nan S.Posen

Proof-of-principle demonstration of Nb3Sn superconducting radiofrequency cavities for high Q0 applications

Many future particle accelerators require hundreds of superconducting radiofrequency (SRF) cavities operating with high duty factor. The large dynamic heat load of the cavities causes the cryogenic plant to make up a significant part of the overall cost of the facility. This contribution can be reduced by replacing standard niobium cavities with ones coated with a low-dissipation superconductor such as Nb3Sn. In this paper, we present results for single cell cavities coated with Nb3Sn at Cornell. Five coatings were carried out, showing that at 4.2 K, high Q0 out to medium fields was reproducible, resulting in an average quench field of 14 MV/m and an average 4.2 K Q0 at quench of 8 × 109. In each case, the peak surface magnetic field at quench was well above Hc1, showing that it is not a limiting field in these cavities. The coating with the best performance had a quench field of 17 MV/m, exceeding gradient requirements for state-of-the-art high duty factor SRF accelerators. It is also shown that—taking i...

Analysis of Nb3Sn surface layers for superconducting radio frequency cavity applications

We present an analysis of Nb3Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb3Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ± 0.4 meV and superconducting critical temperatures (Tc) up to 16.3 K. Scanning transmission electron microscopy performed on cross sections of the samples surface region shows an ∼2 μm thick Nb3Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb3Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low Tc regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb3Sn-coated Nb...

Quench-Induced Degradation of the Quality Factor in Superconducting Resonators

Quench of superconducting radio-frequency cavities frequently leads to the lowered quality factor Q0, which had been attributed to the additional trapped magnetic flux. Here we demonstrate that the origin of this magnetic flux is purely extrinsic to the cavity by showing no extra dissipation (unchanged Q0) after quenching in zero magnetic field, which allows us to rule out intrinsic mechanisms of flux trapping such as generation of thermal currents or trapping of the rf field. We also show the clear relation of dissipation introduced by quenching to the orientation of the applied magnetic field and the possibility to fully recover the quality factor by requenching in the compensated field. We discover that for larger values of the ambient field, the Q-factor degradation may become irreversible by this technique, likely due to the outward flux migration beyond the normal zone opening during quench. Lastly, our findings are of special practical importance for accelerators based on low- and medium-beta accelerating structures residing close to focusing magnets, as well as for all high-Q cavity-based accelerators.

Performance of the Cornell ERL Main Linac Prototype Cryomodule

Cornell has designed, fabricated, and completed initial cool down test of a high current (100 mA) CW SRF main linac prototype cryomodule for the Cornell ERL. This paper will report on the design and performance of this very high Q0 CW cryomodule including design issues and mitigation strategies. INTRODUCTION Cornell University has proposed to build Energy Recovery Linac (ERL) as drivers for hard x-ray sources because of their ability to produce electron bunches with small, flexible cross sections and short lengths at high repetition rates. The proposed Cornell ERL is designed to operate in CW at 1.3GHz, 2ps bunch length, 100mA average current in each of the accelerating and decelerating beams, normalized emittance of 0.3mmmrad, and energy ranging from 5GeV down to 10MeV, at which point the spent beam is directed to a beam stop [1, 2]. The design of main linac prototype cryomodule (MLC) for Cornell ERL had been completed in 2012. The fabrication and testing of MLC components (cavity, high power input coupler, HOM dampers, tuners, etc.,) and assembly of MLC cold mass had been completed in 2014. MLC installation and cooldown preparations began in this summer. We will describe about MLC and initial cool down results in this proceeding. MLC GENERAL LAYOUT The general layout of an ERL main linac cryomodule (MLC) is shown in Fig. 1. It is 9.8 m long and houses six 1.3 GHz 7-cell superconducting cavities with Individual HOM absorbers and one magnet/BPM section. Each cavity has a single coaxial RF input coupler which transfers power from an RF power source to the beam loaded cavity. The specification values of 7-cell cavities are Qo of 2.0e10 at 16.2MV/m, 1.8K. Due to the high beam current combined with the short bunch operation, a careful control and efficient damping of higher order modes (HOMs) is essential. So HOMs are installed next to each cavity. To minimize ambient magnetic field of high-Q 7-cell cavities, MLC has three layers of magnetic shielding; 1) Vacuum Vessel (carbon steel), 2) 80/40 K magnetic shield enclosing the cold mass, and 3) 2 K magnetic shield enclosing individual cavities. All components within the cryomodule are suspended from the Helium Gas Return Pipe (HGRP). This large diameter (280mm) titanium pipe will return the gaseous helium boiled off the cavity vessel to the liquefier and act as a central support girder. The HGRP will be supported by 3 support post. The middle one is fixed; the other side posts are not and will slide by 7-9mm respectively during the cooldown from room temperature to cold. 7-CELL CAVITIES FOR MLC Vertical Test Results All 7-cell cavities for MLC were fabricated in house. Three of six cavities were stiffened cavity and the other three were un-stiffened cavity. Cavity surface preparation recipe consists of bulk Buffered Chemical Polishing (BCP, 140um), degassing (650degC*4days), frequency and field flatness tuning, light BCP (10um), low temperature baking (120degC*48hrs), and HF rinse [3]. Figure 2 shows best Q(E)curve of MLC 7-cell cavities during vertical test (VT) at 1.8K. All 7-cell cavities had surpassed the specification values of Qo=2.0e10 at 16.2MV/m, 1.8K. In fact, average Qo=(3.0±0.3)*1e10 had been achieved during VT at 16.2MV/m, 1.8K. All VT was limited by administrative limit, no radiation or no quench were detected during VT. ____________________________________________ * Work is supported by NSF Grants NSF DMR-0807731 and NSF #ff97@cornell.edu PHY-1002467 Figure 1: Cornell ERL Main Linac Prototype Cryomodule Proceedings of SRF2015, Whistler, BC, Canada FRAA04 SRF Technology Cavity E06-Elliptical performance ISBN 978-3-95450-178-6 1437 C op yr ig ht