Magnetic field management for SRF cavities has grown in importance with recent projects desiring greater Q, implying smaller magnetic fields at the cavities. This requires cryomodules and facilities to be designed with additional measures, such as global and/or local passive magnetic shielding as well as active methods to reduce fields. I will provide a summary of these techniques as applied...
Measurements of the efficiency of the magnetic shield have been carried out for the ESS Spoke cavities. The magnetic shield is made of Cryophy, a Nickel-Iron alloy, and is mounted around the Double-Spoke cavity. It is composed of two 1mm-thick layers, actively cooled by cryogenic circuit (LHe @ 4K). The installation of a mock-up cavity in the prototype cryomodule (made of stainless steel) gave...
Presently, three different techniques are used for deducing the trapped flux of SRF cavities quantitatively based on measurements of flux expulsion at the phase transition moment during the cavity cool-down process. It is still an open question as to whether these techniques give results that are consistent to each other. In this talk, we introduce two formulae that allow direct cross checking...
This presentation will discuss techniques used for magnetic field measurement in SRF cavities as well as samples. Advances, challenges and future prospects will be given.
The efficiency of flux expulsion during the cavity superconducting transition is affected significantly by the properties of the material used to fabricate the cavity itself. Significant variability is indeed observed when comparing cavities fabricated with material coming from different vendors. Here we present results from a recent study in which the microscopic properties of different...
We started EBSD measurements, which can be observed while changing temperature of Nb samples in a chamber. We expect measurements on grain orientation, stress and dislocation can give us important information to understand pinning center and flux expulsion
Initial investigations on magnetic flux expulsion in the CERN vertical test cryostats are discussed, with measurements taken on a bulk Nb 704 MHz 5-cell elliptical cavity. Cool down procedure is assessed in terms of both magnetic field flux expulsion and thermal currents, and the implication to RF performance is discussed.
In addition, mapping of magnetic field inside the cryostat is...
In each VT for QWR at RIKEN, we measure the vertical magnetic field at the upper part of the cavity. We report the obtained data, mainly at a moment the cavity shifts from NC to SC, and from SC to NC.
In this presentation, I will give an overview of the substantial influence of magnetic flux expulsion on the performance of production cavities for LCLS-II, focusing particularly on their performance in cryomodule tests. Strong correlations are observed between Q0 and the temperature at which vacuum furnace treatment occurred.
The SLAC National Accelerator Laboratory is currently constructing a major upgrade to its accelerator, the Linac Coherent Light Source II (LCLS-II). Several Department of Energy national laboratories, including the Thomas Jefferson National Accelerator Facility (JLab) and Fermi National Accelerator Laboratory (FNAL), are participating in this project.The 1.3-GHz cryomodules for this project...
The present status of the measurements and analysis wrt. the residual static magnetic field in the vertical test cryostats and the used test inserts at DESY are presented. First commissiong tests to determine the flux expulsion of 120C electropolished cavities have been done. The more than 1200 vertical tests done on the European XFEL series cavity production gave no indication about a cavity...
The absolute calibration of the AMR sensors used in our flux expulsion experiment will be presented. Furthermore the latest results obtained with the 3D magnetic field mapping system, investigating the flux expulsion efficiency of a 1.3 GHz TESLA-type single cell cavity will be discussed.
This talk will provide a short review of the different pinning mechanisms that operate in useful superconductors. We will consider both high kappa superconducting materials used for high field magnets and low kappa materials used for SRF. We will visualise the different pinning processes that operate and discuss the theoretical descriptions for such processes. Finally we will identify the...
The trapped flux surface resistance dependence on surface treatment, RF field and resonance frequency has been intensively studied at FNAL. The findings of this study are here presented, with a particular focus on the level of sensitivity at high fields given by the state-of-the-art high-gradient treatments such as 120C baking, N-infusion and modified 75-120C baking . Analyzing these results...
We report an overview of Cornell's measurements of residual resistance due to trapped flux in Nb3Sn and impurity-doped niobium in single-cell 1.3 GHz and 2.6 GHz TESLA-shape cavities
Surface resistance arising from trapped flux is experimentally measured, by which the sensitivity to trapped flux is derived. Measurements are carried out with single-cell L-band SRF cavities made of high-purity large-grain niobium materials, immersed in a uniform externally applied magnetic field generated by a solenoid whose axis overlaps the cavity axis. The surface resistance is found by...
"Sensitivity measurements have been performed for several
kinds of surface treatment in SRF cavities. Experimental setup
and results will be shown"
We will describe Fermilab experiments that focus on the optimization of doping parameters to achieve low sensitivity to magnetic flux while maintaining very high Q characteristic of nitrogen doped cavities and same or higher quench fields. One of the directions pursued is using lighter doping recipes which have been shown in the past to increase the mean free path of the resonator and decrease...
In this talk I discuss the RF dissipation of trapped vortices which contribute to the residual surface resistance in SRF cavities. In particular, the power caused by oscillations of flexible pinned vortex segments driven by a weak RF field, and the dependencies of the RF power on frequency, spatial distribution of pinning centers and purity of the material are considered. A brief overview of...
In this talk, I will explain the hysteretic mechanisms that are responsible for field-dependent residual losses of SRF cavities due to the motion of isolated trapped vortex lines under the action of parallel oscillating fields at the surface. By invoking collective weak pinning theory in the context of the Bardeen-Stephen model of vortex dynamics, I will present simple estimates, approximate...
