David Lawrence Nadle

Suppression of resistive wall instabilities with distributed, independently controlled, active feedback coils

External kink instabilities are suppressed in a tokamak experiment by either (1) energizing a distributed array of independently controlled active feedback coils mounted outside a segmented resistive wall or (2) inserting a second segmented wall having much higher electrical conductivity. When the active feedback coils are off and the highly conducting wall is withdrawn, kink instabilities excited by plasma current gradients grow at a rate comparable to the magnetic diffusion rate of the resistive wall.

Active control of 2/1 magnetic islands in a tokamak*

Closed and open loop control techniques were applied to growing m/n=2/1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak-Extended Pulse (HBT-EP) [J. Fusion Energy 12, 303 (1993)]. HBT-EP combines an adjustable, segmented conducting wall (which slows the growth or stabilizes ideal external kinks) with a number of small (6° wide toroidally) driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control from 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplifiers. The phase instability has been observed and is well modeled by the single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equations. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the theory. Suppression of the 2/1 island growth has ...

Observation of wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak*

The High Beta Tokamak‐Extended Pulse (HBT‐EP) experiment [J. Fusion Energy 12, 303 (1993)] combines an internal, movable conducting wall with a high‐power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic (MHD) instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma (minor radius a) resulted in the suppression of β‐limiting disruptions for discharges in which b/a<1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with β values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall‐stabilized discharges was controlled by applying oscillating m=2, n=1 resonant magnetic perturbations. A compact, single‐phase saddle coil system permitted modulation of the rotation velocity of internal m/n=2/1 instabilities by a f...

Stabilization of kink instabilities by eddy currents in a segmented wall and comparison with ideal MHD theory

The characteristics of external kink instabilities observed during wall stabilization studies in the HBT-EP tokamak have been compared with the predictions of ideal MHD theory, in order to examine the stabilizing role of a resistive wall that is segmented both toroidally and poloidally. The reconstructed equilibria, for discharges with different plasma-wall configurations, are consistent with external and internal magnetic measurements, measured soft X ray profiles and measured equilibrium wall eddy currents. The stability analysis of these equilibria predicts patterns of instability induced eddy currents for a model wall that is continuous and perfectly conducting, and these patterns are in good agreement with the ones observed on the HBT-EP segmented wall. These eddy currents account for the observed stabilization of fast ideal modes when the wall is fully inserted, consistent with the prediction of marginal stability.

Initial high beta operation of the HBT-EP Tokamak

HBT-EP is a new research tokamak designed and built to investigate passive and active feedback techniques to control MHD instabilities. In particular, HBT-EP will be able to test techniques to control fast MHD instabilities occurring at high Troyon-normalized beta, βN ≡ βBa/Ip [Tm/MA], since it is equipped with a thick, close-fitting, and adjustable conducting shell. The major goals of the initial operation of HBT-EP have been the achievement of high beta operation (βN ∼ 3) using only ohmic heating and the observation of MHD instabilities. By using a unique fast startup technique, we have successfully achieved these goals. A variety of MHD phenomena were observed during the high beta operation of HBT-EP. At modest beta (βN ≤ 2), discharges have been maintained for more than 10 msec, and these discharges exhibit saturated resistive instabilities. When βN approaches 3, major disruptions occur preceded by oscillating, growing precursors. During start-up, one or more minor disruptions are usually observed. A 1-D transport code has been used to simulate the evolution of the current profile, and these early minor instabilities are predicted to be double tearing modes. The simulation also reproduces the observed high beta operation when saturated neo-Alcator energy confinement scaling is assumed.

Effect of magnetic islands on the local plasma behavior in a tokamak experiment

Effect of Magnetic Islands on the Local Plasma Behavior in a Tokamak Erik Dannel Taylor Experiments on the HBT-EP (High Beta Tokamak-Extended Pulse) tokamak provided local measurements of the pressure and ion velocity perturbations from rotating magnetic island using Mach probes. The presence of magnetic islands created two distinct features in ion fluid velocity measurements. First, the toroidal velocity profile was sharply peaked near the center of the 2/1 magnetic island. Second, the ion velocity near this island was only ~30% of the magnetic island velocity. Measurements of the perturbations from rotating magnetic islands with stationary detectors prompted the development of a new data analysis technique using the Hilbert transform. This method generated plots of the pressure profile co-rotating with the magnetic island, allowing the analysis of the pressure profile behavior at the O and X-points of the magnetic island. Experiments with active rotation control demonstrated that the pressure perturbations followed the magnetic island motion, while simultaneously measuring that the ion velocity and acceleration were less that those of the magnetic island. These observations agreed with predictions from a two-fluid plasma model that included the effect of magnetic islands on the diamagnetic velocity as well as neutral damping effects. Understanding the effect of magnetic islands on the pressure and ion velocity profiles is crucial for both fundamental plasma studies and the development of more efficient tokamaks using advanced tokamak (AT) concepts.

Nonstationary signal analysis of magnetic islands in plasmas

Rotating magnetic islands produce fluctuations on a variety of diagnostics in magnetic fusion energy plasmas. The analysis of these fluctuations requires the calculation of the amplitude, phase, and frequency of the oscillations. These three spectral quantities generally evolve in time, necessitating nonstationary signal analysis techniques. The Hilbert transform offers an efficient and accurate method of calculating these three quantities from one diagnostic signal. This feature allows the Hilbert transform to determine the success of the active rotation control of magnetic islands, and to calculate the profile of the diagnostic measurements in a frame of reference co-rotating with the magnetic island. Comparisons to quadrature and spectrogram techniques demonstrate the accuracy of the Hilbert transform method.

The feedback phase instability in the HBT-EP tokamak

Observations of a performance limiting feedback phase instability in the HBT-EP tokamak are reported. The phase instability consists of a rapid growth of the phase difference between an m/n = 2/1 tearing mode and an external resonant magnetic perturbation. Observations of mode angular dynamics during phase instability test discharges show good agreement with theoretical estimates of the phase instability timescale. The phase instability limits feedback performance in HBT-EP by decreasing the feedback loops phase accuracy as gain increases.