balance building blocks of a burning plasma have been simulated in DIII-D using Xenon and Krypton for Tungsten(W)-like core radiation, NBI and/or ECH for α-heating power (P_α), and independently controlled plasma β_N using NBI. A new algorithm was demonstrated, capable of simulating any desired fraction of P_α, using real-time measurements of T_i and density, a new fusion reactivity model that reproduces the cross-section fractional dependence on T_i at DIII-D levels, and the output to either or both NBI and ECH power. At DIII-D’s ITER Baseline discharges core T_e ~ 2-3 keV, Kr and Xe radiate at equivalent rates to W at the expected ITER and FPP T_e ~ 20 keV. Under these conditions, DIII-D experiments have uncovered non-linear oscillations coupling density, temperature and radiation, and reproduced them with a new coupled model that includes radiation and input power feedback consistent with the experiment. With the addition of the P_α ~ n_e²T_i² typical of fusion reactors, this non-linearity is expected to be enhanced, calling for viable control solutions. The new model reproduces the oscillations and the interplay of the P_α with the background plasma, showing an increase in the oscillations frequency and amplitude proportional to the magnitude of P_α. In the experimental application, both flattop and ramp-down phases were simulated with P_α and ITER-relevant parameters. The interplay between the W-like core radiated power, the simulated P_α, and the β_N feedback shows that P_fus and equivalent Q experiences slow oscillations at fixed P_aux when the response is not controlled. Equivalent values of Q ~ 5-20 were achieved with low P_rad, while higher Kr radiation in nominally Q > 5 conditions becomes uncontrollable due to the sharp reduction of T_i in P_α. These experiments establish a test-bed for simulating fusion burn dynamics and testing burn control techniques needed for long pulse high fusion gain experiments in ITER and FPPs.