Studies on the Fundamental Mechanisms of Niobium Electropolishing
50
E.A.S. Viklund, D.N. Seidman
NU, Evanston, Illinois, USA
L. Grassellino, S. Posen, T.J. Ring
Fermilab, Batavia, Illinois, USA
To improve the superconducting performance of niobium SRF cavities, electropolishing (EP) with a sulfuric and hydroflouric acid mixture is used. The chemistry of this reaction is complex due to the interactions between diffusion mechanisms, surface oxide structure, and multiple chemical species. Past studies on the EP process have produced a certain set of optimum parameters that have been used successfully for a long time. However, two recent developments have called the efficacy of the existing EP process into question. Since the introduction of nitrogen doping the surface quality of some cavities has been very poor. Also, EP performed at colder than standard temperatures leads to an increase in the cavity performance. To understand these questions, we perform a multivariate study on the EP process using niobium test samples electropolished at different temperatures and potentials. We find that electropolishing at lower potentials leads to rough surface features such as pitting and grain etching. Some of the surface features show similarities to features seen in niobium cavities. The effect of electropolishing temperature is not clear based on the results of this study.
J.T. Maniscalco, S. Aderhold, J.D. Fuerst, D. Gonnella
SLAC, Menlo Park, California, USA
T.T. Arkan, M. Checchin, J.A. Kaluzny, S. Posen
Fermilab, Batavia, Illinois, USA
J. Hogan, A.D. Palczewski, C.E. Reece, K.M. Wilson
JLab, Newport News, Virginia, USA
LCLS-II-HE has performance requirements similar to but generally more demanding than those of LCLS-II, with an operating gradient of 21 MV/m (up from 16 MV/m in LCLS-II) and tighter restrictions on field emission and multipacting. In this paper, we outline the requirements for the 1.3 GHz cavities and the plans for qualification of these cavities by vertical test. We discuss lessons learned from LCLS-II and highlight the changes implemented in the vertical test procedure for the new project.
Statistical Modeling of Peak Accelerating Gradients in LCLS-II and LCLS-II-HE
804
J.T. Maniscalco, S. Aderhold, J.D. Fuerst, D. Gonnella
SLAC, Menlo Park, California, USA
T.T. Arkan, M. Checchin, J.A. Kaluzny, S. Posen
Fermilab, Batavia, Illinois, USA
J. Hogan, A.D. Palczewski, C.E. Reece, K.M. Wilson
JLab, Newport News, Virginia, USA
In this report, we study the vertical test gradient performance and the gradient degradation between vertical test and cryomodule test for the 1.3 GHz LCLS-II cavities. We develop a model of peak gradient statistics, and use our understanding of the LCLS-II results and the changes implemented for LCLS-II-HE to estimate the expected gradient statistics for the new machine. Finally, we lay out a plan to ensure that the LCLS-II-HE cryomodule gradient specifications are met while minimizing cavity disqualification by introducing a variable acceptance threshold for the accelerating gradient.
LCLS-II Cryomodules Production Experience and Lessons Learned Towards LCLS-II-HE Project
832
T.T. Arkan, D.J. Bice, J.N. Blowers, C.J. Grimm, B.D. Hartsell, J.A. Kaluzny, M. Martinello, T.H. Nicol, Y.M. Orlov, S. Posen, K.S. Premo, R.P. Stanek
Fermilab, Batavia, Illinois, USA
Funding:DOE LCLS-II is an upgrade project for the linear coherent light source (LCLS) at SLAC. The LCLS-II linac consists of thirty-five 1.3 GHz and two 3.9 GHz superconducting RF (SRF) continuous wave (CW) cryomodules with high quality factor cavities. Cryomodules were produced at Fermilab and at Jefferson Lab in collaboration with SLAC. Fermilab successfully completed the assembly, testing and delivery of seventeen 1.3 GHz and three 3.9 GHz cryomodules. LCLS-II-HE is a planned upgrade project to LCLS-II. The LCLS-II-HE linac will consist of twenty-three 1.3 GHz cryomodules with high gradient and high quality factor cavities. This paper presents LCLS-II-HE cryomodule production plans, emphasizing the improvements done based on the challenges, mitigations, and lessons learned from LCLS-II.
Efforts Towards First Applications of Nb3Sn SRF Cavities
S. Posen
Fermilab, Batavia, Illinois, USA
Research efforts on Nb3Sn SRF cavities have led to substantial advancements over the last decade, including a severalfold increase in maximum gradient with high Q0 at 4 K and scaling up to large structures typically used in applications. In this talk, we overview recent performance advancements achieved in Nb3Sn research and development efforts and describe some first forays into applications for Nb3Sn SRF cavities.
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