Experimental evidence for a reduction in electron thermal diffusion due to trapped particles

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Magnetohydrodynamics

Background:

• Understanding electron thermal diffusion (χ(e)) is crucial for controlling plasma confinement in fusion devices.
• Previous studies lacked the high time resolution needed to capture rapid changes in χ(e) during plasma cycles.
• The Madison Symmetric Torus (MST) provides a unique environment for studying reversed-field pinch (RFP) plasma dynamics.

Purpose of the Study:

• To conduct high time resolution measurements of electron thermal diffusion (χ(e)) during the sawtooth cycle in the MST.
• To compare experimental measurements with predictions from advanced nonlinear resistive magnetohydrodynamic (MHD) simulations.
• To investigate the role of trapped particles in electron thermal transport.

Main Methods:

• Utilized an upgraded multipoint, multipulse Thomson scattering system for high time resolution measurements of electron temperature.
• Performed zero-beta nonlinear resistive MHD simulations at a high Lundquist number (S≈4×10^6), relevant to MST plasmas.
• Compared experimental χ(e) data with simulation predictions, incorporating trapped particle effects.

Main Results:

• Achieved unprecedented time resolution in measuring electron thermal diffusion throughout the sawtooth cycle.
• Simulations at high Lundquist numbers revealed magnetic diffusion contributions to χ(e).
• Experimental and simulated χ(e) values showed agreement only when the reduction of thermal diffusion by trapped particles was included.

Conclusions:

• Trapped particle effects significantly reduce electron thermal diffusion in MST plasmas.
• The study validates advanced MHD simulations by comparing them with high-resolution experimental data.
• Accurate modeling of electron thermal transport, including trapped particles, is essential for future fusion reactor designs.