RF sheath modeling of experimentally observed plasma surface interactions with the JET ITER-Like Antenna

Waves in the Ion Cyclotron Range of Frequencies (ICRF) enhance local Plasma-Surface Interactions (PSI) near the wave launchers and magnetically-connected objects via Radio-Frequency (RF) sheath rectification. ITER will use 20MW of ICRF power over long pulses, questioning the long-term impact of RF-enhanced localized erosion on the lifetime of its Beryllium (Be) wall. Recent dedicated ICRF-heated L-mode discharges documented this process on JET for different types of ICRF antennas. Using visible spectroscopy in JET ICRF-heated L-mode discharges, poloidally-localized regions of enhanced (by ∼2–4x) Be I and Be II light emission were observed on two outboard limiters magnetically connected to the bottom of the active ITER-Like Antenna (ILA). The observed RF-PSI induced by the ILA was qualitatively comparable to that induced by the JET standard, type-A2 antennas, for similar strap toroidal phasing and connection geometries. The Be II line emission was found more intense when powering the bottom half of the ILA rather than its top half. Conversely, more pronounced SOL density modifications were observed with only top array operation, on field lines connected to the top half of the ILA. So far the near-field modeling of the ILA with antenna code TOPICA (Torino Polytechnic Ion Cyclotron Antenna), using curved antenna model, was partially able to reproduce qualitatively the observed phenomena. A quantitative discrepancy persisted between the observed Be source amplification and the calculated, corresponding increases in E// field at the magnetically connected locations to the ILA when changing from only top to only bottom half antenna operation. This paper revisits these current drive phased and half-ILA powered cases using for the new simulations flat model of the ILA and more realistic antenna feeding to calculate the E// field maps with TOPICA code. Further, the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow Wave (SSWICH-SW) code, which couples slow wave evanescence with DC Scrape-Off Layer (SOL) biasing, is used to estimate the poloidal distribution of rectified RF-sheath Direct Current (DC) potential VDC in the private SOL between the ILA poloidal limiters. The approach so far was limited to correlating the observed, enhanced emission regions at the remote limiters to the antenna near-electric fields, as calculated by TOPICA. The present approach includes also a model for the rectification of these near-fields in the private SOL of the ILA. With the improved approach, when comparing only top and only bottom half antenna feeding, we obtained good qualitative correlation between all experimental measurements and the calculated local variations in the E// field and VDC potential.EURATOM 63305

2D mappings of ICRF-induced SOL density modifications on JET

International audienceWaves in the Ion Cyclotron Range of Frequencies (ICRF, 20-80MHz) provide core ion heating in tokamaks and are used to limit impurity accumulation in high-performance JET scenarios. Before reaching the core, these radiofrequency (RF) waves, excited at the Low-Field Side of the torus, interact with the Scrape-Off-Layer (SOL), causing enhanced wall sputtering, heat loads and local density (ne) changes with a complex 3D geometry. Extending previous studies on JET, this paper aims at mapping RF-induced SOL ne patterns in 2D. This puts constraints on SOL RF modelling and provides hints for locating RF-specific W sources on the JET ITER-Like-Wall (ILW), similar to the ITER vessel

Barley starch

This thesis examined barley amylopectin structure and looked for correlations between the structure and physical properties of starch. The structure of amylopectin and gelatinisation and retrogradation of starch were studied in 10 different barley cultivars/breeding lines with differing genetic background. Amylopectin is built up of thousands of chains of glucose monomers, organised into clusters. The detailed fine structure of amylopectin was studied by isolating clusters of amylopectin and their building blocks, which are the tightly branched units building up the clusters. Barley cultivars/breeding lines possessing the amo1 mutation had fewer long chains of DP≥38 in amylopectin and more large building blocks. The structure of building blocks was rather conserved between the different barley cultivars/breeding lines studied and was categorized into different size groups. These different building blocks were shown to be randomly distributed in the amylopectin molecule. The C-chains in amylopectin can be of any length and are a category of chains different from the B-chains. The backbone in amylopectin consists of a special type of B-chains which, when cleaved by α-amylase, become chains of a similar type to C-chains. Gelatinisation and retrogradation (recrystallisation of gelatinised starch) of barley starch was studied by differential scanning calorimetry. The amo1 mutation resulted in a broader gelatinisation temperature range and a higher enthalpy of retrogradation. Other structural features were also found to influence the physical properties of starch. Small clusters and denser structure of the building blocks resulted in higher gelatinisation temperature. Fast retrogradation was observed in barley which had amylopectin with shorter chains and many large building blocks consisting of many chains. Amylopectin structure was also studied in developing barley kernels. Three barley cultivars/breeding lines were grown in a phytotron and kernels were harvested at 9, 12 and 24 days after flowering. The results showed that amylopectin synthesized at later stages of development had a more tightly branched structure. Expression of the enzymes involved in starch biosynthesis is also known to change during endosperm development

