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SRF2003 Show Abstract

Type Poster materials / surface effects
Abstract Field emission (FE) from particulates often limit the performance of superconducting accelerator structures. Therefore we have studied the dc FE properties of Nb samples which were intentionally contaminated with metal particles (Fe,Cu,Al) typical for cavity environments. At first such particulates were randomly deposited on the whole Nb surface. Three samples were investigated with a field emission scanning microscope (FESM) before and after dry ice cleaning (DIC). @ Comparative voltage maps show a drastic reduction of the number of emitters by DIC up to surface fields of 120 MV/m. In order to learn more about the nature and current stability of particulate emitters, a new support system was constructed. It allows the local contamination of samples within six areas (diameter about 0.5 mm) in a 2 x 3 raster with a given sort of particles. This facilitates the relocalisation of emitters found with the FESM and the correlation of their strength and morphology by means of in-situ and high resolution ex-situ SEM investigations. Voltage maps reveal that 35% of the 5-20 mikrom Al particles and 45% of the 1-5 mikrom Fe-particles emit at onset fields E(1nA) up to 75 and 140 MV/m, respectively. During the scans about 50% of the particles were removed from the sample surface by electrostatic forces. Most of the non-emitting particles show charging effects during the SEM investigations, thus suggesting that an insulating oxide layer suppress FE. Three types of current processing effects were observed. Up to currents of 1000 nA approximately 50% of the emitters show strong irreversible emission reduction. Moreover, 26% of emitting particles show stable reversible emission and 24% irreversible emission activation.  @ performed at Fraunhofer IPA, Stuttgart

List of authors...

Principle author first.

Last (Family) Name First Name (Initials only) Affiliation or Organization (abbreviations if possible)
Mueller G. Univ. of Wuppertal
Guenther B. Univ. of Wuppertal
Kaldasch F. Univ. of Wuppertal2
Proch D. DESY Hamburg
Reschke D. DESY Hamburg

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