REVIEW OF SCIENTIFIC INSTRUMENTS 86, 026105 (2015)

Note: Design and development of improved indirectly heated cathode based strip electron gun Namita Maiti,1 Abhijeet Bade,2 G. U. Tembhare,2 D. S. Patil,1 and K. Dasgupta1

1

Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Department of Mechanical Engineering, Veermata Jijabai Technological Institute, Matunga, Mumbai 400 019, India 2

(Received 1 September 2014; accepted 7 February 2015; published online 20 February 2015) An improved design of indirectly heated solid cathode based electron gun (200 kW, 45 kV, 270◦ bent strip type electron gun) has been presented. The solid cathode is made of thoriated tungsten, which acts as an improved source of electron at lower temperature. So, high power operation is possible without affecting structural integrity of the electron gun. The design issues are addressed based on the uniformity of temperature on the solid cathode and the single long filament based design. The design approach consists of simulation followed by extensive experimentation. In the design, the effort has been put to tailor the non-uniformity of the heat flux from the filament to the solid cathode to obtain better uniformity of temperature on the solid cathode. Trial beam experiments have been carried out and it is seen that the modified design achieves one to one correspondence of the solid cathode length and the electron beam length. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4909535]

Several industrial applications find extensive use of linear electron source through evaporation over a large area. Strip beam provides uniform power density over a long and narrow cross section on the target. In case of strip electron gun, in field free region, the electron beam focuses initially at 90◦ and subsequently at 270◦ when deflected by 270◦ in their circular path when subjected to a uniform magnetic field perpendicular to their trajectories. Therefore, the electron gun is the most critical component as linear heat source. Design and performance of strip electron gun using directly heated cathode are reported in Ref. 1. Electron bombardment from a secondary filament has been reported in Ref. 2. The problems of directly heated cathode are taken care by the concept of indirectly heated cathode proposed in Refs. 3–5. An in-depth analysis and design of a high power (200 kW, 45 kV) indirectly heated solid cathode (110 mm length) have been reported in Refs. 6 and 7. An improved emitter as thoriated-tungsten (Th-W) based indirectly heated cathode has been presented in this work. Th-W cathodes (1%–2% Th) have better emissive properties8 compared to pure tungsten and tantalum cathodes under certain conditions. At a temperature of 2000 K, a tungsten cathode can be operated without appreciable evaporation, as its emission at this temperature is only 10−3 A/cm2. Thorium has a low work function of 3.35 V but it melts at 2100 K. Th-W cathodes, on the other hand, can be operated at 2000 K, which is near the melting point of pure thorium, and produce current densities as high as 3.0 A/cm2, 1000 times that of pure tungsten at the same temperature.8 It is well known that the thoriated tungsten has very good electron emission properties. Whereas Th-W as a linear heat source has not been investigated, especially as an indirectly heated linear heat source. In this paper, we focus on (a) one to one correspondence between the Th-W solid cathode and the beam length and (b) uniform current density by achieving uniform temperature on Th-W solid cathode. 0034-6748/2015/86(2)/026105/3/$30.00

In order to design the indirectly heated cathode gun, coupled field analysis is carried out. An electro-mechanical analysis has been carried out to find the temperature distribution on the filament, which is a primary heat source to the indirectly heated cathode. The filament temperature distribution in the form of heat flux has been given as input to the bottom of solid cathode. The thermal-structural coupled field analysis provides the temperature distribution and stress distribution on the solid cathode. A finite element software package ANSYS has been used for coupled field analysis. The electron emission density is reported to be higher in the temperature range of 1900 K–2100 K.8 So the heat flux requirement from the tantalum filament is less, which results in less heat affected problems in maintaining the structural integrity of the gun during high power operation. A systematic design analysis has been carried out in order to use Th-W solid cathode. A potential difference of 7 V is applied across the two terminals of the filament, and the electro-mechanical simulation has been carried out with the given geometrical boundary conditions. The filament has emitting length of 106 mm with 5.5 mm sagging and bent by 45◦ to fix in the holder. Analysis is carried out to observe the temperature distribution on the filament. The long filament has been designed such that the heat flux provided by the filament is more towards the two end sections of the emitting solid cathode length than the center after heating. This differential heat flux to the solid cathode increases the emission length of the cathode, which otherwise gets reduced due to heat conduction to the holder. Also, due to provision of fins, heat loss due to conduction from the solid cathode to the grid cup is reduced and it results into increase in emission length. The Th-W solid cathode is a 3 mm dia-cylindrical rod of 150 mm length. The solid cathode has been fabricated to semi-circular cross section so that heat flux received by the solid cathode is uniform for a particular section. The gun assembly with the filament-cathode

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FIG. 1. (a) Th-W solid cathode. (b) Single long filament and solid cathode assembly.

system has been tested in a vacuum chamber. Fabricated Th-W solid cathode is shown in the Fig. 1(a). This solid cathode has been mounted in gun assembly (Fig. 1(b)). Since Th-W solid cathode works best in the temperature range between 1900 and

FIG. 2. (a) Temperature distribution on Th-W solid cathode with 2 fins. (b) Temperature distribution on the solid cathode.

