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.

In addition, two unknown vibronic bands have been observed in the 12150 cm-1 region and 12450 cm-1 region. From the observed linewidths, isotopic shift of the transition frequency and appearance of intricate spectral pattern, it is plausible that these bands may be due to C2 under the perturbation of a state inaccessible by dipole transition. Further analysis of these bands are underway. / The unprecedented sensitivity and resolution achieved by our spectrometer allows the measurement of the very weak Phillips band system (A 1piu - X1Sigmag +) of C2 in the plasma of methane and helium at a ratio of 1:170 generated under very mild ac hollow cathode discharge conditions. A total of eleven vibronic bands of the Phillips system have been observed in the region from 10300 cm-1 to 14250 cm-1 and analyzed based on the previous work. Combining the observed transition frequencies with those from Douay et al. [J. Mol. Spectrosc. 131, 250 (1988)], a set of spectroscopic constants for both the X1Sigma g+ and A1piu states have been obtained using least-squares fitting. Excellent agreement has been found between determined molecular constants and those found in the literature. The observation of the high upsilon levels in the 1piu state allows the determination of high order anharmonic constants oeze and oeae for the first time. / This thesis reports the construction of a custom-designed experimental setup for the high resolution near infrared spectroscopic studies of molecular ions and radicals generated in gaseous plasma. The home-built near infrared spectrometer system has a frequency resolution of 500 kHz and a frequency accuracy of ∼0.0010 cm-1 with a detection sensitivity of DeltaI/I of 10-6 using zero background concentration modulated phase sensitive detection scheme. An in-house program has been designed for data acquisition, frequency calibration and data processing and storage. / Yeung, Shun Hin. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 129-134). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

This thesis describes physics research carried out at the Columbia Neutral Torus (CNT) stellarator after its adaptation from a non-neutral plasma experiment to a device relevant to magnetic fusion energy research. Results are presented in the areas of plasma heating and related topics (microwave-assisted plasma start-up, overdense heating, inversion of stellarator images), as well as to stellarator stability and related topics (high β, error fields). This thesis also describes the engineering improvements which enabled the said adaptation of CNT. The first step of that process involved the installation of a low-power, pulsed 2.45 GHz magnetron. In those initial experiments it was found that the simultaneous use of microwave start-up and of an emissive hot cathode resulted in non-linearly increased electron densities, implying a synergy between the two start-up methods. Then, a 10 kW, 2.45 GHz heating system was commissioned including a custom-designed transmission line and launch antenna. Highly overdense plasmas (a factor of 4 above the cutoff density) were obtained with this system, both for O-mode and X-mode polarization. The analysis of Langmuir probe profiles of density and temperature required the accurate mapping of the minor radius in the plasma, which motivated a study of CNT error fields. This resulted in a new numerical method for inferring coil misalignments from flux surface measurements. The improved knowledge of the actual magnetic field geometry of CNT permitted to develop and successfully apply an inversion technique to experimental plasma images. This technique (“onion peeling”) reconstructs radial emissivity profiles, and can be considered a 3D generalization of Abel inversion. Finally, simulations of high-β plasma equilibria in different CNT magnetic configurations indicate that (1) ballooning stability limits should be accessible at volume-averaged β as low as 0.9% and (2) ballooning-stable β values as high as 3.0% should be attainable with heating powers as low as 40-100 kW and 1-3 MW respectively, according to stellarator energy confinement scaling laws.

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.

The primary achievement of this study is the development of a new approach for optimizing the plasma shape in a tokamak fusion energy reactor. In the interest of producing the largest possible fusion power output, the shape is optimized to allow for the highest possible beta - the ratio of the fluid to magnetic pressure - that can be sustained without the onset of magnetohydrodynamic (MHD) instabilities. To this end, the study explores the beta-domain that is stabilizable by bulk plasma rotation, with rotation timescales comparable to the resistive dissipation time of the plasma tearing surfaces or of the surrounding vacuum chamber. Modern feedback control systems are able to apply external magnetic fields which are phased to emulate the effect of plasma rotation, making the technique applicable even to large tokamaks with inadequate plasma rotation. In order to explore how the rotationally stabilizable beta-domain is affected by plasma shaping, a new semi-analytic MHD model of a tokamak has been developed. In addition to shaped toroidal tokamak geometry, the model contains dissipative effects resulting from resistivity in both the plasma and in the vacuum-chamber wall. The inclusion of plasma and wall resistivity introduces a lower beta-limit, associated with the onset of an unstable MHD mode, which can become dominated by either resistive-plasma (tearing) or resistive-wall effects in different parts of the parameter space. The computation time for analyzing the mode stability is greatly reduced by approximating the plasma current to reside in a thin layer, a form known as a sharp-boundary model. With fast calculations that focus on the key physics of these MHD instabilities, the model is able to explore qualitative trends of rotational stabilizability over a broad range of plasma shapes. Results of this study predict that varying the elongation or triangularity of the plasma cross-section can lead to qualitatively different beta-limits for the rotationally stabilizable domain. As the shape is varied, the upper bound in beta for rotational stabilization is found to switch from resistive-wall dominated behavior to resistive-plasma dominated behavior. The optimal plasma shape, associated with the highest beta-limit achievable with plasma rotation, is shown to be at the crossing point between the two domains. This discovery provides a basis for understanding existing experimental results and lays the groundwork for more quantitative studies with larger codes.

This thesis presents simulation and experimental work on feedback control of the \emph{phase} of non-rotating magnetic islands (locked modes) in the DIII-D tokamak, as well as its application to synchronized modulated current drive, for stability studies and control of the locked mode \emph{amplitude}. A numerical model has been developed to predict mode dynamics under the effect of various electromagnetic torques, due to the interaction with induced currents in the wall, error fields, and applied resonant magnetic perturbations (RMPs). This model was adapted to predict entrainment capabilities on ITER, suggesting that small (5~cm) islands can be entrained in the sub-10~Hz frequency range. Simulations and subsequent experiments on DIII-D demonstrated a novel technique to prevent locked modes. Preemptive entrainment applies a rotating RMP before a neoclassical tearing mode fully decelerates such that it will be entrained by the RMP and mode rotation can be sustained. A feedback control algorithm was designed and implemented on DIII-D to offer the ability to prescribe any toroidal phase to the mode and to allow for smoother entrainment. Experimental results confirmed simulation predictions of successful entrainment, and demonstrated one possible application to electron cyclotron current drive (ECCD). Feedback-controlled mode rotation and pre-programmed ECCD modulation were synchronized at DIII-D. This allowed a fine control of the ECCD deposition relative to the island O-point. Experiments exhibited a modulation of the saturated island width, in agreement with time-dependent modeling of the modified Rutherford equation. This work contributes to control and suppression of locked modes in future devices, including ITER.

Graduation date: 1990

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 186-193).

Thesis (Ph. D.)--University of Washington, 2001. / Vita. Includes bibliographical references (leaves 164-166).

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 133-135).