Edge Physics Forum 2018




Edge Physics Forum

How to go on with the room temperature solid-state pellet injector

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Edge Physics Forum

PSI poster rehearsals (2/2)

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Edge Physics Forum

PSI poster rehearsals (1/2)

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Edge Physics Forum

Structure and Nonlinear Development of Edge Localized Magnetohydrodynamic Instabilities on the ASDEX Upgrade Tokamak

Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that occur in the high confinement regime (H-mode) of magnetically confined fusion plasmas. ELMs lead to sudden periodic releases of particles and stored energy on a millisecond time scale. These ELM crashes might cause intolerably high heat fluxes onto the divertor target plates or the first wall in future fusion devices. According to the broadly accepted linear peeling-ballooning model these MHD instabilities are driven by edge current density and steep edge pressure gradient, which are characteristic for the H-mode. However, details of the underlying process responsible for ELMs and their nonlinear development during the crashes are not yet fully understood. The focus of this thesis is to determine one of the main characterizing parameters of MHD instabilities, which is the periodic magnetic structure described by the poloidal and toroidal mode numbers m and n. These structures are investigated for ELMs and associated phenomena on the ASDEX Upgrade tokamak. Mode numbers of instabilities are determined by recently upgraded magnetic pick-up coil arrays. It is shown that mode numbers of high frequency oscillations, f > 50 kHz, can only be reliably determined if the frequency dependent phase response of the coils is taken into account. Furthermore, a precise ELM synchronization enables the identification of mode numbers during the fast crash of ELMs, which was never achieved before on ASDEX Upgrade. In addition to that, mode numbers and positions of modes appearing between ELM crashes as well as their connection to the edge gradient development are determined for the first time, which is a big step forward in characterizing them and understanding their role for the ELM itself. Ensembles of modes between ELM crashes are detected with different rotation velocities and thereby different locations at the plasma edge. Modes with higher toroidal mode numbers, n=7-13, appear at the position of fastest poloidal plasma rotation, close to the maximum pressure gradient and might be interpreted as ideal modes without additional phase velocity. Modes with lower toroidal mode numbers, n=2-7, exist further outwards close to the separatrix. A similar low n structure is present during the ELM crash. The detection of this structure and other parameters of the crash such as induced energy losses or duration enables a quantitative comparison to results from modeling with the nonlinear MHD code JOREK for the first time. Here the n=6 component with smaller structure size is linearly dominant, but nonlinear coupling in which n=1 is particularly important leads to the dominance of larger structure sizes with n=3-5 during the ELM crash which is in excellent agreement with experimental observations. Moreover, the scaling of the toroidal and poloidal structure, intensity and duration of the ELM crash with plasma parameters is investigated in a database containing various plasma scenarios. It is found that n increases linearly with the inclination of the magnetic field lines, i.e. decreases with safety factor q. Furthermore, no intense ELMs are found at high edge q and no long lasting ELMs are found at low edge q. Other parameters such as normalized pressure gradient alpha, bootstrap current density jBS or plasma triangularity delta, that should have, according to linear peeling-ballooning theory, an impact on n, do not show clear trends. Introducing a simple geometric model, the scaling of toroidal structure size with q can be explained by the dominance of one poloidal structure. In order to place the nonlinear phase of ELMs into a wider context of other nonlinear edge phenomena, toroidal mode numbers are analyzed between ELM crashes on the JET tokamak and during ELM crashes mitigated with an external magnetic error field, ELM crashes of nitrogen seeded discharges and ELM-like magnetic bursts of the intermediate confinement regime (I-phase). The JET edge modes are found with similar properties as the ones on ASDEX Upgrade. The ELM crash structure is found to adopt to the one of the external error field while nitrogen seeding seems not to change it. I-phase bursts have the same toroidal structure as ELMs. [mehr]

Edge Physics Forum

Characterization of low frequency inter-ELM modes at the pedestal top of H-mode discharges at ASDEX Upgrade

Information from a single diagnostic system is not necessarily sufficient to determine the structure of the mode. This is especially true when diagnosing such a narrow region as the plasma pedestal characterized by steep gradients. In this work, several diagnostics are used to allow for a high degree of characterization of the low frequency modes appearing in between type-I Edge Localizes Modes (ELMs) at the pedestal top of the ASDEX Upgrade plasmas. These modes are measured as temperature (Te) and density (ne) fluctuations, and occasionally as fluctuations in the radial magnetic field (Br). Measured mode velocities are found to be in agreement with the background plasma flow at the position of the modes. The frequency of the mode changes for different heating power levels, which is mainly driven by the change in toroidal rotation. The poloidal structure of the modes is resolved with the Electron Cyclotron Emission Imaging (ECEI) measurements. Bicoherence of the magnetic signal between different frequency branches is evaluated and discussed. It is addressed whether the modes could be resistive in nature. [mehr]

