Identifying and reconstructing fusion plasma boundaries accurately are important research areas in controlled thermonuclear fusion. The traditional electromagnetic measurement methods will inevitably suffer from the accuracy problem arising from neutron radiation and long-term drift. The traditional optical diagnostic methods are non-intrusive and reach a high level of spatial resolution. However, they are commonly limited to two-dimensional imaging. As the processes within the plasma flow are inherently three-dimensional, it is necessary to develop a three-dimensional method for plasma measurement. In order to capture the dynamic information of the plasma and avoid signal distortion, three-dimensional imaging must be achieved at a high speed in parallel or sequential imaging of multiple planes. However, the existing three-dimensional reconstruction methods based on tomography technology are limited by spatial and temporal resolution, and multiple images have to be captured from various angles, or complex experimental setups are needed. All the above methods are not applicable to reconstructing the three-dimensional plasma boundaries in real time. The light field camera is an emerging image acquisition device, in which a microlens array is placed between the main lens and the sensor. With the light field camera, multi-angle information can be captured within a single exposure. Plasma flow is the typical semi-transparent and dispersive media. To date, some studies have used the light field deconvolution algorithm to reconstruct the plasma, but the algorithm requires a long computation time. To this end, we propose a light field deconvolution algorithm based on optical sectioning imaging, which has the advantages of simplicity and speed. We hope that our method can be helpful in the three-dimensional reconstruction of plasma.

The depth information and point spread function are the key parameters of the method in this paper. We obtain these two parameters through experiments. First of all, with the digital refocusing technology, we calculate the relationship between the light-field refocused parameters and real-world depth by using the scale and the image sharpness evaluation algorithm. Then, we determine five points to calculate refocused section images, and by the edge method, the point spread function at these locations is computed for the subsequent iterative deconvolution operation. Finally, we perform the deconvolution operation on the image to be reconstructed and the point spread function to remove the out-of-focus information from the image to be reconstructed.

In order to verify the effectiveness of the proposed method, simulation experiments are conducted. The defocusing effect is simulated by setting different point diffusion functions and image convolution (Fig. 14). The simulation results show that the proposed method can effectively remove the out-of-focus image information. In addition, the effect of the number of sections and section intervals on the reconstruction accuracy is explored, and the structural similarity (SSIM) is used to evaluate the performance (Figs. 16 and 17). The results show that as more sections are involved in the deconvolution, and the spacing gets smaller, the reconstruction performance becomes better. Finally, an experiment with the flame is conducted as the research object. The proposed method recovers the original structure of the section image successfully, and the trend is consistent with the actual flame distribution (Fig. 22), which verifies the experimental efficacy of the proposed reconstruction method.

In order to address the problems in traditional optical diagnostic techniques such as three-dimensional information loss and poor real-time performance, a light field deconvolution method based on optical sectioning imaging is proposed, so as to achieve the three-dimensional reconstruction of plasma boundaries by a single camera without focus adjustment. The three-dimensional reconstruction is transformed into the two-dimensional section reconstruction, which reduces the computational cost greatly. The results show that the original section image of the flame can be reconstructed by the proposed method, which initially demonstrates the feasibility of the three-dimensional reconstruction method of the plasma based on light field imaging. With the optical imaging conditions in this paper as an example, the depth-of-field resolution of the three-dimensional reconstructed object should be close to the depth of the object within the focal plane, and for high depth-of-field resolution, 100 or more focal planes are likely to be required to span the full depth of the object, while only five sections are selected to verify the effectiveness of the proposed method. To further improve the spatio-temporal resolution of the reconstruction, we will make attempts to achieve a more accurate extraction of the point spread function by using a high-precision electrodynamic displacement stage and performing deconvolution operations with more section images.