Tungsten transport in the plasma edge at ASDEX upgrade
The Plasma Facing Components (PFC) will play a crucial role in future deuterium-tritium magnetically\nconfined fusion power plants, since they will be subject to high energy and particle loads,\nbut at the same time have to ensure long lifetimes and a low tritium retention. These requirements\nwill most probably necessitate the use of high-Z materials such as tungsten for the wall materials,\nsince their erosion properties are very benign and, unlike carbon, capture only little tritium. The\ndrawback with high-Z materials is, that they emit strong line radiation in the core plasma, which\nacts as a powerful energy loss mechanism. Thus, the concentration of these high-Z materials has to\nbe controlled and kept at low levels in order to achieve a burning plasma. Understanding the transport\nprocesses in the plasma edge is essential for applying the proper impurity control mechanisms.\nThis control can be exerted either by enhancing the outflux, e.g. by Edge Localized Modes (ELM),\nsince they are known to expell impurities from the main plasma, or by reducing the influx, e.g.\nminimizing the tungsten erosion or increasing the shielding effect of the Scrape Off Layer (SOL).\nASDEX Upgrade (AUG) has been successfully operating with a full tungsten wall for several years\nnow and offers the possibility to investigate these edge transport processes for tungsten. This study\nfocused on the disentanglement of the frequency of type-I ELMs and the main chamber gas injection\nrate, two parameters which are usually linked in H-mode discharges. Such a separation allowed for\nthe first time the direct assessment of the impact of each parameter on the tungsten concentration.\nThe control of the ELM frequency was performed by adjusting the shape of the plasma, i.e. the\nupper triangularity.\nThe radial tungsten transport was investigated by implementing a modulated tungsten source. To\ncreate this modulated source, the linear dependence of the tungsten erosion rate at the Ion Cyclotron\nResonance Heating (ICRH) limiters on the injected ICRH power was used. The phase and amplitude\nof the inwardly propagating tungsten signal was then observed at the erosion site and at three\nradial positions in the main plasma, from which two were identified in the course of this work by a\nthorough investigation of the tungsten radiation features in the Vacuum Ultra-Violet (VUV) spectral\nrange . The newly found observation sites are located right in the steep gradient region, close to\nthe Edge Transport Barrier (ETB) and slightly further inside at the pedestal top of AUG H-mode\ndischarges. Futhermore, the parallel flows in the SOL have been monitored by spectroscopical means\nand Langmuir probes.\nThe experimental results were quite unexpected, since the ELM frequency had no influence on the\ntungsten concentration, and the sole actuator on this quantity was the gas injection rate. The evaluation\nof the modulated tungsten signal revealed that neither gas puffing nor plasma shape had an\nmeasureable influence on the radial tungsten transport processes. In addition, the tungsten erosion\nsources were only partially responsible for the observed tungsten behavior. These observations inspired\na simple model, which balanced the tungsten outflux with the tungsten influx. In this model\nthe impurity exauhst by ELMs is not diffusive, but turbulent and linked to the ELM size. The model\npredicted a linear dependence between the tungsten concentration and the parallel velocity in the\nSOL. This linear dependence was confirmed by the spectroscopical evaluation of the SOL parallel\nflows.