M.C. Choi

Pulsewidth and rising time of relativistic electron beam in gas-filled diode

The pulsewidth and rising time of a mildly relativistic electron beam (300 kV, 1-3 kA) passing through a gas-filled diode region are investigated experimentally under various gas pressures P. The pulsewidth and rising time of a relativistic electron beam (REB) were controlled by adjusting the gas pressures P of the diode region. The pulsewidth and rising time of the relativistic electron beam are experimentally found to scale as P/sup -0.807/spl plusmn/0.054/ and P/sup -0.770/spl plusmn/0.058/, respectively. The REB pulsewidth and rising time are shown to have the same scaling law, within the experimental error range as a function of pressure. In particular, the empirical scaling law of the REB pulsewidth is in remarkably good agreement with the numerical scaling law P/sup -0.809/spl plusmn/0.059/ of the full-space charge neutralization time t/sub n/ at which the ion density n/sub i/ is just equal to the electron beam density n/sub b/ at the diode region under a given gas pressure P. It also is found that ion density n/sub i/ at the full space-charge neutralization time t/sub n/ has quite a similar profile in terms of pressure P to that of the REB peak current detected by a Faraday cup.

Simulation study of high power microwave generation by virtual cathode oscillation in coaxial vircator

Summary form only given, as follows. We have numerically simulated the high power microwave generation by virtual cathode oscillation in a coaxial vircator with 3 dimensional PIC code Magic. It is noted that the coaxial vircator has the advantage of an enhancement of power efficiency and gaining of narrow bandwidth frequency. The study is focused on the design of a diode structure for a coaxial vircator suitable for Chundoong IREB pulser (Max. 600 kV, 85 kA, 60 ns). It consists of a center annular cathode, a cylindrical meshed-anode and a reflector. The simulation results showed that the coaxial vircator can indeed be operated at the selected frequency with the narrow bandwidth and the microwave output power strongly depends on the position and geometry of the reflector. From this simulation results we expect that the maximum microwave output power will be obtained by the reflector of 4 cm in width and also 4 cm in distance away from the IREB, where the resonant frequency of microwave from coaxial vircator is simulated to be 6.577 GHz.

A diode design study of the virtual cathode oscillator with ring-type reflector

Summary form only given, as follows. A numerical simulation study of the high power microwave generation from the virtual cathode oscillator (vircator) is carried out for various diode configurations by a 3 dimensional particle-in-cell (PIC) code called MAGIC. The coaxial vircator has a cylindrical cathode, cylindrical anode-mesh, and reflector, in order to enlarge the active interaction region in the virtual cathode space. The simulation results show that the output microwave frequency has a narrow bandwidth and its output microwave power depends sensitively on the position and width of ring-type reflector. The maximum microwave output power is obtained with a reflector 10mm in width and 40mm in distance from the intense relativistic electron beam. Output microwave mode without the ring-type reflector was mixture of the TM mode, however, it is found to be dominantly TM/sub 01/ mode with a resonance frequency of 22 GHz with the ring-type reflector.

Numerical simulation of the reflex triode virtual cathode with an intense relativistic electron beam

A virtual cathode reflex triode is investigated by numerical simulation with a 3 dimensional particle-in-cell (PIC) code called MAGIC. The reflex triode virtual cathode oscillator has a disk cathode and a disk anode-mesh. Here anode-cathode (A-K) gap distance has been varied from 10 mm to 30 mm. Simulation results show that the output microwave frequency has a narrow bandwidth and its output microwave power depends sensitively AK gap distance. Output microwave mode is mixture of TE mode at A-K gap distance 10 mm, however, TE is to be dominant mode at A-K gap distance 30 mm.

Measurement of electron temperature and plasma density in coplanar AC plasma display panels

The electron temperature and plasma density in coplanar alternating-current plasma display panels (AC-PDPs) have been experimentally investigated by a micro Langmuir probe and the high speed discharge images in this experiment. It is noted in this experiment that the electron temperature obtained from both the micro Langmuir probe and high speed ICCD camera decreases from 2.5 eV to 1.2 eV as the filling Ne+Xe gas pressure increases from 150 Torr to 350 Torr. It is noted that these electron temperatures are in good agreement with each other within 5% error limit. The plasma density at the lateral distance of 125 /spl mu/m away from the center of sustaining electrode gap has been found to be saturated from 0.73/spl middot/10/sup 11/ cm/sup -3/ to 7.54/spl middot/10/sup 11/ cm/sup -3/ at the Ne+Xe filling pressures ranged from 150 Torr to 350 Torr.

Initial microwave generation from the vircator system with various anode-cathode (A-K) gap distances

Summary form only given, as follows. The microwave output of vircator system has been shown to be dependent especially on diode anode-cathode (A-K) gap distance. The initial microwave generation time is delayed when the A-K gap distance has been increased from 2 to 8 mm in this experiment, and also it is shown that the central microwave frequency f is inversely scaled as the A-K gap distance. When the electron beam is generated from the cathode, the electrode sheath plasma is also generated just after the peak diode voltage on the electrodes, hence the perveance of diode is being varied to change the diode output characteristics at this moment. In our pulsed power system chundoong, the perveance characteristics had been experimentally investigated, from which the sheath plasma expansion speed could be found to be about 3 cm//spl mu/s inside the diode. It could be shown in this experiment that the microwave output power becomes to be maximum when the effective A-K gap distance due to the sheath plasma expansion becomes to be around 3 mm from the initial-set A-K gap distances. It means that there might be threshold effective A-K gap condition, at which the maximum microwave power could be achieved by virtual cathode oscillator formed by the injected electron beam current exceeding the space charge limited current.

Plasma propagation speed and electron temperature in surface-discharged alternating-current plasma display panels

Summary form only given, as follows. The space and time-resolved discharge images from an alternating-current plasma display panels (ac-PDPs) has been observed by a high-speed single frame camera, and visualized as motion images. The time duration of the main discharge glow is. also shown to be about 200 ns and a faint light is continuously emitted even after 350 ns from the cathode in this experiment. It is found that glow occurs continuously at the cathode and striations are observed on the anode in a time duration between 100 ns and 300 ns of main discharge in ac-PDP. The propagation speed of surface-discharged plasma on the cathode has been measured to be 0.92 mm/

Characteristic of vircator output at various A-K gap distances with diode perveance

mu

Virtual cathode oscillator under various cathode radii with intense relativistic electron beam

s and 1.34 mn/

Density and velocity measurements of a sheath plasma from MPD thruster

mu