MAGNETIC STRUCTURE FOR FOCUSING OF LINEAR BEAMS

A high convergence magnetically focused linear beam tube includes a pair of magnetic pole piece structures at opposite ends of the interaction circuit. At least one of the magnetic pole piece structures includes a centrally apertured transverse wall and an axial tubular projection extending away from the transverse wall for shaping the magnetic field externally of the region of the interaction circuit. The diameter of the central aperture is predominantly determinative of the gradient of the convergent axial magnetic field in the high field region, i.e., greater than 50 percent of the maximum axial magnetic field especially in the region of the beam near the beam minimum. The beam aperture is surrounded by a non-magnetic gap in the pole piece structure for independently controlling a certain amount of leakage of magnetic field through the pole piece structure to predominantly determine the magnitude of the axial magnetic field in the low field region, i.e., less than 10 percent of the maximum axial magnetic field intensity externally of the interaction circuit. Controlling the gap width is useful in the case of permanent magnet focusing for moving the point of magnetic field reversal away from the interaction circuit at either the collector or cathode end of the tube to obtain a desired shape of the magnetic flux divergence or convergence.