DESY activities on 1.3 GHz tesla shape single cell and nine-cell large grain (LG) resonators are presented; results of the past five years are covered. The R&D program explores the potential for production of elliptical superconducting cavities. The main efforts have been devoted to material investigation, development of LG disk production, cavity fabrication from this material, and a search for appropriate treatment. More than 250 LG disks are manufactured; several single cell and 11 nine-cell resonators are produced and rf tested after buffered chemical polishing and after additional electropolishing. A maximum accelerating gradient of approximately 45 MV/m for this type of cavity was achieved in two resonators. Two of the LG cavities have been installed and are currently being used in the FLASH accelerator operation. Assembly of a cryomodule, consisting of LG cavities only, is in the works. Perspectives of the LG cavity application are discussed. DOI: 10.1103/PhysRevSTAB.16.012003

Since 2009 a series of eight nine-cell cavities (AC151 - AC158) of TESLA shape fabricated of large grain (LG) niobium material is under preparation and test at DESY. In a first step all cavities were tested after a BCP treatment. In a second step additional electro polishing is applied to all cavities. In this paper the treatment will be discussed and a first excellent result of AC155 will be reported. MATERIAL AND FABRICATION

Since 2009 a series of eight nine-cell cavities (AC151 – AC158) of TESLA shape fabricated of large grain (LG) niobium material [1] is under preparation and test at DESY. In a first step all cavities were tested after a BCP treatment. In a second step additional electropolishing is applied to all cavities. In this paper the treatment will be discussed and present results will be reported. MATERIAL AND FABRICATION Development of LG disc production was done within the framework of the European XFEL driven RD the roughness was small compared to conventional fine grain sheets. Three cavities (AC151 – AC153) have been fabricated from material with RRR of about 420. For five cavities (AC154 – AC158) the RRR of the material was about 350. More details about the material are presented in [3]. For the cavities AC155 and AC156 the steps at the grain boundaries, which appeared after deep-drawing, were smoothened by grinding the RF surface of the halfcells. The complete fabrication was done by Research Instruments GmbH (RI).

An overview of the activities within the DESY test program of 1.3 GHz TESLA shape 9‐cell Large Grain (LG) resonators for the European XFEL, which have taken place in last 4 years, is presented. Attention is devoted to development of LG disc production and cavity fabrication from this material, focusing in particular on aspects of production at reasonable accuracy and costs. More than 200 LG discs were manufactured, eleven 9‐cell resonators produced, partially treated at the company Research Instruments (RI) (former ACCEL) and finally treated and RF tested at DESY. Two of the LG cavities are currently used in the FLASH accelerator operation, which is the best demonstration of the feasibility of the LG application. The program compares large grain material with standard sheet niobium. Some data and perspectives of the LG application are discussed.

The accelerating gradient Eacc of X-ray Free Electron Laser (XFEL) prototype cavities manufactured at the industry and treated at DESY demonstrates wide-range scattering from 15 to 41 MV/m. Most cavities satisfy the XFEL specification. Few cavities with low performance (15-17 MV/m) are limited by quench without field emission. The T-map analysis detected quench areas mainly close to the equator. Optical control by a high resolution camera has been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to understand the cause of reduced performance and get more detailed information of the origin of defects, some samples have been extracted from two cavities and investigated by light microscope, digital light microscope with 3D profile measurement, scanning electron microscope SEM, energy dispersive x-ray analysis, and Auger spectroscopy. The electron backscattered diffraction method in a SEM is used to make localized measurements of the lattice curvature. Several surface flaws with sizes from a few mu m to hundreds of mu m detected by microscopy. The defects can be separated into two categories. The first category of defects consists of foreign elements (often an increased content of carbon). Inclusions with increased content of carbon adhere on the surface and presumably have a hydrocarbon nature. Deviation from a smooth surface profile characterizes the second type of defects (holes, bumps, and pits). Some holes and bumps were found directly in the welding seam. The hot spots in the heat-affected zone (HAZ) of the equator welds have been partially associated with pits too. The study correlates the location of pits with the presence of plastic strain found to remain after welding. Pits near the HAZ were found either coincident with or near areas of high strain. Pits away from the weld were often found at grain boundary triple junctions.

