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Applied Surface Science 251 (2005) 151–158

High current density M-type cathodes for

vacuum electron devices

Ji Li *, Zhiqiang Yu, Wensheng Shao, Ke Zhang, Yujuan Gao,Haiqing Yuan, Hui Wang, Kaizhi Huang,

Qilue Chen, Suqiu Yan, Shaolun Cai

Cathode Electronics Laboratory, Beijing Vacuum Electronics Research Institute,

P.O. Box 749, Beijing 100016, China

Available online 20 June 2005

Abstract

We investigated high current density emission capabilities of M-type cathodes used for vacuum electron devices (VEDs).

The experimental results of emission and lifetime evaluating in both close-spaced diode structure and electron gun testing

vehicles are given. Emission current densities measured in the diode structure at 1020 8CBr in the CW mode were above

10 A/cm2; while in electron gun testing vehicles, emission current densities were above 8 A/cm2 in CW mode and above

32 A/cm2 in pulsed mode, respectively. The current density above 94 A/cm2 has been acquired in no. 0306 electron gun

vehicle while the practical temperature is 1060 8CBr. For a comparison some of the data from I-scandate cathodes are

presented. Finally, several application examples in practical travelling wave tubes (TWTs) and multi beam klystrons (MBKs)

are also reported.

# 2005 Published by Elsevier B.V.

Keywords: M-type cathodes; High current density; Lifetime; Emission characteristics; Multi beam klystrons

1. Introduction

The performance and lifetime of most vacuum

VEDs are directly related to the capability of cathodes.

As the main force of thermionic sources, dispenser

cathodes made of a porous tungsten pellet impregnated

* Corresponding author. Tel.: +86 10 64361731x2440;

fax: +86 10 64362878.

E-mail address: liji@vip.163.com (J. Li).

0169-4332/$ – see front matter # 2005 Published by Elsevier B.V.

doi:10.1016/j.apsusc.2005.03.218

with Ba, Ca aluminates, such as M-type cathodes

(coated with osmium, iridium or osmium/ruthenium)

and scandate cathodes play an important role in the

electron beam devices. With VEDs development,

especially for high power MBK and broadband

millimeter wave TWTamplifier, the designers urgently

need cathodes, which are capable of providing high

brightness, stable emission and uniform beam in order

to attain high perveance and low compression ratio.

Moreover, the cathode should be robust to withstand

J. Li et al. / Applied Surface Science 251 (2005) 151–158152

Fig. 1. Standard close-spaced testing diode.

severe operating ambient. As compared with I-scandate

cathodes, M-type cathodes keep one step ahead in

practical application.

In this work, high current density emission

capabilities of M-type cathodes (Os-coated and Ir-

coated) have been studied in both close-spaced diodes

and electron gun testing vehicles. The emphasis of this

study is to understand the practical performance of

M-type cathodes. It is well known that the three factors

of emission current density (Jk), operating tempera-

ture (Tk) and lifetime (L) are mutually related and

mutually restricted. Lifetime tests were conducted for

both Os-coated cathodes and Ir-coated cathodes under

different temperatures as well as different emission

current loading. Our goal is to examine the possible

high current emission abilities of M-type cathodes so

as to guide practical application in tubes production.

2. Experimental procedure [1–3]

2.1. Fabrication of the cathodes

In order to provide a well-defined standard speci-

men for experiment comparison, sufficient cathode

matrix undergoing tests were constructed following

the general technique used for fabricating M-type

Fig. 2. Standard electron

cathodes. The average porosity of tungsten body

impregnated with Ba, Ca aluminates (3:0.5:1) was

controlled to 24% or so. All of the cathode bodies were

machined to corresponding dimensions so as to be used

in diodes or gun vehicles. Finally, the film depositions

were carried out with a thickness of osmium about

5000 A and iridium about 2000 A for Os-coated

cathodes and Ir-coated cathodes, respectively.

gun testing vehicle.

J. Li et al. / Applied Surface Science 251 (2005) 151–158 153

Table 1

Loading during life testing in diodes

No. Diode

S/N

Jk (A/cm2),

CW

Life

(h)

Remark

1 Os-17 10 17322 10% slump

2 Os-18 10 17519 10% slump

3 Os-19 8 23101 Running

4 Os-02 8 5638 Leakage

5 Ir-02 8 7246 Filament fault

6 Ir-17 8 9501 10% slump

7 Ir-14 6 12766 Filament fault

8 Os-08 6 23790 Running

9 Os-11 6 10057 Surface analysis

2.2. Testing vehicles

Both close-spaced diode and electron gun testing

vehicle are used to evaluate the emission performance

during the life test. The schematic drawings of the

diode with water-cooled anode and gun vehicle are

presented in Figs. 1 and 2. The anode in diode is made

of oxygen-free copper. The cathode matrix diameter is

3.0 mm and the space from the cathode to the anode is

about 1.0 mm. For electron gun testing vehicle, the

cathode matrix diameter is 3.6 mm and the perveance

is 2 mp. To measure temperature easily and accurately,

a removable screen and a sapphire viewport is con-

figured in the close-spaced diode and electron gun

testing vehicle, respectively.

