Edge fast-ion transport study using passive FIDA spectroscopy at the ASDEX Upgrade tokamak

Good confinement of fast supra-thermal ions is important in fusion experiments because the fast particles should heat the background plasma and contribute to the current drive. In addition, badly confined fast ions would not only affect the power output of a possible power plant, but may also damage wall components. In particular, good confinement of fast-ions located in the outer region of the plasma is important, since already weak perturbations might be sufficient to redistribute the fast ions onto so-called open field lines. A known instability at the edge of magnetically confined fusion plasmas is the socalled Edge Localised Mode (ELM) which is well known to periodically redistribute heat, particles and momentum. However, it is still not completely clear how strong the in uence of ELMs are on the fast particles. To study fast ions at the plasma edge passive fast-ion deuterium-alpha (FIDA) measurements were developed and applied to the ASDEX Upgrade Tokamak. The FIDA technique is based on the spectroscopic investigation of the Balmer-alpha emission line, which is emitted by fast ions after charge exchange reactions with background neutrals. The high speed of the energetic particles leads to a Doppler shift which separates the FIDA emission from other contributions to the spectrum. The passive FIDA system installed at ASDEX Upgrade consists of two new spectrometers with very high light throughput and lines of sight, which are optimised for the plasma edge. The spectrometers enable FIDA measurements with unprecedented temporal resolution and simultaneously enable the measurement of the emission of thermal background neutrals. The thermal emission contains important information about the neutral particle density, which is crucial for the quantitative interpretation of passive FIDA measurements. Therefore, a new method has been developed, with which the theoretical profiles of neutrals can be verified and, if necessary, scaled. Plasma discharges were carried out on ASDEX Upgrade in which a high density of fast ions outside the plasma centre was generated by means of off-axis neutral particle injection (NBI). Strong passive FIDA signals were observed which show a clear effect of the periodically occurring ELMs. While a clear reduction in the FIDA emission is observed in the outer area of the plasma, measurements within the last closed flux surface show increased signals shortly after the appearance of ELMs. This can be explained by the increased density of background neutrals and is in good agreement with neutral particle measurements (NPA). The decrease in FIDA emissions in the outer area, but also in the inner area after the initial increase, can be attributed to the redistribution of the fast ions. Detailed sensitivity studies regarding changes in plasma position during ELMs show that the result of strong ELM-induced losses is robust. It should also be noted here that small ELMs are observed, which only slightly change the density of the background neutrals and the plasma position and nevertheless cause a significant decrease in FIDA emission. The systematic modification of the theoretical distribution function of fast ions, which is used to predict synthetic FIDA signals, shows the best agreement with the data when about 60% of the fast ions are lost by ELMs outside the last closed flux surface. In addition, a decrease in the fast-ion density of about 20% in a range up to 4 cm within the last closed flux surface can be inferred. This result agrees well with the latest modelling results of ELMs using the non-linear MHD code JOREK and shows that less than 0.3% of all fast ions are lost by ELMs.