Results from the investigation of neoclassical core transport and the role of the radial electric field profile (E r) in the first operational phase of the Wendelstein 7-X (W7-X) stellarator are presented. In stellarator plasmas, the details of the E r profile are expected to have a strong effect on both the particle and heat fluxes. Investigation of the radial electric field is important in understanding neoclassical transport and in validation of neoclassical calculations. The radial electric field is closely related to the perpendicular plasma flow (u ⊥) through the force balance equation. This allows the radial electric field to be inferred from measurements of the perpendicular flow velocity, which can be measured using the x-ray imaging crystal spectrometer (XICS) and correlation reflectometry diagnostics. Large changes in the perpendicular rotation, on the order of ∆u ⊥ ∼ 5 km/s (∆E r ∼ 12 kV /m), have been observed within a set of experiments where the heating power was stepped down from 2 M W to 0.6 M W. These experiments are examined in detail to explore the relationship between heating power, temperature and density profiles and the radial electric field. Finally the inferred E r profiles are compared to initial neoclassical calculations using measured plasma profiles. The results from several neoclassical codes, sfincs, fortec-3d and dkes, are compared both with each other and the measurements. These comparisons show good agreement, giving confidence in the applicability of the neoclassical calculations to the W7-X configuration.
The transport of heavy impurities has been investigated at the Wendelstein 7-X stellarator during core electron root confinement (CERC) experiments. Iron atoms were injected via the laser blow-off technique and analyzed by VUV and x-ray spectrometers. The injected amount of iron does not change the global plasma parameters but yields strong enough line radiation for detailed studies based on the impurity transport code STRAHL. The latter is supplied with neo-classical diffusion and convection profiles from the drift kinetic equation solver (DKES) and has been embedded into a least-squares fit that searches for additional anomalous diffusion and convection profiles, required to explain the measurements. While the resulting convection velocities agree within uncertainties with neo-classical theory, the anomalous diffusion profile exhibits values more than two orders of magnitude larger than the neo-classical one. This significant level of anomalous transport is possibly explained by turbulence. The high ratio and flat density profile present during the experiment yield low thresholds for temperature gradient driven modes that are expected off-axis where the obtained diffusion profile peaks.
Several Collisional-Radiative (CR) models [1, 2, 3] have been developed to calculate the attenuation and the population of excited states of hydrogen or deuterium beams injected into tokamak plasmas. The datasets generated by these CR models are needed for the modelling of beam ion deposition and (excited) beam densities in current experiments, and the reliability of this data will be crucial to obtain helium ash densities on ITER combining charge exchange and beam emission spectroscopy. Good agreement between the different CR models for the Neutral Beam (NB) is found, if corrections to the fundamental cross sections are taken into account. First the H a and H b beam emission spectra from JET are compared with the expected intensities. Second, the line ratios within the Stark multiplet are compared with the predictions of a sublevel resolved model. The measured intensity of the full multiplet is ≈30% lower than expected on the basis of beam attenuation codes and the updated beam emission rates, but apart from the atomic data this could also be due to the characterization of the NB path and line of sight integration and the absolute calibration of the optics. The modelled n = 3 to n = 4 population agrees very well with the ratio of the measured H a to H b beam emission intensities. Good agreement is found as well between the neutral beam power fractions measured with beam emission in plasma and on the JET Neutral Beam Test Bed. The Stark line ratios and s/p intensity ratio deviate from a statistical distribution, in agreement with the CR model in parabolic states from Marchuk et al. [4]. MOTIVATIONPowerful neutral hydrogen or deuterium beams provide the dominant external heating and momentum input in most large scale tokamak experiments. For the interpretation of neutral beam (NB) heated discharges, detailed knowledge is required about the energy distribution of the neutrals (power fractions) and the attenuation of the beams in order to obtain radial proles of the fast ion deposition and hence of the heating, torque and beam driven current. For the quantitative interpretation of Charge eXchange (CX) spectra, the local NB fluxes and population of excited states in the beam are needed to convert CX emissivities into local impurity densities. All these calculations strongly rely on the accuracy of the atomic data for the NB that is provided by Collisional-Radiative (CR) models of the beam [1, 2, 3, 5, 6].When the Beam Emission Spectrum (BES) was recorded for the first time, it was immediately proposed to monitor the beam attenuation, and hence the accuracy of the effective beam stopping cross sections, by using the observed beam emission intensities [7, 8]. This replaces the accumulated error on the beam attenuation along the beam path [9], by a local error in the beam emission rate. Beam emission, when combined with Charge eXchange Recombination Spectroscopy (CXRS), also has the potential of reducing the need of an absolute calibration of the CXRS spectra and a calculation of the intersection integral between a lin...
We study the excitation of parabolic Stark states in hydrogen atoms by collisions with fast ions. It is shown that excitation cross sections are very sensitive to the angle between the electric field and the projectile velocity. The calculated collisional data are implemented in a newly developed collisional–radiative model involving parabolic quantum states of hydrogen. Our simulations are shown to explain the frequently observed non-statistical behaviour of the Hα component intensities under typical conditions of a motional Stark effect (MSE). A good agreement with the MSE data from the Joint European Torus (JET) for emission of the σ and π components (Mandl et al 1993 Plasma Phys. Control Fusion 35 1373) is obtained for the first time.
Most atomic models for neutral hydrogen beams in fusion plasmas assume a statistical (Boltzmann) distribution of populations for excited states with the same principal quantum number n. Here we analyze population distributions for the excited magnetic sublevels of a beam under typical conditions of existing and future fusion devices. The collisional-radiative model NOMAD based on completely m-resolved parabolic states up to n = 10 is used to study this problem. The model utilizes new proton-impact excitation data calculated with the atomic-orbital close-coupling method and the Glauber approximation and takes into account electric-field-induced ionization from highly excited states. Our simulations show that the statistical assumption for a specific n is generally not valid for typical fusion conditions due to radiative processes and strong field ionization. The deviation increases considerably for higher beam energies and stronger magnetic fields. The calculated line intensities of σ and π components and beam-emission parameters are discussed in detail.
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