# High-Speed Tomography--A New Approach to Plasma Bulk Velocity Measurement
## 2 Materials and Methods
### Experimental Setup
#### 2.1.3 Image Recording System  
The image acquisition was realized with a self-developed system, shown in Figure 4, consisting of a rotating mirror, an image intensifier and a high-speed camera. The rotating mirror with an electric motor is a self-assembled component. It consists of a brushless motor controlled by a microcontroller using pulse width modulation to drive the motor shaft. A trigger wheel with a light barrier is responsible for triggering the system at the moment when the mirror is in the desired position for recording. The rotation speed can be adjusted up to 20,000 rpm. The recording technique is described as follows. Figure 4: Arrangement of the image recording system with all components. Figure 3: The deflection mirrors, which provide six different directions of view of the measurement object, were mounted cylindrically in 30-degree increments on a carrier plate. The operation principle of the image recording system is depicted in Figure 5. The image intensifier is operated in multiple exposure mode while the high-speed camera integrates the respective exposures on its chip. The exposure time of the high-speed camera is set to 1.2 ms. The image of the measurement object is shifted by the rotating mirror so that the exposures do not overlap. Thus, it is only necessary to ensure that the exposure time of the high-speed camera is longer than the sum of all exposures of the image intensifier and delays between each of them. To achieve this, the components have to be synchronized. The Quantum Leap image intensifier, manufactured by Stanford Computer Optics, Munich, Germany, can capture up to twenty exposures when operated in multiple exposure mode. The lowest delay value between two exposures can be set to one microsecond and exposure time can be set between one nanosecond and one millisecond. Since the image intensifier is the determining element for the multiple exposure, a frame rate up to one million frames per second can be achieved with this image recording system. The measurement object was reproduced on the photocathode of the image intensifier using a wide-angle AF Nikkor 24 mm F/2.8 D lens manufactured by Nikon, Tokyo, Japan. The output of the image intensifier, on the other hand, was reproduced on the Dimax HS4 high-speed camera from Excelitas PCO GmbH, Kehlheim, Germany, with a Laowa CA-Dreamer 100 mm F/2.8 macro lens manufactured by Venus Optics-Laowa, Tsun Wan, Hong Kong. The high-speed camera is capable of recording 2277 frames per second at full resolution of 2000 \(\times\) 2000 pixels. The challenge is to trigger the image amplifier and the high-speed camera when the rotating mirror is in the recording position, so that the measurement object is displayed on the camera chip and the area of the chip is filled with exposures as best as possible. Figure 6 depicts the trigger scheme for the experiment. The plasma generator was running continuously during the image acquisition. The image recording system is self-triggering. The rotating mirror and a flywheel with a small hole are both mounted on the motor shaft. A light barrier emits a trigger signal when the rotating mirror, or more specifically the hole of the flywheel, is in the light barrier. The flywheel can be adjusted to a desired position with respect to the mirror. The emitted signal triggers both the image amplifier and the high-speed camera. Figure 5: Principle of the multiple exposure of the image recording system. Due to the multiple exposures of the measurement object on the camera chip, the recording system has limitations. Optical imaging must ensure that multiple exposures are placed within the size of the camera chip. To obtain a higher resolution of the object, the temporal resolution has to be reduced, because a longer delay between the exposures is necessary to avoid overlapping of each other, while a high sampling rate of the measurement object limits the spatial resolution. Therefore, a balance has to be found between achieving the highest possible sampling rate without losing too much spatial resolution.