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.

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.

Correction and verification of x-ray imaging crystal spectrometer analysis on Wendelstein 7-X through x-ray ray tracing.

X-ray ray tracing is used to develop ion-temperature corrections for the analysis of the X-ray Imaging Crystal Spectrometer (XICS) used at Wendelstein 7-X (W7-X) and perform verification on the analysis methods. The XICS is a powerful diagnostic able to measure ion-temperature, electron-temperature, plasma flow, and impurity charge state densities. While these systems are relatively simple in design, accurate characterization of the instrumental response and validation of analysis techniques are difficult to perform experimentally due to the requirement of extended x-ray sources. For this reason, a ray tracing model has been developed that allows characterization of the spectrometer and verification of the analysis methods while fully considering the real geometry of the XICS system and W7-X plasma. Through the use of ray tracing, several important corrections have been found that must be accounted for in order to accurately reconstruct the ion-temperature profiles. The sources of these corrections are described along with their effect on the analyzed profiles. The implemented corrections stem from three effects: (1) effect of sub-pixel intensity distribution during de-curving and spatial binning, (2) effect of sub-pixel intensity distribution during forward model evaluation and generation of residuals, and (3) effect of defocus and spherical aberrations on the instrumental response. Possible improvements to the forward model and analysis procedures are explored, along with a discussion of trade-offs in terms of computational complexity. Finally, the accuracy of the tomographic inversion technique in stellarator geometry is investigated, providing for the first time a verification exercise for inversion accuracy in stellarator geometry and a complete XICS analysis tool-chain.

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.

Inference of temperature and density profiles via forward modeling of an x-ray imaging crystal spectrometer within the Minerva Bayesian analysis framework.

At the Wendelstein 7-X stellarator, the X-ray imaging crystal spectrometer provides line integrated measurements of ion and electron temperatures, plasma flows, as well as impurity densities from a spectroscopic analysis of tracer impurity radiation. In order to infer the actual profiles from line integrated data, a forward modeling approach has been developed within the Minerva Bayesian analysis framework. In this framework, the inversion is realized on the basis of a complete forward model of the diagnostic, including error propagation and utilizing Gaussian processes for generation and inference of arbitrary shaped plasma parameter profiles. For modeling of line integrated data as measured by the detector, the installation geometry of the spectrometer, imaging properties of the crystal, and Gaussian detection noise are considered. The inversion of line integrated data is achieved using the maximum posterior method for plasma parameter profile inference and a Markov chain Monte Carlo sampling of the posterior distribution for calculating uncertainties of the inference process. The inversion method shows a correct and reliable inference of temperature and impurity density profiles from synthesized data within the estimated uncertainties along the whole plasma radius. The application to measured data yields a good match of derived electron temperature profiles to data of the Thomson scattering diagnostic for central electron temperatures between 2 and 5 keV using argon impurities.

Stellarators Resist Turbulent Transport on the Electron Larmor Scale.

Electron temperature gradient (ETG)-driven turbulence, despite its ultrafine scale, is thought to drive significant thermal losses in magnetic fusion devices-but what role does it play in stellarators? The first numerical simulations of ETG turbulence for the Wendelstein 7-X stellarator, together with power balance analysis from its initial experimental operation phase, suggest that the associated transport should be negligible compared to other channels. The effect, we argue, originates essentially from the geometric constraint of multiple field periods, a generic feature of stellarators.

Plasma impurities observed by a pulse height analysis diagnostic during the divertor campaign of the Wendelstein 7-X stellarator.

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

Bayesian uncertainty calculation in neural network inference of ion and electron temperature profiles at W7-X.

We make use of a Bayesian description of the neural network (NN) training for the calculation of the uncertainties in the NN prediction. Having uncertainties on the NN prediction allows having a quantitative measure for trusting the NN outcome and comparing it with other methods. Within the Bayesian framework, the uncertainties can be calculated under different approximations. The NN has been trained with the purpose of inferring ion and electron temperature profile from measurements of a X-ray imaging diagnostic at W7-X. The NN has been trained in such a way that it constitutes an approximation of a full Bayesian model of the diagnostic, implemented within the Minerva framework. The network has been evaluated using measured data and the uncertainties calculated under different approximations have been compared with each other, finding that neglecting the noise on the NN input can lead to an underestimation of the error bar magnitude in the range of 10%-30%.

