Abstract
The transfer impedance is a very important parameter of a beam position monitor (BPM) which relates its output signal with the beam current. The coaxial wire method is a standard technique to measure transfer impedance of the BPM. The conventional coaxial wire method requires impedance matching between coaxial wire and external circuits (vector network analyzer and associated cables). This paper presents a modified coaxial wire method for bench measurement of the transfer impedance of capacitive pickups like button electrodes and shoe box BPMs. Unlike the conventional coaxial wire method, in the modified coaxial wire method no impedance matching elements have been used between the device under test and the external circuit. The effect of impedance mismatch has been solved mathematically and a new expression of transfer impedance has been derived. The proposed method is verified through simulation of a button electrode BPM using cst studio suite. The new method is also applied to measure transfer impedance of a button electrode BPM developed for insertion devices of Indus-2 and the results are also compared with its simulations. Close agreement between measured and simulation results suggests that the modified coaxial wire setup can be exploited for the measurement of transfer impedance of capacitive BPMs like button electrodes and shoe box BPM.
Highlights
In particle accelerators, a beam position monitor (BPM) provides position of charged particle beam for various applications like closed orbit measurement, fast orbit feedback system, slow orbit feedback system, coupled bunch instability feedback system etc
The new method is applied to measure transfer impedance of a button electrode BPM developed for insertion devices of Indus-2 and the results are compared with its simulations
Measurement of the transfer impedance is very important for the characterization of the BPM
Summary
A beam position monitor (BPM) provides position of charged particle beam for various applications like closed orbit measurement, fast orbit feedback system, slow orbit feedback system, coupled bunch instability feedback system etc. Conventional expression of transfer impedance [Eq (1)] is not valid in the modified coaxial wire method Considering these reflections, a new expression for the calculation of transfer impedance of a pickup electrode has been derived. To validate the proposed (modified) coaxial wire method, the transfer impedance of a button electrode BPM is calculated by simulating it with particle beam, the conventional wire method and the modified coaxial wire method using CST STUDIO SUITE [13]. This paper presents the modified coaxial wire method and derivation of new expressions of transfer impedance for capacitive pickups like button electrodes and shoe box BPMs. This paper presents validation of the modified coaxial wire method through simulation performed using CST STUDIO SUITE and practical implementation of the modified coaxial wire method to measure transfer impedance of a button electrode BPM designed for Indus-2
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