H-mode access and the role of spectral shift with electrode biasing in the TCABR tokamak

Abstract

The transition from the ohmic heating L-mode to the H-mode in the tokamak TCABR induced by an external electrode biasing is addressed in this work. A gradual improvement in the plasma confinement with the biasing voltage was observed as a result of the enhancement of the E Â B shear flow at the edge. This result was supported by the measurement of plasma parameters in different radial locations by two electrostatic probes. Radial profiles of the electron temperature (from the standard sweeping voltage technique), electron density, and radial electric field, as well as the local turbulence features, were measured. We observed that the electron density profile became steeper as the voltage on the electrode increased, while no significant change in the temperature profile was noticed. In addition, the data were compared with the spectral shift model [G. M. Staebler et al., Phys. Rev. Lett. 110, 055003 (2013)] at different bias voltages, i.e., different E Â B shear flows, showing a good agreement. Particularly, the gradual improvement was held: the higher the biasing voltage, the larger the radial wavenumber shift and so the turbulence suppression. Moreover, by studying the edge radial profile of the mean radial wavenumber, we show that the large shift occurs in the same position where the transport barrier is created.

Turbulent filament properties in L and H-mode regime in the RFX-mod operating as a tokamak

“…They all exhibit a similar behaviour over time: the D α line (Fig. 2 c) drops when biasing is applied, followed by an increase of the central line averaged density -compatible with the typical particle confinement improvement observed in edge biasing experiments [37,1,39]. The plasma current was I p ≤ 50 kA, slightly ramped-up during biasing, which reflects in the decreasing of q 95 (Fig.…”

“…Experimentally, the edge profile of the radial electric field (E r ) can be modified with the aid of an external electrode biasing in colder plasmas [37]. Such a technique has been shown to induce L-H transition in both tokamak [38,39], and stellarator [40], by enhancing E r × B flows in the plasma boundary. In the RFX-mod operating as a tokamak, H-mode has been achieved with an external biasing placed at its edge in both circular and single-null plasmas, where, in some cases, Edge-localized mode (ELM) like structures were observed [41].…”

Modificação do transporte turbulento por polarização eletrostática em plasmas confinados magneticamente

“…No capítulo três serão apresentadas as novas simulac ¸ões realizadas para o modelo de transporte caótico de partículas utilizando dados de potencial do plasma medido por sondas de Langmuir corrigidos pelo perfil de temperatura local obtidos em estudos recentes realizados no TCABR [17,18].…”

“…Essas quebra é devido ao cisalhamento da velocidade do fluxo poloidal gerado pelo gradiente do campo elétrico radial, esse que aumenta com o aumento da tensão aplicada no eletrodo, como mostrado na Figura 2.11. Os resultados experimentais foram compatíveis com a previsão teórica de regime de decorrelac ¸ão das estruturas coerentes [17,18].…”

Busca por evidências de dinâmica caótica nas flutuações turbulentas de densidade em plasmas

“…Por conta de sua aparência, essas estruturas coerentes também são chamadas de blobfilaments [54] (ou apenas blobs). Outras nomenclaturas para essas estruturas existem na literatura, como avaloids [55], e turbulent eddies (cells) [56]. Além disso, alguns autores se referem a grupos dentre essas estruturas com características específicas usando nomenclaturas diferentes.…”