Identification of the beta limit in ASDEX Upgrade

Tokamak plasma is a subject of various resistive and ideal MHD instabilities which restrict the operation space of the device. For largest fusion outcome, it is preferable to operate the tokamak close to the stability limit with maximal possible pressure characterized by the value of normalized beta, , and thus maximal fusion power . In ASDEX Upgrade, the limit for maximal achievable is typically set by the resistive instabilities (tearing modes). If these instabilities are overcome or prevented, higher values of the normalized beta can be reached limited by the onset of the ideal kink instability. The actual limit depends on several factors, including the stabilizing influence of the conducting components facing the plasma surface. At present, the wall elements are far from the plasma and the stability boundary is expected to be close to the “no wall” limit (no stabilizing wall effect). Investigation of maximum achievable βN values for the different operation scenario in ASDEX Upgrade is presented in this work. Two indicators are used to detect the stability boundary:Increase of the resonant field amplification (RFA)Onset of the ideal kink modeRecently installed internal active coils are used to probe stability of the plasma by the RFA technique. A wide range of different MHD diagnostics are used to identify the behaviour and structure of MHD modes in different discharges with high . Experimentally obtained results are compared with the results of the numerical modelling with linear MHD codes CASTOR-FLOW and MARS-K. Such comparison allows to validate the plasma model used in the codes and therefore to make numerical projection for further experimental studies

Identification of the beta limit in ASDEX Upgrade

Tokamak plasma is a subject of various resistive and ideal MHD instabilities which restrict the operation space of the device. For largest fusion outcome, it is preferable to operate the tokamak close to the stability limit with maximal possible pressure characterized by the value of normalized beta, , and thus maximal fusion power . In ASDEX Upgrade, the limit for maximal achievable is typically set by the resistive instabilities (tearing modes). If these instabilities are overcome or prevented, higher values of the normalized beta can be reached limited by the onset of the ideal kink instability. The actual limit depends on several factors, including the stabilizing influence of the conducting components facing the plasma surface. At present, the wall elements are far from the plasma and the stability boundary is expected to be close to the “no wall” limit (no stabilizing wall effect). Investigation of maximum achievable βN values for the different operation scenario in ASDEX Upgrade is presented in this work. Two indicators are used to detect the stability boundary: Increase of the resonant field amplification (RFA) Onset of the ideal kink mode Recently installed internal active coils are used to probe stability of the plasma by the RFA technique. A wide range of different MHD diagnostics are used to identify the behaviour and structure of MHD modes in different discharges with high . Experimentally obtained results are compared with the results of the numerical modelling with linear MHD codes CASTOR-FLOW and MARS-K. Such comparison allows to validate the plasma model used in the codes and therefore to make numerical projection for further experimental studies

Conversion of the dominantly ideal perturbations into a tearing mode after a sawtooth crash

Forced magnetic reconnection is a topic of common interest in astrophysics, space science, and magnetic fusion research. The tearing mode formation process after sawtooth crashes implies the existence of this type of magnetic reconnection and is investigated in great detail in the ASDEX Upgrade tokamak. The sawtooth crash provides a fast relaxation of the core plasma temperature and can trigger a tearing mode at a neighbouring resonant surface. It is demonstrated for the first time that the sawtooth crash leads to a dominantly ideal kink mode formation at the resonant surface immediately after the sawtooth crash. Local measurements show that this kink mode transforms into a tearing mode on a much longer timescale (10(-3)s - 10(-2)s) than the sawtooth crash itself (10(-4)s). The ideal kink mode formed after the sawtooth crash provides the driving force for magnetic reconnection and its amplitude is one of the critical parameters for the length of the transition phase from a ideal into an resistive mode. Nonlinear two fluid MHD simulations confirm these observations. (C) 2014 AIP Publishing LLC

Fast-ion transport in the presence of magnetic reconnection induced by sawtooth oscillations in ASDEX Upgrade

