David Pretty

Suppression of large edge localized modes with edge resonant magnetic fields in high confinement DIII-D plasmas

Large sub-millisecond heat pulses due to Type-I edge localized modes (ELMs) have been eliminated reproducibly in DIII-D for periods approaching nine energy confinement times (τE) with small dc currents driven in a simple magnetic perturbation coil. The current required to eliminate all but a few isolated Type-I ELM impulses during a coil pulse is less than 0.4% of plasma current. Based on magnetic field line modelling, the perturbation fields resonate with plasma flux surfaces across most of the pedestal region (0.9 ≤ ψN ≤ 1.0) when q95 = 3.7 ± 0.2, creating small remnant magnetic islands surrounded by weakly stochastic field lines. The stored energy, βN, H-mode quality factor and global energy confinement time are unaltered by the magnetic perturbation. Although some isolated ELMs occur during the coil pulse, long periods free of large Type-I ELMs (Δt > 4–6 τE) have been reproduced numerous times, on multiple experimental run days in high and intermediate triangularity plasmas, including cases matching the baseline ITER scenario 2 flux surface shape. In low triangularity, lower single null plasmas, with collisionalities near that expected in ITER, Type-I ELMs are replaced by small amplitude, high frequency Type-II-like ELMs and are often accompanied by one or more ELM-free periods approaching 1–2 τE. Large Type-I ELM impulses represent a severe constraint on the survivability of the divertor target plates in future burning plasma devices. Results presented in this paper demonstrate that non-axisymmetric edge magnetic perturbations provide a very attractive development path for active ELM control in future tokamaks such as ITER.

Alfvén eigenmodes measured in the TJ-II stellarator

High frequency modes (150-300 kHz) are found in several magnetic configurations of TJ-II plasmas heated by neutral beam injection (NBI). The clear dependence of mode frequency on plasma density and mass species suggests them to be Alfveigenmodes. The appearance of these modes is linked to the presence of low order rational surfaces close to the rotational transform profile. They can exhibit steady or chirping behaviour depending on the plasma profiles. Frequency chirping is observed in NBI plasmas with broad temperature profiles, but rarely observed with relatively peaked profiles. The Alfvactivity has been characterized in detail with magnetic coils for the standard configuration. Cross analyses with heavy ion beam probe and reflectometer signals have yielded spatial resolution and radial profiles of the perturbation. Correlation between magnetic coil signals and signals from diagnostics sensitive to edge ion losses, namely Langmuir probes and a fast ion loss detector, has been observed in some cases and characterized taking advantage of the chirping nature of the observed Alfvactivity. (Some figures in this article are in colour only in the electronic version)

Fluctuations and stability of plasmas in the H-1NF heliac

The H-1NF heliac is a medium-sized heliac stellarator experiment with major radius R = 1 m, and average plasma minor radius a = 0.15–0.2 m. Its ‘flexible-heliac’ coil set permits precise variation in the value and shape of the rotational transform (ι) profile, with regions of both positive and negative shear. Operation at low fields ( B< 0. 2T ) with argon plasmas heated by helicon waves produces plasmas that have large ion Larmor radii (ρi/a ∼ 0.4) and show confinement transitions at low power like those in the edge of large devices, yielding fundamental measurements concerning electric fields and zonal flows. At a higher field (0.5 T), precise rotational transform scans with H–He plasmas heated by ICRF show resonant equilibrium and stability phenomena which depend on the value of the rotational transform at the radius of zero shear.

A data mining algorithm for automated characterisation of fluctuations in multichannel timeseries

Abstract We present a data mining technique for the analysis of multichannel oscillatory timeseries data and show an application using poloidal arrays of magnetic sensors installed in the H-1 heliac. The procedure is highly automated, and scales well to large datasets. The timeseries data is split into short time segments to provide time resolution, and each segment is represented by a singular value decomposition (SVD). By comparing power spectra of the temporal singular vectors, related singular values are grouped into subsets which define fluctuation structures . Thresholds for the normalised energy of the fluctuation structure and the normalised entropy of the SVD can be used to filter the dataset. We assume that distinct classes of fluctuations are localised in the space of phase differences Δ ψ ( n , n + 1 ) between each pair of nearest neighbour channels. An expectation maximisation clustering algorithm is used to locate the distinct classes of fluctuations and assign mode numbers where possible, and a cluster tree mapping is used to visualise the results.

A multichannel magnetic probe system for analysing magnetic fluctuations in helical axis plasmas

The need to understand the structure of magnetic fluctuations in H-1NF heliac [S. Hamberger et al., Fusion Technol. 17, 123 (1990)] plasmas has motivated the installation of a sixteen former, tri-axis helical magnetic probe Mirnov array (HMA). The new array complements two existing poloidal Mirnov arrays by providing polarisation information, higher frequency response, and improved toroidal resolution. The helical placement is ideal for helical axis plasmas because it positions the array as close as possible to the plasma in regions of varying degrees of favourable curvature in the magnetohydrodynamic sense, but almost constant magnetic angle. This makes phase variation with probe position near linear, greatly simplifying the analysis of the data. Several of the issues involved in the design, installation, data analysis, and calibration of this unique array are presented including probe coil design, frequency response measurements, mode number identification, orientation calculations, and mapping probe coil positions to magnetic coordinates. Details of specially designed digitally programmable pre-amplifiers, which allow gains and filters to be changed as part of the data acquisition initialisation sequence and stored with the probe signals, are also presented. The low shear heliac geometry [R. Jiménez-Gómez et al., Nucl. Fusion 51, 033001 (2011)], flexibility of the H-1NF heliac, and wealth of information provided by the HMA create a unique opportunity for detailed study of Alfvén eigenmodes, which could be a serious issue for future fusion reactors.

