Results of a traveling-wave tube life test program, including the description of a new technique for measuring cathode activity

The results of a life test on 29 TWT\´s all of the same type are reported. The life figures obtained range from 91 to over 19,000 hours with 9 of the tubes still operating. An explanation for this spread was found in the range of cathode temperatures required to give each tube the rated beam current under space change limited emission. The data obtained show a doubling of life for each reduction of 25°C in cathode temperature. In all but one case, the failure was due to exhaustion of cathode material. The results indicate long life is achieved by high cathode activity level since this activity level determines how low an operating temperature can be used. To check the activity, a new method has been developed and is called "dip-testing." While the tube remains under its normal operating condition, the heater power is interrupted for a given time interval. The cathode current undergoes a dip, this dip being a measure of the activity level. Additional accelerated life testing has shown early warning of failure is given by application of this method. Other applications will be mentioned since this activity test is of general use.