Helicity injection and diagnostics for orbitron millimeter wave source

Conventional orbitrons utilize the radially injected electrons for production of electromagnetic waves. In such a scheme, however, more than half of the electrons would not participate in the orbital motion around the anode due to the lack of acceleration. Only the electrons who did not suffer collisions till the radius 2/3 of the outer conductor (cathode) radius are possible to acquire the azimuthal velocity, via collisions, as large as the critical velocity with which the electrons can undergo circular equilibrium orbits. The axisymmetric injection is also a problem; 50% of electrons would be lost directly to the anode by the head-on collisions. This paper discusses various ways to enhance the efficiency and absolute power of an orbitron millimeterwave source. Experimental results are described on employment of a tapered metal-end, tangential injection of a thin electron beam, axial injection of rotating annular electron beams, and application of external magnetic fields. Further problem of conventional orbitrons is in its construction in which the potential-well is prematurely destroyed due to the shortening discharge current. Its diagnostics and consequence are discussed together with a new scheme leading towards the goal, an efficient injection of helicity (or helical electron-beam) into the potential-well conserved orbitron interation region.