Multiple quasicoherent electromagnetic modes with steady-state frequency and different nature and location were observed in the COMPASS tokamak (R = 0.56 m, = 0.2 m) at Bt = 1.14 T with Co-NBI (PNBI = 0.2–0.5 MW, Eb = 32 keV) at frequencies 5 kHz < f < 250 kHz. Modes were observed in both low and high confinement (L- and H-modes) plasmas. Lower frequency modes with f < 50 kHz were identified as low m tearing and kink MHD modes, while higher frequency modes with 50 kHz < f < 250 kHz were considered as having Alfvenic nature. Unexpectedly, such modes were only observed in the H-mode, both in neutral beam injector-assisted and Ohmic, so the mode driving force is not yet clear. Using the linear MHD code KINX, we initially identified the observed mode with a ballooning structure is as beta induced Alfven eigenmode (BAE) with m, n < 5, while an antiballooning mode is initially identified as toroidal Alfven eigenmode (TAE) with m, n < 9.

https://iopscience.iop.org/article/10.1088/0741-3335/57/6/065006/pdf

Quasicoherent modes on the COMPASS tokamak

The control and mitigation of MHD oscillations modes is an open problem in fusion science because they can contribute to the outward particle/energy flux and can drive the device away from ignition conditions. It is then of general interest to extract the mode information from large experimental databases in a fast and reliable way. We present a software tool based on Deep Learning that can identify these oscillations modes taking Mirnov coil spectrograms as input data. It uses Convolutional Neural Networks that we trained with manually annotated spectrograms from the TJ-II stellarator database. We have tested several detector architectures, resultingin a detector AUC score of 0.99 on the test set. Finally, it is applied to find MHD modes in our spectrograms to show how this new software tool can be used to mine other databases.

Automatic Identification of MHD Modes in Magnetic Fluctuations Spectrograms using Deep Learning Techniques

The confinement of suprathermal ions in magnetically confined fusion plasmas is essential to ensure a good fusion performance. Auxiliary heating systems - and fusion reactions themselves - create fast-ion populations, which must be confined for long enough time to transfer their energy to the bulk of the plasma via Coulomb collisions. A good confinement of the fast-ions is needed to ensure a good plasma heating and current drive. Furthermore, if fast-ions are lost to the walls of the machine in a sufficiently intense and localized way, irreversible damage to plasma facing components can be provoked. Therefore, a deep understanding of the mechanisms
 leading to fast-ion transport and eventual losses is of paramount importance.
 The need to develop control tools to avoid these losses is now becoming a priority in the roadmap to future burning plasma experiments. In this sense, scintillator based fast-ion loss detectors (FILD) have been proven to be a powerful diagnostic to study the interaction between fast-ions and magnetohydrodynamic (MHD) instabilities, contributing to unravel the physics underlying the transport mechanisms. In this thesis the study of fast-ion losses in the presence of various MHD instabilities in the ASDEX Upgrade tokamak is presented. A comprehensive description of scintillator based FILDs response is given here for the first time, with a special focus on its velocity-space resolution. As any other instrument in physics, the resolution
 of the system is finite, in this case due to the size of the detector pinhole and the gyrophase distribution of the measured ions. The detector response is described in terms of a simple model based on a weight function formalism. The model allows to calculate synthetic FILD signals given a velocity-space distribution of fast-ions reaching the detector pinhole. This enables a direct comparison between simulations and experimental measurements, taking into account the response of the instrument. Velocity-space tomography techniques have been implemented, which allow to obtain the undistorted velocity-space distribution of fast-ions reaching the detector pinhole. The tool improves the velocity-space resolution of FILD measurements,
 which can potentially reveal additional details in the velocity-space dynamics
 of fast-ion losses.
 These improvements have been applied to the study of different MHD induced fast-ion losses. The first velocity-space resolved absolute measurement of fast-ion losses in the presence of a tearing mode in the ASDEX Upgrade tokamak is presented. An estimate of the different loss channels in absolute terms is given. These measurements, supported by simulations of fast-ion losses including the modelling of ICRF power deposition, suggest that MHDinduced fast-ion losses are responsible
 for the anomalously large heat load measured by the FILD detector, which is then damaged irreversibly. This case represents a perfect example of the potential consequences derived from a bad confinement of the fast-ion population. The velocity-space dynamics of fast-ion losses induced by edge localized modes (ELMs) are investigated. It is observed that, in low collisionallity discharges, a fastion population with energies well above the main neutral beam injection (NBI) - dubbed high-energy feature - is measured. The high-energy feature is correlated with the occurrence of ELMs. The pitch-angle structure of the high-energy feature is observed
 to change with the edge safety factor and the NBI source, which is found to
 be related with the topology of the orbits. The high-energy feature is also observed in mitigated ELM regimes, while not seen in ELM suppressed regimes. This observation is interpreted as the acceleration of beam ions during the ELM crash, when magnetic reconnection is believed to take place. A resonant interaction between the beam-ions and the parallel electric fields emerging during the ELM is proposed as a possible acceleration mechanism, and is observed to qualitatively agree with the main experimental results. The observation motivates a kinetic description of fast-ions in ELM models. Additionally, the finding might also be of interest to the astrophysics community, where acceleration of charged particles in plasmas is ubiquitous,
 in particular in solar flares, which show similarities with ELMs in tokamaks.

