Investigation Of Carbon Fiber Cathode For High Power Diode

As a new type of cathode, the carbon fiber cathode has lots of advantages over others such as lower threshold voltage of emission, lower out-gassing rate, slower expansion velocity of cathode plasma, smaller changes of diode pressure, higher uniformity of electron emission and so on. Therefore, the carbon fiber cathode is a good choice of the high power diode cathode. These properties of the carbon fiber cathode are attributed to the mechanism of its electron emission. So it is important significance to study the electron emission mechanism of carbon fiber cathode.This paper firstly established the model of the plasma formation for the carbon fiber cathode and made related experiments. The surface flashover made important role in the process of electron emission of the carbon fiber cathode. The micro-photos of the carbon fiber cathode after emissions confirmed the established model. And we explained the electron emission process according to the secondary electron emission avalanche and electron triggered polarization relaxations mechanisms. Furthermore, we also made experimental investigations in the cathode lignt spots. The experimental results showed under the condition of using the carbon fiber cathode, compared with the velvet cathode, the threshold voltage of electron emission was lower, the distribution of the light spot was more uniform and electron emission was not obviously concentrated on the edge of the cathode, which proved the plasma uniformity of the carbon fiber cathode.We measured the plasma expansion velocities by the electrical methods. The plasma expansion velocity of the carbon fiber and stainless steel cathodes, through the method of diode impedance fit, were 1.1 cm/μs, 2.5cm/μs respectively. In addition, the expansion velocity of plasma for the carbon fiber and stainless steel cathodes can be obtained qualitatively by the temporal dependence of the diode gap, by which the results of plasma expansion velocity were 0.9 cm/μs, 2.2 cm/μs. And by the method of the diode perveance, we could obtain the result of 0.98 cm/μs which was the plasma expansion velocity for the carbon fiber cathode. Theses results could illuminate that for the carbon fiber cathode, compared with the other cathodes, the plasma expansion velocity was slower.This paper also involved the measurements of the electron beam quality, in allusion tothe electron emission model. By making the bombarded anode experiments, we found that the carbon fiber cathode could provide uniform electron beam, in addition, analyzed the experimental results combined with the mechanism model. As a quantitatively experimental result of the carbon fiber cathode, the divergeance and brightness of the carbon fiber cathode were respectively 1256mrrrmrad, 2*107A/(m-rad)2.As the application of the carbon fiber cathode, the carbon fiber cathode substituted for the stainless steel cathode on the Tomachan experimental device. The experimental results showed: the flattop phenomenon on the diode voltage pulse was very apparent for using the carbon fiber cathode, that is to say, the pulse duration of the diode voltage was widened; moreover, under the condition of the same power of microwave, the pulse duration of the out-put microwave was prolonged by about 30%; the peak power of the microwave was enhanced by 5-7dBm. All these results can show the carbon fiber cathode can have very broad prospect in the field of high power microwave sources.Finally this paper summarizes the works we have done before and also proposes the new ideas and plans in the future investigations.