Using Multispectral Imaging to Measure Temperature Profiles and Emissivity of Large Thermionic Dispenser, Cathodes

Thermionic dispenser cathodes are widely used in modern high-power microwave tubes. Use of these cathodes has led to significant improvement in performance. In recent years these cathodes have been used in electron linear accelerators (LINACs), particularly in induction LINACs, such as the Experimental Test Accelerator at Lawrence Livermore National Laboratory and the Relativistic Test Accelerator at Lawrence Berkeley National Laboratory. For induction LINACs, the thermionic dispenser cathode provides greater reproducibility, longer pulse lengths, and lower emittance beams than does a field emission cathode. Los Alamos National Laboratory is fabricating a dual-axis X-ray radiography machine called dual-axis radiograph hydrodynamic test (DARHT). The second axis of DARHT consists of a 2-kA, 20-MeV induction LINAC that uses a 3.2-MeV electron gun with a tungsten thermionic-dispenser cathode. Typically the DARHT cathode current density is 10 A/cm2 at 1050 C. Under these conditions current density is space-charge limited, which is desirable since current density is independent of temperature. At lower temperature (the temperature-limited regime) there are variations in the local current density due to a nonuniform temperature profile. To obtain the desired uniform current density associated with space-charge limited operation, the coolest area on the cathode must be at a sufficiently high temperature so that the emission is space-charge limited. Consequently, the rest of the cathode is emitting at the same space-charge-limited current density but is at a higher temperature than necessary. Because cathode lifetime is such a strong function of cathode temperature, there is a severe penalty for nonuniformity in the cathode temperature. For example, a temperature increase of 50 C means cathode lifetime will decrease by a factor of at least four. Therefore, we are motivated to measure the temperature profiles of our large-area cathodes