Experimental demonstrations and theoretical analyses of a newelectromechanical energy conversion process which is made feasible onlyby the unique properties of superconductors are presented in thisdissertation. This energy conversion process is characterized by ahighly efficient direct energy transformation from microwave energyinto mechanical energy or vice versa and can be achieved at highpower level. It is an application of a well established physicalprinciple known as the adiabatic theorem (Boltzmann-Ehrenfest theorem)and in this case time dependent superconducting boundaries providethe necessary interface between the microwave energy on one hand andthe mechanical work on the other. The mechanism which brings aboutthe conversion is another known phenomenon - the Doppler effect. Theresonant frequency of a superconducting resonator undergoes continuousinfinitesimal shifts when the resonator boundaries are adiabaticallychanged in time by an external mechanical mechanism. These smallfrequency shifts can accumulate coherently over an extended period oftime to produce a macroscopic shift when the resonator remainsresonantly excited throughout this process. In addition, the electromagneticenergy in s ide the resonator which is proportional to theoscillation frequency is al so accordingly changed so that a directconversion between electromagnetic and mechanical energies takes place.The intrinsically high efficiency of this process is due to the electromechanicalinteractions involved in the conversion rather than aprocess of thermodynamic nature and therefore is not limited by thethermodynamic value.
A highly reentrant superconducting resonator resonating in therange of 90 to 160 MHz was used for demonstrating this new conversiontechnique. The resonant frequency was mechanically modulated at arate of two kilohertz. Experimental results showed that the timeevolution of the electromagnetic energy inside this frequency modulated(FM) superconducting resonator indeed behaved as predicted and thusdemonstrated the unique features of this process. A proposed usageof FM superconducting resonators as electromechanical energy conversiondevices is given along with some practical design considerations.This device seems to be very promising in producing high power (~10W/cm^3)microwave energy at 10 - 30 GHz.
Weakly coupled FM resonator system is also analytically studiedfor its potential applications. This system shows an interestingswitching characteristic with which the spatial distribution of microwaveenergies can be manipulated by external means. It was found thatif the modulation was properly applied, a high degree (>95%) of unidirectionalenergy transfer from one resonator to the other could beaccomplished. Applications of this characteristic to fabricate highefficiency energy switching devices and high power microwavepulse generators are also found feasible with present superconductingtechnology.
