Gurevich A.

Paper	Title	Page
TU104	Dynamics of Vortex Penetration, Jumpwise Instabilities and High-field Surface Resistance
	A. Gurevich FSU
	Penetration and exit of a vortex in a superconductor under strong rf magnetic field B(t), the field and frequency dependences of the dissipated power, and the transient time scales of vortex dynamics are discussed. Vortices breaking through the oscillating surface barrier are driven by extremely high Meissner currents of the order of the depairing current density. As a result, the vortex penetrates a superconductor at supersonic velocities v = 1-10 km/s, then it turns around and annihilates with an incoming antivortex. The decrease of the vortex viscous drag coefficient g(v) at higher velocities v(t) results in a jump-wise vortex penetration and a significant increase of the dissipated power. The effect of dissipation on nonlinear vortex viscosity g(v) and the rf vortex dynamics is quite significant, resulting in jump-wise instabilities, and thermal localization of penetrating vortex channels. We calculate the temperature distribution around a penetrating vortex taking into account retardation of temperature field around rapidly accelerating vortex, and its long-range interaction with the surface. We also address the effect of pinning on the nonlinear rf vortex dynamics and the effect of trapped magnetic flux on the surface resistance Rs which is calculated as a function or rf frequency and field. It is shown that trapped flux can result in a temperature-independent residual resistance Ri at low T, and its hysteretic low-field dependence, so that the residual resistance can decrease as B is increased, reaching a minimum at B much smaller than the thermodynamic critical field. Trapped vortices result in hotspots, which can ignite the global thermal breakdown of the cavity. We propose that slow cycling of rf field can reduce the residual resistance due to rf annealing of magnetic flux, which is pumped out of the cavity by rf field.
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TUP13	Measurement of RF Losses Due to Trapped Flux in a Large-Grain Niobium Cavity	132
	G. Ciovati JLab A. Gurevich NHMFL, FSU
	Trapped magnetic field in superconducting niobium is a well known cause of radio-frequency (RF) residual losses. In this contribution, we present the results of RF tests on a single-cell cavity made of high-purity large grain niobium before and after allowing a fraction of the Earth magnetic field to be trapped in the cavity during the cooldown below the critical temperature Tc. This experiment has been done on the cavity before and after a low temperature baking. Temperature mapping allowed us to determine the location of hot-spots with high losses and to measure their field dependence. The results show not only an increase of the low-field residual resistance, but also a larger increase of the surface resistance for intermediate RF field (higher "medium field Q-slope"), which depends on the amount of the trapped flux. These additional field-dependent losses can be described as losses of pinned vortices oscillating under the applied RF magnetic field.
TUP14	Measurement of the High-Field Q-Drop in a Large-Grain Niobium Cavity for Different Oxidation Processes	137
	G. Ciovati, P. Kneisel JLab A. Gurevich NHMFL, FSU
	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 degree 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 cavity 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 degree C in pure oxygen atmosphere or in air up to 180 degree 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.
WE105	An Investigation of the influence of grain boundaries on flux penetration in high purity large grain niobium for particle accelerators
	Z. H. Sung, P. J. Lee, A. Gurevich, A. A. Polyanskii, D. C. Larbalestier NHMFL, FSU C. Antoine Saclay C. Boffo, H. T. Edwards Fermilab
	Grain boundaries (GBs) in niobium cavities may be one of the important causes of extra power dissipation by reducing the field of first vortex penetration because the superconducting gap and the local depinning current density Jb on the GB are reduced. It is therefore important to measure the critical current density Jb and investigate the microstructure at grain boundaries to better understand whether or how grain boundary weakness can affect SRF cavity performance. Our experiments are currently correlating the global (by magnetometer) and local magnetization (by magneto-optical imaging), transport critical current density and atomic scale structure of Nb samples so that a DC analog of the RF surface currents can be developed for real Nb surfaces prepared using cavity optimization treatments. To measure Jb we apply transport current as a function of perpendicular magnetic field on BCP-treated bi-crystals of as-received, high-purity, large-grain niobium sheet. After measurement, we thin the very same grain boundary so that we image the microstructure of the external surface adjoining each GB by scanning transmission electron microscopy (STEM) in conjunction with EELS (Electron Energy Loss Spectroscopy). EELS has shown the presence of stoichiometric niobium oxide on the topmost layers, well within the typical superconducting niobium penetration depth (~ 50nm). 1. now at SACLAY
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