Prospects of X-ray imaging spectrometers for impurity transport: Recent results from the stellarator Wendelstein 7-X (invited).

This paper reports on the design and the performance of the recently upgraded X-ray imaging spectrometer systems, X-ray imaging crystal spectrometer and high resolution X-ray imaging spectrometer, installed at the optimized stellarator Wendelstein 7-X. High resolution spectra of highly ionized, He-like Si, Ar, Ti, and Fe as well as H-like Ar have been observed. A cross comparison of ion and electron temperature profiles derived from a spectral fit and tomographic inversion of Ar and Fe spectra shows a reasonable match with both the spectrometers. The also measured impurity density profiles of Ar and Fe have peaked densities at radial positions that are in qualitative agreement with the expectations from the He-like impurity fractional abundances, given the measured temperature profiles. Repeated measurements of impurity decay times have been demonstrated with an accuracy of 1 ms via injection of non-recycling Ti, Fe, and Mo impurities using a laser blow-off system.

In situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X.

An in situ wavelength calibration system for the X-ray Imaging Crystal Spectrometer (XICS) on W7-X has been developed to provide routine calibration between plasma shots. XICS is able to determine plasma flow profiles by measuring the Doppler shift of x-ray line emission from highly charged impurity species. A novel design is described that uses an x-ray tube with a cadmium anode placed in front of the diffracting spherically bent crystal. This arrangement provides calibration lines over the full detector extent for both the Ar16+ and Ar17+/Fe24+ spectrometer channels. This calibration system can provide a relative wavelength accuracy of 3 × 10-7 Å across the full spatial extent of the detector, which corresponds to 50 m/s in the W7-X system. An absolute wavelength calibration of 1 × 10-5 Å is expected, corresponding to 1 km/s, based on the current known accuracy of the calibration wavelength and the achievable measurement of the absolute positioning of the hardware. This calibration system can be used to independently calibrate XICS systems on both stellarators and tokamaks, without the need for special plasma conditions often used for calibration, such as locked modes on tokamaks. Experimental and simulated results are shown along with expected results, and the complete design of the calibration hardware that is to be installed in the W7-X XICS system.

Large orbital magnetic moments of small, free cobalt cluster ions Co[Formula: see text] with n [Formula: see text].

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co[Formula: see text] ([Formula: see text]) cluster ions have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of [Formula: see text] per atom was determined for Co[Formula: see text], which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of ≈1 [Formula: see text] were also found for Co[Formula: see text], Co[Formula: see text], and Co[Formula: see text]. The orbital contribution to the total magnetic moment shows a non-monotonic cluster size dependence: The orbital contribution increases from a local minimum at n = 2 to a local maximum at n = 5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that the 3d majority spin states are fully occupied, that is, 3d hole spin polarization is 100%.

Design, capabilities, and first results of the new laser blow-off system on Wendelstein 7-X.

We present a detailed overview and first results of the new laser blow-off system on the stellarator Wendelstein 7-X. The system allows impurity transport studies by the repetitive and controlled injection of different tracer ions into the plasma edge. A Nd:YAG laser is used to ablate a thin metal film, coated on a glass plate, with a repetition rate of up to 20 Hz. A remote-controlled adjustable optical system allows the variation of the laser spot diameter and enables the spot positioning to non-ablated areas on the target between laser pulses. During first experiments, clear spectral lines from higher ionization stages of the tracer ions have been observed in the X-ray to the extreme ultraviolet spectral range. The temporal behavior of the measured emission allows the estimate of transport properties, e.g., impurity transport times in the order of 100 ms. Although the strong injection of impurities is well detectable, the global plasma parameters are barely changed.

Direct observation of high-spin states in manganese dimer and trimer cations by x-ray magnetic circular dichroism spectroscopy in an ion trap.

