Fast Ion Transport Studies in DIII-D High $\beta_N$ Steady-State Scenarios

DIII-D research is identifying paths to optimize energetic particle (EP) transport in high $\beta_N$ steady-state tokamak scenarios. Operation with $q_{min} >2$ is predicted to achieve high $\beta_N$, confinement, and bootstrap fraction. However DIII-D experiments have shown that Alfv\'en eigenmodes (AE) and correlated EP transport can limit the performance of some $q_{min} >2$ plasmas. Enhanced EP transport occurs in plasmas with $q_{min}=\,$2-2.5, $q_{95}=\,$5-7, and relatively long slowing down time. Strong AEs are present, the confinement factor $H_{89}=\,$1.6-1.8 and $\beta_N$ is limited to $\sim$3 by the available power. These observations are consistent with EP transport models having a critical gradient in $\beta_f$. However, adjusting the parameters can recover classical EP confinement or improve thermal confinement so that $H_{89}>2$. One example is a scenario with $\beta_P$ and $\beta_N \approx 3.2$, $q_{min} >3$ and $q_{95}\approx 11$ developed to test control of long pulse, high heat flux operation on devices like EAST. This has an internal transport barrier at $\rho\approx 0.7$, bootstrap fraction $>$75\%, density limit fraction $\approx$1, and $H_{89}\ge2$. In these cases AE activity and EP transport is very dynamic - it varies between classical and anomalous from shot to shot and within shots. Thus these plasmas are close to a threshold for enhanced EP transport. This may be governed by a combination of a relatively low $\nabla\beta_{fast}$ due to good thermal confinement and lower beam power, short slowing down time, and possibly changes to the $q$-profile. Another example is scenarios with $q_{min}\approx$1.1. These typically have classical EP confinement and good thermal confinement. Thus by using its flexible parameters and profile control tools DIII-D is comparing a wide range of steady-state scenarios to identify the key physics setting EP transport