Uniformity for a Magnetron Injection Gun

We report measurements of the azimuthal emission nonuniformity of a gyrotron's thermionic cathode as determined from two experiments, one which determines the total emission and one which scans the emitted beam using a rotating probe. In the total emission experiment, which measured the total collector current over a range of voltages, the cathode's total work function spread was found to be 0.033 eV at T = 1000 'C, centered around 1.88 eV. In the subsequent experiment, which used a mechanically rotatable current probe, global and local effects were investigated. Voltage was varied to examine emission nonuniformities in different regions of cathode operation. While emission became almost completely uniform in the low-voltage space-charge limited regime, emission varied as much as 50 % at high voltages. In another part of the current probe experiment, current-voltage (I-V) curves were measured at azimuthal locations in 300 increments for several cathode temperatures. From this extensive set of data the work function distribution parameters were identified over small sections of the cathode for the entire cathode surface. The efficiency of microwave radiation produced by high power microwave sources is particularly sensitive to the electron beam quality. The beam for these devices must have low momentum and energy spread along with high spatial emission uniformity to ensure the best possible efficiency. High power gyrotrons extract microwave energy from an annular electron beam produced from a thermionic cathode in a magnetron injection gun (MIG). Theoretical studies [1] and experimental research [2], [3] of gyrotrons have shown that the emission uniformity of the annular beam can have a significant effect on the amount of energy available for exciting a particular electromagnetic mode. Mode competition due to an electron beam of poor quality may severely limit the efficiency of a gyrotron. The electron beam quality is largely dependent on the quality of the cathode. For a gyrotron cathode the quality is determined by both its temperature uniformity and its work function uniformity. Cathode emission is generally described by two equations, the Child-Langmuir law, in the low-voltage space-charge limited regime, and the Richardson-Dushman equation, in the high-voltage temperature limited regime. Numerous studies have attempted to model the transition between these two regimes of operation [4][5]. One approach, which is used in this study, assumes distributed parameters to comine the two equations into one expression [6]. For a work function distribution which is Gaussian, with a central work function value 0, and work function spread o, the emission current density JV at a particular voltage value V may be expanded to [7]: KV I/2 21 # o 2 '4 4 Jv (00, a) = 2 +1erf + O erf + x 2 1 (uV 2 a v [-e eE_ e u exp -T 0(1) kT41rEo 2kT where K is the perveance of the beam (dependent on cathode geometry), A0 is the Richardson constant, 120 A/cm deg2 , T is the cathode temperature, e is the electron charge, Eo the free-space permittivity, and k is Boltzmann's constant. E is the electric field which contributes to the Schottky effect. The transitional work function OT is the work function at the threshold voltage where the space-charge limited current equals the temperature-limited current, as described in [7]. Note this expression is derived assuming the cathode has a spread only in its work function, not in temperature. Emission Gate Superconducting Valve Magnet