Systematic study of niobium thermal treatments for superconducting radio frequency cavities employing x ray photoelectron spectroscopy

The structural and chemical composition of the surface layer 100 140 nm of niobium radiofrequency cavities operating at cryogenic temperature has enormous impact on their superconducting characteristics. During the last years, cavities treated with a new thermal processing recipe, so called nitrogen infusion, have demonstrated an increased efficiency and high accelerating gradients. The role and importance of nitrogen gas has been a topic of many debates. In the present work we employ variable energy synchrotron x ray photoelectron spectroscopy XPS , to study the niobium surface subjected to the following treatments vacuum annealing at 800 C, nitrogen infusion, and vacuum heat treatment as for the infusion process but without nitrogen supply. Careful analysis of XPS energy distribution curves revealed a slightly increased thickness of the native oxide Nb2O5 for the infused samples amp; 8764;3.8 nm as compared to the annealed one amp; 8764;3.5 nm which indicates insignificant oxygen incorporation into niobium during 120 C baking and no effect of nitrogen on the formation of oxides or other niobium phases. By conducting an additional in situ annealing experiment and analyzing the niobium after the failed infusion process, we conclude that the vacuum furnace hygiene particularly during the high temperature stage is the prerequisite for success of any treatment recip

Advanced OST System for the Second sound Test of Fully Dressed Cavities

Cavities which exhibit a low field quench are normally discarded from usage in accelerator projects. However, they can be repaired if the exact location of the quench is known. Optical inspection alone cannot reliably locate the source of a quench. Methods that directly measure the quench, such as thermometry or second sound detection, could so far only be performed at undressed cavities. A new, specially designed, second sound system for the first time allows the localization of the quench in multicell cavities equipped with a helium vessel. It can be easily installed in the helium pipe of the cavity. Information on the quench location can be acquired during a standard rf test. A new algorithm localizes the quench based on the real path of the second sound wave around the cavity surface, rather than using simple triangulation. The implemented pathfinding method leads to a high precision and high accuracy of the quench location. This was verified by testing standard dressed 9 cell XFEL cavities. The system can be easily applied to other cavity shapes and size

Electrodeposition of copper on niobium for cryocooler application

The electrodeposition of copper onto niobium using commercial acidic and alkaline electrolytes was tested. The continuous dense polycrystalline copper films were successfully obtained in aqueous alkaline type bath containing copper sulphate, sodium hydroxide and sodium gluconate. The effect of benzotriazole and sodium lauryl sulphate additives on the morphology and crystal structure of the deposited copper was investigated by optical and scanning electron microscopy, and X ray diffraction. No copper oxides were found in the grown films. Copper films had moderate adhesion properties that would be insufficient for cryocooler application. We are currently exploring different com positions of electrolyte baths for obtaining the coatings on niobium with improved adhesio

Progress of R&D on SRF cavities at DESY towards ILC performance goal

An extensive R&D programme on cavity investigationsand treatments towards ILC performance goal has beenestablished at DESY. Aims and details of the program aswell as the detailed results of the optical inspections andreplica investigations of the inner cavities surface will bereported

R amp;D Activities on Centrifugal Barrel Polishing of 1.3 GHz Niobium Cavities at DESY University of Hamburg

In this paper the status of research activities at ILCHiGrade Lab DESY University of Hamburg on Centrifugal Barrel Polishing CBP of 1.3 GHz Niobium Cavities is presented. We focus on CBP based on the polishing recipe reported by Fermi National Laboratory and Jefferson Lab [1]. The aim is to gain a better understanding of the limitations of this technique, detailed characterization of the treated surface after each polishing step using a coupon single cell cavity. Plastic deformations upon initial CBP steps, embedded polishing media and residual damage upon final polishing were investigated at different areas of the cavit

Preparation and Testing of the bERLinPro Gun 1.1 Cavity

For the bERLinPro energy recovery LINAC, HZB is developing a superconducting 1.4 cell electron gun, which, in its final version, is planned to be capable of continuous wave CW 1.3 GHz operation with 77 pC bunch. For this purpose a series of three superconducting cavities, denoted as Gun 1.0, Gun 1.1 both designed for 6 mA and Gun 2.0 100 mA is foreseen. Here the status of the Gun 1.1 cavity is described, including results of the recent vertical testing. Lessons learned from the production and preparation process are summarized, also in order to identify issues critical for the production of Gun 2.

Toward optimization of centrifugal barrel polishing procedure for treatment of niobium cavities

Centrifugal barrel polishing (CBP) is a simple and environmentally friendly method that can be applied for mechanical abrasion of the cavity interior in order to remove the mechanically damaged surface after its production. The CBP recipes described in the literature, however, require CBP to be performed in many stages, require long processing times and nevertheless are unable to provide good cavity RF performance without additional chemical processing. Here, we report new results on characterization of cavity surfaces treated with a typical CBP recipe, including the contamination with abrasive particles, plastic deformation and hydrogen contamination, and critically evaluate it. Methods to reduce the depth of significant plastic deformation as well as the modified commercially viable CBP procedure followed by final electropolishing are proposed and tested on samples

Toward optimization of centrifugal barrel polishing procedure for treatment of niobium cavities

Centrifugal barrel polishing (CBP) is a simple and environmentally friendly method that can be applied for mechanical abrasion of the cavity interior in order to remove the mechanically damaged surface after its production. The CBP recipes described in the literature, however, require CBP to be performed in many stages, require long processing times and nevertheless are unable to provide good cavity RF performance without additional chemical processing. Here, we report new results on characterization of cavity surfaces treated with a typical CBP recipe, including the contamination with abrasive particles, plastic deformation and hydrogen contamination, and critically evaluate it. Methods to reduce the depth of significant plastic deformation as well as the modified commercially viable CBP procedure followed by final electropolishing are proposed and tested on samples

Surface analyses and optimization of centrifugal barrel polishing of Nb cavities

Detailed microscopy investigations of the niobiumsurface quality after centrifugal barrel polishing (CBP)have been performed applying metallographic techniques.The results imply the need for further optimisation of thepolishing procedure, mainly to reduce the thickness of thelayer that is damaged at the surface as well as pollution bythe polishing media. The most realistic application ofCBP is a combination using CBP initially to removesurface defects followed by chemical polishing to obtain achemically clean niobium surface