It is shown that static error fields (EFs) can severely limit the maximum rotation frequency achievable in mode entrainment by applied rotating fields. It is also shown that the rotation ...non-uniformities caused by an EF can be used to diagnose and correct said EF in real time. Simulations using typical DIII-D data show that this can be achieved within a small number of mode rotation periods by an iterative learning control algorithm. In addition to rapidly correcting the EF, this gives access to high entrainment frequencies that would not be accessible otherwise, and paves the way to rotational stabilization and improved mode control.
Abstract
To address the needs for a fusion pilot plan design, DIII-D/EAST joint experiments on DIII-D have demonstrated high normalized beta βN~4.2, toroidal beta βT~3.3% with qmin>2, q95≤8 sustained ...for more than six energy confinement times in poloidal beta regime. The excellent energy confinement quality (H98y2~1.8) is achieved with an internal transport barrier (ITB) at high line-averaged Greenwald density fraction fGr > 0.9. Gyrofluid (TGLF) modeling of the transport characteristics shows that the beam-driven rotation does not play an important role in the high confinement quality. The modeling also captures very well several transport features, giving us confidence in using integrated modeling to project these experimental results to future machines. The high-performance phase is terminated by fast-growing modes triggered near the n=1 ideal-wall kink stability limit. New RF capabilities for off-axis current drive could remove the residual ohmic current to achieve a fully non-inductive state, and improve the mode-wall coupling to increase the ideal-wall βN limit, enabling sustainment of the fully non-inductive high performance plasma in stationary conditions.
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 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.
The toroidal phase and rotation of otherwise locked magnetic islands of toroidal mode number n=1 are controlled in the DIII-D tokamak by means of applied magnetic perturbations of n=1. Pre-emptive ...perturbations were applied in feedforward to "catch" the mode as it slowed down and entrain it to the rotating field before complete locking, thus avoiding the associated major confinement degradation. Additionally, for the first time, the phase of the perturbation was optimized in real-time, in feedback with magnetic measurements, in order for the mode's phase to closely match a prescribed phase, as a function of time. Experimental results confirm the capability to hold the mode in a given fixed-phase or to rotate it at up to 20 Hz with good uniformity. The control coil currents utilized in the experiments agree with the requirements estimated by an electromechanical model. Moreover, controlled rotation at 20 Hz was combined with Electron Cyclotron Current Drive (ECCD) modulated at the same frequency. This is simpler than regulating the ECCD modulation in feedback with spontaneous mode rotation, and enables repetitive, reproducible ECCD deposition at or near the island O-point, X-point and locations in between, for careful studies of how this affects the island stability. Current drive was found to be radially misaligned relative to the island, and resulting growth and shrinkage of islands matched expectations of the Modified Rutherford Equation for some discharges presented here. Finally, simulations predict the as designed ITER 3D coils can entrain a small island at sub-10 Hz frequencies.
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.
ObjectiveThe Truth about Suicide video has been widely used but has never been empirically tested regarding its cultural appropriateness for Asian Americans. The purpose of the study was to determine ...the feasibility of using the video in a web-based suicide awareness programme for Asian American and non-Hispanic white college students.MethodsA cross-sectional, comparative, web-based study was conducted with 227 Asian Americans and 204 non-Hispanic whites at a university in the Midwest region of the USA. Study participants completed a questionnaire measuring their cultural orientation and attitudes towards suicide, watched the 27 min video, completed a debriefing session and evaluated the video's overall suitability.ResultsAsian Americans rated the suicide awareness video significantly lower for cultural relevance than did non-Hispanic whites (F=5.479, p=0.02). Collectivist cultural orientation was a significant predictor for cultural relevance, credibility and appeal; however, evaluation of the video's cultural relevance was negatively affected by Asian ethnicity.ConclusionsCultural orientation and race/ethnicity should be strongly considered when web-based suicide awareness programmes are developed for college students.
The study purpose was to determine the predictors of watching most of a Web-based streaming video and whether data characteristics differed for those watching most or only part of the video. A ...convenience sample of 650 students (349 Asian Americans and 301 non-Hispanic whites) was recruited from a public university in the United States. Study participants were asked to view a 27-minute suicide awareness streaming video and to complete online questionnaires. Early data monitoring showed many, but not all, watched most of the video. We added software controls to facilitate video completion and defined times for a video completion group (≥26 minutes) and video noncompletion (<26 minutes) group. Compared with the video noncompletion group, the video completion group included more females, undergraduates, and Asian Americans, and had higher individualistic orientation and more correct manipulation check answers. The video noncompletion group skipped items in a purposeful manner, showed less interest in the video, and spent less time completing questionnaires. The findings suggest that implementing software controls, evaluating missing data patterns, documenting the amount of time spent completing questionnaires, and effective manipulation check questions are essential to control potential bias in Web-based research involving college students.
Computer technology provides innovations for research but not without concomitant challenges. Herein, we present our experiences with technology challenges and solutions across 16 nursing research ...studies. Issues included intervention integrity, software updates and compatibility, web accessibility and implementation, hardware and equipment, computer literacy of participants, and programming. Our researchers found solutions related to best practices for computer-screen design and usability testing, especially as they relate to the target populations’ computer literacy levels and use patterns; changes in software; availability and limitations of operating systems and web browsers; resources for on-site technology help for participants; and creative facilitators to access participants and implement study procedures. Researchers may find this information helpful as they consider successful ways to integrate informatics in the design and implementation of future studies with technology that maximizes research productivity.
PDF
