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It has been observed in tokamaks that temperature profiles are resilient to changes in heating, and that this effect has not been observed in conventional stellarators. Electron temperature profile resiliency is attributed to anomalous transport driven by turbulent micro-instabilities, and the resulting stiffness in the electron heat flux is measured using a combination of steady-state and perturbative experiments. In this work, stiffness measurements are presented in the quasihelically symmetric configuration of the Helically Symmetric eXperiment (HSX), in which the neoclassical transport is comparable to a tokamak and turbulent transport dominates throughout the plasma. A second gyrotron and transmission line have been installed and tested to facilitate modulated heating experiments on HSX, and a multi-pass absorption model accurately predicts the total absorption and spatial extent of the electron cyclotron resonance heating during a modulation experiment. The electron cyclotron emission measured by an absolutely calibrated 16-channel radiometer is used to measure the local electron temperature and its response to the modulated heating. The amplitude and phase of the heat wave through the foot of the steep electron temperature gradient region of the plasma, 0.2
The Helically Symmetric Experiment (HSX) stellarator has been optimized for low neoclassical transport. Anomalous transport attributed to drift-wave turbulence remains an important loss channel. High-resolution plasma diagnostics can be used to study fluctuations in equilibrium plasma parameters, such as plasma density and temperature, which are sensitive quantities of the underlying turbulence. In this work, core radiation temperature fluctuations were measured in the HSX stellarator using a correlation electron cyclotron emission radiometer. The experimental measurements have been compared with gyrokinetic simulations of plasma turbulence. The HSX correlation electron cyclotron emission diagnostic measures radiation temperature fluctuations from second harmonic X-mode wave emission in optically semi-transparent plasmas. Multiple pass raytracing calculations indicate reinforcement of single pass emission on the high-field side of the magnetic axis, permitting localized measurements. Interpretation of radiation temperature fluctuations as electron temperature fluctuations is within reasonable uncertainty, based on modeling of density fluctuation effects. It is found that long-wavelength radiation temperature fluctuations increase with the inverse scale length of electron temperature. This is consistent with linear gyrokinetic simulations of trapped electron mode turbulence, which show enhanced dominant linear growth rates at higher inverse scale length of electron temperature, and nonlinear gyrokinetic simulations, which when coupled with a synthetic diagnostic, reproduce the experimental trend in fluctuation amplitude. A synthetic frequency spectrum derived from a simulation of the trapped electron mode is similar in shape to the experimental frequency spectrum. The experimental observations and gyrokinetic predictions indicate that electron-temperature-gradient-driven trapped electron modes are destabilized in the core of HSX plasmas. These results improve the understanding of core turbulence in an optimized stellarator and inform optimization strategies for future devices.
Turbulent transport is responsible for much of the energy and particle losses in present-day fusion plasma experiments, and optimization to reduce turbulence will be a major step towards realizing the benefits of fusion energy. Stellarators, with the flexibility afforded by external coils and three-dimensional geometry, may be able to reduce turbulence through careful shaping of the magnetic field. Such optimization relies on the ability of simulations to accurately predict turbulence in real devices, and validation studies are severely lacking for the stellarator. In this dissertation, the magnetic flexibility of the Helically Symmetric eXperiment (HSX) stellarator is exploited to investigate Trapped Electron Mode (TEM) turbulence in quasi-helically symmetric and degraded-symmetry configurations through experimental measurements and gyrokinetic simulation. This work includes the first comparison of nonlinear simulations in the Quasi-Helically Symmetric (QHS) and Mirror configurations, as well as the first comparison of nonlinear simulations at experimental parameters to experimental measurements. A database of archived HSX plasma discharges has enabled the temperature and density profiles to be matched in QHS and Mirror, showing that thermal transport is larger in the Mirror configuration at the mid-radius. Simulations do not reproduce this difference between geometries, but transport is sensitive to whether turbulence is in a ∇[n]-driven or ∇[T][e]-driven regime. More precise gradient measurements would be required for full validation of this geometry dependence. While linear growth rates are not predictive of overall turbulence, general aspects of experimental transport are captured by nonlinear simulations. In both simulation and experiment, the heat flux and density fluctuation amplitude increase more strongly with the density gradient than the temperature gradient, and the simulated heat flux matches measurements within experimental uncertainties for both configurations. This confirms that ∇[n]-driven TEM turbulence is the dominant driver of anomalous transport in HSX. Zonal flows can be important to TEM turbulence saturation, and are present in all nonlinear simulations of HSX. This work includes the first calculation of the linear collisionless zonal flow damping in quasi-symmetric magnetic geometry. Flux-tube, flux-surface, and full-volume calculations of the zonal flow evolution and residual are compared in the QHS and Mirror configurations, as well as the quasi-axial symmetry of the National Compact Stellarator eXperiment (NCSX). Despite quasi-symmetry, the dynamics of the zonal flow in all three configurations are similar to those in a conventional stellarator. The zonal flow oscillation presents another opportunity for comparison between simulation and experiment, but measurement of the zonal flow is left to future work. This dissertation is only the starting point for a validation study on the HSX stellarator. Significant opportunities exist for updated experimental measurements and a deeper investigation into the nonlinear physics responsible for TEM dynamics.
Comprehensive, self-contained, and clearly written, this book describes the macroscopic equilibrium and stability of high temperature plasmas.
DIVOutstanding text for graduate students and research workers proposes improvements to existing algorithms, extends their related mathematical theories, and offers details on new algorithms for approximating local and global minima. /div
The Joint Varenna-Lausanne International Workshop on Theory of Fusion Plasmas takes place every other year in a place particularly favorable for informal and in depth discussions. Invited and contributed papers present state-of-the art researches in theoretical plasma physics, covering all domains relevant to fusion plasmas. This workshop always allows a fruitful mix of experienced researchers and students, to allow for a better understanding of the key theoretical physics models and applications, such as: Theoretical issues related to burning plasmas; Anomalous Transport (Turbulence, Coherent Structures, Microinstabilities) RF Heating and Current Drive; Macroinstabilities; Plasma-Edge Physics and Divertors; Fast particles instabilities.