Synthetic Diagnostic

Fig 1: Direct comparison of experimental results vs theoretical simulation results via synthetic diagnostic is of great significance (Credit to M.Chen)

The correct and clear interpretation of measured experimental imaging signals to provide knowledge about the underlying plasma quantities is essential but rarely simple given that the sophisticated instrumentation and complex physical processes in the plasma serving to generate microwave signals . As a result, the comparison of diagnosed results with the simulation results is tricky as the diagnosed signal (which is usually in the form of power, voltage, etc.) and modeled plasma quantities (density, temperature etc.) are not the same.

Synthetic diagnostics were born to provide an alternative way to facilitate theory- experiment comparisons via forward modeling of the diagnostic process with the full plasma information given by simulation or theory. Thus, the synthetic data and experimental data can be compared in an apples-to-apples way and provide valuable information both for the simulations and experiments

Through close collaboration with the researchers from the Princeton Plasma Physics Laboratory, the University of California, Irvine, and the DIII-D team, the UC Davis Microwave/Millimeter Wave and Plasma Diagnostic Group develops and applies the synthetic diagnostic to the microwave imaging systems --ECEI and MIR, and has already obtained many encouraging results.

Fig 2 (left): Synthetic diagnostic work flow, source: Review of Scientific Instruments 87, 11D303 2016 (Credit to L. Shi)

Synthetic MIR diagnostic

Fig 3: Demonstration of the synthetic MIR diagnostic (Credit to X.Ren & M.Chen)

The synthetic MIR diagnostic (Fig 3) couples quasi-optical modeling of the antennas and lens system, plasma simulation of the time-dependent plasma fluctuations, and 2-D/3-D full-wave modeling of the plasma-wave interaction at the microwave reflection layer.

The synthetic MIR diagnostic has been used extensively to investigate the system operational domain, guide the design of MIR systems (e.g. Fig 4-6) and further our understanding about experimental observed phenomena (e.g. Fig 7).

Fig 4 (right): Synthetically measured phase and prescribed density fluctuation for three comparative cases are shown. As the poloidal mean wavenumber increases, the measured phase signal differs substantially from the density fluctuation. Figure (d) shows that, for a particular value of density fluctuation level, correlation (between synthetically measured phase and prescribed density fluctuation) decreases with increasing wavenumber source: Review of Scientific Instruments 85, 11D863, 2014; (Credit to X.Ren)

Fig 5 (left): Contours of constant correlations between density fluctuations at the cutoff surface and phase fluctuations at the image plane are shown. In (a), the receiver is focused at the midplane and at normal reflection. In (b), contours for the same range of fluctuations are shown, but the receiver is focused 19.3 cm below the midplane with a 10 degree viewing angle. The highlighted curves refer to the minimum acceptable correlation. source:Review of Scientific Instruments 83, 10E338,2012, (Credit to X.Ren)

Fig 6: The correlations are plotted for broadband turbulence with wavenumbers centered at 0 cm−1. The x axis is the Gaussian distribution variation, and the y axis is the fluctuation level. The maximum variation and fluctuation level for correlation values larger than 0.6 are 2.6 cm−1 and 0.3%. (Credit to X.Ren)

Fig 7: Synthetic images of phase perturbation produced by synthetic diagnostic (upper row) and images of density perturbation displacement from M3D-C1 simulation output (lower row). source: 2015 JINST 10 P10036 (Credit to X.Ren & M.Chen)

The application of the synthetic MIR diagnostic has also been used for the study of the edge harmonic oscillation (EHO) on DIII-D. With the comparison of experimental observations with synthetic results based on M3D-C1 simulation outputs, it is confirmed that the MIR optical design, characterized in detail during the first-ever in-vessel calibrations, readily distinguishes the poloidal wavenumber of the mode. Measurement of the mode structure helps to validate the physical picture for EHO stability and control, an important aspect of QH-mode development for ELM avoidance.

Synthetic ECEI diagnostic

Synthetic ECEI diagnostic is also booming and advances our understanding of the plasma physics, especially in the edge pedestal region. The synthetic ECEI code implements a first-of-kind self-consistent reciprocal model that includes not only the emission, reabsorption, and radiation transport, but also simultaneously models refraction and diffraction of the quasi-optical imaging system. This new capability allows for realistic forward modeling of the diagnostic response under a variety of conditions and will be enormously valuable for interpreting data from the plasma edge, where optical thickness varies rapidly along with the plasma’ s refractive index.

Fig 8: Comparison between M3D-C1 simulation (a), synthetic ECEI response (b), and ECEI measurement (c) of electron temperature fluctuations of an EHO (DIII-D, shot #157102 at 2420 ms). source: Review of Scientific Instruments 87, 11D303 2016 (Credit to L. Shi)

Figure 8 shows a comparison between simulated M3D-C1 results, synthetic ECEI signals, and measured ECEI signals. With the synthetic ECEI diagnostic applied to the simulated Te , the response is in qualitative agreement with the measurements, as shown in Figure 8(b) and 8(c). The strong shine through features are reproduced as well as the out of phase intensity fluctuations at the pedestal bottom. source: Review of Scientific Instruments 87, 11D303 2016

Future development for synthetic diagnostics

Roadmap for development of synthetic diagnostic (Credit to Yilun Zhu and Ming Chen)

With the great success that have been achieved, we are excited to explore more with the help of synthetic diagnostics. Further development and application of the synthetic diagnostics might integrate other sophisticated nonlinear simulation models for various plasma physics studies and work towards the auto-feedback for plasma control system.

Synthetic Diagnostic

Synthetic MIR diagnostic

Synthetic ECEI diagnostic

Future development for synthetic diagnostics

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