An Experimental Investigation of High Emission Density Cathodes for High Power Gyrotron Amplifiers

Abstract

The design of a new magnetron injection gun (MIG) and axial energy beam analyzer for the purpose of testing new advanced cathodes for high emission density capability and emission uniformity has been done. The goal of this thin beam design was to reduce magnetic compression in order to reduce the detrimental effects of manufacturing and assembly errors, and test operation at much higher than normal emission densities, e.g. 20 to 50 A/cm2. It should be possible to see in beam analyzer tests, and gyro-TWT experiments, the effects of cathode surface roughness and emission non-uniformity on the electron beam produced by high emission density cathodes. The final result of the design (by EGUN), assuming a perfectly smooth cathode, was an axial velocity spread of 3.90, a transverse spread of 1.7%, for transverse to axial velocity ratio (alpha) of 1.5 with a cathode current density of 30 A/cm2 with a total current of 7.7 amp. Other parameters are a cathode magnetic field of 3.0 kG, final field of 35.6 kG, and voltage of 100 kV. A compression of only -12 is unusually low for a MIG for the final operating magnetic field of 36 kG, compared to 25 to 35 for other MIG designs in use. Other notable features are a relatively steep cathode angle 50 degrees, desirable to reduce transverse misalignments, and a blunt nose. The peak electric field on the cathode is 147 kV/cm and on the edge of the nose is 205 kV/cm, satisfactory for pulse operation, but apparently necessary as efforts to reduce the fields resulted in poorer performance. Present cathodes generally have surface roughness in the range of 2.5-5 microns (particle radius) where estimates show that it is desirable to have roughness of -0.5 microns or less to not introduce significant additional velocity spread. The surface roughness will be inferred by the beam analyzer that directly measures the axial energy distribution of the electrons with a voltage depressing electrode. The mechanical design of the MIG utilizes e-beam heated cathode buttons that can be readily changed to allow fast testing of conventional and the new Scandate types. The new MIG and beam analyzer are under construction. First tests will be of the old gyro-TWT MIG, used to produce 140 kW peak at 94 GHz at only 2.2% bandwidth, which is suspect of very high velocity spread. With the new MIG and improved stability measures being taken in a new W-band gyro-TWT, we anticipate output powers in the range of 250-500 kW.