Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak

Abstract

Abstract. The I-mode regime, routinely observed on the Alcator C-Mod tokamak, is characterized by an edge energy transport barrier without an accompanying particle barrier and with broadband instabilities, known as Weakly Coherent Modes (WCM), believed to regulate particle transport at the edge. Recent experiments on the DIII-D tokamak exhibit I-mode characteristics in various physical quantities. These DIII-D plasmas evolve over long periods, lasting several energy confinement times, during which the edge electron temperature slowly evolves towards an H-modelike profile, while maintaining a typical L-mode edge density profile. During these periods, referred to as I-mode phases, the radial electric field at the edge also gradually reaches values typically observed in H-mode. Density fluctuations measured with the Phase Contrast Imaging diagnostic during Imode phases exhibit three features typically observed in H-mode on DIII-D, although they develop progressively with time and without a sharp transition: the intensity of the fluctuations is reduced; the frequency spectrum is broadened and becomes nonmonotonic; two dimensional space-time spectra appear to approach those in H-mode, showing phase velocities of density fluctuations at the edge increasing to about 10 km/s. However, in DIII-D there is no clear evidence of the WCM. Preliminary linear gyro-kinetic simulations are performed in the pedestal region with the GS2 code and its recently upgraded model collision operator that conserves particles, energy and momentum. The increased bootstrap current and flow shear generated by the temperature pedestal are shown to decrease growth rates, thus possibly generating a feedback mechanism that progressively stabilizes fluctuations.

Validation of nonlinear gyrokinetic simulations of L- and I-mode plasmas on Alcator C-Mod

“…The operating window is much wider at 5.4 T (C-Mod's standard field), and no I-H transitions have been observed at 8 T, even with full input power. 7 Despite this smaller operating window at a lower magnetic field, I-mode has also been observed on both ASDEX Upgrade 8,9 and DIII-D. 10 The properties of I-mode outlined here make it quite a favorable candidate for an operating regime on future fusion devices, such as ITER or another high field burning plasma experiment. For this reason, it is vital to understand the core transport in I-mode plasmas (both experimentally and with models) and, in doing so, enable accurate predictions for future devices.…”

Multi-device studies of pedestal physics and confinement in the I-mode regime

“…The discharge thus shows the key feature of Imode, though it is less robust than on the other tokamaks. As on ASDEX Upgrade, Similar to C-Mod and AUG, E r shear increases in the regions of the T i pedestal, and is also intermediate between L-mode and H-mode levels [7].…”

I-mode investigation on the Experimental Advanced Superconducting Tokamak

“…The I-mode was originally observed as an energy confinement improved mode in 1990s [2]; currently, it has been widely investigated in Alcator C-Mod [3,4,5,6,7,8,9,10], AUG [11,12,13,14,15,16], and DIII-D [17]. In unfavorable configurations [18], the L-mode to I-mode transition occurs when the heating power exceeds a certain threshold [19], and an edge energy transport barrier [7] is constructed accompanied with the weak coherent mode (WCM) [13,8,9,10,17] at the steepest temperature gradient region. In the I-mode operation, an E r well [5,6,17,12] forms at the edge of the plasma, and a stable temperature pedestal is simultaneously established [20,21].…”

“…In unfavorable configurations [18], the L-mode to I-mode transition occurs when the heating power exceeds a certain threshold [19], and an edge energy transport barrier [7] is constructed accompanied with the weak coherent mode (WCM) [13,8,9,10,17] at the steepest temperature gradient region. In the I-mode operation, an E r well [5,6,17,12] forms at the edge of the plasma, and a stable temperature pedestal is simultaneously established [20,21]. The poor particle confinement in the I-mode also contributes to a low core impurity level and sustains the operation performance.…”