Daniel Gonnella

The importance of the electron mean free path for superconducting radio-frequency cavities

Impurity-doping of niobium is an exciting new technology in the field of superconducting radio-frequency accelerators, producing cavities with record-high quality factor Q0 and Bardeen-Cooper-Schrieffer surface resistance that decreases with increasing radio-frequency field. Recent theoretical work has offered a promising explanation for this so-called “anti-Q-slope,” but the link between the decreasing surface resistance and the shortened electron mean free path of doped cavities has remained elusive. In this work, we investigate this link, finding that the magnitude of this decrease varies directly with the mean free path: shorter mean free paths correspond to stronger anti-Q-slopes. We draw a theoretical connection between the mean free path and the overheating of the quasiparticles, which leads to the reduction of the anti-Q-slope towards the normal Q-slope of long-mean-free-path cavities. We also investigate the sensitivity of the residual resistance to trapped magnetic flux, a property that is great...

Performance of the Cornell Main Linac Prototype Cryomodule for the CBETA Project

The Cornell Main Linac Cryomodule (MLC) is a key component in the Cornell-BNL ERL Test Accelerator (CBETA) project, which is a 4-turn FFAG ERL under construction at Cornell University. The MLC houses six 7cell SRF cavities with individual higher order-modes (HOMs) absorbers, cavity frequency tuners, and one magnet/BPM section. Here we present final results from the MLC cavity performance and report on the studies on the MLC HOMs, slow tuner, and microphonics.

Performance of the Novel Cornell ERL Main Linac Prototype Cryomodule

The main linac cryomodule (MLC) for a future energyrecovery linac (ERL) based X-ray light source at Cornell has been designed, fabricated, and tested. It houses six 7cell SRF cavities with individual higher order-modes (HOMs) absorbers, cavity frequency tuners, and one magnet/BPM section. Cavities have achieved the specification values of 16.2MV/m with high-Q of 2.0e10 in 1.8K in continuous wave (CW) mode. During initial MLC cavity testing, we encountered some field emission, reducing Q and lowering quench field. To overcome field emission and find optimal cool-down parameters, RF processing and thermal cycles with different cool-down conditions have been done. Here we report on these studies and present final results from the MLC cavity performance.

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

Niobium Impurity-Doping Studies at Cornell and CM Cool-Down Dynamic Effect on Q0

As part of a multi-laboratory research initiative on high Q0 niobium cavities for LCLS-II and other future CW SRF accelerators, Cornell has conducted an extensive research program during the last two years on impurity-doping of niobium cavities and related material characterization. Here we give an overview of these activities, and present results from single-cell studies, from vertical performance testing of nitrogen-doped nine-cell cavities, and from cryomodule testing of nitrogen-doped nine-cell cavities. We show that 2K quality factors at 16 MV/m well above the nominal LCLS-II specification of 2.7 × 10 can be reached reliably by nitrogen doping of the RF penetration layer. We demonstrate that the nitrogen furnace pressure is not a critical parameter in the doping process. We show that higher nitrogen doping levels generally result in reduced quench fields, with substantial variations in the quench field between cavities treated similarly. We propose that this can be explained by the reduced lower critical field Hc1 in N-doped cavities and the typical variation in the occurrence of defects on a cavity surface. We report on the results from five cryomodule tests of nitrogen-doped 9-cell cavities, and show that fast cooldown with helium mass flow rates above 2 g/s is reliable in expelling ambient magnetic fields, and that no significant change in performance occurs when a nitrogen-doped cavity is installed in a cryomodule with auxiliary components.

Understanding the Field Dependence of the Surface Resistance in Nitrogen-Doped Cavities

An important limiting factor in the performance of superconducting radio frequency (SRF) cavities in medium and high field gradients is the intrinsic quality factor and, thus, the surface resistance of the cavity [1]. The exact dependence of the surface resistance on the magnitude of the RF field is not well understood. We present an analysis of experimental data of LT1-3 and LT1-4, 1.3 GHz single-cell nitrogen-doped cavities prepared and tested at Cornell [2]. Most interestingly, the cavities display anti-Q slopes in the medium field region (i.e. Rs decreases with increasing accelerating field). We extract the temperature dependent surface resistances of the cavities, analyze field dependencies, and compare with theoretical predictions. These comparisons and analyses provide new insights into the field dependence of the surface resistance and improve our understanding of the mechanisms behind this effect.