Progress in extending high poloidal beta scenarios on DIII-D towards a steady-state fusion reactor and impact of energetic particles
J. Huang, A. M. Garofalo, J.P. Qian, Xianzu Gong, S. Ding, J. Varela, Jiale Chen, Wenfeng Guo, K Li, Muquan Wu, Chengkang Pan, Q. Ren, Bin Zhang, L. L. Lao, C. T. Holcomb, J. McClenaghan, David B. Weisberg, V. S. Chan, A.W. Hyatt, Wenhui Hu, G.Q. Li, J.R. Ferron, G. R. McKee, R. I. Pinsker, T. L. Rhodes, G. M. Staebler, D. A. Spong, Z. Yan
Abstract
To prepare for steady-state operation of future fusion reactors (e.g. the International Thermonuclear Experimental Reactor and China Fusion Engineering Test Reactor (CFETR)), experiments on DIII-D have extended the high poloidal beta (<em>β</em><SUB>P</SUB>) scenario to reactor-relevant edge safety factor <em>q</em><SUB>95</SUB> ~ 6.0, while maintaining a large-radius internal transport barrier (ITB) using negative magnetic shear. Excellent energy confinement quality (<em>H</em><SUB>98y2</SUB> > 1.5) is sustained at high normalized beta (<em>β</em><SUB>N</SUB> ~ 3.5). This high-performance ITB state with Greenwald density fraction near 100% and <em>q</em><SUB>min</SUB> ≥ 3 is achieved with toroidal plasma rotation <em>V</em><SUB>tor</SUB> ~ 0 at <em>ρ</em> ≥ 0.6. This is a key result for reactors expected to have low <em>V</em><SUB>tor</SUB>. At high <em>β</em><SUB>P</SUB> (≥1.9), large Shafranov shift can stabilize turbulence leading to a high confinement state with a low pedestal and an ITB. At lower <em>β</em><SUB>P</SUB> (<1.9), negative magnetic shear in the plasma core contributes to turbulence suppression and can compensate for reduced Shafranov shift to continue to access a large-radius ITB and excellent confinement with low <em>V</em><SUB>tor</SUB>, consistent with the results of gyrofluid transport simulations. These high-<em>β</em><SUB>P</SUB> cases are characterized by weak/no Alfvén eigenmodes (a.e.) and classical fast-ion transport. At high density, the fast-ion deceleration time decreases and Δ<em>β</em><SUB>fast</SUB> is lower; these reduce a.e. drive. The reverse-shear Alfvén eigenmodes are weaker or stable because the negative magnetic shear region is located at higher radius, away from the peaked fast-ion profile. Resistive wall modes can be a limitation at simultaneous high <em>β</em><SUB>N</SUB>, low internal inductance, and low rotation. Analysis suggests that additional off-axis external current drive could provide a more stable path at reduced <em>q</em><SUB>95</SUB>. Based on a DIII-D high-<em>β</em><SUB>P</SUB> plasma with large-radius ITB, two scenarios are proposed for CFETR <em>Q</em> = 5 steady-state operation with ~1 GW fusion power: a lower-$l_i$($l_i$ ~ 0.66) and a higher-$l_i$($l_i$ ~ 0.75) case. Using a Landau closure model, multiple energetic particle (EP) effects on the a.e. stability are analyzed modifying the growth rate of the a.e.s triggered by the neutral-beam-injection EPs and alpha particles, although the stabilizing/destabilizing effect is weak for the cases analyzed. The stabilizing effects of the combined EP species <em>β</em>, energy, and density profile in CFETR need further investigation.