High Speed Frame Camera Research Articles

Electrode erosion properties of gas spark switches for fast linear transformer drivers

Fast linear transformer drivers (FLTDs) are a popular and potential route for high-power devices employing multiple “bricks” in series and parallel, but they put extremely stringent demands on gas switches. Electrode erosion of FLTD gas switches is a restrictive and unavoidable factor that degrades performance and limits stability. In this paper, we systematically investigated the electrode erosion characteristics of a three-electrode field distortion gas switch under the typical working conditions of FLTD switches, and the discharge current was 7–46 kA with 46–300 ns rise time. A high speed frame camera and a spectrograph were used to capture the expansion process and the spectral emission of the spark channel was used to estimate the current density and the spark temperature, and then the energy fluxes and the external forces on the electrode surface were calculated. A tens of kilo-ampere nanosecond pulse could generate a 1011 W/m2 energy flux injection and 1.3–3.5 MPa external pressure on the electrode surface, resulting in a millimeter-sized erosion crater with the maximum peak height Rz reaching 100 μm magnitude. According to the morphological images by a laser scanning confocal microscope, the erosion crater of a FLTD switch contained three kinds of local morphologies, namely a center boiling region, an overflow region and a sputtering region. In addition, the crater size, the surface roughness, and the mass loss were highly dependent on the current amplitude and the transferred charge. We also observed Morphology Type I and Type II, respectively, with different pulse parameters, which had an obvious influence on surface roughness and mass loss. Finally, the quantitative relationship between the electrode mass loss and the pulse parameter was clarified. The transferred charge and the current amplitude were proved to be the main factors determining the electrode mass loss of a FLTD switch, and a least squares fitting expression for mass loss was also obtained.

Study on the dark-ring white-eye square super-lattice pattern in dielectric barrier discharge

The dark-ring white-eye square super-lattice pattern was studied in dielectric barrier discharge system for the first time. Each unit cell of the pattern consisted of a central bright spot surrounded by a dark ring. The structure of dark ring around the center bright spot was similar to the white-eye, which was called the dark-ring white-eyes. The spatial-temporal dynamics of the dark-ring white-eye square super-lattice pattern was investigated by high speed frame camera (HSFC). The results showed that the dark-ring white-eye square super-lattice pattern was an interleaving of four different sub-structures. The discharge sequence was center bright dot-bright dot halo-vertex square-vertex connection in each half voltage cycle. The formation mechanism of dark-ring white-eye square super-lattice pattern and dark ring was explained by surface charges theory.

Comparative study on intense emission of velvet and cabon nanotube cathode

So far, the investigations of carbon nanotube (CNT) cathode have been focused on the field emission with low current and voltage. However, the properties of the intense pulsed emission of CNT cathode have not been discussed deeply and comprehensively. In this paper, the intense emission properties of velvet and CNT cathode are studied in various aspects, such as emission capability, cathode plasma expansion, cathode initialization, emission uniformity, operation stability, outgassing property, and so on. Three different CNT cathodes are made by using electrophoresis deposition, chemical vapor deposition and also CNT paper (or buckypaper) gluing. Results show that the emission capability of CNT arrays and CNT paper cathode is definitely better than the velvet cathode. At the same diode voltage (～300 kV), the amplitudes of diode current of CNT array and CNT paper are 2.75 and 3.1 kA respectively, which are bigger than that of the velvet cathode (～1 kA). The orientation of CNT should not affect the emission capability of CNT cathodes. And the small radius of the tube wall and the existence of defects are suggested to be the reasons for the emission of electrons from the body of the tubes. The threshold electric field strength of intense emission of CNTs is about two-thirds of velvet cathode. The onset delay time of CNT cathode is shorter than the velvet cathode by about 12-17 ns at the same electric field growth rate. The time-evolution processes of the plasma expansion velocity of CNT and velvet cathodes are similar, which could be divided into three phases (rapid rising, quick decreasing and stable phase). In summary, the plasma expansion velocity of CNT cathode is less than one fourth that of velvet at the end of the first phase. During the stable phase, which sustains until the end of the voltage pulse, both cathodes have the same plasma expansion velocity (7 cm/s). The emission uniformity of the cathode has been studied by analyzing the distributions of cathode plasma spots and Cherenkov radiation light, which are captured by the high speed frame camera. The emission uniformity of CNT cathode is much better than that of the velvet cathode. Especially, the cathode plasma spots on the whole surface of CNT array cathode are very dense and uniform. The peak outgassing pressure of the CNT paper cathode is 0.3 Pa, which is one fifth that of the velvet cathode; while the peak outgassing pressure of the CNT array cathode is 0.042 Pa, which is the lowest, and the outgassing pressure of the CNT cathode is related to the fabrication methods. Volatile such as epoxy should be avoided in the fabrication processes. This CNT cathode appears to be suitable for intense emission source and high-power microwave device applications.

Experimental measurement of spatially resolved electron density in a filament of a pulsed positive streamer discharge in water

By combining a high-speed frame camera with a monochromator, the spatially resolved optical emission spectrum of hydrogen α line in a single filament of a pulsed positive streamer discharge in water has been experimentally measured. The spatially resolved electron densities in a single filament of a pulsed positive streamer discharge in water with a conductivity of 200 μS/cm were investigated. During the experiment, the average energy per pulse of discharge was 90.6 ± 13.6 mJ. The results show that the electron density in the streamer filament is 1017–18/cm3, and present a decreasing tendency along the axial direction of the streamer filament with increasing distance from the tip of the anode.

