Demonstration of reduced neoclassical energy transport in Wendelstein 7-X.

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator's non-turbulent 'neoclassical' energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

Impact of Magnetic Field Configuration on Heat Transport in Stellarators and Heliotrons.

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator. Similar plasma performances in these experiments suggests that neoclassical and turbulent transport should both be optimized in next step reactor designs.

Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas.

We theoretically assess two mechanisms thought to be responsible for the enhanced performance observed in plasma discharges of the Wendelstein 7-X stellarator experiment fueled by pellet injection. The effects of the ambipolar radial electric field and the electron density peaking on the turbulent ion heat transport are separately evaluated using large-scale gyrokinetic simulations. The essential role of the stellarator magnetic geometry is demonstrated, by comparison with a tokamak.

Calibration techniques for Thomson scattering diagnostics on large fusion experiments.

Larger fusion experiments require long beam paths for laser diagnostics, which requires mechanical stability and measures to deal with remaining alignment variations. At the same time, due to technical and organizational boundary conditions, calibrations become challenging. The current mid-sized experiments face the same issues, yet on a smaller scale, which makes them ideal testing environments for novel calibration methods, since a comparison with the established best practices is still possible. At the stellarator Wendelstein 7-X, the calibration and operation of the Thomson scattering diagnostic is hampered by beam displacements, coating of windows during operation, and access restrictions while the superconducting coils are active. New calibration techniques were developed to improve the profile quality and reduce calibration time. While positional variations of the laser beam have to be minimized, the remaining displacements can be accounted for during the absolute calibration. An in situ spectral calibration has been developed based on Rayleigh scattering, which calibrates the whole diagnostic, including observation windows. In addition, a less accurate but faster method has been developed, which utilizes stray-light of a tunable OPO to perform spectral calibration within minutes and does not require torus hall access. Finally, a workflow has been established to consider finite linewidths of the calibration source in the spectral calibration. While these methods will be used at W7-X to complement existing calibration techniques, they may also solve some of the aforementioned issues expected for even larger and nuclear experiments, where access restrictions are stringent and calibration becomes even more demanding.

Web apps for profile fitting and power balance analysis at Wendelstein 7-X.

Two novel web apps for W7-X are introduced: Profile Cooker and Power House. They are designed to streamline the workflow of profile fitting and power balance analysis while offering a graphical user interface that works in any common browser. This allows us to compile a comprehensive database of experimental power balance results. All fitting functions available in Profile Cooker are presented and compared on the basis of example profiles. The power balance equation assumed in Power House is established and its individual terms are discussed. The main focus of the power balance analysis is on the turbulent transport coefficients. A model for quick calculation of neutral beam power deposition based on experimental profiles is presented. Neoclassical root transition poses an issue for power balance analysis due to the uncertainty of the radial electric field. A global, neoclassical simulation with the code EUTERPE is performed for a set of experimental profiles to gain an understanding of the neoclassical root transition.

A key to improved ion core confinement in the JET tokamak: ion stiffness mitigation due to combined plasma rotation and low magnetic shear.

New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation.

Observation of confined current ribbon in JET plasmas.

We report the identification of a localized current structure inside the JET plasma. It is a field-aligned closed helical ribbon, carrying current in the same direction as the background current profile (cocurrent), rotating toroidally with the ion velocity (corotating). It appears to be located at a flat spot in the plasma pressure profile, at the top of the pedestal. The structure appears spontaneously in low density, high rotation plasmas, and can last up to 1.4 s, a time comparable to a local resistive time. It considerably delays the appearance of the first edge localized mode.

Charge exchange recombination spectroscopy at Wendelstein 7-X.

The Charge Exchange Recombination Spectroscopy (CXRS) diagnostic has become a routine diagnostic on almost all major high temperature fusion experimental devices. For the optimized stellarator Wendelstein 7-X (W7-X), a highly flexible and extensive CXRS diagnostic has been built to provide high-resolution local measurements of several important plasma parameters using the recently commissioned neutral beam heating. This paper outlines the design specifics of the W7-X CXRS system and gives examples of the initial results obtained, including typical ion temperature profiles for several common heating scenarios, toroidal flow and radial electric field derived from velocity measurements, beam attenuation via beam emission spectra, and normalized impurity density profiles under some typical plasma conditions.

