Abstract
We present an analysis of shielding of electromagnetic fields excited by beams of charged particles surrounded by thin conducting layers or metal stripes inside an external structure of finite length. The ability of shielding by a layer thinner than the skin depth is explained and expressions for the impedance are derived. A previous result showing preferential penetration through the shielding layer at the resonant frequencies of the surrounding structure is verified and extended to include finite resistivity of the outer structure. Integration over the spectrum of the beam bunch shows that penetration is (nearly) independent of the quality factors of the resonances. The transition of these results to those for a geometry of infinite length requires numerical evaluation.
Highlights
The shielding of rf fields emanating from coaxial cables was treated in the literature more than 50 years ago [1]
The importance of the finite length of structures surrounding a thin conducting shield has been recognized when investigating the field penetration into the Large Hadron Collider (LHC) kickers [6]. An analysis of such a geometry was made recently [7] which showed that rf fields will penetrate through a thin shielding layer preferentially at the resonant frequencies of the cavity formed by the surrounding structure
Shielding of electromagnetic fields by thin conducting layers or thin wires inside a vacuum chamber of finite resistivity has been analyzed for both cases of infinite or finite lengths of the layer
Summary
The shielding of rf fields emanating from coaxial cables was treated in the literature more than 50 years ago [1]. The wall penetration of rf fields excited by charged particle beams has been analyzed in a number of publications [3 –5] In most of these papers, rotationally symmetric structures or concentric wire cages of infinite extent were assumed for simplicity, as will be done here except for the wire cage. The importance of the finite length of structures surrounding a thin conducting shield has been recognized when investigating the field penetration into the Large Hadron Collider (LHC) kickers [6]. An analysis of such a geometry was made recently [7] which showed that rf fields will penetrate through a thin shielding layer preferentially at the resonant frequencies of the cavity formed by the surrounding structure. The results of a number of bench measurements on such structures have been published [6,8] as well as recent measurements with beam [9]
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