Formation criteria and positioning of internal transport barriers in ASDEX Upgrade

Abstract

Advanced tokamak discharges on ASDEX Upgrade can generate an internal transport barrier (ITB) during the current ramp-up phase early in the discharge. Formation of the ITB has become more reliable with the discovery that a low density is necessary for it to form. These ITBs form in very low or negative central shear regions. There is no clear evidence of integer q magnetic surfaces triggering the ITB phase. The appearance of an integer q surface (usually q = 2) often leads to a second ITB (i.e. another steepening in the ion temperature gradient) inside the first ITB.Investigation of possible turbulence suppression mechanisms suggests that something is missing from the generally accepted model of sufficient E × B sheared flow suppressing the turbulence since the E × B shearing rate was not greater than, or comparable with, the maximum linear growth rate in about half of the ITB discharges analysed. Thus, it does not explain the reduced radial transport observed in these discharges.

Simulation of small size divertor tokamak plasma edge at low density of plasma

“…This delayed heating prevents the neutral beam fuelling from generating higher density before, formation of ITB. This result agrees with the result given by [18].…”

“…2) The radial profile of the plasma density characteristic scale length ( show that, by lowering the plasma density, the strong ITB, steep plasma density characteristic scale length and steep plasma density are formed. According to [18][19][20] the steep plasma density is the main factor for stabilized the turbulence driven by ITG modes. A possible explanation for the necessary condition of lower plasma density for steep plasma density and ITB formation is that, by slightly lowering the plasma density and keeping the temperature plasma heating and toroidal torque the same, the radial electric field shear and toroidal velocity will increase, possibly allowing the turbulence to be suppressed.…”

Thermal ions dilution and ITG suppression in ASDEX Upgrade ion ITBs

“…The density threshold for ITB formation documented in [6] is explained by the variation of the dilution factor n f ast /n e . This parameter explains also the ITB lifetime, of the order of τ sd , as the fast ions get thermalised and enhance the background density.…”

“…The ITG mode is commonly believed to be stabilised by high plasma rotational shearing rate, ω E×B , by weak or negative magnetic shear, by the Shafranov shift and a high ratio T i /T e . In fact, the key parameter for ITB formation in AUG is found to be the low plasma density [6]. The role of ω E×B is also discussed in [6], although more specifically for ITBs relying on NBI pre-heating.…”

Understanding of the density profile shape, electron heat transport and internal transport barriers observed in ASDEX Upgrade

“…The standard paradigm that the shearing rate (ω E $ B ) should exceed the growth rate (γ) of the most unstable mode can, so far, not be established, as shown in Fig. 5 [18] which compares the shearing rate with the growth rate, calculated without (red) and with (blue) the Shafranov shift. On the thick black line the criterion is satisfied.…”