Abstract
Transverse coupling impedance measurements of the Spallation Neutron Source (SNS) beam extraction system were performed and the results are here reported. The SNS beam extraction system is composed from 14 subsystems, each of which consists of a vertical kicker magnet plus a pulse forming network (PFN). Impedance bench measurements were performed on one large and one small aperture magnet, stand-alone as well as assembled with the first-article production PFN. The impedance measuring methods to cover the interesting frequency range from below 1 to 100 MHz are described in considerable detail. The upper frequency range is properly covered by the conventional twin-wire method but it had to be supplemented at the low-frequency end by a direct input impedance measurement at the magnet busbar. Required modifications of the PFN to maintain the impedance budget are discussed. The total impedance estimate was finally obtained by quadratic scaling with vertical aperture from the two tested kicker subsystems.
Highlights
The driving terms of instabilities in accelerator/storage rings depend on the beam surroundings which are conveniently described by impedances
Small stray inductance and capacitance values are not shown in the circuit, length differences between magnet and TW line are ignored, a small difference in inductance and impedance between the line when placed into the reference tube or in the magnet have been neglected
The results presented in this paper were obtained by using the conversion in the network analyzer corresponding to the standard lumped formula
Summary
The driving terms of instabilities in accelerator/storage rings depend on the beam surroundings which are conveniently described by impedances. At the low-frequency end, at 1 MHz and below, the signals become very weak and the noise and drift of the instrument lead to results which may no longer be valid This provided the motivation to extend the study and to search for measurement methods beyond the obvious averaging and smoothing of the data. The low-frequency range is on one hand the cause of a problem, but on the other hand it presents the possibility of treating the beam to magnet interaction as lumped element transformer It was already pointed out by NS that the coupling impedance is generated by the flux coupled to the external system plus a contribution from image currents. Dedicated to the specific SNS kicker magnet system, the detailed discussion of the alternate impedance measurement methods is expected to be of more general interest
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