Multipactor electron gun for millimeter wave tubes

Army millimeter wave radar systems designers impose ever-increasing restrictions on tube volume, weight, and cost while at the same time requiring improved tube life and reliability. The bulkiest portion of a millimeter wave tube is comprised of the thermionic electron gun and the beam focusing magnet. The thermionic electron gun is often the source of tube failure. The magnet accounts for about 30% of the cost and 60% of the tube volume and weight. An RF activated, non-thermionic, electron gun which employs the principle of secondary electron resonance (multipactor) to phase-focus electrons into tight, high density electron bunches is discussed. Since electrons are pre-bunched within the gun itself, long drift lengths and the attendant need for magnetic focusing are eliminated or minimized, thereby decreasing tube size, weight, and cost. Reliability and life are substantially increased by virtue of the fact that thermionic emission is not employed. Capability of multipactor guns to produce significant beam currents has been established by numerous investigators. The practicability and feasibility of such guns for use at millimeter wavelengths, however, depends not only on their electron current capability, but also on their ability to be driven by low power, compact, RF sources. Test vehicles (scaled to 5 GHz) to demonstrate required drive levels have been built and evaluated at ET&DL, ERADCOM. Experimental results show that drive levels on the order of 10 milliwatts are sufficient, demonstrating that small, lightweight, solid-state RF drive sources are practical. Theoretical evidence is presented showing that the level of drive is the same for millimeter frequencies (e.g. 95, 140, 220 GHz) as it is for the 5 GHz scaled test vehicles.