Progress in extending high poloidal beta scenarios on DIII-D towards a steady-state fusion reactor and impact of energetic particles

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 (beta(P)) scenario to reactor-relevant edge safety factorq(95)similar to 6.0, while maintaining a large-radius internal transport barrier (ITB) using negative magnetic shear. Excellent energy confinement quality (H-98y2> 1.5) is sustained at high normalized beta (beta(N)similar to 3.5). This high-performance ITB state with Greenwald density fraction near 100% andq(min)>= 3 is achieved with toroidal plasma rotationV(tor)similar to 0 at rho >= 0.6. This is a key result for reactors expected to have lowV(tor). At high beta(P)(>= 1.9), large Shafranov shift can stabilize turbulence leading to a high confinement state with a low pedestal and an ITB. At lower beta(P)(<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 lowV(tor), consistent with the results of gyrofluid transport simulations. These high-beta(P)cases are characterized by weak/no Alfven eigenmodes (a.e.) and classical fast-ion transport. At high density, the fast-ion deceleration time decreases and Delta beta(fast)is lower; these reduce a.e. drive. The reverse-shear Alfven 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 beta(N), low internal inductance, and low rotation. Analysis suggests that additional off-axis external current drive could provide a more stable path at reducedq(95). Based on a DIII-D high-beta(P)plasma with large-radius ITB, two scenarios are proposed for CFETRQ= 5 steady-state operation with similar to 1 GW fusion power: a lower-li