2.3. Emission measurements

The thermionic emission in planar diodes was

characterized by ‘‘saturated current density’’. Mea-

surements were made using log I versus log V plots.

The point begins to deviate from linearity (the so-

called ‘‘knee curve’’) is defined as saturated emission

current. Nine cathodes (among them six Os-coated

and three Ir-coated) were mounted into cathode CW

life test station. If 10% drop of saturated emission

current occurred, the cathode was presumed to reach

Fig. 3. log I vs. log V characteris

the end of life. The pulsed measurements could be

performed at any time during life test. The tempera-

ture generally was monitored by an optical pyrometer

(Keller, PV-11, made in Germany). Sometimes a dual

color infrared pyrometer was also used as an assistant

to temperature auto record by computer.

3. Results and discussions

Nine cathodes have been testing in diodes with

fixing anode voltage and drawing different current in

CW mode. Table 1 [1–3] provides the current densities

extracted continuously from cathodes during the life

tics of Os-coated cathode.

J. Li et al. / Applied Surface Science 251 (2005) 151–158154

Fig. 4. log I vs. log V characteristics of Ir-coated cathode.

test. An important consideration here is how serious

the anode desorption effect is, which lead cathode to

poisoning. For the water-cooling diode, the space

between cathode and anode generally is controlled

less than 1.0 mm while cathode is in the operating

state. Under CW test condition, if the anode power

assumption more than 180 W, the gases releasing from

anode cannot be negligible. We observed that once the

cathode–anode space less than 0.3 mm, the cathode

usually will be poisoned and the emission slump

obviously. Under this situation, the maximum saturated

emission current density at knee point is only 5–6 A/

cm2 determined by log I versus log V plots.

The similar phenomena were verified in high

vacuum chamber with water-cooling movable anode

setup. In order to decrease the anode effect to cathode,

Table 2

Emission parameters in electron gun vehicles

Gun S/N Cathode type Duty cycle (%) Operating tempe

No. 0306 Os-coated 100 1015

– – 100 1018

– – 0.02 1060

No. 0304 Os-coated 100 1025

– – 2 1070

No. 0301 I-scandate 100 960a Brightness temperature of cathode emission surface (Keller PV-11).

it is necessary to bake the anode to about 600 8C with a

heater so as to remove the absorbed gases thoroughly.

Besides this, the space from cathode to anode must be

adjusted to an appropriate gap in the range 0.4–0.5 mm

(0.3 mm is minimum limit for dc tests). Adopting the

above mentioned method and after several thousand

hours operating, the high current densities were

obtained from Os-coated and Ir-coated cathode as

shown in Figs. 3 and 4. The accumulated lifetime of the

nine cathodes in diodes was 126,940 h by the end of 20

December 2004.

A further Os-coated cathodes evaluation was

carried out in standard electron gun vehicles, which

have a flat cathode (3.6 mm in diameter) in and

perveance is 2 mp [2,3]. According to the design

consideration, it should sustain no less than 10 A/cm2

rature (8CBra) Jk (A/cm2) Life (h) Remark

8.0 >1000 Without magnet

8.5–8.9 – With magnet

94 – No slump

8.0 – –

32 >2000 No slump

8.0 >2000 No slump

J. Li et al. / Applied Surface Science 251 (2005) 151–158 155

Fig. 5. Peak current waveform at the time of 2247 h during life test.

Fig. 6. Upper, without magnetic field; lower, with 1000 Gauss.

J. Li et al. / Applied Surface Science 251 (2005) 151–158156

Fig. 7. Pulsed high current delivery in gun vehicle.

loading in CW mode and 100 A/cm2 in pulsed mode.

Two Os-coated cathode were amounted in electron

gun vehicles for life test either in dc continuous

delivery of 10 A/cm2 loading or in pulsed delivery of

32 A/cm2 loading, respectively. An impregnated

scandate cathode was also put into a gun vehicle for

emission comparison and gun structure verification.

The measurement results are briefly listed in Table 2.

Table 3

Multibeam cathode for MBKs at BVERI

Number of emitters Diameter, F (mm) per emitter Jk (

30 8.0 5

18 0.38 28

19 0.37 28

28 2.5 12

30 4.0 9

24 2.5 12

19 0.56 24

15 1.5 28

30 4.0 9

30 6.0 5a Fundamental mode.b Harmonic mode.

8.0 A/cm2 in CW mode and 32 A/cm2 in pulsed

mode at a 20 ms width and 1000 Hz repetition

frequency rate can be easily drawn from both Os-

coated cathodes. Fig. 5 shows the peak current

waveform during the pulsed life test. The correspond-

ing life was no less than 1000 and 2000 h, respectively.

The question to be pointed out is that the current

densities in gun nos. 0306 and 0304 could not reach

A/cm2) Duty cycle (%) Life (h) Remark

1.1 >1500 FMa

33 >500 FMa

33 >500 FMa

5 >5000 HMb

3.75 >1000 FMa

6 >1500 HMb

6 >1500 FMa

5 >1000 FMa

1 >1500 FMa

1.1 >1500 FMa

J. Li et al. / Applied Surface Science 251 (2005) 151–158 157

Fig. 8. Cathode subassembly (19 beams). Fig. 10. Cathode subassembly (28 beams).