The transport of beam-generated fast ions has been investigated experimentally at the ASDEX Upgrade tokamak in the presence of sawtooth crashes. After sawtooth crashes, phase space resolved fast-ion D-alpha measurements show a significant reduction of the central fast-ion density-more than 50%-together with an increase at larger radii. The corresponding loss of fast particles, detected by a fast-ion loss detector outside the plasma, is marginal during the crash phase, which proves that the sawteeth internally redistribute fast ions. This is also well reproduced by modelling results in which the fast-ion redistribution is assumed to be in line with the sawtooth-induced change of the magnetic field topology. The simulation, however, underestimates the total amount of redistributed fast ions which can be explained by additional drift effects. In the time intervals between the crashes, a neoclassical fast-ion behaviour is observed. Radial profiles and measured decay rates of the redistributed fast ions show that any anomalous fast-ion diffusion in between sawtooth crashes is well below 0.5m(2) s(-1)

14 MeV calibration of JET neutron detectors-phase 1: Calibration and characterization of the neutron source

In view of the planned DT operations at JET, a calibration of the JET neutron monitors at 14 MeV neutron energy is needed using a 14 MeV neutron generator deployed inside the vacuum vessel by the JET remote handling system. The target accuracy of this calibration is 10% as also required by ITER, where a precise neutron yield measurement is important, e.g. for tritium accountancy. To achieve this accuracy, the 14 MeV neutron generator selected as the calibration source has been fully characterised and calibrated prior to the in-vessel calibration of the JET monitors. This paper describes the measurements performed using different types of neutron detectors, spectrometers, calibrated long counters and activation foils which allowed us to obtain the neutron emission rate and the anisotropy of the neutron generator, i.e.The neutron flux and energy spectrum dependence on emission angle, and to derive the absolute emission rate in 4π sr. The use of high resolution diamond spectrometers made it possible to resolve the complex features of the neutron energy spectra resulting from the mixed D/T beam ions reacting with the D/T nuclei present in the neutron generator target. As the neutron generator is not a stable neutron source, several monitoring detectors were attached to it by means of an ad hoc mechanical structure to continuously monitor the neutron emission rate during the in-vessel calibration. These monitoring detectors, two diamond diodes and activation foils, have been calibrated in terms of neutrons/counts within \ub1 5% total uncertainty. A neutron source routine has been developed, able to produce the neutron spectra resulting from all possible reactions occurring with the D/T ions in the beam impinging on the Ti D/T target. The neutron energy spectra calculated by combining the source routine with a MCNP model of the neutron generator have been validated by the measurements. These numerical tools will be key in analysing the results from the in-vessel calibration and to derive the response of the JET neutron detectors to DT plasma neutrons starting from the response to the generator neutrons, and taking into account all the calibration circumstances

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks - A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

This paper presents a survey of the experiments on runaway electrons (RE) carried out recently in frames of EUROFusion Consortium in different tokamaks: COMPASS, ASDEX-Upgrade, TCV and JET. Massive gas injection (MGI) has been used in different scenarios for RE generation in small and medium-sized tokamaks to elaborate the most efficient and reliable ones for future RE experiments. New data on RE generated at disruptions in COMPASS and ASDEX-Upgrade was collected and added to the JET database. Different accessible parameters of disruptions, such as current quench rate, conversion rate of plasma current into runaways, etc have been analysed for each tokamak and compared to JET data. It was shown, that tokamaks with larger geometrical sizes provide the wider limits for spatial and temporal variation of plasma parameters during disruptions, thus extending the parameter space for RE generation. The second part of experiments was dedicated to study of RE generation in stationary discharges in COMPASS, TCV and JET. Injection of Ne/Ar have been used to mock-up the JET MGI runaway suppression experiments. Secondary RE avalanching was identified and quantified for the first time in the TCV tokamak in RE generating discharges after massive Ne injection. Simulations of the primary RE generation and secondary avalanching dynamics in stationary discharges has demonstrated that RE current fraction created via avalanching could achieve up to 70-75% of the total plasma current in TCV. Relaxations which are reminiscent the phenomena associated to the kinetic instability driven by RE have been detected in RE discharges in TCV. Macroscopic parameters of RE dominating discharges in TCV before and after onset of the instability fit well to the empirical instability criterion, which was established in the early tokamaks and examined by results of recent numerical simulations