Edge Physics Forum

Assessment of the strength of kinetic effects of parallel electron transport in the SOL and divertor of JET high radiative H-mode plasmas using EDGE2D-EIRENE and KIPP codes

KInetic code for Plasma Periphery (KIPP) was used to assess the importance of kinetic effects of parallel electron transport in the SOL and divertor of JET high radiative H-mode inter-ELM plasma conditions with the ITER-like wall and strong nitrogen (N2) injection. Plasma parameter profiles along B from one of the EDGE2D-EIRENE simulation cases were used as an input for KIPP, which in turn calculated electron distribution functions, fe, parallel power fluxes, electron-ion thermoforces, Debye sheath potential drops and electron sheath transmission factors at divertor targets. For heat fluxes in the main SOL, KIPP results showed deviations from classical (e.g. Braginskii) fluxes by factors typically ~ 1.5, with the flux limiting upstream and flux enhancement near entrances to the divertor. Inside the divertor, closer to the separatrix, very large heat flux enhancement factors, up to 10 or even higher, indicative of a strong non-local heat transport, were found at the outer target, with fe exhibiting bump-on-tail features at high energies. Under such extreme conditions, however, contributions of conductive fluxes to total power fluxes were strongly reduced, with convective fluxes becoming comparable, or even exceeding, the conductive fluxes. Electron-ion thermoforce, on the other hand, which is known to be determined mostly by thermal and sub-thermal electrons, was found to be in a good agreement with Braginskii formulas, including the Zeff dependence. Overall, KIPP results indicate, at least for plasma conditions used in this modelling, a sizable, but not dominant effect of kinetics on parallel electron transport. [mehr]

Edge Physics Forum

Top launch reflectometer access on AUG

An introduction the proposed new AUG "top-launch" reflectometer antenna access (upper divertor) - its design, its constraints, and what we hope to try and measure. [mehr]

Edge Physics Forum

On the phase velocity in between weak and strong plasma edge turbulence

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Edge Physics Forum

Effects of the density on ECEI measurements at the plasma edge

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Edge Physics Forum

Effective collecting area of a cylindrical Langmuir probe in magnetized plasma

  • Datum: 20.12.2017
  • Uhrzeit: 15:30 - 16:30
  • Vortragender: Mariia Usoltceva
  • Department of Applied Physics, Gent University, 9000 Gent, Belgium Université de Lorraine, 54506 Vandœuvre-lès-Nancy, France Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching, Germany
  • Ort: Garching
  • Raum: Seminarraum D3
  • Gastgeber: IPP

Langmuir probe diagnostic on magnetic plasma devices encounters even more challenges in data processing than in non-magnetized plasmas, the latest itself being far from simple. In this talk a theory of particle collection by a probe at the plasma potential in collisionless weakly-ionized plasma is presented, accounting for velocities distributed according to the Maxwell equation and different mechanisms of particles collection depending on their speed. Experimental validation of the presented theory has been done with 2 cylindrical probe (Rpr = 75 µm, Lpr = 1 cm and Rpr = 0.5 mm, Lpr = 1 cm) parallel to B on a linear plasma device Aline, with magnetic field of 0.0024−0.1 T and plasma density of 10e15−10e17 m^-3 in Helium. Cylindrical probe measurements are compared to data from a planar probe perpendicular to the magnetic field and results for electron density, temperature and plasma potential are presented. The introduced theory has no limits of applicability in terms of probe size, shape or orientation. Alongside the main subject a number of associated issues is addressed with various detailing: a probe design issue relative to magnetized environment, “intersection” method of plasma potential evaluation and robustness of the conventional “1st derivative” method; a current bump near plasma potential for a parallel oriented probe, self-consistent calculation of electron temperature and density. [mehr]

