Core barrier formation near integer q surfaces in DIII-D

Abstract

Recent DIII-D experiments have significantly improved the understanding of internal transport barriers (ITBs) that are triggered close to the time when an integer value of the minimum in q is crossed. While this phenomenon has been observed on many tokamaks, the extensive transport and fluctuation diagnostics on DIII-D have permitted a detailed study of the generation mechanisms of q-triggered ITBs as pertaining to turbulence suppression dynamics, shear flows, and energetic particle modes. In these discharges, the evolution of the q profile is measured using motional Stark effect polarimetry and the integer qmin crossings are further pinpointed in time by the observation of Alfvén cascades. High time resolution measurements of the ion and electron temperatures and the toroidal rotation show that the start of improved confinement is simultaneous in all three channels, and that this event precedes the traversal of integer qmin by 5–20ms. There is no significant low-frequency magnetohydrodynamic activity prior to or just after the crossing of the integer qmin and hence magnetic reconnection is determined not to be the precipitant of the confinement change. Instead, results from the GYRO code point to the effects of zonal flows near low order rational q values as playing a role in ITB triggering. A reduction in local turbulent fluctuations is observed at the start of the temperature rise and, concurrently, an increase in turbulence poloidal flow velocity and flow shear is measured with the beam emission spectroscopy diagnostic. For the case of a transition to an enduring internal barrier the fluctuation level remains at a reduced amplitude. The timing and nature of the temperature, rotation, and fluctuation changes leading to internal barriers suggests transport improvement due to increased shear flow arising from the zonal flow structures.

Internal transport barrier dynamics with plasma rotation in JET

“…This feature, identical to those observed in ref. [8,22], cannot be predicted by transport models since usually no role is given to rational surfaces. On the other hand rarefaction of rational flux surfaces, i.e.…”

The role of zonal flows and predator–prey oscillations in triggering the formation of edge and core transport barriers

“…After the realization that sufficiently large € E × B flow shear can substantially reduce the local turbulence and radial transport level [11], experiments have demonstrated that sheared flow indeed plays a crucial role in triggering edge and core barrier formation, and in maintaining turbulence suppression in the H-mode pedestal and in sustained internal barriers (for example, [7,8,[10][11][12][13][14][15][16][17]). With neutral beam injection and driven ion rotation in the radial momentum balance, or due to the ion pressure gradient, depending on the plasma regime, barrier evolution, and other experimental details.…”

“…This has been attributed to stationary zonal flows (ZFs) near the rational surfaces (in particular the € q = 2 surface [8][9][10]). At higher heating power, the formation radius of transport barriers does not necessarily coincide with the location of rational magnetic surfaces.…”

“…At low or moderate neutral beam or radiofrequency heating power at reversed magnetic shear, internal barriers are often triggered at the radii where low order rational magnetic surfaces enter the plasma [6][7][8].…”

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

“…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].…”