P3-32: Fabrication of the big-size thermionic dispenser cathode

Both kinds of 10 cm diameter thermionic dispenser cathode have been fabricated at BVERI. One is impregnated scandate (I-Sc) cathode and the other is traditional Ir-coated “M” cathode with 411 impregnant. Figure 1 is the photograph of Φ10 cm cathode assembly. The scandate cathode can deliver the same density as “M” cathode but 80-100°C lower in temperature, though it needs higher vacuum than “M” cathode. Structure and heat designs are very important for the manufacture of the big-size cathode assembly. There are only four materials such as tungsten, molybdenum, tantalum, and stainless steel used in structure so as to enhance the structure reliability of the cathode assembly for their high melt point, low evaporation rate as well as excellent mechanical performance. Among them, molybdenum and stainless steel are adopted to support cathode button and heater assembly. Heat conduction and radiation are the major heat loss for the cathode in the vacuum. Yet, under the conditions of high temperature and big area, radiation is the dominant mechanism for heat loss. Some methods were introduced to decrease the heat loss. For instance, a series of slots which distribute in angle and vertical direction regularly are machined along the cathode support stem to reduce the heat conduction loss. Especially, a complicated and reliable thermal shield structure has been designed to reduce the radiation and ensure heat efficiency. More than 2 layers thermal shields surrounding the cathode are used, and 4 layers shields exist behind the cathode. The heater was made of six 20% ReW coiled filaments in parallel and the operation power is below 1500 W. Alumina ceramic insulators are employed between the cathode and heater. The total weight of the entire cathode assembly is 1.95 kilogram. There are some factors influencing on the cathode electron emission performance significantly such as outgas path, vacuum, temperature nonuniformity on cathode surface, air moisture and so forth. The vacuum of the injector test stand at IFP can be maintained in the range of (2~5) × 10 ^-5 Pa, but poor pumping conductance in the region behind the cathode will lead to poor local vacuum nearby the cathode surface. Although the dispenser cathode has the capability of resistance to poisoning, poor vacuum near the cathode surface will still low the emission current dramatically. That is the reason why sometimes the higher temperature is needed to produce the same beam current. Generally the higher temperature tends to decrease the reliability of heater. Thus, keeping good vacuum is very important for normal operating of big-size cathode. Both Ir-coated and I-Sc cathodes have been tested in injector test stand. The applied pulse voltage is not more than 2MV, and pulse width (FWHM) is about 90 ns. The accepted experimental data are as follows (Figure 2, 3). At the pressure of the 4.4 × 10 ^-5 Pa, the I-Sc cathode has the capability to give 1000 ampere in total during 1100~1120°C and the current density is about 12.7A/cm , whereas the total emission current of the "M" cathode is only 720 Ampere because of cathode exposing the poor vacuum during the testing. The optical pyrometer was utilized to monitor the temperature at cathode surface. On the basis of the Φ10 cm cathode assemblies development, the Φ15.5 cm planar cathode assembly has been designed (Figure 4) and fabrication is in process. 3 kinds of cathodes will be attempted for the larger structure. The first is the traditional "M" cathode and the objective is 12A/cm ² ; the second is the I-Sc cathode with the objective of 15A/cm ² ; the third is the sub-micron Sc ₂ O ₃ +W cathode which is expected to offer the current density of 20A/cm ² .