Design of a “slow-wave” ubitron

The NRL fundamental TE11 mode ubitron has been successful in demonstrating high gain (29 dB large signal), broad instantaneous bandwidth (>;20%), high power (>4 MW), and efficient (18%) amplification in Ku band with a 245 kV, 94 A beam [D.E. Pershing, R.D. Seeley, R.H. Jackson and H.P. Freund, Nucl. Instr. and Meth. A 358 (1995) 104]. For a second generation ubitron amplifier design, it is desired to operate at higher frequencies and lower voltages, while preserving the gain, power, and bandwidth characteristics measured with the Ku band amplifier. and ubitron amplifier is currently under development using the basic design concepts of the Ku band amplifier: grazing intersection for wide instantaneous bandwidth, circularly polarized wiggler and microwave fields for high gain and power, and a high quality Pierce gun generating a cylindrical, uniform density electron beam for high efficiency. As has been previously demonstrated [E. Jerby, Phys. Rev. A 44 (1991) 703], the dispersion characteristics of a periodic waveguide can be exploited for low voltage FEL operation. This is the major point of departure from the Ku band design; operation in the HE11 mode of corrugated cylindrical waveguide which permits broader bandwidth amplification at lower voltages than is possible with smooth waveguide. Preliminary calculations indicate that 30% bandwidth operation in Ka band is possible at voltages less than 150 kV with a 8–9 mm period wiggler. Design details will be presented.