2024 – Density limit in low collisionality H-mode plasmas

Density limit in low collisionality H-mode plasmas

2024 Research Campaign, Thrust: High Opacity and Density Operation

Purpose of Experiment

The goal of this experiment is to identify the fundamental mechanism that limit density in H-mode plasmas at reduced pedestal collisionalities, and determine whether the power dependence of H-mode density limit is collisionality-dependent. High density H-mode is essential for burning plasma tokamak concept to maximize fusion gain as fusion gain increases ~<n>^2. However, operating edge density near or above the empirical Greenwald limit can lead to onset of disruption phenomena resulting in unacceptable damage to the fusion device. Understanding the mechanism that limit the density and possibly pushing that limit are thus very important. Besides the only plasma current dependence in the empirical scaling, it has been predicted that a positive power scaling may also be intrinsic to the density limit. How this scaling will look like especially at reduced collisionality is important and high priority to ITER/FPP.

Experimental Approach

The half day experiment will involve two main scans. One is to employ different power levels with both nuetral beams and off-axis ECH to investigate the H-mode density limit power dependence. Gas fueling will be used to increase density. The other is to ramp down plasma current at constant power and density to increase Greenwald density fraction. It will be operated in ITER similar shape with normal toroidal field and plasma current direction. During all these scans, edge turbulence and flows near the H-mode density limit, and across the H-L back transition will be carefully documented. The beam program will be optimized for turbulence measurements. Balanced beams will be used to minimize applied neutral beam torque. The pedestal and edge temperatures, impurity levels and the core-radiative fraction will also be monitored carefully to ensure the pedestal remains in the transport-limited regime.