Microwave Imaging Reflectometry for the study of Edge Harmonic Oscillations on DIII-D

Abstract

Quiescent H-mode (QH-mode) is an ELM free mode of operation in which edgelocalized harmonic oscillations (EHOs) are believed to enhance particle transport, thereby stabilizing ELMs and preventing damage to the divertor and plasma facing components. Microwave Imaging Reflectometer (MIR) enabling direct comparison between the measured and simulated 2D images of density fluctuations near the edge can determine the 2D structure of density oscillation, which can help to explain the physics behind EHO modes. MIR data sometimes indicate a counterpropagation between dominant (n = 1) and higher harmonic modes of coherent EHOs in the steep gradient regions of the pedestal. To preclude diagnostic artifacts, we have performed forward modeling that includes possible optical mis-alignments to show that offsets between transmitting and receiving antennas do not account for this feature. We have also simulated the non-linear structure of the EHO modes, which induces multiple harmonics that are properly charaterized in the synthetic diagnostic. By excluding mis-alignments of optics as well as patially eliminating nonlinearity of EHO mode structure as possible explanation for the data, counter-propagation observed in MIR data, which is not corroborated by external Mirnov coil array measurements, may be due to subtleties of the eigenmode structure, such as an inversion radius consistent with a magnetic island. Similar effects are observed in analysis of internal ECE-Imaging and BES data. The identification of a non-ideal structure motivates further exploration of nonlinear models of this instability.

Rotational shear effects on edge harmonic oscillations in DIII-D quiescent H-mode discharges

“…Although the rotational shear effect can be The correspondence between the phase modulation of the MIR signal and the density fluctuation due to the EHO is complicated. In order to make a truly "apples-to-apples" comparison, we apply the synthetic MIR diagnostic [42] using the full wave reflectometer code FWR2D [43]…”

Estimation of the radial size and density fluctuation amplitude of edge localized modes using microwave interferometer array

“…Microwave Imaging Reflectometry (MIR) is a diagnostic concept based on the principle of conventional reflectometry along with the use of microwave imaging optics to measure the density fluctuations in the plasma. This concept was first proposed by E. Mazzucato [30] based on an extensive numerical study of microwave reflectometry in tokamaks and has been implemented on several fusion devices such as TEXTOR, DIII-D, LHD, TPE-RX, and HL-2A [31][32][33][34][35]. In this study, the MIR system on the KSTAR is reconfigured as MIA as illustrated in figure 1a.…”

“…In order to determine the radial size and density fluctuation amplitude of the observed ELMs, the dependence of the ∆ϕ amplitude on the radial size and fluctuation amplitude of ELMs has been investigated using a two dimensional full-wave simulation code FWR2D [29]. The code simulates electromagnetic wave propagation and reflection through inhomogeneous plasma [34,[45][46][47][48] by solving the time-dependent scalar wave equation for the electric field E(x, y, t),…”

Development and application of radar reflectometer using micro to infrared waves

“…It is seen that a clear correlation is observed in the poloidal direction in the H-mode plasma. MIR systems have been applied to various magnetically confined plasmas in the world such as DIII-D, LHD, KSTAR, ASDEX-U, and EAST, and produces useful information regarding fluctuations [48][49][50][51].…”