Rev. Sci. Instrum. 86, 026105 (2015)

2100 K, the heat flux requirement from the tantalum filament is less compared to the heat flux required when tantalum solid cathode is used. Temperature distribution on solid cathode has been analyzed with fins at the holding section and without fins. Simulated temperature distribution on solid cathode with two fins at the holding section is shown in Fig. 2(a). The temperature uniformity of 170 K has been obtained on the cathode. The emission length is observed to be 75 mm, 85 mm, and 95 mm without fin, with 1 fin, and with 2 fins (Fig. 2(a)) case, respectively. It may be noted that the filament emitting length is 106 mm, whereas the solid cathode emitting length gets reduced due to conduction loss at the two end sections, and fins are provided at the holding section to optimize the emitting length at a desired temperature range. In case of Th-W, the desired operating temperature range is from 1900 K to 2100 K. Therefore, the filament heating is accordingly chosen (7 V for a given filament cross section). The simulated temperature distribution in Fig. 2(a) shows the temperature uniformity (max temp–min temp) of 170 K (2057 K–1887 K) over the solid cathode length of 95 mm. Filament has been operated at 65 A and solid cathode has been applied with 1.5 kV. Fig. 2(b) shows the temperature distribution through simulation as well as actual measurement using two color pyrometer and they are in close agreement in terms of temperature uniformity. The simulated temperature uniformity is 170 K, whereas the measured temperature uniformity is 160 K. Since the shaping of the fabricated filament in the emitting part has been done manually, the measured temperature distribution signature has not followed the simulated one resulting in obtaining the temperature minimum closer to the two ends of the solid cathode. This can be avoided by shaping the filament using a moulded die.

FIG. 3. (a) Electron beam puncture on SS sheet in axial beam. (b) Electron beam puncture at 90◦, 180◦, and 270◦ bent beam.

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Along with temperature distribution, stress analysis of cathode is also carried out. In stress analysis, Von Mises stress is evaluated. In all the three cases (no fin, one fin, and two fins), maximum stress value is less than the Von Mises stress of thoriated tungsten (less than 600 MPa). Trial beam experiment has been carried out on stainless steel with tantalum as backing plate. Puncture (Fig. 3(a)) has been made on a 1 mm thick stainless steel at 11 kV acceleration voltage and beam current of 220 mA with axial beam, and this is an improvement over tantalum cathode7 in which 40 kV acceleration voltage was required to obtain 120 mA beam current. The figure shows the beam puncture length of about 90 mm (Fig. 3(a)) which is in agreement with the simulated result (95 mm Fig. 2(a)) and the beam profile in the Fig. 3(a) is consistent with the measured temperature distribution. The experiment has been conducted by bending the beam by 270◦ at 30 kV acceleration voltage and 110 mA beam

Rev. Sci. Instrum. 86, 026105 (2015)

current (Fig. 3(b)). The puncture has been made on stainless steel at 90◦, 180◦, and 270◦ location. The figure shows that the modified design achieves a one to one correspondence of the emitting solid cathode length and the electron beam length even after bending the beam. 1K.

B. Thakur, G. K. Sahu, R. V. Tamhankar, and K. Patel, Rev. Sci. Instrum. 72, 207 (2001). 2J. Yeheskel, D. Gazit, R. Avida, and M. Friedman, J. Phys. D: Appl. Phys. 16, 499 (1983). 3M. Iqbal, M. A. Chaudhary, M. Rafiq, K. Masud, and F. e Aleem, Rev. Sci. Instrum. 74, 1196 (2003). 4M. Iqbal, K. Masood, M. Rafiq, M. A. Chaudhary, and F.-e Aleem, Rev. Sci. Instrum. 74, 4616 (2003). 5M. Iqbal and F. e Aleem, Rev. Sci. Instrum. 77, 106101 (2006). 6N. Maiti, S. Mukherjee, B. Kumar, U. D. Barve, V. B. Suryawanshi, and A. K. Das, Rev. Sci. Instrum. 81, 013302 (2010). 7N. Maiti, K. Lijeesh, U. D. Barve, N. Quadri, G. U. Tembhare, S. Mukherjee, K. B. Thakur, and A. K. Das, Rev. Sci. Instrum. 84, 083302 (2013). 8I. Langmuir, Phys. Rev. 22, 357 (1923).

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Note: design and development of improved indirectly heated cathode based strip electron gun.

An improved design of indirectly heated solid cathode based electron gun (200 kW, 45 kV, 270° bent strip type electron gun) has been presented. The so...
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