INFN joined the international effort for the PIP-II project at Fermilab and it's going to contribute to the low-beta section of the PIP-II proton linac. In particular, INFN-LASA is finalizing its commitment to deliver in kind the full set of the LB650 cavities, namely 36 plus spares 5-cell cavities at 650 MHz and geometrical beta 0.61. All cavities, designed by INFN-LASA, will be produced and surface treated in industry, qualified through vertical cold test, and delivered as ready for string installation. This paper reports the status of INFN's contribution to PIP-II and of ongoing activities toward the experimental qualifications of infrastructures and prototypes. INFN-LASA CONTRIBUTION The Fermilab Proton Improvement Plan II (PIP-II) Linac [1] is designed to deliver a 1.2 MW Hbeam upgradable to multi-MW to enable LBNF and DUNE neutrino physics projects. The 800 MeV beam will be injected into the upgraded Booster Ring via a linac-to-booster transfer line and it will then proceed to the Main Injector Ring. The PIP-II linac features a flexible time structure for its 0.55 ms beam pulse in order to satisfy different experimental needs, with RF repetition rate spanning from 20 Hz pulsed to continuous-wave (CW). A key section of the linac is the 650 MHz superconducting part with geometric beta factor of 0.61 (LB650) that currently encloses 36 five-cell elliptical cavities in 9 cryomodules, accelerating beam from 177 MeV to 516 MeV. Target cavity accelerating gradient is set at 16.9 MV/m with a quality factor > 2.4 1010, an unprecedented working point for this type of resonators. INFN-LASA firstly provided a novel electromagnetic and mechanical design for the LB650 cavities [2], fully compatible to the performances and technical interfaces posed by the project as well with beam pipes and flanges, power coupler, helium tank, tuner. On December 4th, 2018, the U.S. Department of Energy (DOE) and Italy’s Ministry of Education, Universities and Research (MIUR) signed an agreement to collaborate on the development and production of technical components for PIP-II. Following this milestone, INFN-LASA is finalizing the layout of its in-kind contribution aiming to cover the needs of the LB650 section of the linac, namely:  Grand total of 40 SC cavities (36 plus 2 spares, and 2 initial prototypes) delivered as ready for string assembly, equipping a total of 9 cryomodules.  Qualification via vertical cold-test provided by INFN either through the LASA test stand or through a qualified cold-testing partner infrastructure.  Compliance to the PIP-II Technical Review Plan, the procedure issued by DOE and Fermilab in order to meet PIP-II technical, schedule and budget commitments. PIP-II LB650 CHALLENGES A successful cavity design is the result of an interplay of multiple state-of-the-art competences existing at INFNLASA in electromagnetic, mechanical and technical domains [3]. PIP-II LB650 cavities are themselves among the key scientifical challenges of the whole project, requiring:  An unprecedented quality factor for these resonators, e. g. more than four times higher than that of ESS cavities at a similar gradient.  Assessment of High-Order Modes (HOMs) risks so that neither instabilities nor additional cryogenic losses pose critical issues.  Deep understanding of Lorentz Force detuning, pressure sensitivity and mechanical leading parameters as rigidities, yield limits, stresses. PIP-II operational scenario is actually an uncharted territory in terms of cavity detuning control, especially in view of foreseen pulsed operation of these high loaded-Q cavities.  Mutual compliancy to both European (PED) and U.S. (ASME) pressure vessels codes. R&D AND PROTOTYPE ACTIVITIES In total, seven PIP-II LB650 prototype cavities have been produced counting both single and multi-cell, and three of them are shared with Fermilab since early 2020 for a joint development effort. One of single-cell (B61S_EZ_002) has been surface treated and prepared for vertical test by INFN at Zanon Research and Innovation company [4] and used to put to test a baseline recipe (150 m bulk EP + 800 °C Heat Treatment + final cold EP + 120 °C) together with the upgraded Electro-Polishing plant and with the dedicated process diagnostic [5]. A second INFN single-cell cavity (B61S_EZ_001) has been surface treated and prepared by Fermilab following the state-of-the-art High-Q recipe (180 m bulk EP + 900 °C Heat Treatment + Nitrogen-Doping + final cold EP). In its last cold-test in Fermilab VTS infrastructure this resonator performed excellently and exceeded project requirements [6]. ___________________________________________ * rocco.paparella@mi.infn.it 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-THPAB348 THPAB348 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I 4474 MC7: Accelerator Technology T07 Superconducting RF Plot presented in Fig. 1 reports and compares the quality factor Q versus the accelerating gradient Eacc for both single-cell vertical tests measurements, at LASA and Fermilab VTS. An analytical extrapolation is also shown for the B61S_EZ_002 data set in order to better compare the two different cavity environment during superconducting transition. At Fermilab VTS in fact residual magnetic field is actively tied close to zero by means of Helmholtz coils, flux-gate sensors and a regulating loop [7] while at LASA its value is minimized by passive shielding only and about 8 mG are left in the cavity region. In addition, current cryogenic setup at LASA only allows for a small temperature time rate across transition, about 1 K/min, therefore the reported residual field could be assumed as completely trapped. Figure 1: Q-vs-Eacc results for the two INFN LB650 single cell cavities, compared to PIP-II specifications. More details on the performances of the cavity tested at LASA are reported in [5], together with a description of the assumptions and the procedure for the rescaling of Q-vs-Eacc at zero external magnetic field. Following initial development activities on single cells, two fully compliant, 5-cells LB650 prototype cavities (Fig. 2) have been manufactured by INFN in early 2020 to complete the qualification process for the cavity package. Figure 2: INFN B61_EZ_001 LB650 cavity for PIP-II. An intense development effort is currently ongoing at both INFN and Fermilab premises, sharing one LB650 cavity each, and both prototypes are expected to be qualified via vertical tests at their respective test facilities within few weeks from now [6]. Specifically, for the one to be qualified at LASA, the Electro-Polishing plant at Zanon Research and Innovation is being upgraded and qualified. The LB650 cavity, once treated, will be then immediately prepared for the vertical test, most probably with the baseline recipe used for the single-cell. Innovative diagnostic for the EP process is being developed at INFN and details are reported in [5]. Fluid-Dynamics Test Bench Uncertainties in the solution of fluid simulations are not negligible due to the complex geometry of a SRF cavity and thus without an experimental validation, results from this type of simulations cannot be confidently used to improve the process. To this aim an experimental study has been started at INFN, in the framework of the activities on LB650 prototypes for PIP-II, to investigate the fluid-dynamics of the chemical etching process by means of Particle Image Velocimetry (PIV) technique on a hollow, scaled and transparent model of a LB650 cavity die manufactured in a silicon elastomer. Introduction to this subject and preliminary results are reported in [8]. LASA INFRASTRUCTURE UPGRADE LASA cavity vertical cold-test infrastructure is being upgraded in view of the LB650 cavities qualification and in order to align to state-of-the-art test facilities in the PIP-II collaboration. Inner Magnetic Shielding An inner, cylindrical magnetic shield in cryoperm© has been realized and installed in order to further reduce the residual magnetic field in the cavity region of the test cryostat (Fig. 3). Figure 3: Cryoperm© magnetic shielding in place inside the vertical testing cryostat. A second, external shield at room temperature is placed (not visible here) around the cryostat. 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-THPAB348 MC7: Accelerator Technology T07 Superconducting RF THPAB348 4475 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created. The first operation of the European X-ray free-electron laser facility accelerator based on superconducting technology is reported. The maximum electron energy is 17.5 GeV. A laser average power of 6 W is achieved at a photon energy of 9.3 keV.