On using Rayleigh scattering for in situ spectral calibration of Thomson scattering diagnostics.

A new method for in situ spectral calibration of Thomson scattering diagnostics is proposed. The idea of the method is to apply a wavelength tunable optical parametric oscillator for measurements of Rayleigh scattering at different wavelengths, from which relative sensitivities can be calculated. This extends the usual approach where Rayleigh scattering is used only at a single wavelength for the absolute calibration and spectral sensitivities are obtained separately. With the new approach, the full diagnostic setup is spectrally calibrated at once. Such a calibration can be repeated at regular intervals during an experimental campaign since it does not require a break of the vacuum. In this paper, the Rayleigh scattering calibration is tested in a laboratory setup with a sample Wendelstein 7-X (W7-X) polychromator. It is shown that relative sensitivities of spectral channels can be recovered with a sufficient resolution even under conditions of significant stray light. The stray light is overcome by measuring the linear dependence of the scattered signal on the gas pressure. Good results of laboratory tests motivate the installation of the new calibration system for the Thomson scattering diagnostic at W7-X.

Dual-laser wavelength Thomson scattering at Wendelstein 7-X.

This paper presents the approach of the dual-laser wavelength Thomson scattering (TS) system for the Wendelstein 7-X stellarator. The dual-laser wavelength TS method is based on two lasers with different wavelengths being fired quasi-simultaneously. This method has two advantages compared to a single laser wavelength TS system. First, the dual laser availability allows an in situ spectral calibration, and second, higher electron temperatures can be measured without any change in the spectral filter setup of the polychromators. The W7-X dual-laser wavelength TS concept is based on high power lasers: a set of standard Nd:YAG lasers with λ = 1064 nm wavelength and a Nd:YAG laser with λ = 1319 nm wavelength newly developed for this application. This laser uses a different transition line with 34% efficiency compared to the main 1064 nm Nd:YAG line. Simulations of the expected performance of the new dual-laser wavelength system show that electron temperatures up to Te = 15 keV can be measured compared to the original design parameter up to Te = 10 keV. The in situ spectral calibration can be performed using a range of temperatures from 1 keV to 10 keV using TS measurements of the 1064 nm versus 1319 nm TS simultaneously.

The Thomson scattering system at Wendelstein 7-X.

This paper describes the design of the Thomson scattering system at the Wendelstein 7-X stellarator. For the first operation campaign we installed a 10 spatial channel system to cover a radial half profile of the plasma cross section. The start-up system is based on one Nd:YAG laser with 10 Hz repetition frequency, one observation optics, five fiber bundles with one delay line each, and five interference filter polychromators with five spectral channels and silicon avalanche diodes as detectors. High dynamic range analog to digital converters with 14 bit, 1 GS/s are used to digitize the signals. The spectral calibration of the system was done using a pulsed super continuum laser together with a monochromator. For density calibration we used Raman scattering in nitrogen gas. Peaked temperature profiles and flat density profiles are observed in helium and hydrogen discharges.

Edge profile analysis of Joint European Torus (JET) Thomson scattering data: Quantifying the systematic error due to edge localised mode synchronisation.

