Investigation Of Cathode Plasma And Cathode Surface Damage

Explosive field emission cathode is the key device in the system of high power microwave (HPM) and has thus been the subject of many experimental efforts. The performance of cathode is decisive factor of the output power and pulse width of HPM. The intense electron beam is generated from plasma sheath on cathode. Thus this dissertation chiefly investigated cathode plasma, cathode surface damage and the virtual cathode oscillation in high power diodes.We examined the cathode surface damage after electron emission during the high-voltage pulsed operation of explosive emission cathodes. Measurements of the surface damage for stainless steel and velvet cathodes demonstrated that the smoothing of the cathode surface microstructure obviously occurred on the surface of cathodes subjected to the electron emission. Carbon fiber cathodes after electron emission were found to show the surface erosion on the top and side surface of carbon fiber. There existed the decrease in the amount in emitters on the cathode surfaces of stainless steel and velvet cathodes after electron emission. However, the amount in emitters didn't decrease but increased in the case of the carbon fiber cathode after emission. We measured the electron beam current and the voltage of the diode on condition that the repetition rate was 100Hz. The results showed that the electron beam current and the voltage of the diode were very stable, and air pressure of diode effected cathode lifetime.We measured cathode plasma using high speed camera, and got primary results. By the results we could see the cathode plasma and calculate plasma decay time is about 1.6μs in high power diode. We planed to change the trigger signal of the experiment, and trigger the accelerator and high speed camera using pulse generator. Thus the camera could photo the whole process of plasma expansion from cathode to anode of high power diode. Then the results of the experiment could help us measure the plasma expansion velocity and analyze the electron emission mechanism of cathode.Cesium iodide (CsI) is a good emitter of ultraviolet (UV) radiation. The addition of CsI caused the increase in the voltage pulse duration and reduced the turn-on field for electron emission, thus resulted in the fast rise current. CsI slowed the plasma motion across the anode-cathode gap. We used electrical and optical diagnostics to study the process of plasma formation in high power diode. The result showed that CsI coating effectively inhibited the plasma expansion and, as a result, the diode gap remained unchanged within 200 ns after the beginning of the accelerating pulse.We had established the diode impedance characteristics of carbon fiber cathode and stainless steel cathode in the reflex triode vircator. And we had experimentally investigated the process of virtual cathode formation as the diode impedance collapsed due to plasma within diode expansion from the cathode surface to the anode. The effect of the cathode plasma expansion on the diode impedance based on the Child-Langmuir Law and Parapotential model was discussed. It was observed that the impedance turning occurred as the diode proceeded. The temporal behavior of diode impedance can be divided into five stages. The change of diode impedance can show the time history of virtual cathode oscillation, namely, the startup and cessation of microwave radiation.