The dominant micro-turbulence instabilities in the lower q ₉₅ high β _p plasmas on DIII-D and predict-first extrapolation

Abstract

Large-radius internal transport barriers (ITB) are the signature of high β p scenarios on DIII-D. Previous studies show that a large Shafranov shift, rather the E × B shear, suppresses the turbulence and helps in the formation of the large-radius ITB. New gyrokinetic simulations suggest that the remaining micro-instabilities in lower q 95 (<7.0), high β p ITB plasmas are drift wave instabilities, including the collisionless trapped electron mode in the core and ITB peak gradient region, the electron temperature gradient mode in the ITB peak gradient region and at the ITB foot and the ion temperature gradient mode at the ITB foot. Gyrokinetic simulation results qualitatively agree with the density fluctuation analysis from beam emission spectroscopy, which suggests the existence of a low-k ion mode at the ITB foot. The gyrokinetic simulations also predict that a larger Shafranov shift can overwhelm the driving sources for turbulence from the profile gradients at higher β N , leading to stronger turbulence suppression and stronger ITBs in lower q 95 , high β p plasmas.

DIII-D research advancing the physics basis for optimizing the tokamak approach to fusion energy

“…In separate high β p experiments, large radius ITBs were also obtained with strong deuterium gas injection [52,53]. The high β p configuration lends itself to ITB formation due to a combination of Shafranov shift stabilization of turbulence, high bootstrap current generation at high q 95 and high q min at large radius [54]. Impurity injection and radiation at the plasma edge enhances the redistribution of bootstrap current from near the very edge to a large radius (ρ = 0.7-0.8) location in the core as the pedestal pressure and its gradient are reduced (figure 12(d)).…”

“…Integration of a high-performance core plasma and a low temperature solution for the plasma at the divertor targets has been demonstrated in a high poloidal beta scenario that features large Shafranov shift, internal transport barriers (ITBs) in n e , T e and T i coupled to a detached divertor using active feedback-controlled N 2 or Ne puffing [50][51][52][53][54]. Theory-based modeling suggests that similar plasmas in ITER FPO phase with planned heating systems could be consistent with Q = 10 at reduced plasma current of 7-9 MA [51].…”

Linear simulation of magnetohydrodynamic plasma response to three-dimensional magnetic perturbations in high-β P plasmas

“…A detailed description of this physics module can be found in reference [56]. In this work, the SEGWAY module was particularly useful for theoretically extending the high-β P scenario with a large-radius ITB to edge safety factor values significantly below the q 95 ∼ 6 that has been realized in DIII-D [19,26], which may be of particular interest to the next-step fusion reactors but very challenging in the DIII-D experiment. Specifically, the DIII-D high-β P regime at q 95 ∼ 6 is normally operated very close to the ideal-wall β N limit [13], and β P falls marginally to the summarized β P threshold around 1.9 [16] set by a Shafranov shift essential for turbulence suppression and large-radius ITB formation.…”

“…In this respect, building on earlier research at JT-60U [4,10,11] and DIII-D [12], extensive efforts [13][14][15][16][17][18][19][20][21][22][23][24][25][26] to develop a fully-noninductive high-β P (poloidal beta) scenario have been made by a joint team of researchers from the DIII-D (USA) and EAST (China) tokamaks. In support of future steady-state operations of ITER and the China Fusion Engineering Test Reactor [27], this program aims to develop and to test a possible demonstration scenario for steady-state operation on EAST, compatible with EAST-relevant discharge configurations (divertor shape, elongation, triangularity, etc) and hardware constraints (maximal ramp-up rate of plasma current restricted by superconductive coils, available NBI torque, etc), and advance the physical basis of the high-β P regime by exploiting the operation flexibility and excellent diagnostics of DIII-D.…”

“…In addition, an improved understanding of the physics underlying the formation of the large-radius internal transport barrier (ITB) in electron density n e , electron temperature T e , ion temperature T i , and toroidal rotation v t has been developed. This signature feature of the high-β P scenario in DIII-D is normally located at a large normalized minor radius ρ ∼ 0.7 (ρ is the square root of the normalized toroidal flux) and is associated with high confinement and bootstrap current fraction [20,[23][24][25][26].…”