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Title: Gas cluster ion beam surface treatments for reducing field emission and breakdown of electrodes and SRF cavities.

Abstract

Sub-micron-scale surface roughness and contamination cause field emission that can lead to high-voltage breakdown of electrodes, and these are limiting factors in the development of high gradient RF technology. We are studying various Gas Cluster Ion Beam (GCIB) treatments to smooth, clean, etch and/or chemically alter electrode surfaces to allow higher fields and accelerating gradients, and to reduce the time and cost of conditioning high-voltage electrodes. For this paper, we have processed Nb, stainless steel and Ti electrode materials using beams of Ar, O{sub 2}, or NF{sub 3} + O{sub 2} clusters with accelerating potentials up to 35 kV. Using a scanning field emission microscope (SFEM), we have repeatedly seen a dramatic reduction in the number of field emission sites on Nb coupons treated with GCIB. Smoothing effects on stainless steel and Ti substrates, evaluated using SEM and AFM imaging, show that 200-nm-wide polishing scratch marks are greatly attenuated. A 150-mm diameter GCIB-treated stainless steel electrode has shown virtually no DC field emission current at gradients over 20 MV/m.

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
941497
Report Number(s):
ANL/MCS/JA-62990
Journal ID: ISSN 0168-583X; NIMBEU; TRN: US200825%%554
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Nucl. Instrum. Methods Phys. Res. B; Journal Volume: 261; Journal Issue: 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
36 MATERIALS SCIENCE; BREAKDOWN; CAVITIES; CONTAMINATION; ELECTRODES; FIELD EMISSION; ION BEAMS; MICROSCOPES; POLISHING; ROUGHNESS; STAINLESS STEELS; SUBSTRATES; SURFACE TREATMENTS

