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In this paper, a new carbon fiber based cathode — a low-outgassing-rate carbon fiber array cathode — is investigated experimentally, and the experimental results are compared with those of a polymer velvet cathode. The carbon fiber array cathode is constructed by inserting bunches of carbon fibers into the cylindrical surface of the cathode. In experiment, the diode base pressure is maintained at 1×10−2 Pa–2×10−2 Pa, and the diode is driven by a compact pulsed power system which can provide a diode voltage of about 100 kV and pulse duration of about 30 ns at a repetition rate of tens of Hz. Real-time pressure data are measured by a magnetron gauge. Under the similar conditions, the experimental results show that the outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode and that this carbon fiber array cathode has better shot-to-shot stability than the velvet cathode. Hence, this carbon fiber array cathode is demonstrated to be a promising cathode for the radial diode, which can be used in magnetically insulated transmission line oscillator (MILO) and relativistic magnetron (RM).
Cathodes with ideal shot-to-shot stability, low outgassing and long lifetime have been one of the main pursuits in high power microwave (HPM) field,[1–3] especially in the repetitive operation HPM systems.[4–6] In order to obtain ideal cathodes, researchers have done lots of work to study various cathodes made from different materials,[7–11] among which carbon fiber and velvet are two kinds of the most popular materials.
Carbon fiber cathodes and velvet cathodes have been widely used in HPM tubes,[12–19] especially in tubes where large area emission cathodes are needed, such as magnetically insulated transmission line oscillator (MILO)[12–18] and relativistic magnetron (RM).[19] Correspondingly, a lot of research[20–27] has been done to study the characteristics of these cathodes. As a widely used source of high-current electron beams, the velvet cathode [23–26] has a very uniform emission at a relatively low field level, has a low gap closure velocity, and is inexpensive and widely available. However, the velvet cathode has a short lifetime and suffers from high outgassing rate during the beam pulse because this cathode discharges a significant quantity of matter in the electron emission process.[26] If a velvet cathode is in a repetitive operation, the pressure rise in the background resulting from this outgassing may limit the achievable pulse rep-rate,[25,26] which depends on the vacuum system specifications and the desired pulse duration. Also, high outgassing rate could enhance the plasma formation in the anode–cathode (A–K) region, and the plasma expansion could cause the closure of the A–K gap, thus resulting in the impedance collapse of the diode.[21] The impedance collapse can often lead to the shortening of the radiofrequency (RF) pulse in HPM tube, so the high-outgassing-rate cathode limits the microwave pulse-duration and the achievable pulse repetition rate of the HPM tubes.[25,26] As reported previously,[21,22] the carbon-fiber-based cathode (carbon velvet cathode) not only has a low threshold field and a low gap closure velocity, but also can have a uniform emission. Besides, this cathode has a lower outgassing and longer lifetime than the velvet cathode.[24,27] However, the carbon velvet cathode is not so available as the polymer velvet cathode because of the relatively complicated manufacturing process,[3,7,12,22] especially in the radial diode. The fabrication of radial-emission carbon fiber cathode[3,7,12] has not reached an ideal level and hence more research should be done to apply carbon fibers to radial-emission cathodes.
In this article, aiming at its application in MILO[13–16,18] or in RM,[19,28] a new carbon fiber based cathode — a radial-emission and large area emission carbon fiber array cathode — is constructed and experimentally compared with a velvet cathode in a radial diode. During the experiments, both cathodes are operated in similar conditions. What we are mainly concerned with is the outgassing rates of cathodes, but the shot-to-shot stabilities are also discussed briefly. Some research has been done to study the outgassing rates of carbon fiber cathodes or velvet cathodes. In Ref. [22], the “uncoated carbon-on-epoxy”-type carbon fiber cathode has an outgassing rate of 4.7 atoms/electron; in Ref. [23], the authors found that the velvet cathode has an outgassing rate which is about four times as great as the carbon fiber cathode; in Ref. [29], the carbon nanotube cathode has an outgassing rate of 153 atoms/electron. However, in the previous research, the two kinds of cathodes were not compared with each other directly nor in detail; and the outgassing rate varies with working condition. Here in this paper, we present a direct and detailed comparison between the outgassing rates of the two cathodes.
