1. Introduction

The high-speed imaging technology plays an important role for investigating ultra-fast phenomena in the field of science, industry, defense and biology [1–6]. In recent years, two dimensional single-shot ultrafast imaging methods have been developed, such as the sequentially timed all-optical mapping photography (STAMP) [7] and the compressed ultrafast photography (CUP) [8], which raise the temporal resolution to hundreds of femtoseconds. However, the STAMP can’t be used for investigating self-emission objects and the CUP needs image reconstruction with compressed-sensing algorithms, which have influence upon the quality of the obtained images. Therefore, for investigating the dynamic phenomena with duration times from several nanoseconds to hundreds of nanoseconds in plasma science, pulse power technologies and high energy density physics [9–11], the framing camera is still a preferred tool due to its high photography quality and simple operation. However, the conventional framing cameras are usually established with multiple intensified cameras and a complicated optical structure for splitting the input image, which are not compact for some applications.

Splitting an optical image into several frames, adding a specific delay for each frame and recording all of the frames simultaneously is a strategy for single-shot multi-frame imaging techniques. The multiplexed structured image capture (MUSIC) technique is based on this strategy and is a single-shot multi-frame passive imaging technique. This imaging technique used different optical paths to relay optical images with different delays and then encoded the optical images with Ronchi rulings [12]. The mixed encoded optical images were recorded with a single camera and recovered by the computational algorithm. The plasma emission has been imaged with three frames in a single shot with this technique. However, this method needs image reconstruction.

Fiber bundles play an important role in the area of biomedical imaging, for distal images of internal tissues or organs can be exported with flexible endoscopes and used for clinical diagnosis [13,14]. Based on the characteristic of image delivery, imaging fiber bundles can also be used for delaying optical images. In this paper, a simple method for acquiring multiple frames with a single intensified complementary metal oxide semiconductor (CMOS) camera in a single shot has been proposed. This strategy employs fiber bundles to receive multiple optical images of the optical object and provide a specific delay for each optical image. Then all of the optical images were exported from the fiber bundles and then captured simultaneously with the intensified CMOS camera. The temporal resolution and the inter-frame times are determined by the gating time of the intensified CMOS camera and the optical delays of the fiber bundles, respectively.

2. Experimental setup

The imaging system was established and applied for investigating the dynamic x-ray spot generated by the rod-pinch diode. As shown in Fig. 1, the x-ray spot was generated by the rod-pinch diode and then imaged through a pinhole on to an image plate and an image converter, which was used to acquire the time-integrated x-ray image and convert x-ray images to optical images, respectively. The pinhole was composed of one straight cylindrical hole and two conical holes. The diameter and length of the cylindrical hole is 0.3 mm and 6 mm, respectively. The diameter of each lateral hole of each conical hole is 0.3 mm and 3 mm, respectively, and the length of each conical hole is 27 mm. The pinhole was placed at the distance of 34 mm and 175 mm to the rod and the image converter, respectively. The image plate was placed closely to the image converter. The image converter was prepared with two scintillators, an inorganic scintillator of LYSO (Lu_2(1-x)Y_2xSiO₅) and a plastic scintillator, the decay time of which was evaluated to be about 47 ns and 3 ns, respectively, with a delayed coincidence method under the excitation of 1.25 MeV gamma rays of ⁶⁰Co [15]. Therefore, the imaging system based on the inorganic scintillator and plastic scintillator can be utilized as integrating and impulsive detection system [16] for investigating the rod-pinch diode with several tens of nanoseconds duration time, respectively. The integrating detection system can achieve images with a high signal to noise ratio (SNR) and the impulsive detection system can obtain images with a high temporal resolution.

 

Fig. 1. Experimental setup of the imaging system for investigating the dynamic x-ray spot generated by the rod-pinch diode.

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The imaging system was composed of four lenses, fiber bundles and an intensified CMOS camera. The fiber bundles contained four independent portions with lengths of 0.5 m, 2 m, 4 m and 6 m, which can provide delays of about 2.5 ns, 10 ns, 20 ns and 30 ns for 420 nm light, respectively. Each portion of the fiber bundles contained about 256×256 fibers with 27 µm diameter of each fiber. The image input area of each portion had a dimension of about 6.8 mm×6.8 mm and received optical image independently via a corresponding lens at a small view angle relative to the vertical direction of the image converter. The image output areas of all portions were combined together and coupled to the photosensitive area of the intensified CMOS camera with a fiber-optic faceplate. The intensified CMOS camera can record all of the optical images from the four portions of the fiber bundles.

The rod-pinch diode was installed on the inductive voltage adder (IVA) facility [17], which can provide ∼1.8 MV high voltage and a peak current of ∼36 kA with a rise time of ∼56 ns. The rod-pinch diode consists of a graphite annular cathode and a tungsten cylindrical anode rod with a diameter of 1.2 mm, which extends through the hole in the cathode. When the high voltage is loaded, electrons will emit from the cathode and bombard on the rod anode. Then x-rays will be generated due to bremsstrahlung. The duration time of the x-ray pulse was about 65 ns at full width half maximum (FWHM). The relative x-ray radiation power of the rod-pinch diode was monitored by a PIN x-ray detector, which was placed 2.9 m from the rod to provide the time relationship between the x-ray pulse and the intensified CMOS camera gating signal. The synchronization function was realized with a digital delay and pulse generator. During each experiment, the digital delay and pulse generator was triggered by the IVA facility and then provided a trigger signal for the intensified CMOS camera with a proper delay.

