Plasma image edge detection based on the visible camera in the EAST device

Shu et al. SpringerPlus (2016) 5:2050 DOI 10.1186/s40064-016-3697-9 Open Access RESEARCH Plasma image edge detection based on the visible camera in the EAST device Shuangbao Shu1* , Chongyang Xu1, Meiwen Chen2 and Zhendong Yang2,3 *Correspondence: 1 School of Instrument Science and Opto‑Electronics Engineering, Hefei University of Technology, Tunxi Road 193, Baohe District, Hefei 230009, Anhui Province, People’s Republic of China Full list of author information is available at the end of the article Abstract The controlling of plasma shape and position are essential to the success of Tokamak discharge. A real-time image acquisition system was designed to obtain plasma radiation image during the discharge processes in the Experimental Advanced Superconducting Tokamak (EAST) device. The hardware structure and software design of this visible camera system are introduced in detail. According to the general structure of EAST and the layout of the observation window, spatial location of the discharging plasma in the image was measured. An improved Sobel edge detection algorithm using iterative threshold was proposed to detect plasma boundary. EAST discharge results show that the proposed method acquired plasma position and boundary with high accuracy, which is of great significance for better plasma control. Keywords: EAST, Tokamak, Plasma image, Sobel algorithm, Edge detection Background The Experimental Advanced Superconducting Tokamak (EAST) was built by the Institute of Plasma Physics, Chinese Academy of Sciences, which aims at the demonstration of long pulse stable high-performance plasma operation, and thus provides an important test bed to address key physics and technology issues for next-step fusion devices (Wan 2009; Wan et al. 2013). EAST is the first fully superconducting Tokamak device with advanced divertor configuration and heating scheme similar to the International Thermonuclear Experimental Reactor (ITER). In EAST, 38 poloidally aligned magnetic probes measuring tangential field and 35 flux loops measuring poloidal flux are mounted on the vacuum vessel as shown in Fig. 1. The plasma current and poloidal coil currents are measured by rogowski coils. The EFIT (Equilibrium FITing code) reconstruction provides a least square best fit to the diagnostic data and satisfies the model given by the Grad-Shafranov equation. From such full reconstruction calculation, the plasma pressure, current flux function, internal inductance and the parameters of plasma shape and position can be obtained (Qian et al. 2010; Xiao et al. 2012). However, so heavy computation is not fast enough to plasma control. Thus, the RTEFIT (Real-Time EFIT) algorithm modified from this offline EFIT is used for the fast equilibrium solution in the plasma feedback control. The real-time reconstruction algorithm consists of a fast loop and a slow loop running on two CPUs separately. The fast loop does the fitting calculation for © The Author(s) 2016. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Shu et al. SpringerPlus (2016) 5:2050 Fig. 1 EAST cross section and magnetic diagnostics distribution with flux loops (circle) and magnetic probes (square) poloidal current source including external coils and plasma current in each grid. And the slow loop completes the steps required in the reconstruction iteration to prepare a new data set including response vector and normalized flux for fast loop. In RTEFIT code, the most important modifications to offline EFIT are the one iteration calculation and the reuse of the data set in fast loop. Each time the fast loop is executed with a new set of diagnostic data, the same data set is reused until a new one is updated by the slow loop. The plasma current and position control generally used is RZIP control. The control parameters are feedback controlled by adjusting the current in poloidal field (PF) coils. The requested PF coil current is composed of feed-forward part and the feedback part. In a control cycle, the plasma current Ip is measured directly from rogowski coil, but the radial and vertical position of plasma current center can be calculated by the estimator using magnetic diagnostic data. With PID (Proportion Integration Differentiation) operation and decoupling calculation, the requested value for PF coil current is achieved. Then the corresponding command for power supply is generated with PF current by PID calculation. The calculation of EFIT (including offline EFIT and RTEFIT) requires a large number of electromagnetic measurement data from magnetic probes, flux loops, etc. The plasma-forming process is quite complex in the Tokamak start-up phase. The startup plasma is not in equilibrium state, and the result of magnetic measurement hardly Page 2 of 13 Shu et al. SpringerPlus (2016) 5:2050 reflects the real situation of plasma current because of the existence of induced eddy current in the vacuum chamber wall (Liu et al. 2008; Zhou et al. 2015). Thus, the calculated result from EFIT is inaccurate and unusable for reliable plasma current and its position control. In addition, during the discharge process, the distance from the outer closed magnetic surface to the inner wall of mid-plane of the high field side (also called as Gap) is a very important parameter. During the preliminary discharge stage, the Gap influences the initiation of plasma, and accurate Gap control becomes helpful for reliable plasma current and position control. EFIT is also one of the methods to get Gap (Qian et al. 2009). Because the Gap will influence the initiation of plasma, it is more meaningful to obtain the accurate distance between plasma and inner wall of the EAST. A fast visible camera, as a kind of imaging device, can obtain the radiation images in real-time during the plasma discharge (Jia et al. 2015; Yuan et al. 2013; Chapman et al. 2014). This paper mainly discussed how to acquire the plasma image and detect the spatial position of plasma boundary in EAST. A fast image acquisition system for EAST was introduced in this paper. A Sobel edge detection method with improved iterative thresholding algorithm was proposed in real-time plasma boundary detection. According to the EAST device structure, spatial location in the image was calibrated. Gap can also be obtained. It has considerably practical meaning for further plasma control. The rest of the present paper is organized as follows. According to the structure of the EAST device and the position of the observation window, a fast camera image acquisition system and the camera calibration were presented in second section. In third section, the plasma image edge (...truncated)