Verification of the ted cathode models by longterm lifetest data

Abstract

Travelling Wave Tubes are one of the dominant components on satellites, which have highest demand in performance, reliability and stability during their 15-20 years mission in space. Actually the cathode is always considered as a "wear-out" item for the TWT, although the usual definition of "End of Life" (EOL) for a cathode describes a decrease of 10% of emission current during lifetime, which can usually be compensated by an increasing anode voltage. The more and more complex operation of the tubes require the understanding of the affecting mechanisms that drive the lifetime of the cathode. As such information is of essential interest for the customer as well as the manufacturer, the verification of the cathode emission stability over the required long operation time has been demonstrated. Different strategies have been developed for both types of cathodes, used at TED: a) the Os-coated tungsten matrix M-type cathode and b) the mixed metal dispenser cathodes ("MM-type") with a Tungsten- Osmium matrix.. Usually, early life measurements provide the only information a user has, before releasing the tube to its final application. In combination with an explanatory theoretical life model these early data can be accurately analysed and extrapolated to their required lifetime. The comparison to measured in orbit data of operating TWTs may be further used to manifest the EOL behaviour. Considering the M-type cathode, the production is consequently monitored by life testing - either in specific test vehicles or in travelling wave tubes. Recent results show excellent stability of the cathode behaviour for a lifetime of more than 20 years for space cathodes in diodes (J0 = 7 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 1000deg Cb) and >18 years in tubes (Delta Jn ~ - 3% at 985degCb, for Jn = 0,6 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). These results are used to demonstrate excellent agreement with the semi-empirical model for cathode lifetime which is in use at TED, established by A.M. Shroff and D. Dieumegard. As the variation of J0 and Jn are already well predicted by the model, the empirical parameters of the activation energy E (=Vthetas ) are now deduced directly from the life test results. Today, the "MM-type" cathodes, are also operating in orbit for two decades, showing stable emission currents of up to 1.6 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in the frame of space requirements. Utilizing the advantage of life testing at extended temperatures (i.e. at 1170degCb) the MM-type cathode allows monitoring the entire cathode life of 15 years within about 4 months. This yields into an effective control of the production process parameters, as well as an early reliability assessment of the entire cathode lot. The comparison of the accelerated life test data to the actual in-orbit data, released by different customers and describing the cathode behaviour of several satellites in orbit up to 8 years will be reported. Based on these recent data, the agreement between accelerated on ground measurement and in orbit behaviour of the TWTs can be recognized, leading to an updating of the theoretical model for the MM-type cathodes, which has been generated rather conservative in the past. This model is including all different types of MM-type cathodes from L-,S- to C- and Ku- up to Ka-band.