C. Cates

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 When the active feedback coils are off and the highly conducting wall is withdrawn, kink instabilities excited by plasma current gradients grow at a rate comparable to the magnetic diffusion rate of the resistive wall.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 ...

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.

High-speed optical diagnostic that uses interference filters to measure Doppler shifts

A high-speed, noninvasive velocity diagnostic has been developed for measuring plasma rotation. The Doppler shift is determined by employing two detectors that view line emission from the identical volume of plasma. Each detector views through an interference filter having a passband that varies linearly with wavelength. One detector views the plasma through a filter whose passband has a negative slope and the second detector views through one with a positive slope. Because each channel views the same volume of plasma, the ratio of the amplitudes is not sensitive to variations in plasma emission. With suitable knowledge of the filter characteristics and the relative gain, the Doppler shift is readily obtained in real time from the ratio of two channels without needing a low throughput spectrometer. The systematic errors—arising from temperature drifts, stability, and frequency response of the detectors and amplifiers, interference filter linearity, and ability to thoroughly homogenize the light from the f...

Real-time measurement of toroidal rotation (abstract)

One of the important goals in Columbia’s HBT-EP tokamak program is the improvement in the stability of tokamak plasmas by controlling the bulk plasma flow relative to the conducting wall. The method for active plasma flow control in HBT-EP is the application of oscillating resonant magnetic perturbations to oppose the velocity of magnetic islands at the q=2 surface. Real time (10 kHz) feedback control without inserting a material probe necessitates the use of an optical toroidal rotation measurement whose data is available during the shot. This is being accomplished in a novel way by seeding the deuterium plasma with 5%–10% helium and measuring the Doppler shift of the chord-integrated emission of the He II (n=4→3) line at 4686 A. Since the electron temperature is expected to be about 30 eV at the q=2 surface, helium is not fully stripped. The shift in wavelength is calculated by measuring the change in intensity as the line moves across the passband of an interference filter that varies linearly. Filters...