Edge Physics Forum 2018

Effects of the density on ECEI measurements at the plasma edge

Edge Physics Forum

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

Edge Physics Forum

Top launch reflectometer access on AUG

Edge Physics Forum
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]
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]

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

Edge Physics Forum
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 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]

PSI poster rehearsals (1/2)

Edge Physics Forum

PSI poster rehearsals (2/2)

Edge Physics Forum

Power Threshold Studies on ASDEX Upgrade and the Influence of the SOL on the H‐mode Onset

Edge Physics Forum

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

Edge Physics Forum
At JET high radiaton, high power discharges were achieved by neon seeding. The increasing radiation causes these discharges to transit from ELMy H-mode into a regime with degraded pedestal profiles, an X-point radiator, and detached divertor targets. The confinement time is not affected by the increased radiation. Depending on the heating power, the plasma transits into L-mode, M-mode, or ELM-free H-mode. These heating power dependent modes and the stability of the discharges will be investigated in this presentation. [mehr]
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