10 A/cm2 in CW mode, even increasing applied

voltage. We thought this maybe attribute to the

affection of ‘‘collector effect’’. Similar results were

attained from I-scandate cathode in gun no. 0301. All

the cathodes operate in fully space charge limited

region since emission current increased little when

increasing heater voltage.

During the test, we found anode beam interception

and partial ‘‘oxidization’’ of collector (whose end was

close to the anode) even with water-cooling. To avoid

Fig. 9. Cathode subassembly (15 beams).

above disadvantages, an annular magnet (Sm–Co) was

fixed at the front end of collector in gun no. 0306 (after

1000 h dc life test at 8 A/cm2). Beam simulations

without/with annular magnets are illustrated in Fig. 5.

Compared to the gun without magnet, it can be seen

there exists partial intercross of beam flow but electron

beam travel longer path with 1000 Gauss magnet

(Fig. 6). The annular magnet will be helpful to fast

dissipate the electron beam energy through water-

cooling collector. The practical test data verified the

Fig. 11. Cathode subassembly (30 beams).

J. Li et al. / Applied Surface Science 251 (2005) 151–158158

simulation results. The anode beam interception

decreased from 5.7 to 2.4 mA. The maximum current

density has been extracted to 8.9 A/cm2 instanta-

neously. After several hours of operation, the emission

became stable at 8.5 A/cm2. It still has potential to

improve the current density by increasing magnet field.

The extracted pulsed (1 ms, 200 Hz) emission

current density up to 94 A/cm2 was also achieved in

gun vehicle no. 0306. The details in current and

perveance are shown in Fig. 7. The perveance was a

little down since the practical cathode temperature is

only 1060 8CBr. It is possible to improve emission

uniformity and stabilize perveance by increasing

cathode-operating temperature. More than 100 A/cm2

can be expected by using a new power supply.

4. Application examples [1–4]

At BVERI, requirements of high current density

mainly focus on MBKs and dual mode TWTs

application. For MBKs, cathodes generally can

provide current densities from 5 to 28 A/cm2 with

lifetime ranging from several hundreds to ten thousand

hours. MBKs listed in Table 3 cover the spectrum from

L-band to Ku-band. Some of multi beam cathode

subassemblies are illustrated in Figs. 8–11. For TWT

application, 8 A/cm2 was drawn under duty cycle of

25% (pulse width is 125 ms) in a real beam focused

dual mode TWT (without RF interaction). After

2500 h life test, no any emission degradation was

observed. The predicted lifetime is above 5000 h.

5. Conclusions and future work

(a) In close-spaced diodes, more than 10 A/cm2

current densities can be drawn from both Os-

coated and Ir-coated cathode at practical tempera-

ture of 1020 � 10 8CBr under CW mode, subject to

an appropriate space between cathode and anode

as well as thoroughly degassing of anode.

(b) I

n present standard electron gun testing vehicles,

8 A/cm2 dc current density can be acquired at

practical temperature of 1020 � 10 8CBr. The

measure results are pretty stable and repeatable;

As for seeking more higher current density, the gun

vehicle must be modified by adding magnet poles.

(c) I

t has been verified that Os-coated cathodes have

capability to provide 94 A/cm2 current density at

temperature of 1060 8CBr. It can be expected to

acquire more than 100 A/cm2 current density at

1100 8CBr. While cathodes operate in tight pulse

width and low repeat frequency, it is hopeful to get

several thousands hours cathode lifetime.

(d) F

or MBKs and high power dual mode TWTs

application, high current densities are generally

required, and both Os-coated and Ir-coated

cathodes can provide the emission current densities

more than 10 A/cm2. Compared with I-scandate

cathodes, M-type cathodes are more robust to resist

poor ambient environment and ion bombardment.

(e) A

s increasing with current density, the cathode

emission uniformity, which affects the beam

quality, is becoming a vital factor in application.

So future work will focus on two aspects: one is to

study cathode emission uniformity with a com-

puter controlled movable pin-hole anode in high

vacuum chamber; another is to keep on exploring

new type thermionic cathode which should

provide high brightness, high emission uniformity

at a relative lower temperature.

Acknowledgements

The authors would like to express their gratitude to

the National High Power Key Laboratory for funding of

studies and to Professor Dingyi Yang (from Microwave

Tube Laboratory, BVERI) for the helpful discussions.

We would also like to thank Professor Fujiang Liao

(from National High Power Key Laboratory) and

Professor Genyu Ying (from College of Electronics and

Communication Engineering, Tsinghua University) for

their encouragement and guidance.

References

[1] J. Li, et al., GF Report, Cathode Assemblies, 2002 (in Chinese).

[2] J. Li, et al., Technical Report No. AG11061 (Interior Report),

2003 (in Chinese).

[3] J. Li, AG11061 Annual Report No. (Interior report), 2004 (in

Chinese).

[4] W. Shao, et al. The design of multi beam cathode for MBKs, in:

Proceedings of the Cathode Electronics Seminar, Xiamen, 2002

(in Chinese).