Coupled RF wave propagation and DC plasma biasing in the SOL of tokamaks

International audienceIn the prospect of future long-pulse magnetic fusion devices with high-Z plasma facing materials, the interaction of waves in the Ion Cyclotron Range of Frequencies (ICRF, 30-80MHz) with the Scrape-Off Layer (SOL) remains a challenge. The intense time-harmonic RF electric fields E emitted in the SOL at frequency ω0 are suspected to cause RF oscillations VRF exp(-iωt ) of the sheath voltage at plasma-wall interfaces. Non-linear rectification of these oscillations then produces a Direct-Current (DC) self-biasing of the SOL plasma. Ion acceleration across the larger DC potential V_DC is suspected to enhance the plasma-surface interactions locally. Sheaths also modify the RF wave reflection at material boundaries, in a way depending on V_DC. The first part of the talk will present the simulation tools developed to address these issues. Simulations over spatial scales comparable to ICRF antenna dimensions couple simple models of RF full-wave propagation for E and DC SOL biasing for V_DCvia non-linear sheath boundary conditions applied at plasma-facing material boundaries. An intermediate step computes the complex oscillating voltages V_RF at sheath boundaries. The coupled model is implemented using the Newton-Raphson scheme in the multiphysics Finite Element solver COMSOL. Open questions on mathematical and numerical aspects of the code will be reviewed. At the inner part of the simulation domain, we emulate radiation to infinity using Perfectly Matched Layers (PMLs) adapted for gyrotropic media. Inside PMLs, spatial coordinates are stretched into the complex plane, turning propagative waves into evanescent ones. The main restriction is to avoid coexisting forward and backward waves. PMLs are implemented as lossy inhomogeneous materials with artificial dielectric and magnetic tensors depending on the plasma parameters, the stretching function and the stretched coordinates. In toroidal devices it might be clever to stretch the “natural” coordinates of the plasma geometry instead of Cartesian ones. The second part of the talk therefore focuses on a PML formulation generalized to any orthogonal system of coordinates. Numerical tests of the method were performed for radial PMLs of cold magnetized plasmas in cylindrical geometry. In this case cylindrical waves play a similar role as plane waves with Cartesian coordinates. Specificities associated with the local curvature of the geometry will be highlighted. The real part of the radial coordinate stretch affects the reflection of propagative waves via an artificial displacement of the inner metallic boundary towards regions of lower or higher curvature. This radial stretch can therefore reduce curvature effects, at the expense of refined meshing. The PML loses efficiency when the real part of the stretched radius becomes negative. This is probably related to the crossing of a singular point of the coordinate system inside the PML domain. For given plasma and fixed PML settings, a critical azimuthal mode number m of the cylindrical waves always exists above which the radial PML loses efficiency. The critical m value can be made arbitrarily high by increasing the real or imaginary radial stretching, so that all m values relevant for a realistic simulation can behave correctly. The associated numerical cost depends on the requirements about azimuthal resolution

Progress in controlling ICRF-edge interactions in ASDEX upgrade

RF measurements during variation of the strap voltage balance of the original 2-strap ICRF antenna in ASDEX Upgrade at constant power are consistent with electromagnetic calculations by HFSS and TOPICA, more so for the latter. RF image current compensation is observed at the antenna limiters in the experiment at a local strap voltage of about half of the value of the remote strap, albeit with a non-negligible uncertainty in phasing. The RF-specific tungsten (W) source at the broad-limiter 2-strap antenna correlates strongly with the RF voltage at the local strap at the locations not connected to opposite side of the antenna along magnetic field lines. The trends of the observed increase of the RF loading with injection of local gas are well described by a combined EMC3-Eirene - FELICE calculations, with the most efficient improvement confirmed for the outer-midplane valves, but underestimated by about 1/3. The corresponding deuterium density tailoring is also likely responsible for the decrease of local W sources observed in the experimen