TESLA 9-cell cavity [1] was designed a decade ago for pulse operation at duty factor of a few percents and is successfully used for the FLASH linear accelerator at DESY [2], [3] and will be used for planned XFEL [4]. Recently, numerous coherent and synchrotron light sources projects base their driving superconducting linacs on this design assuming operation in a continuous wave (CW) mode at rather high gradients. We have performed CW tests of a standard 9-cell TESLA cavities installed in helium vessel and fully equipped with the standard TESLA-TTF auxiliaries, main coupler and both Higher Order Mode (HOM) couplers in the horizontal test cryostat to find out a limit in the CW operation.

TESLA 9-cell cavity (1) was designed a decade ago for pulse operation at duty factor of a few percents and is successfully used for the FLASH linear accelerator at DESY (2), (3) and will be used for planned XFEL (4). Recently, numerous coherent and synchrotron light sources projects base their driving superconducting linacs on this design assuming operation in a continuous wave (CW) mode at rather high gradients. We have performed CW tests of a standard 9-cell TESLA cavities installed in helium vessel and fully equipped with the standard TESLA-TTF auxiliaries, main coupler and both Higher Order Mode (HOM) couplers in the horizontal test cryostat to find out a limit in the CW operation.

The conceptional design of the proposed linear electron-positron collider TESLA is based on 9-cell 1.3 GHz superconducting niobium cavities with an accelerating gradient of Eacc ≥ 25 MV/m at a quality factor Q0 ≥ 5 · 10 . The design goal for the cavities of the TESLA Test Facility (TTF) linac was set to the more moderate value of Eacc ≥ 15 MV/m. In a first series of 27 industrially produced TTF cavities the average gradient at Q0 = 5 · 10 9 was measured to be 20.1 ± 6.2 MV/m, excluding a few cavities suffering from serious fabrication or material defects. In the second production of 24 TTF cavities additional quality control measures were introduced, in particular an eddy-current scan to eliminate niobium sheets with foreign material inclusions and stringent prescriptions for carrying out the electron-beam welds. The average gradient of these cavities at Q0 = 5·10 9 amounts to 25.0±3.2 MV/m with the exception of one cavity suffering from a weld defect. Hence only a moderate improvement in production and preparation techniques will be needed to meet the ambitious TESLA goal with an adequate safety margin. In this paper we present a detailed description of the design, fabrication and preparation of the TESLA Test Facility cavities and their associated components and report on cavity performance in test cryostats and with electron beam in the TTF linac. The ongoing R&D towards higher gradients is briefly addressed.

The Tera Electronvolt Superconducting Linear Accelerator (TESLA) is the only linear electron–positron collider project based on superconductor technology for particle acceleration. In the first stage with 500GeV center-of-mass energy an accelerating field of 23.4MV/m is needed in the superconducting niobium cavities which are operated at a temperature of 2K and a quality factor Q0 of 1010. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800GeV requires accelerating gradients of 35MV/m. Using an improved cavity treatment by electrolytic polishing, it has been possible to raise the gradient to 35–43MV/m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently 4 nine-cell cavities have achieved 35MV/m at Q0⩾5×109, and a fifth cavity could be excited to 39MV/m. In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler. One cavity has been operated for 1100h at the TESLA-800 gradient of 35MV/m and 57h at 36MV/m without loss in performance.

After three years of preparation, two superstructures, each made of two superconducting 7-cell weakly coupled subunits, have been installed in the TESLA Test Facility linac (TTF) for the cold- and beam test. The energy stability, the HOMs damping, the frequency and the field adjustment methods were tested. The measured results confirmed expectation on the superstructure performance and proved that alternative layout for the 800 GeV upgrade of the TESLA collider, as it was proposed in TDR, is feasible. We report on the test and give here an overview of its results which are commented in more detail elsewhere in these Proceedings.

