ITB formation in terms of ω_E×Bflow shear and magnetic shearson JET

Abstract

A linear empirical threshold condition ω E×B /γ ITG > 0.68s − 0.095 has been found for the onset of the ion internal transport barriers in the JET optimised shear database. Here, s is the magnetic shear, ω E×B the flow shearing rate and γ ITG is an approximate of the linear growth rate of the ion temperature gradient instability. The present empirical threshold condition for the ITB formation will provide a first clear indication of the strong correlation of s and ω E×B at the ITB transition. The empirical analysis consists of ITB discharges from a wide plasma parameter range; the toroidal magnetic field varies between 1.8-4.0 T, the auxiliary heating power between 10-30 MW and the diamagnetic energy between 3-12 MJ. The predictive simulations of several ITB discharges with the empirical ITB threshold condition reproduce the experiments with time averaged prediction errors of the order of 10-25% in T i and T e profiles and 10-15% in n e profiles as well as the toroidal flow velocity with errors of the order of 10-20%. The simulated times of the onset of the ITB compared to the experimental ones are typically within 0.4 s and the simulated ITB widths within 0.1 in r/a throughout the whole simulations.

Compatibility of internal transport barrier with steady-state operation in the high bootstrap fraction regime on DIII-D

“…Notice that, in the prediction without E × B shear [figure 4(e)], the foot of the ITB has moved slightly inward. It is not clear whether this small effect would be significant enough to cause progressive shrinking of the ITB radius, nevertheless it is consistent with the speculation by Tala et al [18] discussed above. However, analysis of the E × B shear components in these plasmas suggests that the E × B shear affecting the ITB foot can be produced by the pressure gradient associated with the ITB itself, and does not rely on externally driven large toroidal rotation shear.…”

“…The observation of ITBs extending into the positive magnetic shear region has been made on several tokamaks, with monotonic q-profiles such as in [16,17]; and in q-profiles with strong shear reversal in the core, such as in [2]. On JET, it was argued by Tala et al [18] that essential to these type of ITBs is the presence of high values of rotational shear, raising questions concerning the relevance of ITBs in the positive magnetic shear region to future devices where rotation effects are likely to be smaller than in present tokamaks (at least those with unbalanced NBI). However, in these high-β P discharges the ITB foot in the rotation profile is observed at smaller minor radius than in all other profiles, as shown by the vertical dashed line in figures 3(a) and (b).…”

Internal transport barrier dynamics with plasma rotation in JET

“…[11,25], under the assumption of neo-classical poloidal rotation, determined by NCLASS [24]. The JET plasmas, discussed here, are in the low collisionality regime and it has been demonstrated that in this case the dominant contribution to the rotational shear is usually the gradient in the toroidal rotation profile [11,13].…”

“…At JET internal transport barriers are usually formed in discharges with predominant NBI and EX/8-3 this system provides a large toroidal torque and consequently a large rotational shear. Studies have shown the effect of rotational shear on ITBs in JET discharges with small but positive magnetic shear [11]. Experiments to produce ITBs with low torque have been performed previously by simply increasing the fraction of Ion Cyclotron Resonance Heating (ICRH).…”

Two-fluid tokamak equilibria with reversed magnetic shear and sheared flow

“…Long quasi-steady or steady ITB states have been obtained in different tokamaks, e.g ASDEX Upgrate [1,2], JT-60U [3], Tore-Supra [4], and JET [5] where ITBs were maintained for up to 11 s. The ITBs usually are associated with reversed magnetic shear profiles [6], [7] and their main characteristics are steep pressure profiles in the barrier region [8] and radial electric fields associated with sheared flows [9,10]. The mechanism responsible for the formation of ITBs and the underlying physics is not completely understood.…”