The rest of the present paper is organized as follows. In section
The experimental configuration is shown in Fig.
The pulsed power system can provide a diode voltage of about hundreds of kV and pulse duration of about 30 ns at a repetition rate of tens of Hz. In the experiments, the diode voltage was about 100 kV and the system worked at repetition rate of 10 Hz or 20 Hz. The base pressure in the diode chamber was maintained at 1×10−2 Pa–2×10−2 Pa by a vacuum system which consisted of a turbo molecular pump and a backing pump. The pumping rate of the turbo molecular pump was 240 l/s, and the cylindrical tube which connected the diode chamber to the vacuum system had a diameter of 4 cm and a length of 80 cm. Pressures were measured by a magnetron gauge. The magnetron gauge remained switched on in repetitive operation process and its output voltage data was used to acquire time-resolved pressure data. The diode voltage and current were measured by a capacitance voltage divider and a Rogowski coil respectively, and both were recorded by an oscilloscope. The micrographs (Figs.
Figures
In the experiments, the diode voltage is about 100 kV and the system works at a repetition rate of 10 Hz or 20 Hz. Typical waveforms of diode voltage and current in a single pulse of the two cathodes are shown in Fig.
Typical waveforms of the diode voltage and diode current in a repetitive operation of the carbon fiber array cathode and the velvet cathode are shown in Figs.
The pressure evolution data are obtained from the output voltage of the magnetron gauge, and the outgassing characteristic of cathode can be analyzed from pressure evolution data of the diode chamber. It is found that the outgassing rate of the cathode decreases gradually in the initial tens to hundreds of pulses of the experiments, but tends to reach a relatively stable value after a number of pulses. This may be due to the fact that some contaminants or water vapors are absorbed on a fresh cathode surface, and in the initial pulses, these absorbed contaminants or water vapor were desorbed and then extracted out by the vacuum pump; these contaminants or water vapor get “cleared up” to some extent after a number of pulses, at which time, the outgassing rate tends to reach a stable value. In a stable state, typical variations of the pressure data of carbon fiber array cathode and velvet cathode in experiments are shown in Figs.
According to Ref. [30], if the base pressure is sufficiently low, then the equilibrium pressure will be approximately given by
In experiment, it is impossible to precisely measure the outgassing singly from the cathode because there is outgassing from other parts of the chamber inevitably (outgassing from anode surface caused by the electron bombardment, leakage, etc.) and we cannot discriminate them from the outgassing of the cathode either. Thus the measured outgassing rate is usually the whole outgassing in the diode chamber. However, because the anode and other parts of the diode are the same for the two cathodes, the reason for the difference in outgassing rate should be attributed to the cathodes themselves. The outgassing from the cathode is determined by the original materials and the fabrication technique. When cathodes are exposed to atmosphere, a large number of neutrals can be absorbed on their surfaces, of course, the specific numbers are different for surfaces of different materials and surfaces with different planeness. When cathodes are put into work, there will be high temperature around them because of the current heating, and the material that cannot withstand high temperature will tend to release matters easily. The micrographs of the surface morphologies of carbon fiber and velvet fiber are taken by an SEM (Figs.
A low-outgassing-rate carbon fiber array cathode is fabricated and experimentally compared with a polymer velvet cathode in a radial diode. Experimental results show that the shot-to-shot stability of the carbon fiber array cathode is better than that of the velvet cathode when the cathodes work under a diode voltage of around 100 kV. The outgassing rate of the carbon fiber array cathode is an order smaller than that of the velvet cathode (the outgassing rates are 6.9 atoms/electron and 85.8 atoms/electron for the two kinds of cathodes, respectively). In general, the carbon fiber array cathode is a promising substitution for the velvet cathode for MILO repetitive operation. To make sure the performance of carbon fiber array cathode is better than that of traditional velvet cathode, our further work will concentrate on the applications of this carbon fiber array cathode in a MILO for repetitive operation.
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