Achieving phase and amplitude stability in pulsed superconducting cavities

Superconducting cavities exhibit a high susceptibility to mechanical perturbations due to the narrow bandwidth of the cavities. Significant phase and amplitude errors can be induced by the frequency variations driven by microphonics and Lorentz force detuning especially in the case of pulsed operation where mechanical resonances of the cavity can be excited. While the relativistic electron and positron beams in the TESLA linacs will permit the control of the vector-sum of many cavities driven by one klystron, the non-relativistic proton beam in the linacs for SNSand the JAERI/KEK Joint Project limits the number of cavities that can be controlled by one klystron. Considerable experience of RF control at high gradients (>15 MV/m) with pulsed RF and pulsed beam has been gained at the TESLA Test Facility in which presently 16 cavities are driven by one klystron. The RF control system employs a completely digital feedback system to provide flexibility in the control algorithms, precise calibration of the vectorsum, and extensive diagnostics and exception handling capabilities. Presently under study is a piezotranslator based active compensation scheme for the time varying Lorentz force detuning which if successful will reduce RF power requirements at gradients >25 MV/m considerably and provide improved field stability

Design of low level RF control for the TESLA superstructure

The superstructure is a viable option for the TESLA linear collider because of a high effective gradient and a reduced number of rf components. However, the high number and close proximity of passband modes impose challenging demands on the rf control system. The control problem is complicated by the fact that the cavity probe signal does not exactly reflect the actual accelerating voltage experienced by the beam due to the different coupling of the generator, pickup probe and beam to the FM passband modes. The digital control system developed for the standard 9-cell cavity is not adequate for operation of the superstructure. As discussed in this paper, an additional filter is needed to guarantee robustness and stability. Based on rf control simulations the filter characteristic is optimized

Control theory

In engineering and mathematics, control theory deals with the behaviour of dynamical systems. The desired output of a system is called the reference. When one or more output variables of a system need to follow a certain reference over time, a controller manipulates the inputs to a system to obtain the desired effect on the output of the system. Rapid advances in digital system technology have radically altered the control design options. It has become routinely practicable to design very complicated digital controllers and to carry out the extensive calculations required for their design. These advances in implementation and design capability can be obtained at low cost because of the widespread availability of inexpensive and powerful digital processing platforms and high-speed analog IO devices

Dynamic Lorentz force compensation with a fast piezoelectric tuner

Superconducting cavities are highly susceptible to small changes in resonance frequency due to their narrow bandwidth. At the proposed linac for the TESLA Linear Collider [1] the frequency changes resulting from mechanical deformations caused by Lorentz force detuning of the pulsed cavities will be of the order of the cavity bandwidth at the design operating gradient close to 25 MV/ m. The additional power required for field control is of the order of 10 % and will be intolerably high for the planned upgrade to 35 MV/m which appears to be feasible in the near future. While passive stiffening of the cavities is already applied to the present cavity design, the further reduction of the Lorentz force detuning constant is technically challenging. Therefore we propose an active scheme which reduces the timevarying Lorentz force detuning to much less than one cavity bandwidth. If successful, the scheme will improve the power efficiency of the TESLA linac significantly