Intermittent electron density and temperature fluctuations and associated fluxes in the Alcator C-Mod scrape-off layer

The Alcator C-Mod mirror Langmuir probe system has been used to sample data time series of fluctuating plasma parameters in the outboard mid-plane far scrape-off layer. We present a statistical analysis of one second long time series of electron density, temperature, radial electric drift velocity and the corresponding particle and electron heat fluxes. These are sampled during stationary plasma conditions in an ohmically heated, lower single null diverted discharge. The electron density and temperature are strongly correlated and feature fluctuation statistics similar to the ion saturation current. Both electron density and temperature time series are dominated by intermittent, large-amplitude burst with an exponential distribution of both burst amplitudes and waiting times between them. The characteristic time scale of the large-amplitude bursts is approximately 15{\mu}s. Large-amplitude velocity fluctuations feature a slightly faster characteristic time scale and appear at a faster rate than electron density and temperature fluctuations. Describing these time series as a superposition of uncorrelated exponential pulses, we find that probability distribution functions, power spectral densities as well as auto-correlation functions of the data time series agree well with predictions from the stochastic model. The electron particle and heat fluxes present large-amplitude fluctuations. For this low-density plasma, the radial electron heat flux is dominated by convection, that is, correlations of fluctuations in the electron density and radial velocity. Hot and dense blobs contribute approximately 6% of the total fluctuation driven heat flux

Fluctuation statistics in the scrape-off layer of Alcator C-Mod

We study long time series of the ion saturation current and floating potential, sampled by Langmuir probes dwelled in the outboard mid-plane scrape off layer and embedded in the lower divertor baffle of Alcator C-Mod. A series of ohmically heated L-mode plasma discharges is investigated with line-averaged plasma density ranging from n_e/n_G = 0.15 to 0.42, where n_G is the Greenwald density. All ion saturation current time series that are sampled in the far scrape-off layer are characterized by large-amplitude burst events. Coefficients of skewness and excess kurtosis of the time series obey a quadratic relationship and their histograms coincide partially upon proper normalization. Histograms of the ion saturation current time series are found to agree well with a prediction of a stochastic model for the particle density fluctuations in scrape-off layer plasmas. The distribution of the waiting times between successive large-amplitude burst events and of the burst amplitudes are approximately described by exponential distributions. The average waiting time and burst amplitude are found to vary weakly with the line-averaged plasma density. Conditional averaging reveals that the radial blob velocity, estimated from floating potential measurements, increases with the normalized burst amplitude in the outboard mid-plane scrape-off layer. For low density discharges, the conditionally averaged waveform of the floating potential associated with large amplitude bursts at the divertor probes has a dipolar shape. In detached divertor conditions the average waveform is random, indicating electrical disconnection of blobs from the sheaths at the divertor targets.Comment: 45 pages, 20 figure

Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer

This is the peer reviewed version of the following article: Theodorsen, A., Garcia, O.E., Kube, R., LaBombard, B. & Terry, J.L. (2017). Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer. Nuclear Fusion, 57(114004). https://doi.org/10.1088/1741-4326/aa7e4c, which has been published in final form at https://doi.org/10.1088/1741-4326/aa7e4c.Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data from a set of Ohmically heated plasma density scan experiments. It is found that the relative fluctuation amplitudes are modest and close to normally distributed at the separatrix but become increasingly larger and skewed towards the main chamber wall. The frequency power spectra are nevertheless similar for all radial positions and line-averaged densities. Predictions of a stochastic model, describing the plasma fluctuations as a super-position of uncorrelated pulses, are shown to be in excellent agreement with the measurements. This implies that the pulse duration is the same, while the degree of pulse overlap decreases radially outwards in the scrape-off layer. The universal frequency power spectral density is thus determined by the shape and duration of the large-amplitude bursts associated with blob-like structures. The model also describes the rate of threshold level crossings, for which the exponential tails underline the intermittency of the fluctuations in the far scarpe-off layer

Fast imaging of filaments in the X-point region of Alcator C-Mod

A rich variety of field-aligned fluctuations has been revealed using fast imaging of Dαemission from Alcator C-Mod's lower X-point region. Field-aligned filamentary fluctuations are observed along the inner divertor leg, within the Private-Flux-Zone (PFZ), in the Scrape-Off Layer (SOL) outside the outer divertor leg, and, under some conditions, at or above the X-point. The locations and dynamics of the filaments in these regions are strikingly complex in C-Mod. Changes in the filaments’ generation appear to be ordered by plasma density and magnetic configuration. Filaments are not observed for plasmas with n/nGreenwald≲ 0.12 nor are they observed in Upper Single Null configurations. In a Lower Single Null with 0.12 ≲ n/nGreenwald ≲ 0.45 and Bx∇B directed down, filaments typically move up the inner divertor leg toward the X-point. Reversing the field direction results in the appearance of filaments outside of the outer divertor leg. With the divertor targets “detached”, filaments inside the LCFS are seen. These studies were motivated by observations of filaments in the X-point and PFZ regions in MAST, and comparisons with those observations are made. Keywords: Alcator C-Mod; Turbulence; Divertor; X-point; Filament

