2024 – Understanding the Physics of Density Limit in L-mode plasmas

Understanding the Physics of Density Limit in L-mode plasmas

2024 Research Campaign, Thrust: High Opacity and Density Operation

Purpose of Experiment

The purpose of this half-day experiment is to investigate the physical mechanisms underlying the density limit (DL) in L-mode plasmas. Determine if the edge shear flow collapse at high density is the trigger for the enhanced edge transport and further DL disruption. Identify the role of turbulence spreading from the plasma edge to the Scrape-Off-Layer (SOL) and towards the core plasma, while approaching the DL. Study the effects of plasma shaping (triangularity) on the DL onset. The results of this experiment will help to validate the existing models of the DL. Understanding the DL mechanism is essential for optimizing the plasma performance in higher density regimes. Background: Fusion reactor will necessarily operate at high plasma density as the fusion gain scales as the square of density 𝑃fus~𝑛^2. However, tokamak plasmas exhibit MARFEs, MHD instabilities or disruptions when a line-averaged density reaches a certain value, which is reflected in an empirical Greenwald density limit scaling [1]: 𝑛Gr[10^20𝑚^-3] = 𝐼p[𝑀𝐴]/𝜋𝑎^2[𝑚^2], where 𝐼p is the plasma current and 𝑎 is the plasma minor radius. Microscopic transport physics based model [2,3] considers the collapse of the edge shear layer as an initiator for the density limit disruption. Another approach connects the density limit with the changes in the dominant turbulent transport mechanism [4]. Electron adiabaticity 𝛼 is a key parameter, and the onset of the density limit is associated with 𝛼<<1 [3,5,6]. Experiments at HL-2A [6], J-TEXT [7], and ASDEX-U [8] showed the reduction of the shear flow as the Greenwald density limit is approached. The correlation was observed between the shear layer degradation and the drop in electron adiabaticity 𝛼.

Experimental Approach

To investigate the physics of the density limit we utilize 2 techniques to reach 𝑛Gr: plasma current ramp down at constant density and slow density ramp-up at constant 𝐼p. The experiment will use positive triangularity L-mode plasmas with USN configuration with the direction of ion ∇𝐵 drift away from the X-point to have higher LH power threshold. The first shot will have a slow NBI power ramp with 𝑛3~0.5 𝑛Gr to find the LH power threshold 𝑃LH. Further discharges will have power below 𝑃LH to keep plasma in L-mode. The second step focuses on the effects of shaping (triangularity scan, δ > 0). Triangularity δ is changed shot to shot, and in every shot, after 𝐼p reaches flat-top, the plasma density is slowly increased with gas puffing until disruption. Next, we will run 𝐼p ramp down with varying q95 (fixed toroidal field Bt) and fixed q95 (Bt changing). 𝐼p will be decreased until 𝑛Gr is reached. In all the discharges we will use a powerful set of DIII-D diagnostics to measure turbulence properties as plasma reaches the DL.

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