Dynamic frequency-domain interferometer for absolute distance measurements with high resolution.

A unique dynamic frequency-domain interferometer for absolute distance measurement has been developed recently. This paper presents the working principle of the new interferometric system, which uses a photonic crystal fiber to transmit the wide-spectrum light beams and a high-speed streak camera or frame camera to record the interference stripes. Preliminary measurements of harmonic vibrations of a speaker, driven by a radio, and the changes in the tip clearance of a rotating gear wheel show that this new type of interferometer has the ability to perform absolute distance measurements both with high time- and distance-resolution.

Application of high speed frame camera on the intense electron beam accelerator: An overview

AbstractHigh speed framing camera (HSFC) could be used to capture the image of the electron beams generated by the intense electron-beam accelerator (IEBA), and it is useful to visualize the evolution of discharging and plasma generation phenomenon. So an overview of the application of HSFC on the IEBA is presented. First, we introduce the synchronization problem of HSFC and IEBA, and a synchronization trigger system which could provide a trigger signal with rise time of 17 ns and amplitude of about 5 V is presented. Second, an imaging system based on IEBA, HSFC, and the synchronization trigger system is developed, and it can be used to image the developmental process of plasma in the output vacuum chamber of IEBA and to measure the electrical parameter of IEBA and electrical trigger signal in real time. Furthermore, the imaging system is used to investigate the developmental process of the electron beam of the A-K gap in vacuum under 180 nanosecond quasi-square pulses. It is obtained that the short A-K gap is closed prematurely under long pulse operation with plasma expansion velocity of about 6.25 cm/µs and the light emission in the A-K gap region has the characteristics of “re-ignition” with light duration time about 3800 ns. At last, the discharging process of surface flashover channel of poly-methyl methacrylate (PMMA) insulator with gap spacing of 170 mm in vacuum under nanosecond quasi-square pulses is studied by the imaging system, and the change of luminosity is analyzed during the surface flashover process.

Dynamic Behavior of a Porous Brittle Material: Experiments and Modeling

The purpose of our research is to study porous polycrystalline graphite under various loading conditions. New experimental data are provided. Some of them concern impacts of a 500μm diameter steel sphere at velocities above 4000 m/s on thick carbon targets, leading to strong debris ejection and cratering. A high speed frame camera showed the debris velocity distribution to lie in the 10-200 m/s range. Post-mortem tomographies have also been performed. They reveal some subsurface cracks, but also provide some evidence that the fragmented sphere lies below the target surface. Dynamic loadings involving similar energy densities (above 2000J/cm2) can also be reached through the interaction of a nanosecond intense laser focalized on a carbon target. An experimental result obtained on a laser facility is presented. Numerical simulations have been performed in order to explain the observed results. An Eulerian hydrocode has been chosen because of the large deformation occurring under considered experiments. We have used a classical model to describe the porous behavior, including equation of state, elasticity, shear strength and densification. In this paper, we focus on the effect of the addition to this porous model of a failure criterion relying on the Weibull theory. The selection of the parameters is based on one set of data and the fit is demonstrated against another test. The overall agreement with the experimental data is good.

Leader propagation speed and the final jump distance of 8 m long laboratory sparks

Vertical profile of the leader in long laboratory sparks was analyzed using high speed video photographs. Sparks were generated in an 8 m gap of sphere – plane electrodes with positive impulse voltage of 250/2500 μs wave form. The spatial resolution of the high speed frame camera was 19.6 mm per pixel with a time resolution of 50 μs. It was found that the average final jump distance of five sparks was about 4 m and the average electric field in the final jump region is about 5 × 10 5 V/m the leader speed varies from 1.3 × 10 4 m/s to 3.1 × 10 4 m/s as it propagates toward the ground. The average leader speed of the leader was in the order of 1.8 × 10 4 m/s. The observations show that there is a significant increase in the leader speed just before the final jump.

Characteristics of long gap surface flashover channel in vacuum under nanosecond quasi-square pulses

High-voltage vacuum insulator failure is generally due to surface flashover rather than insulator bulk breakdown. To study the characteristic of surface flashover channel, a diagnosis system to obtain the images of flashover channel is fabricated using a high speed frame camera, and results are presented by an investigation into the flashover of poly-methyl methacrylate insulator with gap spacing of 170 mm in vacuum under 180 ns quasi-square pulses. It is obtained that after formation of the flashover channel, the impedance of the flashover channel is inductive and the waveform of the measured voltage is determined by the inductance of the flashover channel directly.

Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.

Investigations of discharge phenomena in IPD coaxial accelerator with squirrel cage electrodes

The paper presents recent experimental studies of plasma dynamics in a coaxial accelerator used for impulse plasma deposition (IPD) [1–4]. During the IPD process plasma is generated in the working gas due to a high-voltage high-current discharge, ignited within an interelectrode region of a coaxial accelerator. Usually a device is composed of an internal rod and an external tube, which are insulated from one another by a ceramic insulator. In the present work we describe the investigations of discharge phenomena in the IPD accelerator with the outer electrode system composed of stainless steel rods, fixed symmetrically around the internal rod in the form of a squirrel cage. Structure of the current sheet, as well as its dynamics, has been examined experimentally in a visible spectrum with a high—speed frame camera. It has been found that in the considered geometry plasma dynamics has different characteristics when compared with discharge within the tube electrode.