Strongly intermittent far scrape-off layer plasma fluctuations in Alcator C-Mod plasmas close to the empirical discharge density limit

Intermittent plasma fluctuations in the boundary region of the Alcator C-Mod device were comprehensively investigated using data times-series from gas puff imaging and mirror Langmuir probe diagnostics. Fluctuations were sampled during stationary plasma conditions in ohmically heated, lower single null diverted configurations with scans in both line-averaged density and plasma current, with Greenwald density fractions up to $0.85$. Utilizing a stochastic model, we describe the plasma fluctuations as a super-position of uncorrelated pulses, with large-amplitude events corresponding to blob-like filaments moving through the scrape-off layer. A deconvolution method is used to estimate the pulse arrivals and amplitudes. The analysis reveals a significant increase of pulse amplitudes and waiting times between pulses as the line-averaged density approaches the empirical discharge density limit. Broadened and flattened average radial profiles are thus accompanied by strongly intermittent and large-amplitude fluctuations. Although these filaments are arriving less frequently at high line-averaged densities, we show that there are significant increases in radial far-SOL particle and heat flux which will further enhance plasma--wall interactions. The stochastic model has been used as the framework for study of the scalings in the intermittency, flux and mean waiting times and mean amplitudes and is being used to inform predictive capability for the effects of filamentary transport as a function of Greenwald fraction

Smaller & Sooner: Exploiting High Magnetic Fields from New Superconductors for a More Attractive Fusion Energy Development Path

The current fusion energy development path, based on large volume moderate magnetic B field devices is proving to be slow and expensive. A modest development effort in exploiting new superconductor magnet technology development, and accompanying plasma physics research at high-B, could open up a viable and attractive path for fusion energy development. This path would feature smaller volume, fusion capable devices that could be built more quickly than low-to-moderate field designs based on conventional superconductors. Fusion’s worldwide development could be accelerated by using several small, flexible devices rather than relying solely on a single, very large device. These would be used to obtain the acknowledged science and technology knowledge necessary for fusion energy beyond achievement of high gain. Such a scenario would also permit the testing of multiple confinement configurations while distributing technical and scientific risk among smaller devices. Higher field and small size also allows operation away from well-known operational limits for plasma pressure, density and current. The advantages of this path have been long recognized—earlier US plans for burning plasma experiments (compact ignition tokamak, burning plasma experiment, fusion ignition research experiment) featured compact high-field designs, but these were necessarily pulsed due to the use of copper coils. Underpinning this new approach is the recent industrial maturity of high-temperature, high-field superconductor tapes that would offer a truly “game changing” opportunity for magnetic fusion when developed into large-scale coils. The superconductor tape form and higher operating temperatures also open up the possibility of demountable superconducting magnets in a fusion system, providing a modularity that vastly improves simplicity in the construction, maintenance, and upgrade of the coils and the internal nuclear engineering components required for fusion’s development. Our conclusion is that while tradeoffs exist in design choices, for example coil, cost and stress limits versus size, the potential physics and technology advantages of high-field superconductors are attractive and they should be vigorously pursued for magnetic fusion’s development

Statistical properties of fluctuation driven flows in the outboard mid-plane SOL of Alcator C-Mod

The scale length of the radial density profile in scrape-off layer plasmas has been shown to depend on the line-averaged plasma density. While first reported on AlcatorC-Mod, recent work verifies this phenomena in JET and ASDEX Upgrade. In low density plasmas with a sheath-limited SOL, the density profiles present a two-scale structure which allows the separation of the SOL into two distinct regions. While the near SOL presents a small profile length scale, the far-SOL presents a flat density profile length scale. Transitioning into detached divertor conditions by increasing the line-averaged plasma density, the density profile features a shallow length scale. In this contribution, AlcatorC-Mods Mirror Langmuir Probe system is used to investigate the radial profiles as well as the statistical properties of flucutations in the electron density, temperature, as well as the fluctuation driven fluxes

Blob shapes in the scrape-off layer: Comparison of measurements to simulations

Poster fra konferanse

Universality of intermittent fluctuations in the Alcator C-Mod scrape-off layer

Conference presentatio

Heat-flux footprints for I-mode and EDA H-mode plasmas on Alcator C-Mod

IR thermography is used to measure the heat flux footprints on C-Mod’s outer target in I-mode and EDA H-mode plasmas. The footprint profiles are fit to a function with a simple physical interpretation. The fit parameter that is sensitive to the power decay length into the SOL, λ[subscript SOL], is ~1–3× larger in I-modes than in H-modes at similar plasma current, which is the dominant dependence for the H-mode λ[subscript SOL]. In contrast, the fit parameter sensitive to transport into the private-flux-zone along the divertor leg is somewhat smaller in I-mode than in H-mode, but otherwise displays no obvious dependence on I[subscript p], B[subscript t], or stored energy. A third measure of the footprint width, the “integral width”, is not significantly different between H- and I-modes. Also discussed are significant differences in the global power flows of the H-modes with “favorable” ∇B drift direction and those of the I-modes with “unfavorable” ∇B drift direction.United States. Dept. of Energy (Cooperative Agreement DE-FC02-99-ER54512