Ideal MHD Limited Electron Temperature in Spherical Tokamaks

Area of Science:

• Plasma Physics
• Fusion Energy Research
• Magnetohydrodynamics

Background:

• Central electron temperatures in the National Spherical Torus Experiment (NSTX) are observed to remain constant despite increasing heating power and plasma pressure (β).
• This phenomenon challenges conventional understanding of plasma confinement and heating dynamics in toroidal devices.

Purpose of the Study:

• To investigate the underlying physical mechanisms responsible for the observed flattening of central electron temperature profiles in NSTX.
• To test the hypothesis that non-disruptive, low toroidal mode number (n), pressure-driven ideal magnetohydrodynamic (MHD) instabilities play a crucial role.

Main Methods:

• Utilizing a 3D resistive magnetohydrodynamic (MHD) simulation of a specific NSTX discharge.
• Systematically varying parameters such as toroidal magnetic field strength and heating power within the simulation.

Main Results:

• The simulations demonstrate that low-n, pressure-driven ideal MHD instabilities can indeed disrupt magnetic surfaces in the plasma core.
• A critical threshold for plasma pressure (β) was identified; above this value, the central electron temperature profile ceases to peak on the magnetic axis.
• The simulations successfully replicate the experimental observation of a flattened central electron temperature profile.

Conclusions:

• Nondisruptive, low-n, pressure-driven ideal MHD instabilities provide a plausible explanation for the anomalous flattening of central electron temperature profiles in NSTX.
• These instabilities, by breaking magnetic surfaces, act as an effective mechanism limiting the achievable on-axis electron temperature at high plasma pressures.
• The findings suggest that understanding and controlling these MHD instabilities is critical for optimizing heating and achieving higher plasma performance in spherical torus devices.