Design and Scaled Experimental Investigation of a 95 GHZ Slotted ThirdHarmonic GyroTwt Amplifier.
Abstract
A highly efficient and compact millimeterwave gyroTWT 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 fastwave interaction, it does not require fragile and complicated structures as in linearTWTs and can, therefore, yield high power at millimeterwave 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 highharmonic (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 thirdharmonic 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 Laplacetransformed analytical linear theory based on the Vlasov equation is developed in the slotted geometry to determine the startoscillation conditions for possible threats, such as harmonic gyro BWOs, harmonic penioBWOs and absolute instability. In addition, a slowtimescale particletracing nonlinear theory is used to study the nonlinear performance of the three section slotted thirdharmonic gyroTWT where each section is shorter than the critical startoscillation 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 axisencircling 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 constantdrive bandwidth of 2%. A number of scaled 10 GHz devices have been tested with the axisencircling electron beam provided by a gyroresonant RF accelerator to verify this 95 GHz design in both the singlesection and twosection configurations. A twosection device has been tested to study the stability characteristics of multisection device with respect to the harmonic backwardwave oscillations and to compare the nonlinear saturation performance (saturation efficiency and gain) of the twosection 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% smallsignal bandwidth. Unfortunately, the device did not reach saturation. Additionally, a low level of the fourthharmonic gyroBWO 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.).
 Publication:

Ph.D. Thesis
 Pub Date:
 1996
 Bibcode:
 1996PhDT........24C
 Keywords:

 Physics: Fluid and Plasma; Engineering: Electronics and Electrical