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Title: Measurement of the high-field Q drop in the TM010 and TE011 modes in a niobium cavity

Abstract

In the last few years superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards achieving reproducibly higher fields is represented by ''anomalous'' losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field (Bp) is above about 90 mT, in the absence of field emission. This effect, called ''Q drop'' has been measured in many laboratories with single- and multicell cavities mainly in the gigahertz range. In addition, a low-temperature (100-140 C) ''in situ'' baking of the cavity was found to be beneficial in reducing the Q drop. In order to gain some understanding of the nature of these losses, a single-cell cavity has been tested in the TM010 and TE011 modes at 2 K. The feature of the TE011 mode is to have zero electric field on the cavity surface, so that electric field effects can be excluded as a source for the Q drop. This article will present some of the experimental results for different cavity treatments and will compare them with existing models.

Authors:
;
Publication Date:
Research Org.:
Thomas Jefferson National Accelerator Facility, Newport News, VA
Sponsoring Org.:
USDOE - Office of Energy Research (ER)
OSTI Identifier:
885348
Report Number(s):
JLAB-ACP-06-478; DOE/ER/40150-3958
Journal ID: ISSN 1098-4402; TRN: US0603821
DOE Contract Number:
AC05-84ER40150
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Special Topics. Accelerators and Beams; Journal Volume: 9
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; SUPERCONDUCTING CAVITY RESONATORS; ELECTRIC FIELDS; FIELD EMISSION; MAGNETIC FIELDS; NIOBIUM; QUALITY FACTOR; PERFORMANCE

