•Aim to develop designs for performing thermo-current measurements in DEMO.•Experiments with shunts and Rogowski coils were performed in AS-DEX Upgrade.•Rogowski coil measurements require stray field ...compensation of poloidal and toroidal field coil currents.•Shunt measurements with a signal level of 0.75 mV can provide a useful feedback control signal.
Detachment control in DEMO is a fundamental requirement to prevent damage to the plasma facing components. Thermo-currents flowing through the plasma facing components of the divertor cassette are driven by the thermoelectric voltage generated by the plasma temperature difference between the inner and outer target plates. Shunt and Rogowski coil measurements to measure thermo-currents for detachment control on ITER are planned. Thermo-current measurements on ASDEX Upgrade have been carried out in support of establishing designs considered suitable to measure thermo-currents in DEMO.
ASDEX Upgrade (AUG) is the only tokamak in Europe to have low activation ferritic steel as the inner vessel wall facing component. Together with the massive tungsten tiles in the lower divertor, AUG ...is the tokamak with the closest DEMO wall. The project is a first step towards the extensive use of ferritic steel in future fusion reactors. For example, the test blanket module of ITER is planned to have a ferritic steel wall and thick tungsten tiles as a plasma facing component.
The ‘ad hoc’ ferritic steel built with low activation capability is known as Eurofer. As the low activation property is not a requirement for AUG, the material selected for the project is the martensitic steel P92 which is the most similar material to Eurofer from a magnetic point of view. The purpose of the project is to improve understanding of the magnetic perturbation of the ferritic steel both on the plasma and magnetic probes, evaluating and controlling these effects. Additionally, the effect of the additional forces on the supporting structure has been addressed.
Bearing this in mind, in 2013 a step wise program has been started and part of the W coated graphite tiles in the region of the inner column were replaced by steel tiles 1. The first campaign did not suffer any particular issue related to the new material. According to the calculations, the plasma was almost unperturbed, thanks also to the toroidal symmetry of the tiles inside the vessel, and the magnetic probe measurements were properly corrected 2.
Inspection of the machine pointed out some hardware problems. The graphite tiles adjacent to the steel tiles were damaged. The graphite tiles had broken edges in 5 from 64 positions and notches in many others. The coating of the graphite and steel tiles, made of tungsten and TiO respectively, was damaged. At first glance it was clear that the steel tiles were moving but it was definitely unexpected. In understanding the process, the location of the damage was the crucial hint. In fact all failures were located at the boundary between 2 vacuum vessel octants. To justify this failure mode inside the vessel, a hypothesis (about current flowing in the heat shield supporting structure) was made and FEM analyses were carried out in this direction. With extreme caution, in 2015 just one additional row of steel tiles was added together with diagnostics that confirmed the hypothesis. Now that a clear understanding of the problem has been reached, the project to add further rows of steel tiles can be continued. For the next campaign it is planned to replace all the tiles in the middle region of the heat shield together with stiffening and modification of the supporting structure.
In this paper the learning process from the damage of the tiles and its causes, from the FEM analysis results to the data diagnostics will be reported. The future plans for steel tiles in AUG will be discussed.
The present paper describes the effect of magnetic pick-up coil transfer functions on mode number analysis in magnetically confined fusion plasmas. Magnetic probes mounted inside the vacuum chamber ...are widely used to characterize the mode structure of magnetohydrodynamic modes, as, due to their relative simplicity and compact nature, several coils can be distributed over the vessel. Phase differences between the transfer functions of different magnetic pick-up coils lead to systematic errors in time- and frequency resolved mode number analysis. This paper presents the first in situ, end-to-end calibration of a magnetic pick-up coil system which was carried out by using an in-vessel driving coil on ASDEX Upgrade. The effect of the phase differences in the pick-up coil transfer functions is most significant in the 50-250 kHz frequency range, where the relative phase shift between the different probes can be up to 1 radian ( 60°). By applying a correction based on the transfer functions we found smaller residuals of mode number fitting in the considered discharges. In most cases an order of magnitude improvement was observed in the residuals of the mode number fits, which could open the way to investigate weaker electromagnetic oscillations with even high mode numbers.
