Canted antiferromagnetic order in the kagome material Sr-vesignieite

We report 51 V NMR, muon spin rotation, and zero-applied-field 63 , 65 Cu NMR measurements on powder samples of Sr-vesignieite, SrCu 3 V 2 O 8 ( OH ) 2 , a S = 1 / 2 nearly kagome Heisenberg antiferromagnet. Our results demonstrate that the ground state is a q = 0 magnetic structure with spins canting either in or out of the kagome plane, giving rise to weak ferromagnetism. We determine the size of ordered moments and the angle of canting for different possible q = 0 structures and orbital scenarios, thereby providing insight into the role of the Dzyaloshinskii-Moriya interaction in this material

Magnetic and electronic ordering phenomena in the Ru2O6 layer honeycomb lattice compound AgRuO3

The silver ruthenium oxide AgRuO3 consists of honeycomb Ru5 2O 6 layers and can be considered an analogue of SrRu2O6 with a different intercalation. We present measurements of magnetic susceptibility and specific heat on AgRuO3 single crystals, which reveal a sharp antiferromagnetic transition at 342 3 K. The electrical transport in single crystals of AgRuO3 is determined by a combination of activated conduction over an intrinsic semiconducting gap of almost equal to 100 meV and carriers trapped and thermally released from defects. From powder neutron diffraction data a N el type antiferromagnetic structure with the Ru moments along the c axis is derived. Raman spectroscopy on AgRuO3 single crystals and muon spin rotation spectroscopy on powder samples indicate a further weak phase transition or a crossover in the temperature range 125 200 K. The transition does not show up in the magnetic susceptibility, and its origin is argued to be related to defects but cannot be fully clarified. The experimental findings are complemented by density functional theory based electronic structure calculations. It is found that the magnetism in AgRuO3 is similar to that in SrRu2O6, however, with stronger intralayer and weaker interlayer magnetic exchange interaction

Macroscopic phase separation of superconductivity and ferromagnetism in Sr_0.5Ce_0.5FBiS_2-xSe_x revealed by μSR

The compound Sr0.5Ce0.5FBiS2 belongs to the intensively studied family of layered BiS2 superconductors. It attracts special attention because superconductivity at T sc = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T C = 7.5 K. Recently it was reported that upon replacing S by Se T C drops and ferromagnetism becomes of an itinerant nature. At the same time T sc increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr0.5Ce0.5FBiS2-xSex with x = 0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x = 0.5 and x = 1.0 we find a ferromagnetic volume fraction of ∼8 % and ∼30 % at T = 0.25 K, well below T C = 3.4 K and T C = 3.3 K, respectively. For x = 1.0 (T sc = 2.9 K) the superconducting phase occupies most (∼64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

International audienceAbstract An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low- Z impurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β . The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts

Characteristics, management, and outcomes of patients with left‐sided infective endocarditis complicated by heart failure: a substudy of the ESC‐EORP EURO‐ENDO (European infective endocarditis) registry

International audienc

Runaway electron beam control

Post-disruption runaway electron (RE) beams in tokamaks with large current can cause deep melting of the vessel and are one of the major concerns for ITER operations. Consequently, a considerable effort is provided by the scientific community in order to test RE mitigation strategies. We present an overview of the results obtained at FTU and TCV controlling the current and position of RE beams to improve safety and repeatability of mitigation studies such as massive gas (MGI) and shattered pellet injections (SPI). We show that the proposed RE beam controller (REB-C) implemented at FTU and TCV is effective and that current reduction of the beam can be performed via the central solenoid reducing the energy of REs, providing an alternative/parallel mitigation strategy to MGI/SPI. Experimental results show that, meanwhile deuterium pellets injected on a fully formed RE beam are ablated but do not improve RE energy dissipation rate, heavy metals injected by a laser blow off system on low-density flat-top discharges with a high level of RE seeding seem to induce disruptions expelling REs. Instabilities during the RE beam plateau phase have shown to enhance losses of REs, expelled from the beam core. Then, with the aim of triggering instabilities to increase RE losses, an oscillating loop voltage has been tested on RE beam plateau phase at TCV revealing, for the first time, what seems to be a full conversion from runaway to ohmic current. We finally report progresses in the design of control strategies at JET in view of the incoming SPI mitigation experiments

Real-time-capable prediction of temperature and density profiles in a tokamak using RAPTOR and a first-principle-based transport model

The RAPTOR code is a control-oriented core plasma profile simulator with various applications in control design and verification, discharge optimization and real-time plasma simulation. To date, RAPTOR was capable of simulating the evolution of poloidal flux and electron temperature using empirical transport models, and required the user to input assumptions on the other profiles and plasma parameters. We present an extension of the code to simulate the temperature evolution of both ions and electrons, as well as the particle density transport. A proof-of-principle neural-network emulation of the quasilinear gyrokinetic QuaLiKiz transport model is coupled to RAPTOR for the calculation of first-principle-based heat and particle turbulent transport. These extended capabilities are demonstrated in a simulation of a JET discharge. The multi-channel simulation requires ∼0.2 s to simulate 1 second of a JET plasma, corresponding to ∼20 energy confinement times, while predicting experimental profiles within the limits of the transport model. The transport model requires no external inputs except for the boundary condition at the top of the H-mode pedestal. This marks the first time that simultaneous, accurate predictions of Te, Tiand nehave been obtained using a first-principle-based transport code that can run in faster-than-real-time for present-day tokamaks

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