1,727,352 research outputs found
Pressure driven high-n/high-m tearing modes in low shear regions with high pressure gradients and high resistivity
Full text linkThe effect of non-axisymmetric wall geometry on 13C transport in ASDEX Upgrade
Full text linkWe present the first results of 3D simulations of global 13C transport in ASDEX Upgrade (AUG) indicating that the deposition profile of 13C exhibits toroidal asymmetry in the main chamber. In 2007, the migration of carbon in AUG was studied with a methane (13CH4) injection experiment (A. Hakola et al and the ASDEX Upgrade Team 2010 Plasma Phys. Control. Fusion 52 065006). The total amount of deposited 13C was estimated by assuming toroidally symmetric deposition. Remarkably, the total number of deposited atoms was observed to be less than 10% of the number of injected atoms. The experiment has been simulated with the 3D orbit-following Monte Carlo code ASCOT using both a realistic 3D wall geometry of AUG and a 3D magnetic field with toroidal ripple. The simulations indicate that the non-axisymmetric wall geometry causes notable toroidal asymmetry in the deposition profile in the outer (low-field side) midplane region which can provide a partial explanation for the missing carbon inferred from post-mortem analysis of 13C deposition.</p
Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team (vol 81, 033507, 2010)
Full text linkThis addendum applies to papers authored by contributors from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team published in Review of Scientific Instruments. This addendum provides the full list of ASDEX Upgrade Team contributors and their affiliations
Tungsten spectra recorded at the LHD and comparison with calculations
Full text linkWe have measured extreme ultraviolet (EUV) spectra from highly charged tungsten ions in low-density and high-temperature plasmas produced in the Large Helical Device at the National Institute for Fusion Science. The EUV spectra emitted after injection of a tungsten pellet into a hydrogen plasma were recorded at plasma temperatures of 1.5 and 3 keV and were dominated by an intense transition array in the 4.5–6.5 nm region, the profile and extent of which was different in both spectra. Some discrete lines present were identified by comparison with existing spectral data while atomic structure calculations showed that the dominant emission in both arose from Δn = 0, n = 4–n = 4 transitions and the main differences could be attributed to the appearance of the 4p–4d and 4s–4p transitions from W XXXIX to W XLVI in the higher temperature spectrum. Comparison with calculations showed that the dominant emission in both temperature regimes arose from stages where the 4f subshell was either almost or completely stripped. We also investigated if the effect of low density favours transitions to the lowest level as observed in recently reported results.Science Foundation IrelandOther funderJapan Society for the Promotion of Scienceau, sp, ab, st, en, li - TS 30.03.1
Spectra of magnetic perturbations triggered by pellets in JET plasmas
Full text linkAiming at investigating edge localised mode (ELM) pacing for future application on ITER, experiments have been conducted on JET injecting pellets in different plasma configurations, including high confinement regimes with type-I and type-III ELMs, low confinement regimes and Ohmically heated plasmas. The magnetic perturbations spectra and the toroidal mode number, n, of triggered events are compared with those of spontaneous ELMs using a wavelet analysis to provide good time resolution of short-lived coherent modes. It is found that—in all these configurations—triggered events have a coherent mode structure, indicating that pellets can trigger an MHD event basically in every background plasma. Two components have been found in the magnetic perturbations induced by pellets, with distinct frequencies and toroidal mode numbers. In high confinement regimes triggered events have similarities with spontaneous ELMs: both are seen to start from low toroidal mode numbers, then the maximum measured n increases up to about 10 within 0.3 ms before the ELM burst
ASDEX Upgrades New Plasma Control Scheme
Full text linkASDEX Upgrade is a medium sized tokamak experiment investigating highly shaped plasma and advanced scenarios to be extrapolated for ITER. Eleven independent magnetic coils allow for proper shaping and plasma current control. For plasma heating and current drive eight NBI beam lines, two ICRH antenna pairs and four ECRH gyrotrons are available. Five channels for controlling gas valves and a pellet injector serve for fuelling. All actuators are driven by a digital discharge control system. One basic enhancement of the latest generation is a unified framework for all feedforward and feedback control tasks in a discharge. The framework consists of two layers. The core layer implements wind-up safe feedback controllers with a collection of overlayed output limitations. Each controller is dynamically switchable in references, controlled variables, control law and control parameters via a control mode. The coordination layer implements intelligent discharge protection or optimisation algorithms which synchronously can change control modes and dynamically can generate reference waveforms adapted to the discharge's state and goal. The core layer comprises the backbone of plasma control. Current, shape, heating and fuel control all use a library of highly configurable single- and multivalriable control laws. P, PI and PID controllers are standard components but state space and sliding mode policies can easily be supplemented, too. Likewise, a broad selection of output limiters is available in the library. It ranges from constant values to rate limiters, and multi-signal dependent polynomial characteristics. The controller is aware of any output limitation and can take anti-wind-up measures. Furthermore, a feedforward policy allows to tune the behaviour upon mode transitions, like smooth adaptation or freezing the last output. With the coordination layer, tasks like marfe protection, power exhaust protection and soft pulse termination are accomplished. These specialised algorithms are plugged into the framework using a common interface. The framework approach easily allows for further extensions and opens a door for future experimental investigations
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