MHD activity and formation of the electron internal transport barrier in the T-10 tokamak

Abstract

The plasma stability and confinement have been investigated through control of the safety factor profile q(r) by the electron cyclotron current drive in the T-10 tokamak. The regimes with dq/dr ∼ = 0 and dq/dr < 0 in the plasma core were obtained. Various types of MHD activity were observed: ordinary sawtooth, saturated sawtooth, humpbacks, hills etc. It was shown that when the minimal value q min increases from q min < 1 to q min = 2 the plasma becomes strongly unstable due to the corresponding MHD activity or passes to the steady-state improved confinement mode. The latter is realized when the electron internal transport barrier (EITB) is formed. The condition for the appearance of the EITB is dq/dr ∼ = 0, where q = m/n lies near a rational value for low m and n.

A review of experimental drift turbulence studies

“…This result is reminiscent of the radial correlation length in the L-H transition, but occurs over a much longer time period. We note that similar studies and results have been published from JET [338,339], ASDEX-UG [340] and T-10 [341] in both ion ITB and electron ITB conditions (which were defined by reductions in the respective thermal diffusion coefficients for the two heat transport channels). Furthermore, improved confinement regimes have also been demonstrated in stellarator devices and have been associated with the formation of sheared radial electric fields and plasma flows as well.…”

Detailed measurements of the electron cyclotron current drive efficiency on DIII-D

“…For example, electron cyclotron current drive (ECCD) near the q = 3/2 surface, where q is the safety factor, has been used to stabilize the (m, n) = (3, 2) neoclassical tearing mode on several tokamaks [1,2,3,4,5]. Modification of the current profile using ECCD also has been demonstrated for reversal of the magnetic shear [6,7,8] and sawtooth suppression [9,10,11]. More global applications for ECCD have been fully non-inductive current drive in tokamaks [11,12,13,14] and bootstrap current compensation in stellarators [15].…”

Sustainment of the Reversed ShearMagnetic Field Configuration with Near-Rational q-min Value with the Help of the Rotating Magnetic Field

“…Due to the low variation of q with radius near the shear reversal point (q-minimum point), the distance between the neighboring resonant magnetic surfaces is increased which may prevent overlapping of resonant turbulent modes with different helicities and may lead to the local reduction of transport. It has been observed experimentally [1] and also found in global turbulence modelling [2] that when the q-minimum value, q min , is close to a low order rational number, the reduction of anomalous transport becomes stronger. This is due to the fact that there are no other low order rational numbers in a certain vicinity of a given low order rational number as shown in Fig.…”