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3D Microwave Camera for MHD Instabilitiesin High Temperature Plasmas at KSTAR
HYEONK PARK
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3D Microwave Camera for MHD Instabilities in High Temperature Plasmas at KSTAR

HyeonK. Park
POSTECH

ABSTRACT

Full understanding and control of the magnetohydrodynamic (MHD) instabilities that are harmful in magnetically confined plasmas demand first principle based theoretical models that can only be achieved through visualization of their dynamics. 2-D microwave cameras with high spatial and temporal resolution based on electron cyclotron emission imaging (ECEI) techniques [1] successfully characterized various MHD instabilities of high temperature plasmas (several hundred million degrees) at Korean Superconducting Tokamak Advanced Research (KSTAR).

fig. 1: A schematic of the two ECEI systems on KSTAR. The ECEI-I system has been successfully operated for the 2010 and
2011 campaigns. The second system (ECEI-II) is commissioned in the 2012 campaign.

The ECEI technique is based on established radiometry for the measurement of the electron cyclotron emission (ECE) intensity from the magnetically confined plasmas, in which the emission intensity can be treated as a "black body." The microwave camera at KSTAR consists of two independent receiver arrays forming an image on the detector plane and large aperture optics with zooming capability as illustrated in Fig. 1, which provide a simultaneous measurement of two independent regions along the field of view. The two viewing areas, which correspond to the individual receiver arrays as denoted by HFS (high-field side) and LFS (low-field side) in the figure, can be placed anywhere in the poloidal cross-section with a variable vertical coverage from ~30 to ~90 cm. This flexibility has allowed various combinations of HFS and LFS view positions, providing excellent opportunities to study a variety of plasma instabilities and turbulence phenomena in 2D. Each detector array provides 24 (vertical) x 8 (radial) = 192 local emission measurements with a spatial resolution ~1?2 cm and a time resolution down to ~1 ms. Ultimately, in order to develop a fully comprehensive physical model and control method for the harmful MHD instabilities, it is essential to extend to 3D images with adequate temporal and spatial resolution. This challenge has been achieved in the 2012 KSTAR campaign with the aid of the second ECEI-II system which is located at an adjacent port (22.50 apart) as illustrated in Fig.1. The 3D images will provide the critical physics information which was not available by conventional diagnostic methods.

fig. 2: Images of the instabilities from the two ECEI systems: (a) Sawtooth oscillation in the core (b) Tearing mode in the middle (c) ELM at the edge of the KSTAR H-mode plasma.

During the KSTAR campaigns of 2010 and 2011, images from a variety of instabilities such as the sawtooth crash, tearing mode, and the edge-localized modes (ELM) were successfully documented [2]. In particular, ELMs are ubiquitous in the high confinement mode (H-mode) of a tokamak plasma. The H-mode was adopted as a standard mode of operation in ITER, yet large ELM bursts can severely damage the fist wall; thus, understanding and control of this instability became an essential research subject for all tokamaks. The phenomenology of the instabilities that are potentially harmful for tokamak operation is multi-dimensional (a combination of global symmetry and local asymmetric transient behaviors) and the response to various control methods is even more complex. A control system that requires actively cooled current carrying in-vessel structures is challenging to engineer not only for ITER but also for future devices, such as the DEMO facility. Therefore, it is desirable to develop a control technique that is both effective and relatively simple to engineer. This goal can only be achieved from a full understanding of the physics of these instabilities. The 2D images (radial and poloidal) of these instabilities from the microwave camera revealed new and unambiguous detailed physics of these instabilities and additional toroidal information would perfect the understanding of these instabilities. In the 2012 KSTAR campaign, 3D visualization of these instabilities was demonstrated for the first time by adding the ECEI-II system as illustrated in Fig. 2. These are examples from the sawtooth instabilities residing in the core region, ELMs at the edge region and tearing instabilities in between the core and edge of the toroidal plasmas. The preliminary analysis revealed unprecedented new features of physics which have never been clarified before and the result will positively contribute to enhancing the physics modeling of these instabilities in the future. This work is supported by NRF of Korea contract No. 20090082507

References

[1] Park, H.K., et al., "Simultaneous microwave imaging system for density and temperature fl uctuation measurements on TEXTOR", Rev. Sci. Instrum. 74, 3787
(2004).

[2] yun, G.S., et al., "Two-Dimensional Visualization of Growth and Burst of the Edge-Localized Filaments in KSTAR H-mode Plasmas", Phys. Rev. Lett. 107, 045004 (2011)

hyeon K. Park is a professor of Physics at the Pohang University of Science and Technology. He earned his Ph. D. from the University of California at Los Angeles in 1984. He joined the Princeton Plasma Physics Laboratory at Princeton University as a principal research physicist in 1997 and became a professor of the Pohang University of Science and Technology in 2007.

 
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