Internal transport barrier dynamics with plasma rotation in JET

Abstract

Abstract. At JET the dynamics of Internal Transport Barriers (ITBs) has been explored by trying to decouple the effects of heating on one hand and torque on the other with the ultimate objective of identifying the minimum torque required for the formation of transport barriers. The experiments shed light on the physics behind the initial trigger for ITBs, which often shows to be linked to the shape of the q profile and magnetic shear, while the further development was influenced by the strength of the rotational shear. In discharges with a small amount of rotational shear ITBs were triggered, which suggest that the rotational shear is not the dominant factor in the triggering process. However, the subsequent growth of the barrier was limited if the rotational shear was too low at the time of triggering. This growth phase may be highly non-linear, with several possible positive feedback loops, such as the increases of the toroidal and poloidal component of the rotational shear caused by the ITB itself.

Zero-Turbulence Manifold in a Toroidal Plasma

“…We note that the dependence of this optimum value of u ′ on q/ǫ is not as strong as the dependence of the optimum value of γ E on q/ǫ (clearly this must be so because u ′ = (q/ǫ)γ E ). In a device with an optimised value of q/ǫ, a near maximum critical temperature gradient would be achievable for u ′ > ∼ 5, shears comparable to those observed in experiment [3,16,17].…”

“…Note, however, that Ref. [3] reports that the shear was dominated by an enhanced poloidal flow, an effect which is not included in our numerical model.…”

“…This quenching is most effective at zero magnetic shear [12][13][14], a regime which has been associated in experiments with high confinement of energy in the presence of strongly sheared flows [3,15]. Refs.…”

“…Firstly, and principally, we have calculated the shape of the zero-turbulence manifold, the surface that divides the regions in the parameter space (γ E , q/ǫ, R/L T ) where subcritical turbulence can and cannot be nonlinearly sustained. We have described the shape of this manifold and its physical origins, and presented its two implications for confinement in toroidal plasmas: that reducing the ratio q/ǫ, i.e., increasing the ratio of the poloidal to the toroidal magnetic field, improves confinement at ev- ery nonzero value of γ E , and that at fixed q/ǫ, there is an optimum value of γ E (that is, an optimum value of the toroidal flow shear u ′ = dRω/dr/(v thi /R)) at which the critical temperature gradient is maximised, in some instances to values comparable to those observed in internal transport barriers [3,17]. How to calculate the heat and momentum fluxes that would need to be injected in order for such optimal temperature gradients to be achieved was discussed in Ref.…”

“…The heat loss that occurs as a result of turbulence driven by the ion temperature gradient (ITG) is one of the main obstacles to a successful fusion reactor. A large body of experimental work has demonstrated the effectiveness of strongly sheared equilibrium-scale flows in reducing this turbulence [1][2][3]. Numerical models [4][5][6][7] have demonstrated that by reducing the strength of the ITG instability, which drives the turbulence, and by shearing apart the turbulent structures, the radial gradient of the flow component perpendicular to the magnetic field can indeed lead to a great reduction in the heat loss that results from a given temperature gradient.…”