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B. Vanovac, E. Wolfrum, S. Denk, F. Mink, F. Laggner, G. Birkenmeier, M. Willensdorfer, E. Viezzer, M. Hoelzl, S. Freethy, M. Dunne, A. Lessig, N. Luhmann Jr., . ASDEX Upgrade Team, -. EUROfusion MST1 Team:
"Effects of density gradients and fluctuations at the plasma edge on ECEI measurements at ASDEX Upgrade";
Plasma Physics and Controlled Fusion, 60 (2018), 4; 04500201 - 04500211.

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.

ECEI, pedestal, optical depth, density fluctuations, mode velocity