The diftron - A new low-noise cyclotron wave amplifier

An experimental cyclotron wave amplifier has been built which achieves an excess noise temperature of about 100°K without the aid of a pump. In this new tube, the electron beam is launched in a divergent magnetic field. A pole piece behind the cathode produces 1400 gauss at the emissive surface; the axial field then decreases adiabatically to an asymptotic lower limit of 140 gauss in the interaction space. The noise temperature of the fast and slow cyclotron wave, for an electron beam generated in this manner, is equal to the cathode temperature times the ratio of the two magnetic fields. For the 10:1 field reduction used in this experiment, the noise temperature of the beam is only 100°K. The low noise gun is followed by a pair of Cuccia couplers tuned to 400 mc, serving as input and output devices. The two couplers are separated by a twisted d.c. quadrupole which amplifies the fast cyclotron wave signal; at the same time, beam noise from the slow cyclotron wave is converted into the fast mode. Data on this 400 mc tube will be presented. At the present time a terminal noise figure of 2 db has been measured with 15 db of signal gain. Work is continuing and more experimental data will be reported at the meeting. This experimental amplifier opens the way to the design of slow wave amplifiers, e.g. transverse field TWT's, or of devices which use D.C. pumping to actively couple fast and slow waves, enabling them to achieve without an r.f. pump the low-noise performance normally expected of parametric devices.