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Prospects of Pasotron Development in the Country

It is well-known that vacuum microwave tubes are leading generators of electromagnetic energy for applications in which medium or high RF powers are required. Although the conventional microwave tubes appear to be suitable for such applications however, the performance of a large number of systems that is determined by a combination of the bandwidth, efficiency and microwave power is limited by the capabilities of such conventional vacuum microwave tubes. Some research work has been done in the recent past clearly showing that the presence of controlled amount of ionized gas (plasma) inside the microwave tubes can significantly improve their characteristics beyond what is currently available in evacuated devices. The presence of plasma can increase the beam current transport and relax the required magnetic field in microwave tubes, and significantly improve their performance with respect to RF power, bandwidth, efficiency, compactness, and long-pulse as well as high Pulse Repetition Frequency (PRF) operation capability [1]. The plasma-assisted devices can be especially attractive for systems and applications where size and weight reductions are important (such as satellite and airborne). For high peak and high average power microwave generation, a small size, light weight source is a plasma-assisted slowwave oscillator called Pasotron, which has drawn considerable attention of the researchers for the last couple of decades [1-2]. The Pasotron can operate without or minimum external magnetic field in contrast to other high-power microwave sources where the beam is focused by a strong external magnetic field, because in the Pasotron, the presence of ions neutralizes the beam space charge and allows for radial motion of electrons under the action of transverse fields of the wave called ‘Bennett pinch’ [2]. The Pasotron device mainly consists of the plasma cathode electron gun (PCE-Gun), the periodic Slow Wave Structure (SWS) and the radiating antenna. The PCE-Gun has capabilities to generate highcurrent, broad and focused electron beams for plasma-assisted microwave sources. Generally, two types of the PCE-Gun namely differential pressure based hollow cathode and pseudospark type are used for the generation of intense electron beams. A single gap Pseudospark based 5-25kV/50-160A PCE-Gun has already been developed at CSIR-CEERI, Pilani [3-4]. A pseudospark discharge is a viable possibility due to the emitted linear beam’s characteristic properties, such as, high current density and high brightness as well as self-focusing during its propagation. The developed PCE-Gun is capable of producing beam of high current density and electron beam is found to be focused and propagated for more than 250 mm [4-5]. To design the slow wave structure for microwave sources, the current density evaluation of the electron beam is an important parameter which would provide an insightful measurement. The predictive capabilities of PCE-Gun for Pasotron development are underway at CSIR-CEERI, Pilani. CSIR-CEERI has initiated a research programme for the technology development of Pasotron under 12th Five year Network Project. The aim of the programme is to design and develop the relevant technology for high power plasma-assisted microwave tubes which are capable of performing better than their conventional vacuum counterparts. It is envisaged to design and develop improved version PCE guns and slow-wave interaction structures with particular reference to their employment in Pasotron. Recently an effort has been made to design and develop the Slow Wave Structure (SWS) for X-band Pasotron. The initial design has been taken as rippled wall cylindrical waveguide having radial profile given by R (z) = R0 + hcos (k0z), where R0, h, and k0 are mean radius, rippled amplitude and spatial periodicity, respectively. The design and technology established will be exemplified by the development and testing of an integrated Pasotron for the first time in the country. In addition, the relevant technology developed can pave the way for plasma-assisting the other microwave tubes for improving their performance.