Equilibrium and global MHD stability study of KSTAR high beta plasmas under passive and active mode control

Abstract

The Korea Superconducting Tokamak Advanced Research, KSTAR, is designed to operate a steady-state, high beta plasma while retaining global magnetohydrodynamic (MHD) stability to establish the scientific and technological basis of an economically attractive fusion reactor. An equilibrium model is established for stability analysis of KSTAR. Reconstructions were performed for the experimental start-up scenario and experimental first plasma operation using the EFIT code. The VALEN code was used to determine the vacuum vessel current distribution. Theoretical high beta equilibria spanning the expected operational range are computed for various profiles including generic L-mode and DIII-D experimental H-mode pressure profiles. Ideal MHD stability calculations of toroidal mode number of unity using the DCON code shows a factor of 2 improvement in the wall-stabilized plasma beta limit at moderate to low plasma internal inductance. The planned stabilization system in KSTAR comprises passive stabilizing plates and actively cooled in-vessel control coils (IVCCs) designed for non-axisymmetric field error correction and stabilization of slow timescale MHD modes including resistive wall modes (RWMs). VALEN analysis using standard proportional gain shows that active stabilization near the ideal wall limit can be reached with feedback using the midplane segment of the IVCC. The RMS power required for control using both white noise and noise taken from NSTX active stabilization experiments is computed for beta near the ideal wall limit. Advanced state-space control algorithms yield a factor of 2 power reduction assuming white noise while remaining robust with respect to variations in plasma beta.

Plasma stability theory including the resistive wall effects

“…This can be illustrated by the difference between and , the stability limits for the same configuration computed, respectively, with ideal wall and without wall, in terms of the frequently used parameter where is the minor radius of the plasma, is the magnetic field and is the net current in the plasma. The representative examples are and in ITER [2], and in the DIII-D tokamak [12], and in JT-60SA [26] and with in KSTAR [82]. For more data and details see table II in [21], figures 3 and 4 in [83] and discussion in [65].…”

“…Such simplification is not an attribute of some cylindrical models only. In the thin-wall code VALEN, the perturbation produced by the plasma is also modelled as a single mode described by a real [41, 45, 82].…”

“…For example, in [41] the results are presented up to for the DIII-D tokamak, while in [49] the thin-wall modelling of RWM dynamics in the JT-60SA tokamak was performed with reaching 80. In [82], the VALEN computational results are shown with a growth rate of while the open loop growth rate of the RWM was estimated as . In [119], the wall forces produced during ITER disruptions have been calculated up to still using a thin-conducting-wall model.…”

Resistive wall mode active control physics design for KSTAR

“…1. 3 Figure 3 also illustrates the two-dimensional geometry of the KSTAR conducting structure including passive stabilizer plates and IVCCs along with the magnetic sensors 37 analyzed. The design of the existing copper passive stabilizing plates in KSTAR was determined by considering its impact on RWM passive growth rates.…”

“…The design of the existing copper passive stabilizing plates in KSTAR was determined by considering its impact on RWM passive growth rates. 3,7 The passive plates are positioned to effectively couple to RWMs which are ballooning in nature with the highest mode perturbation on the outboard side. Each of the up-down symmetric stabilizers is segmented into four toroidal quadrants, and connected by gap resistors to produce a toroidal resistance accommodating controllability of both vertical instabilities (n ¼ 0) and FIG.…”