Abstract
Within the framework of the CERN program on electron cloud effects in accelerators, a coaxial multipacting test stand was built. In order to simulate bunched beam, the test stand is subjected to short rf pulses. The field strength in a traveling wave mode is sufficient to trigger multipacting in ``as received'' surfaces, but not in chambers treated to reduce the secondary emission yield. Thus a number of upgrades in the bench setup have been pursued, mainly in two directions. The first one is a general reduction in mismatching (i.e., electrical losses) amongst the different parts of the setup. Secondly, instead of dumping the pulsed power into a load, it is recirculated by means of a broadband working regime resonant ring. This ring required the design of a directional coupler with up to 1 kV dc isolation, very low transmission losses, and a four octave bandwidth. This paper reports on the steps required to build this traveling wave resonant ring (improvements on the chamber and implementation of the coupler) and includes an appendix on the main properties of the setup that relate to electron multipacting studies.
Highlights
Multipacting is an electron multiplication resonance, which develops in rf devices when periodic field strength is maintained between two opposite surfaces and certain energy and resonant conditions for electron kinetics are met
The resonant ring concept has been used in the microwave range in a simultaneous multifrequency mode of operation to upgrade the standard TW setup for electron cloud studies
Improvements on the original TW chamber and for the coupler are in good agreement with the initial calculations and objectives
Summary
Multipacting is an electron multiplication resonance, which develops in rf devices when periodic field strength is maintained between two opposite surfaces and certain energy and resonant conditions for electron kinetics are met. In order to study those phenomena in a laboratory, a bench test setup [2] was built where six wires are inserted in a circular vacuum chamber and subjected to short rf pulses the transverse electromagnetic (TEM) field produced by a bunched beam (Fig. 1). The achievable electric field strength is mainly limited by the output power of the wide band amplifier. On the 50 load, the initial (i.e., before improvements) output voltage circulating through the wires, VW, is limited to 100 V (base line peak), for which electrons get an energy Eien 75 eV, according to both simulations and measurements in Ref. The dependence of the pulse amplitude required to trigger multipacting in the TW chamber as a function of electron dose is studied in Ref. After the bakeout of a stainless steel surface and a small electron dose, the minimum pulse amplitude required to trigger multipacting exceeds 200 V. The RR has stringent requirements: low reflection from the traveling wave transmission line and an rf
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