The electronic structure and magnetic moments of free Mn2 (+) and Mn3 (+) are characterized by 2p x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron radiation beamline. Our results directly show that localized magnetic moments of 5 µB are created by 3d(5)((6)S) states at each ionic core, which are coupled ferromagnetically to form molecular high-spin states via indirect exchange that is mediated in both cases by a delocalized valence electron in a singly occupied 4s derived antibonding molecular orbital with an unpaired spin. This leads to total magnetic moments of 11 µB for Mn2 (+) and 16 µB for Mn3 (+), with no contribution of orbital angular momentum.

Magnetic moments of chromium-doped gold clusters: the Anderson impurity model in finite systems.

The magnetic moment of a single impurity atom in a finite free electron gas is studied in a combined x-ray magnetic circular dichroism spectroscopy, charge transfer multiplet calculation, and density functional theory study of size-selected free chromium-doped gold clusters. The observed size dependence of the local magnetic moment can be understood as a transition from a local moment to a mixed valence regime. This shows that the Anderson impurity model essentially describes finite systems even though the discrete density of states introduces a significant deviation from a bulk metal, and the free electron gas is only formed by less than 10 electrons. Electronic shell closure in the gold host minimizes the interaction of localized impurity states with the confined free electron gas and preserves the magnetic moment of 5 µ_{B} fully in CrAu_{2}^{+} and almost fully in CrAu_{6}^{+}. Even for open-shell species, large local moments are observed that scale with the energy gap of the gold cluster. This indicates that an energy gap in the free electron gas stabilizes the local magnetic moment of the impurity atom.

Electronic ground states of Fe2(+) and Co2(+) as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy.

The (6)Π electronic ground state of the Co2 (+) diatomic molecular cation has been assigned experimentally by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. Three candidates, (6)Φ, (8)Φ, and (8)Γ, for the electronic ground state of Fe2 (+) have been identified. These states carry sizable orbital angular momenta that disagree with theoretical predictions from multireference configuration interaction and density functional theory. Our results show that the ground states of neutral and cationic diatomic molecules of 3d transition elements cannot generally be assumed to be connected by a one-electron process.

Status of the diagnostics development for the first operation phase of the stellarator Wendelstein 7-X.

An overview of the diagnostics which are essential for the first operational phase of Wendelstein 7-X and the set of diagnostics expected to be ready for operation at this time are presented. The ongoing investigations of how to cope with high levels of stray Electron Cyclotron Resonance Heating (ECRH) radiation in the ultraviolet (UV)/visible/infrared (IR) optical diagnostics are described.

Spin coupling and orbital angular momentum quenching in free iron clusters.

Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.

Communication: Highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of doped silicon clusters from core level spectroscopy.

A method to determine band gaps of size-selected and isolated nanoparticles by combination of valence band and core-level photoionization spectroscopy is presented. This approach is widely applicable and provides a convenient alternative to current standard techniques for the determination of band gaps by optical or photoelectron spectroscopy. A first application to vanadium doped silicon clusters confirms a striking size-dependence of their highest occupied-lowest unoccupied molecular orbital gaps.

Rapid and frequent induction of protective immunity exceeding UK recommendations for healthcare settings by MF59-adjuvated hepatitis B vaccine.

The ability of three doses of a novel MF59-adjuvanted hepatitis B virus (HBV) vaccine containing surface and pre-S2 antigens given at 0, 1, and 6 months to induce levels of HBV surface antibody (sAb) > or = 100 mIU/ml was compared with a UK licensed alum-adjuvanted yeast-derived HBV vaccine in HBV-naive healthcare workers (HCWs). One month after second immunization with HBV/MF59, 100% of HCWs had sAb > or = 100 mIU/mL, compared with only 11% and 85% after two or three immunisations with Engerix-B. The sAb GMT of the Engerix B immunised group remained below 100 mIU/mL until month seven, (compared with month one for HBV/MF59), and was 123-fold lower at this time (208,561 vs. 1,686 mIU/mL). In our subjects HBV/MF59 vaccine rapidly induced sAb to levels far in excess of those recommended by the Department of Health for high-risk situations (e.g. HCWs and patients on dialysis). It has the potential for shorter schedules and reduced need for serology and boosters.