Abstract
Many proposals for next generation light sources based on single pass free electron lasers or energy recovery linac facilities require a continuous wave (cw) driven superconducting linac. The effective beam loading in such machines is very small and in principle the cavities can be operated at a bandwidth of a few Hz and with less than a few kW of rf power. However, a power reserve is required to ensure field stability. A major error source is the mechanical microphonics detuning of the niobium cavities. To understand the influence of cavity detuning on longitudinal beam stability, a measurement program has been started at the horizontal cavity test facility HoBiCaT at HZB to study TESLA-type cavities. The microphonics detuning spectral content, peak detuning values, and the driving terms for these mechanical oscillations have been analyzed. In combination with the characterization of cw-adapted fast tuning systems based on the piezoelectric effect this information has been used to design a detuning compensation algorithm. It has been shown that a compensation factor between 2--7 is achievable, reducing the typical detuning of 2--3 Hz rms to below 0.5 Hz rms. These results were included in rf-control simulations of the cavities, and it was demonstrated that a phase stability below 0.02\ifmmode^\circ\else\textdegree\fi{} can be achieved.
Highlights
In the past decade many proposals were made for medium energy linear accelerator facilities with beam energies below 5 GeV covering facilities such as free electron lasers (FEL), energy recovery linacs (ERL) foreseen as generation synchrotron light sources and drivers for nuclear fission to enhance the capabilities of rare isotope experiments [1,2,3]
The task of field control may be less complex allowing for high field stability and a low energy spread of the beam
One has to understand and measure the amount of the expected microphonics detuning in the operating range of cavity rf bandwidth optimized for FEL or ERL applications
Summary
In the past decade many proposals were made for medium energy linear accelerator facilities with beam energies below 5 GeV covering facilities such as free electron lasers (FEL), energy recovery linacs (ERL) foreseen as generation synchrotron light sources and drivers for nuclear fission to enhance the capabilities of rare isotope experiments [1,2,3]. ERLs and FEL linacs operate at a very low beam loading. This allows for optimizing the cavity bandwidth with respect to the external perturbations detuning the cavity eigenfrequency. Half-bandwidth of less than 20 Hz at 1.3 GHz operating frequency have to be considered for cavity field control. For a given accelerator it has to be determined what level of field stability would be needed to achieve the desired stability in energy and arrival time jitter of the bunched beam. It can be calculated in rf field simulations [5]. The generalizations of the results presented in this paper are valid for all similar high 1⁄4 v=c % 1 multicell elliptical cavities
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