Effect of plasma shape on confinement and MHD behaviour in TCV

Abstract

The TCV tokamak (B T < 1.5 T, R ≈ 0.88 m, a < 0.25 m) has produced a wide variety of plasma configurations, both diverted and limited, with elongations κ a ranging from 0.9 to 2.58, triangularities δ a from −0.7 to 1 as well as discharges with nearly rectangular cross sections. Plasma currents of 1 MA have been obtained in elongated discharges (κ a ≈ 2.3). Ohmic discharges with δ a < 0 have smaller sawteeth and higher levels of MHD mode activity than plasmas with δ a > 0. The main change in MHD behaviour when elongation is increased beyond two is an increase in the relative importance of modes with m, n < 1 and a reduction of sawtooth amplitudes. Confinement is strongly dependent on plasma shape. In ohmic limiter L-modes energy confinement times improve typically by a factor of two as the plasma triangularity is reduced from 0.5 to 0 at constant q a . There is also an improvement of confinement as the elongation is increased. In most discharges the changes in confinement are explained by a combination of geometrical effects and power degradation. A global factor of merit H s (shape enhancement factor) has been introduced to quantify the effect of flux surface geometry. The introduction of H s into well known confinement scaling expressions such as Neo-Alcator and Rebut-Lallia-Watkins scaling leads to improved descriptions of the effect of shape for a given confinement mode. In some cases with κ a 1.7 limited ohmic L-modes undergo a slow transition to a confinement regime with an energy confinement improved by a factor of up to 1.5 and higher particle confinement. First experiments to study the effect of shape in ECRH at a frequency of 83 GHz (second harmonic) have been undertaken with 500 kW of additional power.

Optimisation of confinement in a fusion reactor using a nonlinear turbulence model

“…The final fusion power in this study is small compared to the input heating power (on the ions) of 20 MW, but that is because the parameters in this study (table 1) are not reactor-like, but rather of the same order of magnitude as those of JET, the largest existing device: a choice made to allow assessment of the credibility of the initial solution. Even more encouragingly, the qualitative result of this optimisation -that core confinement improved at lower triangularity and higher elongation -is directly corroborated by experimental measurements on the TCV tokamak (Weisen et al 1999) and theoretical studies (Marinoni et al 2009), although as TCV is a much smaller device than that we consider here, with important effects arising from kinetic electron modes and finite gyroradius, quantitative comparisons cannot be justified.…”

“…Even more encouragingly, the qualitative result of this optimisationthat core confinement improved at lower triangularity and higher elongation-is directly overview. The field of nonlinear multi-dimensional optimisation is large and increasing, with both commercial (ESTECO 2018) and freely available Abramson et al (2006) corroborated by experimental measurements on the TCV tokamak (Weisen et al 1999) and theoretical studies (Marinoni et al 2009), although as TCV is a much smaller device than that we consider here, with important effects arising from kinetic electron modes and finite gyroradius, quantitative comparisons cannot be justified.…”

“…The two control parameters chosen were the elongation and triangularity of the outer flux surface (as defined in Lütjens et al (1996)). The shape of the magnetic field has long been known to have a marked effect on the turbulence, and the choice of these two shaping parameters in particular was motivated by pioneering experimental work on the Tokamak à Configuration Variable (TCV), which was designed to allow large variation in both -demonstrating a consequent large variation in performance (Weisen et al 1999).…”

International Stellarator/Heliotron Database progress on high-beta confinement and operational boundaries

“…The Shafranov shift is associated with flux compression, which leads to a steepening of surface averaged profile gradients and hence increased transport fluxes [46]. Therefore, this effect of the flux surface geometry could contribute to the limitation of β without invoking flux surface destruction.…”

“…Nevertheless, at maximum β, it appears that the confinement deteriorates when the Shafranov shift reaches some critical value. The Shafranov shift is associated with flux compression, which leads to a steepening of surface averaged profile gradients and hence increased transport fluxes [46]. Therefore, this effect of…”