Effects of strong density fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

EUROfusion Consortium 63305

Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (Te) and its fluctuations (δTe). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%–15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s–1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s–1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed.EUROfusion Consortium 63305

The role of radial electric field and neoclassical transport in the establishment and sustainment of the edge transport barrier in the ASDEX Upgrade tokamak

EUROfusion Consortium 63305

Investigation of inter-ELM ion heat transport in the H-mode pedestal of ASDEX Upgrade plasmas

The ion heat transport in the pedestal of H-mode plasmas is investigated in various H-mode discharges with different pedestal ion collisionalities. Interpretive modelling suggests that in all analyzed discharges the ion heat diffusivity coefficient, χ i , in the pedestal is close to the neoclassical prediction within the experimental uncertainties. The impact of changing the deposition location of the electron cyclotron resonance heating on the ion heat transport has been studied. The effect on the background profiles is small. The pre-ELM (edge localized modes) edge profiles as well as the behaviour of the electron temperature and density, ion temperature and impurity toroidal rotation during the ELM cycle are very similar in discharges with on- and off-axis ECRH heating. No significant deviation of χ i from neoclassics is observed when changing the ECRH deposition location to the plasma edge.European Commission (EUROfusion 633053)European Union (EUROfusion Grant WP14-FRF-IPP

Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (T e) and its fluctuations (δT e). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%-15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (T rad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s-1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s-1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed

On the ion and electron temperature recovery after the ELM-crash at ASDEX upgrade

The access to fast measurements, i.e. Δt ≈ 100 µs, of the ions and the electrons during an entire edge localized cycle (ELM) reveals asymmetries in the recovery of the maximum edge gradients. Different magnetic fluctuations are found to correlate with the saturation of the edge ion temperature (Ti), electrons temperature (Te) and density (ne) gradients. In particular, while ∇Ti and ∇ne clamp roughly 3.0 ms after the ELM-crash together with the onset of mid-frequency (f ≲ 50 kHz) magnetic fluctuations, ∇Te recovers to the pre-ELM conditions only after 7.0 ms and saturates with the appearance of high frequency fluctuations (f ≈ 200 kHz). The effect of electron temperature gradient modes (ETGs) and of energy losses induced by ionization of neutrals are discussed as possible reasons for the delayed recovery of ∇Te. The onset and the suppression of ETGs qualitatively follow the requirements of an increased electron heat transport. However, gyro-kinetic simulations are necessary to quantify the impact of ETGs. On the other hand, the impact of the neutral ionization during the density build-up as an electron energy loss channel is measured to be small compared to the total electron energy. The dominant terms in the electron energy balance are instead the radiative power and the ion-electron heat exchange. Keywords: ELM, Ion temperature, ETG, Neutral ionization, Fast Charge Exchang

Ion heat transport dynamics during edge localized mode cycles at ASDEX Upgrade

The edge ion heat transport is analyzed in ASDEX Upgrade (AUG) by combining a comprehensive set of pedestal measurements with both interpretive and predictive modelling. The experimentally determined ion heat diffusivities, $\chi_{\rm i}$ , are compared with neoclassical theory and the impact of edge localized modes (ELMs) on the edge ion heat transport level is studied in detail. Pedestal matching experiments in deuterium and hydrogen plasmas show that the inter-ELM pedestal $\chi_{\rm i}$ remains close to the neoclassical value. The additional power needed in hydrogen to get similar pedestal temperatures as in deuterium plasmas mostly affects the electron heat channel, i.e. the electron heat diffusivity increases while the ion heat diffusivity stays at the same level within the uncertainties. Sub-ms measurements of the edge ion temperature allows us to extend the analysis to the entire ELM cycle. During the ELM crash, the ion heat transport is increased by an order of magnitude. The perturbed heat flux increases first at the separatrix, i.e. first the separatrix ion temperature increases, leading to a flatter ion temperature gradient, followed by a decrease of the whole pedestal profile. The ion heat transport returns to its pre-ELM neoclassical level 3–4 ms after the ELM crash.Peer reviewe