Bifurcation of poloidal rotation and suppression of turbulent fluctuations: A model for the L–H transition in tokamaks

Abstract

The poloidal momentum balance equation in tokamaks is shown to have bifurcated solutions; the poloidal flow velocity Up can suddenly become more positive when the ion collisionality decreases. The corresponding radial electric field Er becomes more negative, suppresses turbulent fluctuations, and improves plasma confinement. A heuristic argument is employed to illustrate the effects of Er on turbulent fluctuations. A more negative value of Er and/or a more positive value of dEr /dr can suppress the fluctuation amplitudes, if dP/dr<0 (with r the local minor raidus and P the plasma pressure). The theory is employed to explain the L–H transition observed in tokamaks.

Drift waves and transport

“…The phenomena of turbulence-driven flows has not been identified in plasma experiments due to competing mechanisms such as the torque produced by radial currents (Diamond and Kim, 1991) and ion orbit losses (Shaing et al, 1990;Shaing, 1993). Ida et al (1992) compare their tokamak experiments with five theories without finding substantial agreement.…”

“…The nonlinear theory for the suppression of the turbulent diffusivity D gives the reduction factor of D/͓1 ϩ c 2 k y 2 ⌬x 2 (v E Ј ) 2 ͔ for broad band ⌬ drift-wave turbulence with short correlation time c ϭ1/⌬ (Shaing et al, 1990;Biglari et al, 1990;and Zhang and Mahajan, 1992). The typical mode width ⌬x and the mean k y have ⌬xk y ϳ1.…”

Role of flow shear in enhanced core confinement regimes

“…It is well known that the E x B flow shear plays a crucial role in the L to H transition of many tokamak plasmas [1][2][3][4]. In this paper, the importance of E x B flow shear in various enhanced core confinement regimes is discussed.…”

Ion drifts in a snowflake divertor

“…This yields the following estimate of the loss time: In summary: the mechanism of prompt ion losses considered in Refs. [2,3,6] for the X-point divertor is analyzed for the snowflake configuration. The general classification of the ion trajectories in the vicinity of the null-point is presented.…”

“…In the case of the standard X-point geometry the prompt losses have been studied in a number of publications (e.g., [2][3][4] and references therein), whereas the snowflake geometry has not been explored in this context.…”