Pre-ionization by AC Ohmic coil operation in the TST-2 spherical tokamak

Abstract

AC Ohmic coil operation experiments with frequencies up to 10 kHz were performed on the TST-2 spherical tokamak device, and the pre-ionization process was studied in detail. The minimum loop voltage for pre-ionization was 0.4 V, which corresponds to 0.5 V m −1 at the inboard limiter. Dependences of growth rate and saturation level of the process on various parameters were obtained, and they are compared with a time-dependent 0-dimensional model based on Townsend avalanche and loss along field lines. Most of the dependences are reproduced qualitatively by the model, and quantitative differences are within a factor of several. However, the external vertical field dependence of the appearance time, which is defined as the time to observe a plasma, and the isotope effect cannot be reproduced by the model. An ambipolar diffusion state which is predicted theoretically but mitigated experimentally is discussed. It was found that secondary electron emission at the limiter surfaces is a candidate mechanism to mitigate the state.

Theoretical study of pre-ionization by inductive field in tokamaks

“…This is what we call an ambipolar diffusion state, and such a state was found in the experiments described in [7], and a similar phenomenon is reproduced by a two-dimensional (2D) PIC simulation [6]. In contrast, the AC Ohmic coil operation experiments showed exponential growths and instantaneous growth rates can be as fast as an ideal rate, implying the loss term should be negligible [5]. Thus, we need to find some mechanisms destroying or mitigating the ambipolar diffusion state.…”

“…The AC Ohmic coil operation is a good experimental system, in which quasi steady state conditions are achieved, and we can clearly observe the exponential growth of density during pre-ionization [4]. A time dependent zero-dimensional model, which is an extension of the previous models, has been developed, and the model can explain various experimental results qualitatively [5]. The differences between the predicted and experimental quantities, such as growth rates, are within an order of magnitude.…”

“…One of the main targets of this study is the theoretical interpretation of the results of the AC Ohmic coil operation, in which thin cylindrical plasma is generated on the inboard side [4,5]. The toroidal field strength B t was much higher than the vertical and the radial field strengths B z and B R .…”

“…Since the electrons travel along a field line and are lost at the end of the field line, the actual loss is more like a convection, and a one-dimensional (1D) model is necessary to describe the loss properly. A 1D fluid model was introduced in [5], and variables, such as electron density and velocity, are represented as functions of the coordinate along a field line. Particles are lost when they reach the boundary (i.e.…”

“…In this paper, the 1D fluid model presented in [5] is modified to include several additional effects. The model system consists of a field line and two solid boundaries.…”