The new generation of hadron SRF linac projects to produce rare isotopes and high-intensityneutrinos require developing a new elliptical cavity design optimized for medium beta = 0.65 and operating at relatively low frequency, at 650 MHz. During operation, vibration noise (microphonics) causes the cavity to deform, shifting its resonance frequency. Slight deformations of elliptical cavities can significantly detune the cavity frequency from the resonance. A frequency shift in the cavity lowers the nominal accelerating field, which can be compensated either by using additional RF power or both a dedicated fast mechanical tuner and more RF power. In addition, cavities must be designed to reduce sensitivity to microphonics. Microphonics compensation by increased RF power is not economical, especially in low-intensity linacs.The first part of this thesis explores a cost-effective method that uses a tuner consisting of apiezoelectric actuator. The piezoelectric actuator can elastically deform the cavity to track the resonance frequency, thus allowing for efficient use of RF power towards beam acceleration. The development and testing of a new combined slow and fast tuner for the 650 MHz cavity are presented in this thesis. The slow tuning component uses a stepper motor and can compensate for frequency detuning on the order of kHz on the order of seconds. The fast tuning component uses a piezoelectric actuator that can compensate for frequency detuning on the order of Hz and ms. A lumped circuit model to study the resonance response of the cavity in the presence of microphonics is presented. This model incorporates the experimental results of the mechanical modes of the 650 MHz cavity for the first time. Lower frequency cavities are larger and more sensitive to microphonics than widely used 1.3 GHz elliptical cavities for the acceleration of electrons. We had an opportunity to study the microphonics properties in the cryomodule setting with eight 1.3 GHz cavities experimentally before the development of a tuner for 650 MHz cavities. A new correlation between various cryogenic parameters and the frequency detuning of 1.3 GHz cavities is presented for multiple cavities in different cryomodules. A tuner that incorporates a piezoelectric actuator is used for the active compensation of microphonics. The piezoelectric properties of actuators for operation in a CW linac are available in the scientific literature. This thesis provides new results for piezo actuators operated at a high voltage needed for large gradient pulsed linacs. This work led to the discovery of large dielectric heating of the piezo and deviation from the dielectric heating formula found in the literature. Large heating of the piezo will result in a reduced lifetime. A discussion on why this deviation occurs is presented. A novel piezoelectric actuator was designed and built, yielding a reduction of heating by a factor of 14 at liquid helium temperatures. The properties of lithium niobate for use in SRF cavity resonance control were tested for the first time. Lithium niobate shows no heating but with a compromise of a smaller stroke. The results pave the way for future optimization of materials with larger stroke but smaller dielectric heating.In the second part of this thesis, two resonance control algorithms were implemented on a single-cellelliptical 1.3 GHz cavity at room temperature. The proportional-integral (PI) loop algorithm was used to compensate for slow varying vibrations (less than 5 Hz). The successful implementation of this algorithm with the single-cell elliptical cavity at room temperature demonstrates that it can control liquid helium pressure variations and other slow varying vibrations of the cavity in the cryogenic environment. For vibrations above 5 Hz, the least mean square (LMS) algorithm was used. The implementation of the LMS algorithm in the FPGA successfully suppressed 2 sinusoidal vibrations by a factor of 3 with white Gaussian noise (0.07 Hz detuning) in a matter of ms. The use of active resonance control will reduce the required cavity RF power and increase the reliability of the linac.
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