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• W1570662984 abstract "Introduction Fluctuationsand turbulencearebelievedtoplay an importantroleinanomalous transport of heat and particles in magnetic fusion devices, and hence their characterization is important. Specifically, the radial and poloidal correlation lengths of turbulence (Lr and Lp) may be used to define the spatial size of turbulent eddies. Lr has been measured using a variety of diagnostics, the most common being correlation reflectometry. In this paper, Lr measurements obtained using the newly developed technique of correlation Doppler reflectometry are presented. This technique, where two microwave beams are launched into the plasma from the same tilted antenna, has been developed on ASDEX Upgrade to provide simultaneous measurement of the perpendicular velocity of the turbulence (u ), the radial electric field (Er), its shear (∂Er ∂r) and the radial correlation length of the turbulence (Lr). Previous work has focused on the former three measurements [1, 2, 3, 4] while the current work concentrates on Lr measurements. Since the Doppler reflectometer probes a specific turbulence wavenumber k , it is necessary to demonstrate that the diagnostic measures the true Lr. This has been investigated using a two-dimensional Finite Difference Time Domain (FDTD) code to simulate the Doppler reflectometer response and to recreate the experimental Lr measurements. This paper presents examples of experimental Lr measurements from correlation Doppler reflectometry at the plasma edge along with simulation results from the 2D code. Technique The correlation Doppler reflectometry system on ASDEX Upgrade consists of two identical V-Band heterodyne reflectometers. The two Doppler reflectometer channels are connected to the same antenna pair so that they launch microwaves with the same line of sight simultaneously into the plasma. The microwaves have different launch frequencies (f1 and f2) and therefore reflect from different radial positions in the plasma (r1 and r2). For Lr measurements, the frequency of one reflectometer channel f1 is held constant while the frequency of the second f2 is stepped away. Here, a stair case every 50ms with a frequency difference between the two channels starting at 0.1GHz (to avoid any cross talk between the two channels) and increasing logarithmically was found to be sufficient. Cross correlating the two Doppler shifted reflectometer signals (i.e. the complex Doppler reflectometer signals I iQ Ae if , see [3]) gives the coherence between the two signals as a function of their frequency separation. The density profile ne is required to translate the frequency separation to a radial separation r. The radial separation when the coherence drops to 1/e gives a measure of the spatial correlation of the turbulence (designated the radial correlation length, Lr) for the particular turbulent wavenumber k studied. (The wavenumber is selected by the tilt angle between the plasma flux surface normal and the incident microwave beam.) Experimental Results The first example examines the effect of plasma heating on Lr. During Ohmic (1 5 1 6s), L-mode ECRH heated (2 5 2 6s) and L-mode NBI heated" @default.
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• W1570662984 date "2006-01-01" @default.
• W1570662984 modified "2023-09-26" @default.
• W1570662984 title "Radial Correlation Length Measurements in ASDEX Upgrade" @default.
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