Design and Scaled Experimental Investigation of a 95 GHZ Slotted Third-Harmonic Gyro-Twt Amplifier.

A highly efficient and compact millimeter-wave gyro-TWT at 95 GHz has been designed for airborne applications, and tested as a scaled 10 GHz device for verification of the design. Since the device employs a fast-wave interaction, it does not require fragile and complicated structures as in linear-TWTs and can, therefore, yield high power at millimeter-wave frequencies. In order to reduce the magnetic field strength to the value that can be supplied by a copper solenoid for the 95 GHz operation, a high-harmonic (s >= 3) interaction can be utilized. High -harmonic interactions in a smooth cylindrical waveguide, however, requires a relativistic energy electron beam ({gg}100kV) for an efficient interaction, therefore inhibiting the realization of a compact and light weight system. A third-harmonic interaction in a slotted circuit with radial walls resembling the anode block of the conventional magnetron is utilized to achieve strong low voltage amplification while reducing the magnetic field strength requirement by a factor of three. A Laplace-transformed analytical linear theory based on the Vlasov equation is developed in the slotted geometry to determine the start-oscillation conditions for possible threats, such as harmonic gyro -BWOs, harmonic penio-BWOs and absolute instability. In addition, a slow-timescale particle-tracing nonlinear theory is used to study the nonlinear performance of the three -section slotted third-harmonic gyro-TWT where each section is shorter than the critical start-oscillation length. The nonlinear theory predicts that the 95 GHz device, which utilizes an 11.6 kG magnet and a 50 kV, 3 A, alpha equiv v_|/v_| = 1.4 axis-encircling electron beam with an axial velocity spread of 6%, will yield an stable output power of 30 kW with an efficiency of 20%, a saturated gain of 40 dB and a constant-drive bandwidth of 2%. A number of scaled 10 GHz devices have been tested with the axis-encircling electron beam provided by a gyroresonant RF accelerator to verify this 95 GHz design in both the single-section and two-section configurations. A two-section device has been tested to study the stability characteristics of multi-section device with respect to the harmonic backward-wave oscillations and to compare the nonlinear saturation performance (saturation efficiency and gain) of the two-section device with the theoretical predictions. The current device using the 10 dB coupler pair with a 75 kV, 2.6 A, alpha = 1.4 electron beam yielded a peak linear gain of approximately 25 dB and 5% small-signal bandwidth. Unfortunately, the device did not reach saturation. Additionally, a low level of the fourth-harmonic gyro-BWO oscillation power in the 2pi/3 mode at 10.5 GHz was observed, contradicting the theoretical prediction. It is suspected that a finite coupling between the two sections lengthened the effective interaction length and thereby caused it to oscillate. (Abstract shortened by UMI.).