182 leaves : ill. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Mawson Institute for Antarctic Research, 1972

Thesis (Ph.D.) -- University of Adelaide, Mawson Institute for Antarctic Research, 1972.

Thesis (M.S.)--University of Michigan, 1964.

Thesis (M.S.)--University of Michigan, 1977.

Thesis--University of Wisconsin--Madison. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis--University of California. / Includes bibliography.

Experimental studies of MHD modes, including dynamics and stability, using a biased electrode for rotation control on the High Beta Tokamak –- Extended Pulse (HBT-EP) are presented. When the probe is inserted into the edge of the plasma and a voltage applied, the rotation of long-wavelength kink instabilities is strongly modified. A large poloidal plasma flow results at the edge, measured with a bi-directional Mach probe with changes in edge kink mode rotation at different biases. This poloidal plasma rotation cannot fully account for the large mode rotation frequency on HBT-EP. By including the electron fluid motion, the mode rotation predictions agree with measurements, indicating that the modes travel with the electron fluid. A GPU-based digital feedback system is used to adjust the probe voltage in real time for controlling both the plasma flow and mode rotation. This active mode rotation control is desirable because it allows for MHD stabilization, as well as studies under conditions of varying mode rotation rates. Mode dynamics were studied using various diagnostics to understand how plasma conditions fluctuate during mode activity and to understand the interaction of the bias probe with the plasma during this activity. Phase-dependent mode behavior was observed, especially at slow mode rotation, which might be attributed to an intrinsic error field or a nonlinear interaction between the bias probe and the mode. Applied resonant magnetic perturbations were used to study the dynamic response of a stable plasma with different mode rotations. At slower rotation, the plasma had a greater response to the perturbations and the plasma reached a saturated response with large perturbations, similar to previous results. At large positive biases, the probe current induces a torque that opposes the natural direction of mode rotation. By applying a sufficiently large torque, a transition is induced into a fast rotation state (both mode and plasma rotation). High poloidal shear flows at the edge were measured in this state, similar to conditions in H-mode plasmas on other devices. The bias required to induce the transition is shown to depend on an applied error field. A technique was established using this transition to determine the natural error field on HBT-EP.

The first study of magnetohydrodynamic (MHD) equilibria and external kink modes in shaped plasmas on the High Beta Tokamak - Extended Pulse (HBT-EP) is described. A new poloidal field coil and high-current, low-voltage capacitive power supply was designed and installed. The new coil significantly modifies the shape of the plasma cross section and provides a new research tool for the study of kink mode structure and control. When fully energized, the coil creates a magnetic separatrix, which defines the boundary between confined and unconfined plasma. The separatrix is set by a poloidal field null called an “X-point”, which is on the inboard side of the torus, above the midplane. Several arrays of magnetic sensors observe and characterize the plasma equilibrium and the MHD fluctuations from kink modes. Free-boundary plasma equilibria are reconstructed using standard methods that minimize the mean-square error between the numerically reconstructed equilibria and various measurements. Reconstructions of shaped plasma equilibria show the creation of fully diverted plasmas with shaped outer boundaries. The reconstructions are confirmed by direct measurements using arrays of magnetic sensors and a moveable Langmuir probe to measure the outermost closed flux surface. Measurements of individual kink modes are obtained from the magnetic fluctuations using a technique known as biorthogonal decomposition. External kink modes that naturally arise in shaped plasmas are observed and described. The poloidal structure of modes in shaped plasmas are found to be similar to those that arise in circular plasmas, except near the X-point. The magnetic signature of kink modes on the surface of the plasma are calculated using the ideal MHD code DCON. For plasmas with an X-point, DCON shows a short-wavelength, low amplitude structure near the X-point. The code VALEN is used to calculate the perturbed magnetic field measured at the sensors due to the DCON mode at the plasma surface. VALEN includes the effects of sensor/plasma separation and eddy currents induced in conducting structures by rotation of the modes. Good agreement is found between the measured mode structures and the ideal kink mode structures calculated at the sensors by VALEN. A distributed array of forty active control coils was used to perturb the plasma equilibria, and for both shaped and circular equilibria, the structure of the response to the perturbation was found to be the same as the that of the dominant naturally occurring mode in that equilibrium. Finally, the magnitude of the plasma’s response to applied magnetic perturbations was found to be comparable between shaped and unshaped plasmas, even though separation between the sensors and the boundary of the shaped plasmas increases relative to circular plasmas with the same plasma current and radial positions. In addition to demonstrating a new research tool for study of kink modes on HBT-EP, this research demonstrates the importance of accurate electromagnetic calculations, including eddy currents, when comparing measured and predicted mode structure.

Thesis--University of Wisconsin-Madison. / Includes bibliographical references.

The first large-scale study of magnetically-confined plasma for the production of fusion energy is scheduled to begin this decade and will answer many questions. Two critical issues are: (1) how to control and prevent non-axisymmetric magnetic perturbations that may drive harmful current and plasma energy into the surrounding walls, and (2) how to understand the relationship between plasma rotation, plasma confinement, and plasma stability. To address both, this dissertation reports research with biasable electrode arrays in the HBT-EP tokamak. This work conducts systematic studies of driven current and achieves the first active control of plasma rotation and rotating magnetic instabilities with a toroidal electrode array. Electrode-driven current impacts the plasma in several ways. First, it can increase, decrease, and reverse plasma rotation as measured by Mach probes, which results in an altered radial electric field. By controlling the electrode voltage with an active feedback system, plasma rotation is controlled between 4 and 8 kHz. Second, by modulating the driven electrode current at fixed frequencies, spontaneous magnetic perturbations develop at the plasma’s edge. These distortions are field aligned, do not rotate, and match the magnetic helicity of the scrape-off-layer (SOL). Direct measurement of SOL current to collectors mounted on the wall, show that the SOL current is field-aligned with a filamentary structure. When a naturally-occurring rotating m=2 mode is present, magnetic measurements show that the two structures are superimposed with no obvious indication of coupling. Third, when the electrode current is driven at the natural frequency of rotating magnetic perturbations, the plasma’s proportional response increases, indicating a resonance at 9 kHz. Resonance is observed in the radial electric field, floating potential profile, plasma rotation, and magnetic measurements. Finally, when the electrode array is biased in quadrature and actively controlled, driven currents modify the rotation and amplitude of the long-wavelength rotating magnetic modes. When the quadrature electrode array is phase locked to the n=1 mode rotation, mode amplitudes are suppressed by as much as 50%. Suppression shows a clear dependence on a phase between the rotating mode and the driven current. These experiments show that the structure of SOL currents are field-aligned and demonstrate a clear relationship between biased-electrode driven current and the rotation and amplitude of helical magnetic perturbations.