Superstructures, chains of superconducting multi-cell cavities (subunits) connected by e/2 long tube(s) have been proposed as an alternative layout for the TESLA main accelerator [1]. After three years of preparation, two superstructures, each made of two weakly coupled superconducting 7-cell subunits driven by a single Fundamental Power Coupler (FPC), have been installed in the Tesla Test Facility linac for beam tests. Energy stability, HOM damping, frequency and field adjustment methods were tested. The measured results confirmeSuperstructures, chains of superconducting multi-cell cavities (subunits) connected by e/2 long tube(s) have been proposed as an alternative layout for the TESLA main accelerator [1]. After three years of preparation, two superstructures, each made of two weakly coupled superconducting 7-cell subunits driven by a single Fundamental Power Coupler (FPC), have been installed in the Tesla Test Facility linac for beam tests. Energy stability, HOM damping, frequency and field adjustment methods were tested. The measured results confirmed expectation on the superstructure performance and proved that an alternative layout for the 800 GeV upgrade of the TESLA collider is feasible. We report on the test and give here an overview of its results. The tests confirmed very good damping of HOMs in superstructures and thus has openedmore » a possible new application of this concept to high current energy recovery machines. We have built two 1.5 GHz copper models of two superstructures: 2x5-cells and 2x2-cells to prove further improvement of HOM damping. This contribution presents also measured results on these models. d expectations on the superstructure performance and proved that an alternative layout for the 800 GeV upgrade of the TESLA collider is feasible. We report on the test and give here an overview of its results.« less

Several bimetallic NbCu single cell cavities of the TESLA shape have been fabricated at DESY. The cavities have been hydroformed by expansion of seamless tubes with internal water pressure while simultaneously swaging them axially. Tube radius and axial displacement are being computer controlled according to results of FEM simulations and experimentally obtained strainstress curve of tube material. The tube ends were necked before hydroforming. The seamless tubes have been produced by explosion bonding and flow forming. The thickness of the Nb and Cu layers in the tube is about 1 mm and 3 mm, respectively. Surface treatments such as buffered chemical polishing, (BCP), high pressure ultra pure water rinsing (HPR), annealing at 800°C and baking at ca. 150°C have been applied. The RF performance of NbCu clad cavities is similar to that of bulk Nb cavities. The highest accelerating gradient achieved was Eacc = 40MV/m with a Qo-value of ~ 10 10 after ca.180μm BCP, annealing at 800°C and baking at 140°C for 30 hours. The degradation of the quality factor Qo after “quenches” of the cavity is moderate; after ca. 150 quenches it reaches the saturation point of Qo = 1.4 10 10 at low field. This indicates that on the basis of RF performance and material costs the combination of hydroforming with tube cladding is a very promising option.

Abstract Three electropolished one-cell cavities were,measured before and after in-situ bakeout under ultra-high vacuum conditions. Before bakeout the cavities showed,a strong reduction in quality factor at fields above 25 MV/m. After the bakeout,the Q drop,was,no,longer present and gradients of up to 39 MV/m were achieved. This indicates that electropolishing yields highest accelerating gradients in niobium,cavities only in combination,with in-situ bakeout. 1 ELECTROPOLISHING OF NIOBIUM

The recent multipacting observations for 9-cell Tesla cavities are presented. Investigated cavities were prepared by BCP (buffered chemical polishing) or EP (electropolishing) after 800 C or 1350 C furnace treatment. An influence of low temperature heat treatment (120-140 C) on multipacting process for EP cavities is analysed. An attempt to correlate multipacting in 9-cell Tesla cavities with method of preparation, a chemical and thermal treatment is done.

Cavity production for the European XFEL was recently started with first niobium sheets arriving. From this stage to the accelerating module being ready for the linac installation, many critical RF measurements are necessary. During the mechanical cavity fabrication the cavity half-cells, dumb-bells and end-groups are measured and sorted. The cavity spectrum and field profiles are measured and tuned. The HOM (Higher Order Modes) couplers filter tuning, vertical cavity RF tests, cavity checks during the string assembly and final cavity performance measurements in the module as well as the fundamental mode and HOM RF spectra measurements complete the sequence. We present the procedures of the RF measurements and discuss the first results for the XFEL prototype modules with special attention for the cavity tuning.

Few cavities of the 4th and 6th cavity generationtreated accordingly the XFEL recipe have shown performance of ca. 15 MV/m caused by thermal break down without field emission. Effort to post purify some cavities with titanium, that was successfully applied for FLASH cavities, did not improve the performance. The T-map analysis detected the quenchareas mainlyclose to the equator. Optical controlby high resolution camera and non-destructive X-Ray radiography have been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to get more detailed information of defects some samples have been extracted from cavity and investigated by light microscope, SEM, EDX and Auger spectroscopy. The detected distinctions are discussed