A method of bunch by bunch measurement at nanoseconds interval by using cavity beam position monitors

Abstract

The application of RF cavity beam position monitors (CBPM) is essential for precise beam position measurements in X-ray Free-Electron Lasers (XFEL) and linear colliders. This paper addresses the solution to the inter-bunch signal interference when the bunch separation is as short as nanoseconds. The signals can overlap from bunch to bunch, which can induce appreciable error in evaluating beam position. We propose a conceptual method to eliminate the dominant signal overlap under a quadrature phase offset condition. Using this method, the elimination automatically occurs in the process of signal digitalization. The dominant interference that is contributed by the last bunch can be reduced over an order of magnitude in a conventional CBPM system. In addition, the required bandwidth can be effectively reduced by a factor of 2, and the design challenge for a wideband CBPM is relaxed. The method was demonstrated in an analytic approach by simulating the signal evolution in the RF front end of electronics built in MATLAB code. In the practical concern of applying this method, the factors that perturb the elimination have been examined as well. Based on the numerical result, the CBPM prototype has been designed for the pre-research plan of future high repetition rate hard XFEL at Chinese Academy of Engineering Physics. The combined simulations in CST and MATLAB on the CBPM prototype indicate agreement with the result obtained in the analytic approach.