Increased understanding of the dynamics and transport in ITB plasmas from multi-machine comparisons

Abstract

Our understanding of the physics of internal transport barriers (ITBs) is being furthered by analysis and comparisons of experimental data from many different tokamaks worldwide. An international database consisting of scalar and 2-D profile data for ITB plasmas is being developed to determine the requirements for the formation and sustainment of ITBs and to perform tests of theory-based transport models in an effort to improve the predictive capability of the models. Analysis using the database indicates that: (a) the power required to form ITBs decreases with increased negative magnetic shear of the target plasma, and: (b) the ExB flow shear rate is close to the linear growth rate of the ITG modes at the time of barrier formation when compared for several fusion devices. Tests of several transport models (JETTO, Weiland model) using the 2-D profile data indicate that there is only limited agreement between the model predictions and the experimental results for the range of plasma conditions examined for the different devices (DIII-D, JET, JT-GOU). Gyrokinetic stability analysis (using the GKS code) of the ITB discharges from these devices indicates that the ITG/TEM growth rates decrease with increased negative magnetic shear and that the ExB shear rate is comparable to the linear growth rates at the location of the ITB.

Control of internal transport barriers on Alcator C-Mod

“…It has also been demonstrated that reversal of magnetic shear by use of current modification by lower hybrid current drive 7 or electron cyclotron current drive 8 encourages ITB development. These are thought to occur when the effects of pressure profile gradient driven instabilities are reduced by the introduction of rotational shear into the unstable region or by providing a region of reversed magnetic shear 9 .…”

Chaotic transport in Hamiltonian systems perturbed by a weak turbulent wave field

“…The transport barrier formation near the low-order rational tori found numerically in Hamiltonian systems perturbed by a weak turbulent field at the large Kubo numbers, probably, may have some sort of relation to the transport barriers in fusion experiments (see, e.g., a review [4]). As was mentioned above there is a possible relation between the density of rational magnetic surfaces (or tori) and the transport of particles and energy, particularly, the transport barrier formation in fusion experiments, which has been already discussed in a number of papers [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. Unlike qualitative arguments given in those works we have explicitly shown the formation of transport barriers in Hamiltonian systems by direct numerical calculations and explored the conditions at which such barriers are created.…”

“…7 in the whole region 0 < J < 1 for the large Kubo number K = 100 (a) and in the region 0.5 < J < 0.55 located near the rational tori ω = 1/3 for the several values of K = 1,2,5, and 100 (b). The values for the other parameters are chosen: q 0 = 0.8, q a = 5, γ ≡ 1, 20 n 50, and α = 5/6, which corresponds to the power spectrum |H mn | 2 ∼ n −5/3 , and the 1 The existence of gaps in the density of rational tori has been pointed out already in many above-mentioned works [15][16][17][18][19][20][21][22][23][24][25], however, no quantitative estimations have been presented. Moreover, I could not find the results obtained here in existing literature, although one could expect that this problem has been treated already in number theory or in other fields of physics.…”

“…At present several possible mechanisms, like an ExB flow shear, a zonal flow, or a magnetic shear, have been proposed. One of these issues is the role of low-order magnetic surfaces in the formation transport barriers which have been discussed in a number of works [12][13][14][15][16][17][18][19][20][21][22][23][24][25].…”

The ecology of flows and drift wave turbulence in CSDX: A model

“…Scalings of turbulent diffusion in both CSDX and larger devices characterized by higher temperature follow the same trend. 46,47 When the axial to azimuthal flow coupling is weak, the axial flow is mainly driven by the turbulent Reynolds stress, particularly by the parallel residual part. The reduced 4-field model can thus be simplified to a 2-field predator-prey model which evolves v y and n. In CSDX, probe measurements show that the magnitude of v z is moderate, and that the parallel Reynolds power is much less than that in the perpendicular direction.…”