A PM focused ultra-low-noise TWT for X-band

This paper describes the development of an ultra-low-noise TWT, using permanent magnet focusing, that operates over the 8 to 12 Gc frequency band. The terminal noise figure is less than 5 db with spot noise figures on a few developmental models approaching 4 db. Gain across the frequency band is greater than 25 db and the saturated power output is approximately 1 mw. The unique features of this tube and the key to obtaining the ultra-low-noise performance lie in the systematic development of 1) the electron gun-- where the potential distribution in the cathode-first anode region is independently and optimally adjusted; 2) the cathode--where a novel, very thin emitting annulus signficantly increases the electron density and forces the normalized beam impedance at the cathode plane to approach unity; 3) the helix assembly--where the copper-plated quartz rod-supported helix and the concommitant thin annular beam provide high gain and low loss per unit length; and 4) the permanent magnet focusing structure--where a high energy product, hybrid Alnico magnet produces a magnetic field of 1300 oersteds in the gun region. Additional subtle but critical factors such as tube alignment, cathode coating and base nickel, copper plating of the helix, magnetic field straightening, and low loss rf coupling are also described.