High Voltage Cathode Temperature Measurement

Abstract

The problem of making accurate measurement of temperature of a high voltage cathode in an emission microscope is discussed. Temperatures to 1300°C with voltages to 50 kV were measured within approximately ±5°C. Radiation sensing measurement schemes were dismissed, primarily on the basis of limited accuracy due to emissivity variations. In the developed system the entire temperature measurement and control system consisting of a Pt/Pt-10 Rh thermocouple, a recorder, controller and power amplifier, are floated at cathode potential to 50 kV. The presence of high voltage, about 109 times greater than the permissible thermocouple error voltage, produces negligible effect on indicated temperatures; the entire thermocouple circuit is enclosed in a shield at cathode potential. A high voltage insulated isolation transformer provides line power to the measurement and control system and insulated shaft extensions permit safe manipulation of all controls with high voltage applied. The accuracy of ±5°C achieved for temperature measurements was determined by utilizing the known temperature of metallurgical phase transformations. The recorder is about as safe and convenient to use as a normal grounded recorder.