The Joint European Torus (JET) high resolution Thomson scattering (HRTS) system measures radial electron temperature and density profiles. One of the key capabilities of this diagnostic is measuring the steep pressure gradient, termed the pedestal, at the edge of JET plasmas. The pedestal is susceptible to limiting instabilities, such as Edge Localised Modes (ELMs), characterised by a periodic collapse of the steep gradient region. A common method to extract the pedestal width, gradient, and height, used on numerous machines, is by performing a modified hyperbolic tangent (mtanh) fit to overlaid profiles selected from the same region of the ELM cycle. This process of overlaying profiles, termed ELM synchronisation, maximises the number of data points defining the pedestal region for a given phase of the ELM cycle. When fitting to HRTS profiles, it is necessary to incorporate the diagnostic radial instrument function, particularly important when considering the pedestal width. A deconvolved fit is determined by a forward convolution method requiring knowledge of only the instrument function and profiles. The systematic error due to the deconvolution technique incorporated into the JET pedestal fitting tool has been documented by Frassinetti et al. [Rev. Sci. Instrum. 83, 013506 (2012)]. This paper seeks to understand and quantify the systematic error introduced to the pedestal width due to ELM synchronisation. Synthetic profiles, generated with error bars and point-to-point variation characteristic of real HRTS profiles, are used to evaluate the deviation from the underlying pedestal width. We find on JET that the ELM synchronisation systematic error is negligible in comparison to the statistical error when assuming ten overlaid profiles (typical for a pre-ELM fit to HRTS profiles). This confirms that fitting a mtanh to ELM synchronised profiles is a robust and practical technique for extracting the pedestal structure.

Overview of diagnostic performance and results for the first operation phase in Wendelstein 7-X (invited).

Wendelstein 7-X, a superconducting optimized stellarator built in Greifswald/Germany, started its first plasmas with the last closed flux surface (LCFS) defined by 5 uncooled graphite limiters in December 2015. At the end of the 10 weeks long experimental campaign (OP1.1) more than 20 independent diagnostic systems were in operation, allowing detailed studies of many interesting plasma phenomena. For example, fast neutral gas manometers supported by video cameras (including one fast-frame camera with frame rates of tens of kHz) as well as visible cameras with different interference filters, with field of views covering all ten half-modules of the stellarator, discovered a MARFE-like radiation zone on the inboard side of machine module 4. This structure is presumably triggered by an inadvertent plasma-wall interaction in module 4 resulting in a high impurity influx that terminates some discharges by radiation cooling. The main plasma parameters achieved in OP1.1 exceeded predicted values in discharges of a length reaching 6 s. Although OP1.1 is characterized by short pulses, many of the diagnostics are already designed for quasi-steady state operation of 30 min discharges heated at 10 MW of ECRH. An overview of diagnostic performance for OP1.1 is given, including some highlights from the physics campaigns.

Preoperative CA-125 levels predict poor prognostic pathologic features in early stage, FIGO grade 1 and 2 endometrial adenocarcinoma.

Preoperative CA-125 levels were studied in patients with favorable histology and early clinical stage endometrial adenocarcinoma to determine its ability to predict the presence of poor pathologic prognostic features on final pathology. One hundred and one patients with clinical stage I (N = 65) or II (N = 19) or diagnosed by endometrial curettage (EMC) only (N-17) with grade 1 or 2 endometrial adenocarcinoma without gross cervical involvement underwent preoperative CA-125 levels. Final pathology was reviewed for five poor prognostic pathologic features: FIGO grade 3 histology, unfavorable histologic type (sarcoma, clear cell, or papillary serous), invasion into the outer third of the myometrium, extension to the cervix, and extra-uterine metastases. Fifteen patients (14.9%) had CA-125 levels greater than 30 IU ml-1. Of these 15 patients, 12 had one or more of the five poor prognostic pathologic features (positive predictive value 80.0%, specificity 95.8%, P < 0.0001). However, since 30 of the 101 patients were found to have one or more of these poor prognostic pathologic features the sensitivity was only 40.0%. When clinical stage I patients were analyzed separately three patients (4.6%) had CA-125 levels greater than 30 IU ml -1 (positive predictive value 100%, specificity of 100%, sensitivity of 21.4%, P = 0.008). For patients with clinical stage II carcinoma, CA-125 was not predictive of pathologic findings except as a negative predictor of disease in a subgroup of patients whose endocervical curettage (ECC) demonstrated carcinoma unattached to endocervical tissue. In patients diagnosed by EMC only, an elevated CA-125 level was associated with poor prognostic pathologic features (P = 0.001). An elevated preoperative CA-125 reliably predicts advanced disease even in patients with apparently favorable histology and clinical stage, however the sensitivity of this method remains low.

[Subfertility in South Limburg: calculation of incidence and appeal for specialist care]. / Subfertiliteit in Zuid-Limburg: berekening van incidentie en van beroep op specialistische zorg.

