Characterization of Multi-Beam CathodeUsing Thermionic Emission Microscope

Sushil K. Shukla, Suryanarayana R. Ruddarraju, Rahul Prajesh and Niharika Rajoriya* * Banasthali University, Rajasthan

Microwave Tubes Division CSIR-Central Electronics Engineering Research Institute (CEERI)

Pilani-333031, Rajasthan, India

Abstract: Thermionic emission microscope (THEM) is an analytical research tool to study the emission uniformity of a thermionic cathode. In this paper, the design of electrostatic lens for a multi-beam cathode (MBC) has been given using 2-D TRAK and Omni-TRAK codes. The design of single beam lens system, available with us, containing a small cathode has been modified to study large size MBC using scaling laws. The present MBC contains 19 protruding buttons contributing to 19 beamlets whose image is projected onto the screen for study. The influence of various parameters such as protrusion, diameter of button, and their interspacing has been studied. The studies prove that THEM could be used as an effective tool for the characterization of MBC.

Keywords: MBC; scaling laws; THEM; 2-D TRAK; Omni-TRAK; immersion lens

Introduction THEM is a useful tool to study the spatial emission distribution of a thermionic cathode [1]. A schematic diagram showing various subsystems of THEM is shown in Figure 1. The system comprises of: (a) an electrostatic immersion lens to produce the image of cathode emission; (b) deflection plates to deflect the beam; and (c) Faraday cage to collect the elemental currents. The image of the cathode is magnified by applying suitable voltages to the cathode, which operates under temperature limited conditions and, the image is formed at the screen due to the electrons emitted by the cathode surface.

Figure 1. Schematic layout of THEM

The single beam cathode has a smooth planar surface while the MBC has several miniature buttons, contributing to the

multiple beamlets emanating from individual buttons, which are projected from the main body of cathode. The objective of the study is to find out whether THEM can be utilized for the characterization of MBC at all. At CSIR-CEERI, the MBC work has been initiated as an academic activity for which THEM would be utilized for characterization.

Design of Electrostatic Lens The design available with us [1] for a single beam cathode of diameter 3.2 mm has been modified to handle multi-beam cathode containing 19 buttons each of diameter 0.6 mm using 2-D TRAK and Omni-TRAK. For a clear understanding, and for a better explanation of results, the MBC containing 5 buttons has been taken up. The results of 19 buttons are also presented. Figure 2 shows various electrodes of the lens system used for the simulation.

Figure 2. (a) MBC with five protruded buttons and (b) lens showing the electrodes with respective voltages

The MBC has a base metal of diameter ‘D’ with five emitting buttons each of diameter ‘d’, spacing ‘g’ and height ‘h’. The lens in our existing THEM has a planar cathode of diameter 3.2 mm. This design has been modified by applying scaling laws [2] to study larger cathode of diameter 5.4 mm. The main formulation used in scaling is

(1) where V is potential applied to the electrode and L is the spacing.

The simulation has been carried out for a planar geometry to ensure the utilization of scaling laws for the present lens system. The results of small cathode and large cathode are shown in Figure 3. The buttons are introduced subsequently.

Deflection plates: X & Y Accelerating. electrode Wehnelt

electrode

Cathode

Electrostatic lens

Faraday cage

345978-1-4673-0369-9/12/$31.00 © 2012 IEEE

The results corresponding to 5 buttons (using 2-D TRAK) are shown in Figure 4 while the results corresponding to 19 buttons (using Omni-TRAK) are shown in Figure 5 and 6.

Results and Discussion The trajectory plots of small cathode (Figure 3(a)) are similar to those of large size cathode (Figure 3(b)) due to the application of equation (1). This implies that the lens for a large diameter cathode has to be different accordingly, if a complete image is to be viewed. Our further interest is to study the influence of protrusions. The multi-beams emanated from 5-button cathode are shown in Figure 4 together with the set of parameters chosen in simulation.

Figure 3. Trajectory plots of (a) 3.2 mm dia. cathode and (b) 5.4 mm dia. cathode using 2-D TRAK

Simulations have been performed to study the influence of changing ‘d’, ‘g’ and ‘h’ on the image formation. The results corresponding to a particular set of values are shown in Figure 4. It has been observed that for ‘d/g’>1, the spacing between the beamlets is low while the spreading of beamlet is high (Figure 4(a)). On the contrary, for ‘d/g’<1, the spacing between the beamlet is high and the spreading of beamlet is low (Figure 4(c)). This phenomenon is attributed to the penetration of the field of accelerating electrode into the inter-button regions. However, it has also been observed that the influence on the variation in ‘h’ is not significant.

Figure 4. Trajectory plots of MBC,obtained from 2-D TRAK,for different sets of button parameters:‘d’,‘g’ & ‘h’

To study a 19-button cathode, which is of our practical interest, a half-plane has been chosen, due to cylindrical symmetry, to minimize computational time. The geometrical details are shown in Figure 5. The results obtained using Omni-TRAK are given in Figure 6. Due to the overlapping of trajectories, obtained from 9-1/2 button cathode, the individual beamlets in 2-D are not well distinguished; however, the individual beamlets are visible to some extent in the 3-D plot.

Figure 5. Geometries of: (a) MBC and (b) THEM comprising of lens, deflection plates and screen

Figure 6. Trajectory plots in: (a) 2-D and (b) 3-D of 9-1/2 -button MBC using Omni-TRAK

Conclusions 2-D TRAK and Omni-TRAK are effective tools in the study of electrostatic lens and deflection systems of THEM. The variation in button height has least influence on image formation while the variations in its diameter and interspacing alter the image size. To visualise the complete picture of MBC, the electrostatic lens has to be tailored according to the diameter of the MBC.

Acknowledgements The authors wish to thank Director, CSIR-CEERI for supporting the work.

References1. R. K. Barik, R. S. Raju, S. Das, and S. Rathore, “Design

and Development of Thermionic Emission Microscope (THEM)”, IVEC-2011, Bangalore, pp71-72.

2. H. Moss, “Narrow Angle Electron Guns and Cathode Ray Tubes”, Academic Press

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