Low-voltage (≤ 20 kV) sheet beam with solenoidal focusing for w-band extended interaction klystron (EIK)

Low-voltage, high-perveance sheet beams are attractive for compact, high-power millimeter wave (MMW) amplifiers, because significantly higher beam current and RF power can be accommodated in the larger cross sectional area (as compared to conventional pencil beam devices). Furthermore, solenoidal focusing offers important advantages over the more popular periodic permanent magnet (PPM) focusing because of the higher field (and hence higher current density) that can be achieved with existing permanent magnets and the absence of orbital stability limitations. To demonstrate these advantages, a beam stick (gun, magnet, beam tunnel, and collector) is being fabricated to generate and transport a 19.5-kV, 3.5-A sheet electron beam (0.32 mm x 4 mm) in a uniform 8.5-kG magnetic field. This beam is designed to power a 94-GHz extended interaction klystron (EIK) at ~10-kW peak power. Simulations using MICHELLE show that this beam can be transported with substantially no wall interception through a 0.4-mm-high beam tunnel over distances of 4-5 cm, approximately 4 times the length of the EIK interaction circuit. A fundamental requirement for achieving this stable transport in a uniform magnetic field is avoidance of ExB drift effects (e.g., diocotron instability and beam tilting). We have suppressed these effects by utilizing an essentially planar magnetic field topology and by injecting a very uniform beam into the magnetic field in such a way that the electron guiding centers lie along the midplane and the beam edge is approximately matched to a self-electric- field potential contour in the beam tunnel. Key features of the beam stick and our design methodology will be described.