OBJECTIVE: To estimate the prevalence of subfertility in a region of the Netherlands clearly defined as to medical care. DESIGN: Descriptive. SETTING: Six regional hospitals and one university medical centre. METHODS: Prospective registration of all 1060 new patients attending one of the seven hospitals and complaining of subfertility, in 1992. RESULTS: After exclusion of patients living outside the health care region, and/or having pursued pregnancy for less than 12 months, and/or having visited one of the other hospitals (with the same complaint) before, 536 patients remained. For this region, with a population of 641,876 at the time of the study, we estimated the cumulative incidence of subfertility in 1992 to be 10.4% (95% confidence interval: 9.6-11.2%). Between 14 and 16.5% of couples sought specialist medical care for their fertility problem. CONCLUSION: Between 14 and 16.5% of couples sought specialist medical care for fertility problems during their reproductive life. More than 10 percent fulfilled the criteria for subfertility.

Note: statistical errors estimation for Thomson scattering diagnostics.

A practical way of estimating statistical errors of a Thomson scattering diagnostic measuring plasma electron temperature and density is described. Analytically derived expressions are successfully tested with Monte Carlo simulations and implemented in an automatic data processing code of the JET LIDAR diagnostic.

Electron cyclotron emission measurements on JET: Michelson interferometer, new absolute calibration, and determination of electron temperature.

At the fusion experiment JET, a Michelson interferometer is used to measure the spectrum of the electron cyclotron emission in the spectral range 70-500 GHz. The interferometer is absolutely calibrated using the hot/cold technique and, in consequence, the spatial profile of the plasma electron temperature is determined from the measurements. The current state of the interferometer hardware, the calibration setup, and the analysis technique for calibration and plasma operation are described. A new, full-system, absolute calibration employing continuous data acquisition has been performed recently and the calibration method and results are presented. The noise level in the measurement is very low and as a result the electron cyclotron emission spectrum and thus the spatial profile of the electron temperature are determined to within ±5% and in the most relevant region to within ±2%. The new calibration shows that the absolute response of the system has decreased by about 15% compared to that measured previously and possible reasons for this change are presented. Temperature profiles measured with the Michelson interferometer are compared with profiles measured independently using Thomson scattering diagnostics, which have also been recently refurbished and recalibrated, and agreement within experimental uncertainties is obtained.

Spatial resolution of the JET Thomson scattering system.

The instrument function of the high resolution Thomson scattering (HRTS) diagnostic in the Joint European Torus (JET) has been calculated for use in improved pedestal profile analysis. The full width at half maximum (FWHM) of the spatial instrument response is (22 ± 1) mm for the original HRTS system configuration and depends on the particular magnetic topology of the JET plasmas. An improvement to the optical design of the laser input system is presented. The spatial smearing across magnetic flux surfaces is reduced in this design. The new input system has been implemented (from JPN 78742, July 2009) and the HRTS instrument function corresponding to the new configuration has been improved to approximately FWHM = (9.8 ± 0.8) mm. The reconstructed instrument kernels are used in combination with an ad hoc forward deconvolution procedure for pedestal analysis. This procedure produces good results for both the old and new setups, but the reliability of the deconvolved profiles is greatly reduced when the pedestal width is of the same order as, or less than the FWHM of the instrument kernel.

Deconvolution of Thomson scattering temperature profiles.

Deconvolution of Thomson scattering (TS) profiles is required when the gradient length of the electron temperature (T(e)) or density (n(e)) are comparable to the instrument function length (Δ(R)). The most correct method for deconvolution to obtain underlying T(e) and n(e) profiles is by consideration of scattered signals. However, deconvolution at the scattered signal level is complex since it requires knowledge of all spectral and absolute calibration data. In this paper a simple technique is presented where only knowledge of the instrument function I(r) and the measured profiles, T(e, observed)(r) and n(e, observed)(r), are required to obtain underlying T(e)(r) and n(e)(r). This method is appropriate for most TS systems and is particularly important where high spatial sampling is obtained relative to Δ(R).