Citation Formats

Swenson, D. R., Wu, A. T., Degenkolb, E., Insepov, Z., Mathematics and Computer Science, Epion Corp., and Jefferson National Lab. Gas cluster ion beam surface treatments for reducing field emission and breakdown of electrodes and SRF cavities.. United States: N. p., 2007. Web. doi:10.1016/j.nimb.2007.04.277.
Swenson, D. R., Wu, A. T., Degenkolb, E., Insepov, Z., Mathematics and Computer Science, Epion Corp., & Jefferson National Lab. Gas cluster ion beam surface treatments for reducing field emission and breakdown of electrodes and SRF cavities.. United States. doi:10.1016/j.nimb.2007.04.277.
Swenson, D. R., Wu, A. T., Degenkolb, E., Insepov, Z., Mathematics and Computer Science, Epion Corp., and Jefferson National Lab. Mon . "Gas cluster ion beam surface treatments for reducing field emission and breakdown of electrodes and SRF cavities.". United States. doi:10.1016/j.nimb.2007.04.277.
@article{osti_941497,
title = {Gas cluster ion beam surface treatments for reducing field emission and breakdown of electrodes and SRF cavities.},
author = {Swenson, D. R. and Wu, A. T. and Degenkolb, E. and Insepov, Z. and Mathematics and Computer Science and Epion Corp. and Jefferson National Lab.},
abstractNote = {Sub-micron-scale surface roughness and contamination cause field emission that can lead to high-voltage breakdown of electrodes, and these are limiting factors in the development of high gradient RF technology. We are studying various Gas Cluster Ion Beam (GCIB) treatments to smooth, clean, etch and/or chemically alter electrode surfaces to allow higher fields and accelerating gradients, and to reduce the time and cost of conditioning high-voltage electrodes. For this paper, we have processed Nb, stainless steel and Ti electrode materials using beams of Ar, O{sub 2}, or NF{sub 3} + O{sub 2} clusters with accelerating potentials up to 35 kV. Using a scanning field emission microscope (SFEM), we have repeatedly seen a dramatic reduction in the number of field emission sites on Nb coupons treated with GCIB. Smoothing effects on stainless steel and Ti substrates, evaluated using SEM and AFM imaging, show that 200-nm-wide polishing scratch marks are greatly attenuated. A 150-mm diameter GCIB-treated stainless steel electrode has shown virtually no DC field emission current at gradients over 20 MV/m.},
doi = {10.1016/j.nimb.2007.04.277},
journal = {Nucl. Instrum. Methods Phys. Res. B},
number = 2007,
volume = 261,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Sub-micron-scale surface roughness and contamination cause field emission that can lead to high voltage breakdown of electrodes, and these are limiting factors in the development of high gradient RF technology. We are studying various Gas Cluster Ion Beam (GCIB) treatments to smooth, clean, etch and/or chemically alter electrode surfaces to allow higher fields and accelerating gradients, and to reduce the time and cost of conditioning high voltage electrodes. For this paper, we have processed Nb, Stainless Steel, and Ti electrode materials using beams of Ar, O2, or NF3 +O2 clusters with accelerating potentials up to 35 kV. Using a Scanningmore » Field Emission Microscope (SFEM), we have repeatedly seen a dramatic reduction in the number of field emission sites on Nb coupons treated with GCIB. Smoothing effects on Stainless steel and Ti substrates have been evaluated using AFM imaging and show that 200-nm wide polishing scratch marks are greatly attenuated. A 150-mm diameter GCIB treated stainless steel electrode has now shown virtually no DC field emission current at gradients over 20 MV/m.« less
  • Sub-micron-scale surface roughness and contamination cause field emission that can lead to high voltage breakdown of electrodes, and these are limiting factors in the development of high gradient RF technology. We are studying various Gas Cluster Ion Beam (GCIB) treatments to smooth, clean, etch and/or chemically alter electrode surfaces to allow higher fields and accelerating gradients, and to reduce the time and cost of conditioning high voltage electrodes. For this paper, we have processed Nb, Stainless Steel, and Ti electrode materials using beams of Ar, O2, or NF3 +O2 clusters with accelerating potentials up to 35 kV. Using a Scanningmore » Field Emission Microscope (SFEM), we have repeatedly seen a dramatic reduction in the number of field emission sites on Nb coupons treated with GCIB. Smoothing effects on Stainless steel and Ti substrates have been evaluated using AFM imaging and show that 200-nm wide polishing scratch marks are greatly attenuated. A 150-mm diameter GCIB treated stainless steel electrode has now shown virtually no DC field emission current at gradients over 20 MV/m.« less
  • Recently, it was demonstrated that significant reductions in field emission on Nb surfaces could be achieved by means of a new surface treatment technique called gas cluster ion beam (GCIB). Further study as shown in this paper revealed that GCIB treatments could modify surface irregularities and remove surface asperities leading to a smoother surface finish as demonstrated through measurements using a 3D profilometer, an atomic force microscope, and a scanning electron microscope. These experimental observations were supported by computer simulation via atomistic molecular dynamics and a phenomenological surface dynamics. Measurements employing a secondary ion mass spectrometry found that GCIB couldmore » also alter Nb surface oxide layer structure. Possible implications of the experimental results on the performance of Nb superconducting radio frequency cavities treated by GCIB will be discussed. First experimental results on Nb single cell superconducting radio frequency cavities treated by GCIB will be reported.« less
  • Field emission is one of the key issues in superconducting RF for particle accelerators. When present, it limits operating gradient directly or via induced heat load at 2K. In order to minimize particulate contamination of and thus field emission in the CEBAF SRF cavities during assembly, a cold ceramic RF window was placed very close to the accelerating cavity proper. As an unintended consequence of this, the window is charged by field-emitted electrons, making it possible to monitor and model field emission in the CEBAF cavities since in-tunnel operation began. From January 30, 1995, through February 10, 2003, there weremore » 64 instances of spontaneous onset or change in cavity field emission with a drop in usable gradient averaging 1.4 ({sigma} 0.8) MV/m at each event. Fractional loss averaged 0.18 ({sigma} 0.12) of pre-event gradient. This event count corresponds to 2.4 events per century per cavity, or 8 per year in CEBAF. It is hypothesized that changes in field emission are due to adsorbed gas accumulation. The possible implications of this and other observations for the International Linear Collider (ILC) and other future accelerators will be discussed.« less
  • No abstract prepared.