Ciovati G.

Paper	Title	Page
TU102	Review of High Field Q-Slope, Cavity Measurements	70
	G. Ciovati JLab
	One of the most interesting phenomenon occurring in superconducting radio-frequency (SRF) cavities made of bulk high RRR niobium is represented by a sharp decrease of the quality factor above peak surface magnetic field of about 90 mT and is referred to as "high field Qslope" or "Q-drop". This phenomenon was observed first in 1997 and since then some effort was devoted to the understanding of the causes behind it. Still, no clear physical interpretation of the Q-drop has emerged, despite several attempts. In this contribution, I will review the experimental results for various cavities measured in many laboratories and I will try to identify common features and differences related to the Q-drop.
	Slides(PDF)
TUP01	A Coaxial TE011 Cavity and a System to Measure DC and RF Properties of Superconductors	98
	G. Ciovati, P. Kneisel, G. R. Myneni, M. Morrone, R. Bundy, B. Clemens, T. Elliott, G. Slack, L. Turlington JLab J. Mondal BARC
	A coaxial niobium cavity has been designed and built where the center conductor consists of a removable sample. In addition, a system to measure properties such as magnetization, penetration depth, critical temperature and thermal conductivity on the same cylindrical sample has been designed and built. The purpose of this effort is to investigate possible correlations between DC and RF properties of superconductors. In this contribution, the design of the various components is discussed and the test results on a niobium sample obtained so far are presented.
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.
TUP17	Preliminary Results on "Polarized" Buffered Chemical Polishing of a Large Grain Niobium Cavity	154
	G. Ciovati JLab S. G. Corcoran Virginia Tech J. Halbritter Forschungszentrum Karlsruhe
	In order to further understand the relation between the high-field Q-drop and the native oxide layer on the surface of SRF niobium cavities, we tried to alter the oxidation of niobium by applying a small voltage between a large-grain niobium cavity and a niobium rod inserted in the center, during buffered chemical polishing (BCP). The cavity RF test results at 1.7 K and 2.0 K did not show any major difference in the Q-drop behavior, compared to a standard BCP treatment. In one case, dark gray regions were visible inside the cavity and were responsible for additional losses, as seen with temperature maps. In order to better understand the electrochemical process occurring during the "polarized" BCP treatment, measurements of the polarization curve have been made on a cylindrical niobium sample, with a cylindrical niobium rod in the center.
TUP65	JLAB CW Cryomodules for 4th Generation Light Sources	288
	R. A. Rimmer, R. Bundy, G. Cheng, G. Ciovati, W. Clemens, E. F. Daly, J. Henry, W. R. Hicks, P. Kneisel, S. Manning, R. Manus, F. Marhauser, J. Preble, C. Reece, K. Smith, M. Stirbet, L. Turlington, H. Wang, K. M. Wilson JLab
	Fourth generation light sources hold the prospect of unprecedented brightness and optical beam quality for a wide range of scientific applications. Many of the proposed new facilities will rely on large superconducting radio frequency (SRF) based linacs to provide high energy, low emittance CW electron beams. For high average power applications there is a growing acceptance of energy recovery linac (ERL) technology as the way to support large recirculating currents with modest RF power requirements. CW SRF and high current ERLs are two core competencies at Jefferson Lab. JLab has designed and built a number of CW cryomodules of several different types starting with the original CEBAF design, with variations for higher current in the two generations of JLab's free-electron laser (FEL), through two intermediate prototypes to the final high-performance module for the 12 GeV upgrade. Each of these represent fully engineered and tested configurations with a variety of specifications that could be considered for possible use in fourth generation light sources. Furthermore JLab has been actively pursuing advanced concepts for highcurrent high-efficiency cryomodules for next generation ERL based FEL's. These existing and proposed designs span the range from about 1mA single-pass to over 100 mA energy recovered current capability. Specialized configurations also exist for high-current non-energy recovered sections such as the injector region where very high RF power is required. We discuss the performance parameters of these existing and proposed designs and their suitability to different classes of fourth generation light sources.
WEP66	High pressure rinsing system studies	664
	D. Sertore, M. Fusetti, P. Michelato, C. Pagani INFN Milano-LASA T. Higo, J. Hong, K. Saito KEK G. Ciovati, T. Rothgeb JLab A. Matheisen, N. Krupka DESY
	High pressure rinsing (HPR) is a key process for the surface preparation of high field superconducting cavities. HPR water jets used in different laboratories have been characterized measuring the transferred momentum between the water jet and a target connected to a load cell. The information taken during these measurements, combined with HPR process parameters, allow calculating new significant measurable variables such as the jet power, the deposited energy on the cavity surfaces and the pressure.