3. Results and discussion

In order to evaluate the imaging quality of the system, the image converter was replaced by a luminated plate with resolution bars. The raw image was recorded by the intensified CMOS camera and is shown in Fig. 2(a). The edge spread function of each frame along the cut in Fig. 2(a) was obtained by averaging ten samples and is shown in Fig. 2 (b, c, d, e), respectively. The spatial resolution of each imaging channel is evaluated to be about 0.76mm, 0.66mm, 0.75mm and 0.65mm, respectively, which is indicated by the distance between the lateral positions with intensities of 10% and 90% [18].

 

Fig. 2. (a) The raw image of the luminated plate with resolution bars recorded by the intensified CMOS camera. This image is composed of four frames, marked with F1, F2, F3 and F4. The edge spread functions of F1 (b), F2 (c), F3 (d) and F4 (e), which are used to estimate the spatial resolution of the imaging system.

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The temporal resolution of the x-ray imaging system was determined by two key portions, the employed scintillator plate and the intensified CMOS camera. For the current intensified CMOS camera, the highest temporal resolution was about 3 ns. However, the temporal resolution of the LYSO and plastic scintillator was about 47 ns and 3 ns, respectively, due to their fluorescence decay times. During the following experiments, the exposure time of the intensified CMOS camera was set to about 10 ns. Therefore, if the plastic scintillator was employed, the x-ray imaging system can be seen as an impulsive detection system [16] with a temporal resolution of 10 ns. However, if the LYSO scintillator plate was employed, the x-ray imaging system can only be seen as an integrating system [16] due to the long decay time of the LYSO.

The imaging system was applied for the diagnosis of the rod-pinch diode. The physical mechanism of the rod-pinch diode consists of three stages [19]. At the first stage, electrons move along the radial direction of the annular cathode and directly across the anode-cathode gap to bombard on the anode rod. The electrons flow in this stage is called space-charge-limited (SCL) flow. At the second stage, the electrons flow is called weakly pinched (WP) flow, which is deflected by the self-magnetic field. In this stage, electrons bombard on the whole cylindrical surface of the rod. At the third stage, the self-magnetic field force electrons to move along the surface of the rod and bombard on the tip of the rod, leading to the appearance of pinch. The electrons flow in this stage is called magnetically limited (ML) flow.

In order to experimentally investigate the development of the electrons flow of the rod-pinch diode, the view direction of the x-ray imaging system was set at an angle of 80 degrees relative to the axis of the rod. The temporal resolution of the intensified CMOS camera was set to about 10 ns. One raw time-resolved x-ray image, which was recorded with the camera via combination of the inorganic scintillator, is shown on the top left of Fig. 1. The raw image contained four sequential x-ray framing images, which are shown in Fig. 3 (a) with the image processing of rotation and rearrangement according to the time relationship. The x-ray framing images employing the plastic scintillator in another shot were also obtained, which are shown in Fig. 3 (b). Two time-integrated x-ray images recorded by the image plate in shots mentioned above are shown in Fig. 3 (c, d), respectively. The experimental results show the dynamic x-ray spot generated by the rod-pinch diode. The first frames of Fig. 3 (a, b) indicate the SCL stage, for the whole rod emit spatially homogeneous radiation from the rod. The second to fourth frames of Fig. 3 (a, b) indicate the ML stage, for the x-ray radiation was mainly emitted from the tip and the position at 3-4 mm relative to the tip. These experimental results agree roughly with the theoretical results [19]. The phenomenon of two spots at the tip of the rod was experimentally obtained, which is similar with the reported results [20,21]. However, due to the ability of the imaging system for recording sequential frames with a high temporal resolution in a single shot, the simultaneous development of the two x-ray spots at the tip of the rod is observed in our experiments and shown in Fig. 3 (a, b).

 

Fig. 3. The experimental results of the investigation of the rod-pinch diode. (a), (b) represent the results captured by the imaging system in shot #37 with the LYSO scintillator and in shot #39 with the plastic scintillator, respectively. The exposure time of the intensified CMOS camera was about 10 ns. (c), (d) show the results recorded by the image plate in shot #37 and shot #39, respectively.

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4. Conclusion

In summary, a high-speed imaging system with multiple frames in a single shot has been developed, which utilizes fiber bundles to receive and delay multiple optical images and then captures the multiple delayed optical images with a single intensified CMOS camera. The single-shot framing quantity is four in current scheme, which can be increased by increasing the fiber bundles, but leading to the loss of spatial resolution or field of view area. In addition, for the fiber bundle was shielded by flexible pipes, the framing light path is easy to configured with the increase of the framing number. The temporal resolution and the inter-frame separation time is determined by the gating time of the intensified CMOS camera and the length of the fiber bundles, respectively. Therefore, the ultimate temporal resolution of this method is about hundreds of picoseconds. This imaging system has been applied in investigating the dynamics of x-ray spot generated by the rod-pinch diode. Scintillators have been used to converter x-ray images to optical images, which can be recorded with the imaging system. It should be noted that the image converter can be changed by scintillators with other spectra for some specific applications. Sequential images, representing the distribution of the x-ray spot, have been obtained, which indicate the mechanism of the electrons flow attaching the rod. The method provides a promising way for recording multiple frames with a single intensified camera.