A 5 MW peak 300 kW average power L-band klystron

Abstract

This paper discusses design considerations and performance characteristics of a 5 MW peak, 300 kW average power L-band klystron. The tube to be discussed is a 4 cavity klystron tunable over a 10% frequency range centered at 1320 Mc. Nominal operating parameters are a beam voltage of 135 kv and a beam current of 120 amp. The rf pulse length is 40 microseconds, the rf duty factor .06. The tube utilizes a non-intercepting modulating anode which permits the application of sophisticated beam modulation techniques. The crucial design problems on this program can be classified into 3 major areas. They are: the generation, focusing and collection of an electron beam with an average power in excess of 1 MW; the transmission of 300 kW of average rf power through an output window and the beam modulation by a non-intercepting control electrode. The very large beam power required extreme care in the design of the beam forming and beam focusing. Beam powers up to 1.3 MW of average power are focused through a drift tube with 1 1/8 inches I. D. The interception without rf drive is below 1%. The design of a collector capable of dissipating this amount of power with surface collection densities low enough and water cooling surface large enough for reliable long life operation was a major problem which had to be solved. The power handling capacity of the output window was a limiting factor in achieving higher output powers. Because of the comparatively low peak power of this tube the window problem was an average power problem only. Thermal stresses caused by excessive and non-uniform heating of the window are the limiting factor. Excess heating of the window due to single surface multipactoring has been eliminated and with proper heat transfer design the heat generated by unavailable rf losses in the window material can be handled easily. The final window design has been tested up to a power level of 380 kW without failing. The third problem of switching a modulating anode with a gain of one at voltages up to 140 kv has also been successfully solved on this tube. A special design of the modulating anode area completely eliminated the so-called post pulse arcing phenomena which so far has been a limiting factor in using modulating anodes at these high voltages.