Nonlinear modeling of the scaling law for the m/n=3/2 error field penetration threshold

The scaling law for the n=2 error field (EF) penetration threshold is predicted numerically based on nonlinear single-fluid and two-fluid modeling using the TM1 code.

The simulated penetration threshold of radial magnetic field br at the plasma edge is scaled to the electron density ne, temperature Te, viscous time, toroidal field Bt and the natural frequency by scanning these parameters separately.

Single fluid modeling shows that the EF threshold scaling is similar with the analytical scaling law in both the Rutherford and visco-resistive regimes.

However, two-fluid modeling shows that the scaling law differs significantly in particular regarding the dependence on plasma rotation.

In detail, the scaling coefficient on density decreases from 0.67 to 0.56 and on temperature decreases from 0.67 to 0.32, while on viscous time is around -0.45 and on toroidal field decreases slightly from -1.15 to -1, when the ratio ExB over electron diamagnetic drift frequency varies from 0 to 10.

Scans of the plasma rotation reveals that the penetration threshold linearly depends on the perpendicular rotation frequency (or natural frequency), and there is a minimum in the required field amplitude when electron fluid frequency near 0.

In addition, the enduring mystery of non-zero penetration threshold at zero plasma natural frequency in EF experiments is resolved by two-fluid simulations.

We find that the very small island and smooth bifurcation in EF penetration near zero frequency is hard to detect in the experiment, leading to a finite penetration threshold within the capability of the experimental measurements.