Edge Physics Forum

Proto-MPEX Overview and Power Balance

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Edge Physics Forum

Edge density and radiation measurements on medium-sized tokamaks

  • Datum: 06.12.2017
  • Uhrzeit: 15:30 - 16:30
  • Vortragender: Balazs Tal
  • Wigner Research Centre for Physics, Hungarian Academy of Sciences
  • Ort: Garching
  • Raum: Seminarraum D3

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Edge Physics Forum

Predictions of HFS SOL flow behavior and inner divertor detachment by SOLPS 5.0

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Edge Physics Forum

Edge coherent modes observed with the ultra-fast swept reflectometer

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edge physics forum: Edge Physics Forum

Status of the Divertor Thomson Scattering System

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Edge Physics Forum

Turbulence Nonlinearities Shed Light on Geometric Asymmetry in Tokamak Confinement Transitions

  • Datum: 08.11.2017
  • Uhrzeit: 15:30 - 16:30
  • Vortragender: Cziegler, Istvan
  • York Plasma Institute, Department of Physics, University of York, Heslington YO10 5DD, United Kingdom
  • Ort: Garching
  • Raum: Seminarraum D3

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Edge Physics Forum

Edge Physics Forum

Korrektur von IR-Thermographiedaten mit IR-Volumenemission

Für den sicheren Betrieb einer Fusionsanlage ist es unerlässlich die Abfuhr der erzeugten Wärme zu kontrollieren und zu charakterisieren. Infrarot Thermographie wird zur Bestimmung der Oberflächentemperatur der Wand und der damit verbundenen Wärmeflüsse benutzt. Dabei wird die Planck Strahlung der Oberfläche mit Sichtlinien durch das Plasma gemessen. Besonders in für zukünftige Reaktoren vorhergesehenen Operationsbereichen mit erhöhter Randdichte wird die Temperaturbestimmung durch vom Plasma emittierte Bremsstrahlung in ASDEX Upgrade erschwert.Diese Bachelorarbeit zeigt, dass anhand der vorhandener Daten eine nachträgliche Korrektur der infrarot Thermographiedaten möglich ist und charakterisiert diese.Dazu werden die Temperaturdaten gefiltert und der Wärmefluss mithilfe des THEODOR Codes neu berechnet.Unterschiedliche Filtermöglichkeiten werden anhand des resultierenden Wärmeflussses charakterisiert. [mehr]

Edge Physics Forum

Influence of external magnetic perturbations on the plasma boundary in L-mode and on the plasma position control system of ASDEX Upgrade

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Edge Physics Forum

Inter-ELM pedestal evolution in AUG - Impact of the triangularity on the pedestal structure and dynamics

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Edge Physics Forum

Neutrals in the tokamak edge: neoclassical ions and momentum transport

Due to their high cross-field mobility, neutral particles interacting with the plasma through charge-exchange can contribute to the radial transport of toroidal angular momentum even at low relative densities. The contribution of non-Maxwellian parts of the ion distribution function is particularly interesting, as it can generate a momentum flux through the neutrals even in the absence of rotation. The neutrals can therefore contribute to intrinsic rotation. We model neutrals using a short mean-free-path approximation, in a background of neoclassical ions having arbitrary collisionality, on closed flux surfaces near the edge. As an application, we will show the effect of flux surface geometry on plasma rotation in 'predictive' mode where the neutrals are assumed to dominate the angular momentum transport. We will also show the momentum flux through neutrals in 'interpretive' mode, where plasma profiles from an L-mode AUG discharge are used to estimate the magnitude of the angular momentum flux through neutrals, finding that it may be a significant fraction of the angular momentum input from NBI. [mehr]

Edge Physics Forum

Investigation of the influence of the ion heat flux on access to regimes of improved energy confinement in Alcator C-Mod.

The role of the edge ion heat flux for transitions from L-mode to H-mode in Alcator C-Mod is investigated. Several selected discharges from the low to medium density range in C-Mod were analyzed by time dependent power balance analysis using the transport code TRANSP. The ion heat flux was calculated by adding the power from the ion cyclotron resonance heating deposited in the ion channel and the electron-ion heat exchange. These calculations showed that the surface integrated edge ion heat flux obtained just prior to the L-H transition increases linearly with line average density, which agrees with previous results from AUG. It appears, as if a critical amount of heat flux per particle is necessary for the transition. The comparison of the results of this work and previous research at AUG lead to a simple linear scaling law for the critical edge ion heat flux necessary for the transition into H-mode. [mehr]

Edge Physics Forum

Dependence of Tokamak Confinement on ELM Power Losses

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