A reduced global Alfven eigenmodes model for Mirnov array data on the H-1NF heliac

We extend a reduced-dimension stellarator ideal-MHD normal-mode model to include a vacuum region, and describe magnetic fluctuations in the H-1 heliac. We apply the reduced model to a magnetic field configuration with high density and compute the two lowest frequency global Alfven eigenmodes (GAEs): (m, n) = (4, 5) and (7, 9). The poloidal mode number, predicted frequency and radial attenuation profile are then compared with measurements from a poloidal Mirnov array. Of the two candidates, the (7, 9) mode has a closer match in frequency and radial attenuation profile, and its narrower radial eigenfunction is a better match to recent optical-emission measurements [18]. Measurements of the temporal evolution of the mode are also consistent with the inferred Alfven frequency scaling at the radial localization of the (7, 9) mode. Combined, these observations suggest the measured fluctuation is a (7, 9) GAE. A wider benefit of our work includes the potential of a reduced-dimension normal-mode MHD model, with negligible computational needs, to characterize and identify mode activity in real time based on frequency, phase information, internal measurements and vacuum region attenuation. Such a model, which is likely to improve in accuracy for devices with simpler configuration such as a tokamak, may be useful as a tool for real time MHD spectroscopy, and to predict and control global eigenmodes associated with fast ion loss in burning plasmas.

The use of Bayesian inversion to resolve plasma equilibrium

Recently, bayesian probability theory has been used at a number of experiments to fold uncertainties and interdependencies in the diagnostic data and forward models, together with prior knowledge of the state of the plasma, to increase accuracy of inferred physics variables. A new probabilistic framework, MINERVA, based on bayesian graphical models, has been used at JET and W7-AS to yield predictions of internal magnetic structure. A feature of the framework is the bayesian inversion for poloidal magnetic flux without the need for an explicit equilibrium assumption. Building on this, we discuss results from a new project to develop bayesian inversion tools that aim to (1) distinguish between competing equilibrium theories, which capture different physics, using the MAST spherical tokamak, and (2) test the predictions of MHD theory, particularly mode structure, using the H-1 Heliac. Specifically, we report on correction of the motional Stark effect, pickup coils, flux-loop constrained bayesian inferred equilibrium for varying toroidal flux.

Clustering of periodic multichannel timeseries data with application to plasma fluctuations

Abstract A periodic datamining algorithm has been developed and used to extract distinct plasma fluctuations in multichannel oscillatory timeseries data. The technique uses the Expectation Maximisation algorithm to solve for the maximum likelihood estimates and cluster assignments of a mixture of multivariate independent von Mises distributions (EM-VMM). The performance of the algorithm shows significant benefits when compared to a periodic k-means algorithm and clustering using non-periodic techniques on several artificial datasets and real experimental data. Additionally, a new technique for identifying interesting features in multichannel oscillatory timeseries data is described (STFT-clustering). STFT-clustering identifies the coincidence of spectral features over most channels of a multi-channel array using the averaged short time Fourier transform of the signals. These features are filtered using clustering to remove noise. This method is particularly good at identifying weaker features and complements existing methods of feature extraction. Results from applying the STFT-clustering and EM-VMM algorithm to the extraction and clustering of plasma wave modes in the time series data from a helical magnetic probe array on the H-1NF heliac are presented.

Three view electronically scanned interferometer for plasma electron density measurements on the H-1 heliac

We report the development of a three view electronically scanned millimeter-wave interferometer for plasma electron density profile measurement on the H-1 heliac. The system utilizes an electronically tunable backward-wave oscillator whose output is incident on a fixed blazed diffraction grating such that sweeping the source frequency effects a spatial scan of the plasma cross section. Two diagonal views essentially span most of the plasma cross section, while the horizontal arm views the lower half of the plasma. The diffracted beams traverse the plasma in <1ms with a spatial resolution ∼20mm. A study of the density projection dependence on magnetic configuration shows that the presence of low-order rational surfaces in the plasma gives rise to sharp density gradients in the vicinity of the surface.

Studies of resonantly produced plasmas in the H-1NF heliac using a far-infrared scanning interferometer

The H-1NF heliac regularly operates using the ion cyclotron range of frequencies at 0.5 T to produce plasmas with a mixture of hydrogen and helium gases. Due to the complex three-dimensional structure of the magnetic fluxsurfaces, these plasmas require sophisticated diagnostic systems, with good spatial coverage, to extract meaningful physical information. This article presents a study of the dependence of the plasma density profile on resonant heating conditions and magnetic configuration, using a far-infrared scanning interferometer. Recent modifications to the system and data that illustrate the performance of the interferometer will be discussed.