Velocity-space resolved measurements of fast-ion losses due to magnetohydrodynamic instabilities in the ASDEX Upgrade tokamak

Recent advances in the modeling, analysis, and measurement of fluctuations have significantly improved the diagnosis and understanding of Alfv?n eigenmodes in the strongly shaped H-1NF helical axis stellarator. Experimental measurements, including 3D tomographic inversions of high resolution visible light images, are in close agreement with beta-induced Alfv?n eigenmodes (BAEs) calculated using the compressible ideal MHD code, CAS3D. This is despite the low ? in H-1NF, providing experimental evidence that these modes can exist due to compression that is induced by the strong shaping in stellarators, in addition to high ?, as is the case in tokamaks. This is confirmed using the CONTI and CAS3D codes, which show significant gap structures at lower frequencies which contain BAE and beta-acoustic Alfv?n eigenmodes (BAAEs). The BAEs are excited in the absence of a well confined energetic particle source, further confirming previous studies that thermal particles, electrons, or even radiation fluctuations can drive these modes. Datamining of magnetic probe data shows the experimentally measured frequency of these modes has a clear dependence on the rotational transform profile, which is consistent with a frequency dependency due to postulated confinement related temperature variations.

Experiment-theory comparison for low frequency BAE modes in the strongly shaped H-1NF stellarator

Turbulence is considered to play an important role in the edge cross field heat and particle transport in fusion devices. Scrape-off layer (SOL) turbulence characteristics were measured by the combined probe mounted on the multi-purpose manipulator during the first experimental campaign of W7-X. An electromagnetic coherent mode (EMCM) at 7 kHz has been observed by multiple diagnostics in both the plasma core and the SOL and exhibits a strong dependence of the magnetic topology. As demonstrated by the measurements of the combined probe, the EMCM starts to appear at a radius of R = 6.15 m along the path of probe measurement and this location is shifted inwards in higher iota configurations. It propagates along the direction of electron diamagnetic drift in the far SOL with a poloidal velocity about 0.6 km/s while it turns to the opposite direction gradually in the near SOL in the laboratory frame, but keeps a velocity of about 0.6-0.7 km/s along the direction of electron diamagnetic drift in the plasma frame. This mode can be induced by raising the ECRH heating power in similar discharge conditions, which is probably linked to the gradient of electron temperature and pressure. The EMCM is enhanced significantly in the edge magnetic island with long connection length where the EMCM can grow up due to the long particle confinement time.

/pdf/observations-of-the-effects-of-magnetic-topology-on-the-sol-340bgi1fi6.pdf

Observations of the effects of magnetic topology on the SOL characteristics of an electromagnetic coherent mode in the first experimental campaign of W7-X

High frequency modes (150-300 kHz) are found in several magnetic configurations of TJ-II plasmas heated by neutral beam injection (NBI). The clear dependence of mode frequency on plasma density and mass species suggests them to be Alfveigenmodes. The appearance of these modes is linked to the presence of low order rational surfaces close to the rotational transform profile. They can exhibit steady or chirping behaviour depending on the plasma profiles. Frequency chirping is observed in NBI plasmas with broad temperature profiles, but rarely observed with relatively peaked profiles. The Alfvactivity has been characterized in detail with magnetic coils for the standard configuration. Cross analyses with heavy ion beam probe and reflectometer signals have yielded spatial resolution and radial profiles of the perturbation. Correlation between magnetic coil signals and signals from diagnostics sensitive to edge ion losses, namely Langmuir probes and a fast ion loss detector, has been observed in some cases and characterized taking advantage of the chirping nature of the observed Alfvactivity. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0029-5515/51/3/033001/pdf

Alfvén eigenmodes measured in the TJ-II stellarator

The International Stellarator/Heliotron Profile Database (ISHPDB) is a community effort aiming at a concise documentation of experimental data and modeling of stellarator reactor relevant issues. ISHPDB extends the global stellarator confinement database ISCDB. ISHPDB is jointly hosted by IPP and NIFS [1]. The effort is structured by physics topics led by the acknowledged experts in their respective fields. A first inter-machine study of 1-d data as part of ISHPDB was given in [2] proving the concept of ISHPDB to result in significant progress. More extended inter-machine studies on high-β physics, impurity transport, edge physics, aspects of neoclassical transport and configuration dependencies will be given at this conference. This contribution focuses on a survey of the ISHPDB topical structure. The resulting requirements for physics oriented documentation will be summarized. A physics motivated quality assurance of the database will be discussed. A proposal for the implementation of an ISHPDB database schema compliant with existing tokamak databases and its interfaces will be introduced.

Status of the International Stellarator/Heliotron Profile Database

Status of the Inter national Stella r ator /Heliotr on Pr ofile Database

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Alfvén eigenmodes measured in the TJ-II stellarator

Status of the International Stellarator/Heliotron Profile Database

Status of the Inter national Stella r ator /Heliotr on Pr ofile Database