Abstract
A laser blow-off (LBO) system has been installed in the HSX stellarator to investigate impurity transport. A Nd:YAG laser is used to ablate a target area on a glass slide that has been ...coated with a thin film of aluminum and rapidly deposits a small, controlled quantity of aluminum atoms into HSX plasma. To study the radial propagation and confinement of the injected aluminum impurities, time-resolved emission measurements are made using a 20 channel AXUV photodiode detection system. The LBO system has achieved its design purpose, most notably the ability to make controlled impurity injection without significant perturbation to the background plasma. A detailed description of the setup as well as initial results of operation are presented.
The edge topology of helically symmetric experiment (HSX) in the quasi-helically symmetric configuration is characterized by an 8/7 magnetic island remnant embedded in a short connection length ...scrape-off layer (SOL) domain. A 2D mapping of edge plasma profiles within this heterogeneous SOL has been constructed using a movable, multi-pin Langmuir probe. Comparisons of these measurements to edge simulations using the EMC3-EIRENE 3D plasma fluid and kinetic neutral gas transport model have been performed. The measurements provide strong evidence that particle transport is diffusive within the island region and dominantly convective in the SOL region. Measurements indicate that phenomenological cross-field diffusion coefficients are low in the SOL region between the last closed flux surface and edge island (i.e. D⊥≈0.03 m2 s−1). This level of transport was found to increase by a factor of two when a limiter is inserted almost completely into the magnetic island. A reduction in gradients of the edge electrostatic plasma potential was also measured in this configuration, suggesting that the reduced electric field may be linked to the increased cross-field transport observed.
To increase flexibility in ECRH experiments on the helically symmetric experiment (HSX), a second gyrotron and transmission line have been installed. The second antenna includes a steerable mirror ...for off-axis heating, and the launched power may be modulated for use in heat pulse propagation experiments. The extraordinary wave at the second harmonic of the electron gyrofrequency or the ordinary wave at the fundamental resonance are used for plasma start-up and heating on HSX. The tracing visualized ray tracing code (Marushchenko et al 2007 Plasma Fusion Res. 2 S1129) is used to estimate single-pass absorption and to model multi-pass wave damping in the three-dimensional HSX geometry. The single-pass absorption of the ordinary wave at the fundamental resonance is calculated to be as high as 30%, while measurements of the total absorption indicate that 45% of the launched power is absorbed. A multi-pass ray tracing model correctly predicts the experimental absorption and indicates that the launched power is absorbed within the plasma core ().
Abstract
Spontaneous plasma flows have been observed in electron cyclotron heating (ECH) plasmas in the helically symmetric experiment. A smaller parallel flow was observed in the quasi-helically ...symmetric configuration compared with that observed in the Mirror configuration, although the mirror configuration has a larger neoclassical viscosity. Using the GNET code, we evaluate the electromagnetic and collisional forces induced by ECH. We also evaluate the parallel flow driven by the ECH force, by solving the momentum balance equations and Ampère law. The obtained flows show reasonable agreement with results obtained from experiments. This work indicates that the radial electron current generated by ECH could drive the parallel flow.
Differences in the electron particle and thermal transport are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The ...thermal diffusivity is reduced in the QHS configuration, resulting in higher electron temperatures than in the nonsymmetric configuration for a fixed power input. The density profile in QHS plasmas is centrally peaked, and in the nonsymmetric configuration the core density profile is hollow. The hollow profile is due to neoclassical thermodiffusion, which is reduced in the QHS configuration.
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Energetic electrons generated by electron cyclotron resonance heating are observed to drive instabilities in the quasihelically symmetric stellarator device. The coherent, global fluctuations peak in ...the plasma core and are measured in the frequency range of 20-120 kHz. Mode propagation is in the diamagnetic drift direction of the driving species. When quasihelical symmetry is broken, the mode is no longer observed. Experimental observations indicate that the unstable mode is acoustic rather than Alfvénic.
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This Letter presents theory-based predictions of anomalous electron thermal transport in the Helically Symmetric eXperiment stellarator, using an axisymmetric trapped-electron mode drift wave model. ...The model relies on modifications to a tokamak geometry that approximate the quasihelical symmetry in the Helically Symmetric eXperiment (particle trapping and local curvature) and is supported by linear 3D gyrokinetic calculations. Transport simulations predict temperature profiles that agree with experimental profiles outside a normalized minor radius of rho>0.3 and energy confinement times that agree within 10% of measurements. The simulations can reproduce the large measured electron temperatures inside rho<0.3 if an approximation for turbulent transport suppression due to shear in the radial electric field is included.
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Inboard/outboard asymmetry in the impurity ion parallel flow is observed in the HSX stellarator using charge exchange recombination spectroscopy (CHERS). This observation shows the presence of ...counter-streaming Pfirsch-Schlüter flow predicted by neoclassical theory. The asymmetry of the flow is used to calculate the magnitude and direction of the radial electric field (Er), as well as the mean flow, using computed magnetic geometry factors. This method enables calculation of Er near the core of the HSX plasma where the Er obtained from the radial force balance equation has large uncertainties due to the relatively large width of the diagnostic neutral beam with respect to the plasma minor radius.
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