Global gyrokinetic simulations of kinetic ballooning mode in NSTX-U plasmas

Abstract Understanding the kinetic ballooning mode (KBM) is crucial for optimizing plasma performance in high- β spherical tokamaks. Global gyrokinetic simulations of KBMs in a projected NSTX-U shot are presented using realistic magnetic geometry and plasma profiles. Linear simulations across varying plasma β values show that the KBM is unstable in NSTX-U at significantly higher plasma β than conventional tokamaks due to the plasma shaping effects. Isotope effects on KBM are more pronounced at higher plasma β but remain relatively weak near the KBM stability threshold. Without flow shear, KBMs are unstable in both the core and pedestal regions. In the core, the investigated toroidal mode numbers in the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>n</mml:mi> <mml:mo>=</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mn>15</mml:mn> <mml:mo>,</mml:mo> <mml:mo>…</mml:mo> <mml:mo>,</mml:mo> <mml:mn>31</mml:mn> <mml:mo stretchy="false">]</mml:mo> </mml:mrow> </mml:math> are observed to be unstable. In the pedestal, on the other hand, the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>n</mml:mi> <mml:mstyle scriptlevel="0"/> <mml:mo>=</mml:mo> <mml:mn>15</mml:mn> </mml:mrow> </mml:math> mode is the most unstable mode due to local β values lower than those in the core. Parameter scans of plasma profile gradients and plasma β demonstrate that the linear KBM in the core exhibits a high sensitivity to β , whereas KBM in the pedestal shows a lower sensitivity. A reduction of β by approximately 15% from the projected value stabilizes KBMs in the core. Nonlinear simulations reveal that self-generated zonal flows play a crucial role in regulating KBM-driven turbulence, reducing the size of turbulent eddies, shortening the radial correlation length by nearly threefold, and decreasing turbulent transport by approximately 35%. These findings provide valuable insights into KBM stability and turbulent transport, offering guidance for optimizing operational scenarios in future experiments.