Turbulence characteristics of the I-mode confinement regime in ASDEX Upgrade

Abstract

The I-mode is an improved confinement regime of tokamak plasmas operating in the unfavourable ion ∇B-drift direction combining H-mode-like energy confinement with L-mode-like particle and impurity transport r1s. To qualify the I-mode as an operating scenario for ITER threshold and accessibility studies r2s also on a multimachine basis r3s are needed. Even if the I-mode might not be considered as a promising scenario for ITER, studies of turbulence in I-mode may offer a better understanding of the physics of the interaction of energy and particle transport barriers in general. The mechanism which selectively reduces only one of the transport channels is not understood. In the present contribution the turbulence characteristics of I-mode plasmas in ASDEX-Upgrade (AUG) have been studied in detail. In AUG the WCM appear at frequencies around 100 kHz with a width of a few 10 kHz r4s. The GAM advects the WCM and leads to the broadening through the Doppler effect r4s. In AUG magnetic fluctuations already present in L-mode close to the WCM frequency are amplified during the I-mode. Using the measured background profiles the gyrokinetic eigenvalue solver LIGKA is used to determine the kinetic continuum branches of the GAM and the geodesic Alfvénic mode (GAlf). The frequency of the magnetic fluctuations close to the WCM frequency coincides with the GAlf frequency r4s. Comparisons with the Mirnov coils show the characteristic toroidal and poloidal mode numbers of the GAlf of zero and one, respectively r4s. The origin of the WCM lies in the intermittent events, which exhibit a wavelet-like temporal structure where the characteristic frequency coincides with the centre-of-mass frequency of the WCM r5s. Possible generation scenarios for the bursts are discussed. The nonlinear interaction between GAM, WCM and the bursts will be discussed based on a detailed bispectral analysis of nonlocal interactions in frequency space of the experimental data. The importance of the solitary events for a selective density transport remains an open issue.Published as a journal article in Nuclear Fusion http://iopscience.iop.org/article/Abstract:Turbulence in the I-mode confinement regime of ASDEX Upgrade exhibits beside strong geodesic acoustic mode (GAM) activity two prominent features, the weakly coherent mode (WCM) and strongly intermittent solitary density perturbations. The nonlinear interaction between these structures is studied in detail by means of a conditional averaged waveletbicoherence analysis. The wavelet analysis reveals that the WCM appears as these density bursts. The GAM is coupled to all frequency scales of the velocity fluctuations via a modulational instability. The WCM is broadened through the Doppler effect by the GAM flow. The WCM shows coupling to higher frequencies prior to the bursts indicating a wavesteepening-like process.

Comparison of Conditional Average Using Threshold and Template Methods for Quasi-Periodic Phenomena in Plasmas

“…It is noted that in Figs. (a)-(c), the template method provides the increase and decrease of high-frequency fluctuation components accompany with the bursts, which is consistent with the previous work [11]. Template method can conserve even high frequency components clearly.…”

“…The other technique developed recently is called the template method [7,8], in which the clocks are determined as the times when the cross-correlation between the template and the signal is sufficiently high, in other words, when the signal resembles to some degree of the standard pattern defined as 'template'. This article presents the results of conditional averaging applied on the intermittent bursts observed during the I-mode plasma of ASDEX Upgrade (AUG) [9][10][11], and shows the excellent features of the template methods by the comparison to the threshold methods.…”

Microwave beam broadening due to turbulent plasma density fluctuations within the limit of the Born approximation and beyond

“…Note that the absolute values of the frequency, the plasma density, and of the background magnetic field correspond to the values of the ASDEX-Upgrade tokamak [7,43] reduced by approximately a factor 2.5. The reduced frequency and hence increased vacuum wavelength allows for a coarser numerical grid to be used decreasing the required computational resources.…”

“…Note that the values used for major and minor radii correspond to the ASDEX Upgrade tokamak [43]. Figure 2 shows as an example one sample for the actual 2D density profile used in the full-wave simulations.…”

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.…”