High power cusp gun

Abstract

Summary form only given. Certain gyrotron-type devices, including harmonic gyrotrons and peniotrons, require an axis encircling electron beam. To advance from achieving mere proof of principle to developing practical and efficient devices, it is necessary to generate a high quality beam with low velocity spread and ripple. The main thrust of this program is to develop a high quality electron gun to operate at 70 kV with 3.5 A beam current. Other goal specifications are magnetic focusing field =6.5 kG, beam alpha =1.5, beam ripple </spl plusmn/10%, and axial velocity spread <5%. The overall gun design concept retains the simplicity of the Northrop Grumman novel gun, with the beam velocity spread minimized through the addition of a magnetic ring at the cathode (floating pole piece) and a partially shielded emitting surface. Using an in-house code, the gun electrodes and magnetic pole piece electrodes were varied systematically to achieve a rough order design. A tolerance analysis showed that pole piece dimensions are not critical. Then using the Herrmannsfeldt electron optics code and the Maxwell magnetostatic code to simulate beam trajectories, shaping the cathode ring to suppress the field at the cathode, recessing the cathode to suppress edge emission, and taking into account cathode surface roughness and the high (sub-Curie) temperature of the cathode ring, all goal specifications were achieved and the total beam envelope thickness, including ripple, was about /spl plusmn/10%. The cathode magnetic ring turned out to be the same as the outer focus electrode. The gun was fabricated and tested in a bell jar to demonstrate that the focus electrode was below the Curie temperature.