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• W2090170742 abstract "CPD cathode, made out of tungsten wire, using active sintering resulted in uniform and two-fold emission as compared to B-type cathode. Studies on wire diameter using MATLAB code suggests that 20 μm diameter provides optimum barium coverage. Detailed experimental results will be presented. A conventional dispenser cathode has an emitting button, made out of porous tungsten. The pores are distributed in random sizes and shapes across the surface. As a result the barium diffusion from bulk to the pore end and the subsequent migration into the inter-pore region are not uniform all across the surface, due to which the emission uniformity is affected. Figure 1. SEM image of CP pellet A cathode wherein the distribution and size of the pores can be directly controlled would represent a controlled porosity dispenser (CPD) cathode. One of the techniques to fabricate a CPD cathode pellet is by slicing a pellet out of sintered bunch of tungsten (W) wires [1]. A simple technique has been developed by us [2] to fabricate the pellet out of “activated sintered bunch of tungsten (W) wires”. In this approach the activation energy for sintering is lowered by an addition of a small amount of active material thereby avoiding the need to reach elevated temperatures. A compression fixture has been designed and fabricated to carry out sintering operation under pressure. The fabrication of CPD cathode assembly, mainly, involved: (a) making bunch, out of bits of W-wire of diameter 50 μm and compressing under hot condition, (b) machining of bunch using wire cutting to obtain pellets, (c) vacuum firing of pellets to remove the excess active material, (d) impregnating the pellets with barium-calcium-aluminates in the ratio of 4:1:1, and (e) integration of the impregnated pellets with potted heater assembly using laser tagging. Figure 2. Pulsed I-V characteristics of CPD Cathode The SEM image the cathode pellet is shown in Fig. 1.The pores have a triangular geometry with hexagonal distribution 978-1-4799-5772-9/14/$31.00 ©2014 IEEE around the circumference of each wire. The average pore size is found to be about 11μm when the triangle is replaced with an effective circle of same area. The cathodes were tested in an analytical system comprising of: (a) anode for emission measurements and (b) Auger electron spectroscopy (AES) system for surface analysis. The I-V plots are shown in Fig. 2. The emission is about twice that of a conventional B-type cathode for the same temperatures. This is attributed to high Ba/W and O/W ratios on the surface as compared to those of a B-type. Miram curves generated, out of pulse emission data, exhibited a better temperature limited and fully space charge limited (TL-FSCL) transition at lower FSCL current densities and, also, a lower patchiness of emission as compared to those of a B-type. Figure 3. Histograms representing percentage area (of different coverage) with wire radius A code “CPD CALCULATOR” which is a graphical user interface (GUI) has been developed using MATLAB software, incorporating the diffusion dynamics, to graphically appreciate the steady state Ba spread. These studies show that the barium coverage falls exponentially from the pore edge [3]. To obtain a large fraction of area with the required minimum coverage (θ), the inter-pore distance should be comparable to the diffusion length. A model has been developed to estimate the optimum wire size for a good performance. The results are shown in Fig. 3 in the form of histogram. It can be seen that there is no added advantage if we go for wire size lower than that 20 μm. For wire size greater than 40 μm, though a large area is well covered (θ>0.7), the pore size would be greater than that of B-type, resulting in high evaporation." @default.
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• W2090170742 date "2014-06-01" @default.
• W2090170742 modified "2023-09-25" @default.
• W2090170742 title "Active sintered controlled porosity dispenser (CPD) cathode" @default.
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• W2090170742 doi "https://doi.org/10.1109/ivesc.2014.6892079" @default.
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