Elimination Effects of Surface Carbon Contaminations on Field Emission in Superconducting RF Cavities PDF Download
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Author: Kiyomitsu Asano
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Category :
Languages : en
Pages : 6
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Author: Kiyomitsu Asano
Publisher:
ISBN:
Category :
Languages : en
Pages : 6
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Author: H. A. Schwettman
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Category :
Languages : en
Pages : 44
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Measurements of the x-radiation and the electron loading in a niobium cavity resonant in the TM(010) mode at 1208 MHz have been made in order to study enhanced electron field emission in superconducting cavities. Measurements have also been made of the low temperature dc electron field emission from bismuth, niobium, copper and tungsten using a configuration which approximates the point-to-plane geometry. In the rf cavity measurements, it is found that the x-radiation count rate data follows a modified Fowler-Nordheim equation which takes into account the time dependent rf fields, the electron dynamics in the rf fields, the emission of secondary electrons, and the production and absorption of x-radiation. Further, it is found that the x-radiation from the cavity can be reduced by a factor of 10,000 to 1,000,000 by helium-ion sputter processing at low temperature. (Modified author abstract).
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Category : Electric discharges through gases
Languages : en
Pages : 270
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Category :
Languages : en
Pages :
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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 were 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.
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Languages : en
Pages :
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The higher energies planned for the next generation of particle accelerators and storage rings makes the use of superconducting hi-Q RF cavities highly desirable. Past efforts to produce reliable cavities for such projects have met with limited success. Among the barriers to achieving the maximum electric field gradient are oxide layer charging, single surface multipactoring and field emission. These are suface effects. At SLAC, a multi-technique surface analysis system has been constructed to examine possible sources of these problems and to suggest processes or surface coatings which will reduce or eliminate them. As one component of this analysis, we have investigated the time evolution of species on anodized Nb2O5-on-Nb surfaces as a function of electron bombardment. The surface concentration of C increases at an anomalously high rate under the exciting electron beam. Examination of the surface and gas phase indicate that the C source is in the underlying material. Estimates of the penetration depth of the beam are in agreement with the fact that there is a significant rate increase in the surface C buildup when the beam penetrates the anodized layer into the Nb bulk. Bulk analytical methods indicate, however, that the C concentration in the Nb is very low. Grain boundary diffusion of C to the surface and/or an enhancement of the C diffusion coefficient due to localized beam pipe heating are examined as possible explanations of this effect.
Author: H. Schopper
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Category :
Languages : en
Pages : 26
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Author: Peter Hutson Ceperley
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Category :
Languages : en
Pages : 266
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Category :
Languages : en
Pages :
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Radio-frequency superconducting (SRF) cavities are widely used to accelerate a charged particle beam in particle accelerators. The performance of SRF cavities made of bulk niobium has significantly improved over the last ten years and is approaching the theoretical limit for niobium. Nevertheless, RF tests of niobium cavities are still showing some ''anomalous'' losses that require a better understanding in order to reliably obtain better performance. These losses are characterized by a marked dependence of the surface resistance on the surface electromagnetic field and can be detected by measuring the quality factor of the resonator as a function of the peak surface field. A low temperature (100 C-150 C) ''in situ'' bake under ultra-high vacuum has been successfully applied as final preparation of niobium RF cavities by several laboratories over the last few years. The benefits reported consist mainly of an improvement of the cavity quality factor at low field and a recovery from ''anomalous'' losses (so-called ''Q-drop'') without field emission at higher field. A series of experiments with a CEBAF single-cell cavity have been carried out at Jefferson Lab to carefully investigate the effect of baking at progressively higher temperatures for a fixed time on all the relevant material parameters. Measurements of the cavity quality factor in the temperature range 1.37 K-280 K and resonant frequency shift between 6 K-9.3 K provide information about the surface resistance, energy gap, penetration depth and mean free path. The experimental data have been analyzed with the complete BCS theory of superconductivity. The hydrogen content of small niobium samples inserted in the cavity during its surface preparation was analyzed with Nuclear Reaction Analysis (NRA). The single-cell cavity has been tested at three different temperatures before and after baking to gain some insight on thermal conductivity and Kapitza resistance and the data are compared with different models. This paper describes the results of these experiments and comments on existing models to explain the effect of baking on the performance of niobium RF cavities.
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Category :
Languages : en
Pages : 15
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A model for the onset of the reduction in SRF cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Since magnetic fields at the cavity equator are tied to accelerating electric fields by a simple geometric factor, the onset of magnetic flux penetration determines the onset of Q-drop. We consider breakdown of the surface barrier at triangular grooves to predict the magnetic field of first flux penetration H{sub pen}. Such defects were argued to be the worst case by Buzdin and Daumens, [1998 Physica C 294 257], whose approach, moreover, incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter [kappa]. Since previous Q-drop models focused on either topography or contamination alone, the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of H{sub pen} when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. Still lower H{sub pen} was predicted when both effects were combined, i.e. contamination should exacerbate the negative effects of roughness and vice-versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of [kappa]. This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ≈20%, and that that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.
Author: United States. Energy Research and Development Administration
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Category : Medicine
Languages : en
Pages : 772
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