Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra

Abstract

Backscattering of a microwave beam close to the cut-off allows for measurement of density fluctuations ñ( k ⊥ ) at a specified wave-number, selected by the scattering geometry k ⊥ = −2 k i , where k i is the beam wave-number at the reflection layer. On the Doppler reflectometry system installed on Tore Supra, both the scattering wave-number k ⊥ and the scattering localization (r/a) can be changed during the shot owing to the steppable probing frequency and the motorized antenna. Operating in O mode, the spatial and wave-number ranges depend essentially on density profile, typically probing 0.5 < r/a < 0.95 and 2 < k < 15 cm −1 . Wave number spectra are similar to those obtained with conventional scattering systems. The perpendicular fluctuation velocity in the laboratory frame is obtained from the Doppler shift of the frequency spectrum ω = k ⊥ v ⊥ . It is dominated by the plasma E r × B velocity. In the core, the latter is mainly due to the projection of the toroidal velocity, as this is shown by comparison with measurements by charge exchange recombination spectroscopy. In the set of analysed Tore Supra ohmic and ICRH plasmas, the observed rotation is consistent with a poloidal velocity in the electron diamagnetic direction and/or a toroidal velocity in the counter current direction. The detailed structure of the velocity profile, at the edge and in different plasma regimes, allows us then to get information on the radial electric field distribution. The dynamics of the fluctuation velocity can be studied from the time frequency analysis of the signal, for investigating intermittent behaviour and transient regimes.

Interaction of Mean and Oscillating Plasma Flows Across Confinement Mode Transitions

“…fig. 1) then radiation will be scattered back to the reflectometer receiver antenna [11,12]. If the fluctuations move relative to the diagnostic antennas the backscattered signal spectrum is Doppler frequency shifted by ¾ Ù¡ Ù ·Ù ·Ù Ö Ö , where Ù is the mean velocity of the density fluctuations and the Bragg matched wavenumber.…”

“…the Doppler shift is directly proportional to the phase velocity of the fluctuations Ú Ô moving in the plasma ¢ (guiding centre) frame, Ù Ú ¢ · Ú Ô . If Ú Ô is known, or small Ú ¢ Ú Ô , then the ¢ velocity, and hence the radial electric field Ö , can be extracted directly from with good accuracy [12][13][14]. The magnitude of Ú Ô has been investigated using linear and non-linear gyrokinetic turbulence codes for realistic tokamak conditions [15].…”

“…In both the steep edge gradient region (dominant toroidal electron drift wave EDW turbulence) and shallower core gradient region (Ion-temperature gradient ITG or trapped electron mode TEM turbulence) phase velocities much smaller than the diamagnetic drift velocity, Ú £ × × Ä Ò , are obtained (also confirmed by experimental core measurements [16] The backscattered power is proportional to the turbulence spectral power Ë´ µ AEÒ µ ¾ at the probed . Thus the turbulence spectrum can be measured simply by scanning the tilt angle Ó to vary the probed [12,18,19]. Also, since Ö fluctuations AE Ö translate directly to AEÚ ¢ which in turn appear as AE , this allows direct access to long wavelength Ö flow oscillations such as GAMs and zonal flows [7].…”

Density Fluctuations Measurements on Tore Supra Tokamak Plasma: Radial Profiles and Spectra

“…Spectra shown in Fig 1 b exhibit different behaviours, according to their radial position : slopes for low k r are almost identical, S δn (k r , r) ∼ k −2 r , for 1 cm −1 < k r < 10 cm −1 ( 0.1 < k r ρ s < 1, ρ s being the electronic Larmor gyroradius ). Some discrepancies appear while comparing with k θ spectra previously extracted from the coherent forward Thomson scattering diagnostic [7], scaling with k −3.5 ⊥ for 6 cm −1 < k ⊥ 14 cm −1 ( 0.5 < k ⊥ ρ s < 2.5 ) and the Doppler reflectometry [8] , scaling with…”

<i>Ka</i>-band Microwave Frequency Comb Doppler Reflectometer System for the Large Helical Device

“…Doppler reflectometry can measure the perpendicular velocity of electron density fluctuations v ⊥ , the radial electric field E r , and the perpendicular wavenumber spectrum S (k ⊥ ) in magnetized confinement plasmas. As a result, a number of systems have been used in worldwide fusion plasma devices, such as tokamaks (Tuman-3M [2], ASDEX Upgrade [3][4][5], Tore Supra [6,7], DIII-D [8,9], JT-60U [10]) and helical/stellarators (Wendelstein 7-AS [1,11], TJ-II [12], LHD [13]). …”