2023 – Detailed characterization of neg-D turbulent transport properties

Detailed characterization of neg-D turbulent transport properties

2023 Research Campaign, Thrust: Negative Triangularity

Purpose of Experiment

An attractive feature of negative triangularity (NT) plasmas is that they exhibit very good energy confinement properties without the edge transport barrier normally excited in high confinement discharges. The edge barrier can lead to pressure build up there that occasionally collapses and deposits damaging energy to the wall. Also, NT plasmas do not hold in impurity elements that can be picked up at the plasma edge and convected into the core where they radiate a large fraction of the plasma energy away. It is important to understand the fundamental physics behind negative triangularity’s favorable characteristics. This experiment explores NT properties by perturbing the plasma in a regular, proscribed way. By creating repeating pulses of energy and particles at specific radial locations in the discharge, and following their movement through the plasma, the characteristics of diffusion and convection across the plasma can be discovered.

Experimental Approach

High power microwaves (approximately 2 MW) that heat the plasma at specific, relatively small locations near the mid radius are launched into the plasma in bursts, typically of a frequency around 25 Hz. Those bursts cause increases in temperature that then propagate both inward and outward in radius. Following those bursts as they move radially with a diagnostic that measures temperature at many spatial locations versus time allows one to determine the diffusivity, which is a measure of how fast energy leaks out. If the pulses travel very fast, that is high diffusivity and equates to poor energy confinement. But if the pulses travel very slow, that is low diffusivity and indicates good confinement. Similarly, gas puffs from the normal fueling valves at the edge of the plasma are fired in a repeating way, typically much slower than the microwaves, about 3 Hz. The resulting pulses of particles are tracked by a diagnostic that measures plasma density at many locations and their movement into the core is measured and particle transport is determined. Pulses of impurities can be injected as well and the impurity bursts can be monitored to see if they move in and stay (undesirable) or whether they come back promptly to the edge (desirable).

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).

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