Citation Formats

Gianluigi Ciovati, and Peter Kneisel. Measurement of the high-field Q drop in the TM010 and TE011 modes in a niobium cavity. United States: N. p., 2006. Web. doi:10.1103/PhysRevSTAB.9.042001.
Gianluigi Ciovati, & Peter Kneisel. Measurement of the high-field Q drop in the TM010 and TE011 modes in a niobium cavity. United States. doi:10.1103/PhysRevSTAB.9.042001.
Gianluigi Ciovati, and Peter Kneisel. Sat . "Measurement of the high-field Q drop in the TM010 and TE011 modes in a niobium cavity". United States. doi:10.1103/PhysRevSTAB.9.042001. https://www.osti.gov/servlets/purl/885348.
@article{osti_885348,
title = {Measurement of the high-field Q drop in the TM010 and TE011 modes in a niobium cavity},
author = {Gianluigi Ciovati and Peter Kneisel},
abstractNote = {In the last few years superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards achieving reproducibly higher fields is represented by ''anomalous'' losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field (Bp) is above about 90 mT, in the absence of field emission. This effect, called ''Q drop'' has been measured in many laboratories with single- and multicell cavities mainly in the gigahertz range. In addition, a low-temperature (100-140 C) ''in situ'' baking of the cavity was found to be beneficial in reducing the Q drop. In order to gain some understanding of the nature of these losses, a single-cell cavity has been tested in the TM010 and TE011 modes at 2 K. The feature of the TE011 mode is to have zero electric field on the cavity surface, so that electric field effects can be excluded as a source for the Q drop. This article will present some of the experimental results for different cavity treatments and will compare them with existing models.},
doi = {10.1103/PhysRevSTAB.9.042001},
journal = {Physical Review Special Topics. Accelerators and Beams},
number = ,
volume = 9,
place = {United States},
year = {Sat Apr 01 00:00:00 EST 2006},
month = {Sat Apr 01 00:00:00 EST 2006}
}
  • The most challenging issue for understanding the performance of superconducting radio-frequency (rf) cavities made of high-purity (residual resistivity ratio > 200) niobium is due to a sharp degradation (“Q-drop”) of the cavity quality factor Q0(Bp) as the peak surface magnetic field (Bp) exceeds about 90 mT, in the absence of field emission. In addition, a low-temperature (100 – 140 C) “in-situ” baking of the cavity was found to be beneficial in reducing the Q-drop. In this contribution, we present the results from a series of rf tests at 1.7 K and 2.0 K on a single-cell cavity made of high-puritymore » large (with area of the order of few cm2) grain niobium which underwent various oxidation processes, after initial buffered chemical polishing, such as anodization, baking in pure oxygen atmosphere and baking in air up to 180 °C, with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system allows measuring the local temperature rise of the cavity outer surface due to rf losses, which gives information about the losses location, their field dependence and space distribution. The results confirmed that the depth affected by baking is about 20 – 30 nm from the surface and showed that the Q-drop did not re-appear in a previously baked cavity by further baking at 120 °C in pure oxygen atmosphere or in air up to 180 °C. These treatments increased the oxide thickness and oxygen concentration, measured on niobium samples which were processed with the cavity and were analyzed with Transmission Electron Microscope (TEM) and Secondary Ion Mass Spectroscopy (SIMS). Nevertheless, the performance of the cavity after air baking at 180 °C degraded significantly and the temperature maps showed high losses, uniformly distributed on the surface, which could be completely recovered only by a post-purification treatment at 1250 °C. A statistic of the position of the “hot-spots” on the cavity surface showed that grain-boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field and the low-field energy gap, which supports the hypothesis of thermo-magnetic instability governing the Q-drop and the baking effect.« less
  • In the last few years superconducting radio-frequency cavities made of high purity (RRR>200) niobium achieved accelerating gradients close to the theoretical limits. An obstacle towards reproducibly achieve higher fields is represented by some ''anomalous'' losses causing a sharp degradation of the cavity quality factor when the peak surface magnetic field is above about 90 mT, in the absence of field emission. This effect, called ''Q-drop'' has been measured in many laboratories on single- and multi-cell cavities mainly in the gigahertz range. In order to gain some understanding of the nature of these losses, a CEBAF single cell cavity has beenmore » tested in the TM010 and TE011 modes at 2 K. The feature of the TE011 mode is to have zero electric field on the cavity surface, allowing to exclude any electric field effect on the Q-drop. This paper will present some of the experimental results for different cavity treatments and will compare them with existing models for the Q-drop.« less
  • In this contribution, we present the results from a series of RF tests at 1.7 K and 2.0 K on a single-cell cavity made of high-purity large (with area of the order of few cm2) grain niobium which underwent various oxidation processes. After initial buffered chemical polishing, anodization, baking in pure oxygen atmosphere and baking in air up to 180 °C was applied with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system was used allowing to measure the local temperature rise of the cavitymore » outer surface due to RF losses, which gives information about the losses location, their field dependence and space distribution on the RF surface. The results confirmed that the depth affected by baking is about 20 – 30 nm from the surface and showed that the Q-drop did not re-appear in a previously baked cavity by further baking at 120 °C in pure oxygen atmosphere or in air up to 180 °C. A statistic of the position of the "hot-spots" on the cavity surface showed that grain-boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field and the low-field energy gap, which supports the hypothesis of thermomagnetic instability governing the Q-drop and the baking effect.« less
  • In this contribution, we present the results from a series of RF tests at 1.7 K and 2.0 K on a single-cell cavity made of high-purity large (with area of the order of few cm2) grain niobium which underwent various oxidation processes. After initial buffered chemical polishing, anodization, baking in pure oxygen atmosphere and baking in air up to 180 °C was applied with the objective of clearly identifying the role of oxygen and the oxide layer on the Q-drop. During each rf test a temperature mapping system was used allowing to measure the local temperature rise of the cavitymore » outer surface due to RF losses, which gives information about the losses location, their field dependence and space distribution on the RF surface. The results confirmed that the depth affected by baking is about 20 – 30 nm from the surface and showed that the Q-drop did not re-appear in a previously baked cavity by further baking at 120 °C in pure oxygen atmosphere or in air up to 180 °C. A statistic of the position of the “hot-spots” on the cavity surface showed that grain-boundaries are not the preferred location. An interesting correlation was found between the Q-drop onset, the quench field and the low-field energy gap, which supports the hypothesis of thermo-magnetic instability governing the Q-drop and the baking effect.« less
  • We propose a scheme for the measurement of a two-mode entangled field-state in a high-Q cavity. The scheme utilizes the momentum distribution spectrum in the Raman-Nath regime of a three-level atom in V configuration. Due to the two modes of the electromagnetic field the atom may have x interactions with mode A, and y interactions with mode B, causing a complex momentum distribution. The momentum distribution of the atom after interaction with the quantized cavity fields contains the information of the field photon statistics. We reconstruct the joint photon statistics of the entangled field with the help of recorded momentummore » spectrum. We also propose to reconstruct the Wigner function of a two-mode entangled field state by injecting two coherent states resonant to each mode into the cavity and then measuring the joint photon statistics of the displaced field.« less