Abstract
The reproducibility of diamagnetic flux measurements in vacuum field shots for the internal and external diamagnetic flux measurements in the ASDEX Upgrade tokamak is investigated. A ...comparison of diamagnetic flux measurements and predictions from equilibrium reconstruction, including pressure constraints from thermal and fast-ion contributions, is reported. The ideal magnetohydrodynamic model involved in interpreting diamagnetic flux measurements in a tokamak with anisotropic pressure is summarised. The plasma energy calculated from equilibrium reconstruction and inferred from diamagnetic flux measurements is compared with the respective values calculated by modelling with TRANSP and IDE/RABBIT. It is found that at low densities, in discharges with parallel neutral beam sources, the plasma energy inferred from the diamagnetic flux measurements is smaller than the plasma energy calculated from the equilibrium reconstruction. In addition, details of the modelling are discussed to stress the interdependence of the various modelling inputs and the interpretation of the results. A comparison of diamagnetic flux measurements with calculations from equilibrium reconstruction by IDE and fast-ion profiles from RABBIT and TRANSP simulations can identify anisotropic plasma discharges and provide a consistency check of the input data for modelling.
The existing VxWorks real-time system for the position and shape control in ASDEX Upgrade has been extended to calculate magnetic flux surfaces in real-time using a multi-core PCI Express system ...running LabVIEW RT 8.6. real-time signal processing of bolometers and manometers is performed with the on-board FPGA to calculate the measured radiated power flux and particle flux respectively from the raw data. Radiation feedback experiments use halo current measurements from the outer divertor with real-time median filter pre-processing to remove the excursions produced by ELMs. Integration of these plasma diagnostics into the control system by the exchange of XML sheets for communicating the real-time variables to be produced and consumed is in operation. Reflective memory and UDP are employed by the LabVIEW RT plasma diagnostics to communicate with the control system and other plasma diagnostics in a multi-platform real-time network.
•Spline basis current functions with second-order linear regularisation.•Perturbations of magnetic probe measurements due to ferromagnetic tiles on the inner wall and from oscillations in the fast ...position coil current are corrected.•A constraint of the safety factor on the magnetic axis is introduced. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction.•External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity.
Real-time magnetic equilibria are needed for NTM stabilization and disruption avoidance experiments on ASDEX Upgrade. Five improvements to real-time magnetic equilibrium reconstruction on ASDEX Upgrade have been investigated. The aim is to include as many features of the offline magnetic equilibrium reconstruction code in the real-time equilibrium reconstruction code. Firstly, spline current density basis functions with regularization are used in the offline equilibrium reconstruction code, CLISTE 1. It is now possible to have the same number of spline basis functions in the real-time code. Secondly, in the presence of edge localized modes, (ELM's), it is found to be necessary to include the low pass filter effect of the vacuum vessel on the fast position control coil currents to correctly compensate the magnetic probes for current oscillations in these coils. Thirdly, the introduction of ferromagnetic tiles in ASDEX Upgrade means that a real-time algorithm for including the perturbations of the magnetic equilibrium generated by these tiles is required. A methodology based on tile surface currents is described. Fourthly, during current ramps it was seen that the difference between fitted and measured magnetic measurements in the equilibrium reconstruction were larger than in the constant current phase. External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity. This is the first step towards including vacuum vessel currents during the plasma current ramp in the real-time equilibrium reconstruction. Fifthly, the introduction of a constraint of the safety factor on the magnetic axis is found to be a helpful method to improve the prediction of the location of rational surfaces for NTM stabilization and disruption avoidance experiments. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction using this internal constraint.
The existing real time system for the position and shape control in ASDEX Upgrade has been extended to calculate magnetic flux surfaces in real time using a multi-core PCI Express system running ...LabVIEW RT. The availability of reflective memory for LabVIEW RT will allow this system to be connected to the control system and other diagnostics in a multi-platform real time network. The measured response of each magnetic probe to the individual poloidal field coil currents in the absence of plasma current is compared to the calculated value. Prior to a tokamak discharge this comparison can be used to check for failure of the magnetic probe, flux loop or integrator.
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