HoBiCaT - Results
Recent tests of a fully dressed TESLA cavity (input coupler, liquid helium tank, HOM couplers, tuner) have demonstrated that CW operation up to 20 MV/m is possible - in collaboration with DESY and Jefferson Lab.
Wavelet analysis of time domain microphonics detuning.
Time domain open loop microphonics detuning (upper plot).
The lower plot shows the time-frequency map of the detuning by Morlet Wavelet analysis. The color code represents the detuning amplitude. The frequency span is zoomed to the excited first mechanical eigenmode.
Microphonics compensation with piezo tuner.
Compensation of microphonics at a TESLA cavity with a loaded quality factor of 6.4e7 using the integrated piezo tuning system. The result was achieved by combining feedback and adaptive feedforward techniques.
The adaptive feedforward uses the information of the piezo-to-RF detuning transfer function to take the exact amplitude and phase information into account.
Tuners play an important part in cavity operation: Their purpose is to modify the resonant frequency by changing the length (or equator diameter, resp.) of the cavity. Tuners are electro-mechanical devices fixed to the Helium tank that can apply stretching forces on the flexibly attached cavity body. Coarse and slow frequency changes up to 1Mhz in seconds to minutes are imposed with a stepper motor. Fine and fast changes of less than 1kHz in milliseconds are being applied with piezoelectric elements.
Cavity quality vs. solenoid current
Investigation of magnetic contamination
We have investigated the influence of an external magnetic field during superconducting phase transition on the cavity quality factor. It is apparent that a small fraction of the field is frozen in the cavity surface due to flux pinning, degrading the surface resistance and thus the quality factor, indicating an incomplete Meissner transition. No saturation has been observed for all solenoid currents used in our experiment.