Control of 3D equilibria with resonant magnetic perturbations in MST

Abstract

To aid in diagnosis of 3D equilibria in the Madison Symmetric Torus, it has become necessary to control the orientation of the equilibria. In reversed field pinch experiments a transition to a 3D equilibrium is common with sufficiently large plasma current (and Lundquist number). Diagnosis of this state is hampered by the fact that the helical structure is stationary but with an orientation that varies shot-to-shot. A resonant magnetic perturbation (RMP) technique has been developed to vary controllably the orientation of the 3D equilibria and optimized to minimize the plasma wall interaction due to its use. Application of an RMP now allows alignment of the structure with key diagnostics, including Thomson scattering and an interferometer-polarimeter.

Tearing mode dynamics and locking in the presence of external magnetic perturbations

“…A possibility is that the difference in RMP waveforms (ramp and square) could result in a difference in the plasma-wall interaction. 10 This interaction could lead to a torque at the edge, which is not included in the model. However, the plasma braking due to the RMP is mainly in the core-region, where the dominant TMs (n ¼ 6 À 10) are located.…”

“…The experimental data in this work were measured in the MST 32 reversed-field pinch. Essential for this study is the magnetic feedback system, 10 which is used to perturb the plasma. The resonant part of the perturbation interacts with the tearing modes, whose dynamics are measured by poloidal magnetic field pick-up coils.…”

“…At the poloidal gap, the radial component of the error field is measured and corrected for by magnetic coils in a feedback system (32 sensors and 38 actuators). 10 The feedback system was used to apply magnetic perturbations with poloidal harmonics m ¼ 1 and several toroidal n. The m ¼ 1 radial component of the applied field is measured by the sensors, but the n spectrum cannot be resolved due to the coil geometry. However, the poloidal gap is too small compared to the torus circumference that it is basically a spatial delta function in toroidal angle.…”

“…8 In reversed-field pinches (RFPs), RMP application has been used to, for example, induce slow rotation to reduce localized plasma wall interaction 9 and to control the orientation of 3D equilibria. 10 The interaction between a resonant error field and a tearing mode (TM) can cause braking of the TM itself and the plasma rotation. [11][12][13] This can eventually lead to a wall locked mode and consequently reduced confinement and possibly, in tokamaks, a disruption.…”

“…The error field at the poloidal gap is corrected for by external saddle coils in a feedback system. 10 In addition to the error field correction, the feedback system can be used to produce RMPs with a pre-set amplitude and poloidal mode number m. Due to the coils limited toroidal extent, they produce a broad spectrum in toroidal mode number n. MST has several TMs that are naturally rotating together with the plasma fluid. Hence, MST is a suitable device to study the simultaneous braking of several TMs, due to a multi-harmonic RMP.…”