PPM focused TWT using a field emitter array cold cathode

Abstract

Summary form only given. With the recent advances in field emitter array (FEA) technology, current densities as well as total currents from planar arrays have reached values meeting or exceeding many moderate power TWT beam requirements. The development of a robust high current density (>10 A/cm/sup 2/) cold cathode source, such as the FEA, would open new vistas in the power-bandwidth performance of microwave devices, particularly those operating at millimeter wavelengths. Extremely compact, lightweight, low frequency designs may also result with the advent of FEA gated emission cathodes. Before the development of such technologies, however, the challenges of FEA beam generation, focusing and confinement in a TWT PPM configuration must be solved. The SRI field emitter arrays (known as Spindt cathodes) consist of microfabricated arrays of metal tips on a conducting electrode (base). Each tip has its own concentric aperture in an accelerating electrode (or gate) which is insulated from the base by a thin dielectric layer. Typically, the tip height and thickness of the dielectric layer are about 1 pm, the tip radius about 200 /spl Aring/, the aperture diameter about 0.5 /spl mu/m, and the tip-to-tip spacing from 1 to 5 /spl mu/m. However, all the dimensions can be controlled over a wide range. In particular, the aperture can be as small as 0.1 /spl mu/m and the tip radius 50 /spl Aring/. For Northrop Grumman's C-Band TWT application, the Spindt cathode is of 1 mm diameter with 50000 tips. These tips are 4 /spl mu/m apart which reduces the probability of failure of an entire chip due to a neighboring individual tip blow-up. In order to obtain the required. 146 A for this C-Band TWT, the average current loading is less than 3 /spl mu/A/tip. SRI International has routinely demonstrated 10 /spl mu/A per tip from such cathodes over the last two decades.