P1-31: Manufacturing for a Sheet Beam M-type Cathode Yin Shengyi, Wang Yu, Wang Xinxin, Meng Mingfeng, Ruan Cunjun

Key Laboratory of High Power Microwave Sources and Technologies Institute of Electronics, Chinese Academy of Sciences

Beijing, China, 100190 ysy210@sohu.com Abstract: In order to meet the need of a sheet beam electron gun testing, we have developed a new sheet beam M-type cathode in elliptic cylinder shape with major axis 10 mm，stub axle 4 mm and radius 17 mm. In this paper, the main manufacturing procedures of the cathode are discussed. The test results of the sheet beam electron gun indicate, the beam transmission rate is up to 98.6%, the current density of the cathode reaches 8.9A/cm2. These results provide a good foundation for developing higher performance sheet cathodes in the future.

Keywords: sheet beam; cathode; elliptic cylinder shape; electron gun; manufacturing

Introduction The sheet beam klystron is an important development direction for klystron research because it can realize high power microwave output in high frequency range. In the initial stage of this klystron in study, it is necessary to obtain an electron gun that can produce a high-quality electron beam. As a key part of the electron gun, the sheet beam cathode has become the important studying content in this stage.

According to the need of sheet beam electron gun research in our research institute , we have developed a new sheet beam M-type cathode in elliptic cylinder shape with major axis 10 mm, stub axle 4 mm , radius 17 mm. As to a sheet beam electron gun, the most important data is the beam transmission rate. In order to get high value, the suitable electronic optical system is essential. As regards cathode, in order to obtain the high-quality sheet electron beam, it should be relatively uniform in emission capability , otherwise small disparity can cause electron beam lose centre symmetrical, and cause beam transmission rate to drop.

In order to make emission capability especially emission homogeneity meeting the request of the electron gun , we carry on optimization in the manufacturing respects, such as the tungsten sponge, heater, coating alloy system, etc.. The actual test of the electron gun shows, in a situation that the electronic optical system is got and optimized, the beam transmission rate can be up to 98.6%. The relevant results will be reported as follows.

Tungsten sponge preparing The porous tungsten matrix with uniform pores is the foundation of a cathode to obtain uniform emission and high current density. For this reason, we adopt the graded

tungsten powder to prepare the tungsten sponge [1-2]. Fig. 1(a) provides the photo of broken surface of tungsten sponge prepared with 5μm graded tungsten powder; Fig.1 (b) provides the photo of broken surface of tungsten sponge prepared with 7μm not-graded tungsten powder.

Compare Fig. 1(a) with Fig. 1(b) , it is easy to find that the tungsten particle grain size and the micro-hole size distribute much more uniformly with graded tungsten powder than with not-graded tungsten powder.

(a) 5μm graded (b) 7μm not-graded

Fig. 1 The tungsten sponge broken surface

Cathode machining Fig. 2 shows shape and size of the sheet beam cathode. Because of in elliptic cylinder shape, the cathode is unable to adopt a turning procedure. The machining process that we adopted is: at first milling the 3 ellipse side surface sequentially, then processing the cylinder surface with line cutting machine.

Fig. 2 Sheet beem cathode schematic

Heater design Considering the cathode section as ellipse shape, if adopt an ordinary circular heater, the temperature of both ends of major axis direction of the cathode would be lower than other area, cause here to emit smaller. We design a heater with the structure of a pair of helix to connect at both ends of major axis direction. Fig. 3 shows the heater's sketch map. In order to fix the nude WRe heater, we fill the WRe heater and alumina powder into molybdenum tube that brazed to the cathode back, and sinter them together at high temperature (1750°C).

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Fig. 3 Heater schematic

Surface chemical cleaning After impregnation, there is a large amount of residual salt on the surface of the cathode. In order to remove the residual salts, a chemical cleaning procedure is adopted [2]. After chemical cleaning, water on the cathode surface is taken off by putting cathode into ethanol, and then the cathode is reduced in a hydrogen stove with heat treatment standard of 1200�× 10min.

Coating 5W3Re2Os film According to the emission testing results in reference [2], the current density of the cathode coated with 5W3Re2Os ternary alloy film increases by more than 60% as compared with that of the cathode coated with Os film. We deposit 5W3Re2Os film on the sheet beam cathode, the film thickness is 0.4～0.5μm.

Emission homogeneity testing The cathode emission homogeneity can be compared with “slope" of space-charge-limited region [1, 3]. The slope is close to the theory value 1.5, the cathode emission the more uniformly. The slope of cathode with W-Ir base or coated with Os film is generally 1.33-1.38[3].

Fig. 4 shows a curve of logI-logU for a Φ2.5mm flat cathode. From Fig. 4, it is easy to know that the slope of space-charge-limited region is up to 1.45, it is the highest to be relative.

300 400 500 600

10

20

30

40

Voltage (V)

Cur

rent

den

sity

(A

/cm

2 )

T(℃ B) Slope J(A/cm2) 1000 1.45 26.18

Fig. 4 Current density versus voltage

Electron gun test results Assembling the cathode into a sheet beam electron gun，the volt-ampere characteristic for the electron gun is tested. The tested data are listed in table 1.

It can be found that, during the process of testing, the voltage rising from 30kV to 65kV, the collector current increases in step with total current, and both are nearly the same. When reach 65kV in voltage, the collector current is slightly smaller than total current, the beam transmission rate is up to 98.6%. The cathode current density is 8.9A/cm2 at this moment.

The above-mentioned higher data prove that our developed sheet beam cathode could meet the request of this sheet electron gun.

Conclusions We have developed a new sheet beam M-type cathode in elliptic cylinder shape with major axis 10 mm and stub axle 4 mm, radius 17 mm. It is manufactured by using graded tungsten powder, milling and line cutting machining, a double helix heater, surface chemical cleaning, ternary alloy coating and so on.

The cathode possesses good emission homogeneity. With this cathode, the beam transmission rate of the sheet beam gun is up to 98.6%.

Acknowledgements This work is supported by the Chinese National Natural Science Foundation ( 60871053).

References 1. Yin Shengyi, Wang Yu, “A method for manufacturing

tungsten sponge,” Chinese patent, patent number ZL 2006 1 0086550.5

2. Yin Shengyi, Zhang Honglai, Dingaogen, “Manufacturing for high-performance multibeam cathodes,” Vacuum Electronics Conference, 2009. IVEC apos: 09. IEEE international volume, issue, April 28-30, pp:287- 288, 2009

3. Yu Zhiqiang, Li Ji, “Reserch of emission properties for W-Ir alloy cathodes,” The 16th academic military collection of thesis of microwave tube seminar of annual meeting, Baotou, China, August 1-3, pp:373-380, 2007

Table 1 Sheet electron gun testing results

Test item Data Voltage (V) 30.21 35.52 40.03 45.00 49.50 54.61 60.22 64.93 Total current (A) 0.93 1.18 1.4 1.66 1.91 2.20 2.50 2.80 Collector current (A) 0.92 1.18 1.4 1.66 1.90 2.20 2.48 2.76 Beam transmission rate（％） 98.9 100 100 100 99.5 100 99.2 98.6

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