The 3.9 GHz cryomodule and RF system for the XFEL Injector is being assembled and delivered to the underground building in summer 2015, for the injector commissioning in Fall 2015. This contribution outlines the status of the activity and reports the preparation stages of the technical commissioning of the system.

A post is an assembly of a low thermal conduction composite material pipe (fiberglass pipe) and some shrink-fit aluminium and steel discs and rings, designed to provide a mechanical support and a thermal insulation to the cold mass of the long cryomodules of the TTF, which are foreseen also for the XFEL and ILC. We review here the production, testing and qualification for the production of post supports, which have been successfully provided for the cryomodules of the TTF in DESY, the STF in KEK and ILCTA in FNAL.

TRASCO (the acronym for TRAsmutazione di SCOrie, which stands for waste transmutation) is a joint INFN/ENEA program aiming at the design and the technological investigation of the main components of an accelerator driven system (ADS) for nuclear waste transmutation. We discuss here the status of the R&D activities on the high current proton accelerator, under INFN responsibility. PACS.: 29.17.+w, 29.27.-a Published by SIS–Pubblicazioni Laboratori Nazionali di Frascati

The Tera-eV Energy Superconducting Linear Accelerator (TESLA), a 32 km long superconducting linear electron/positron collider of 500 GeV (upgradeable to 800 GeV) centre of mass energy, presently in the planning phase at DESY, will consist of about 21000 superconducting RF 9-cell cavities of pure Niobium. Each cavity of about 1 m length has to be cooled in a 2.0 K helium bath and operated at 1.3 GHz with 5 Hz beam repetition rate. The cavities, equipped with their individual helium vessels made from titanium, RF input couplers , tuners and magnetic shielding, will be assembled in groups of 12 in about 16 m long cryostats (cryomodules) with thermal radiation shielding at 5-8 K and 40-80 K. Part of the cryomodules will also be equipped with superconducting quadrupoles and steering coils. About 10% of the cryomodules will contain cryogenic valves and instrumentation required for cooling purposes. The paper presents design, construction and assembly details of the cryomodules, which are based on the experience with somewhat shorter cryomodules, already in use at the TESLA Test Facility (TTF) at DESY.

The experience gained in the commissioning and operation of the first and second generation of the TTF cryostat lead to a new and improved design which should fit the requirements of the TESLA collider in terms of cost and performance. The redistribution of the components in the cryostat cross-section allows us a reduction of 15% the vacuum vessel diameter and the use of a standard 38 pipe. The thermal shields have been adapted to fit the new vacuum vessel, while the finger-welding technique has become a standard. A more stable quadrupole package position (in spite of the possible asymmetrical forces acting on the Helium Gas return Pipe (HeGRP) edges during pumping and cooldown) has been obtained by moving the three post positions. To allow the possible use of rigid couplers and superstructures, a sliding support scheme has been developed for the cavities. In connection with a reference Invar bar it lets the cavities stay fixed and aligned while the HeGRP slides over them during cooldown and warmup.

One of the principal goals of the ongoing TTF project is to produce prototype low cost and reliable cryomodules meeting the stringent requirements for TESLA. The first cryomodule was assembled and tested at DESY. The experience gained during the design and the commissioning suggested new solutions to improving the technical design.The most important improvement is the design of the new thermal shields at 4.5 K and 70 K. In this design the cooling helium pipes are directly welded to the aluminum shields. This solution simplified the technical design of the entire cryomodule and indicated some issues that needed study to verify the behavior of the new structure during cooldown and operation. The principal aspect studied has been the procedures necessary to avoid stresses and deformations produced during the cooldown which might damage the shields. The solution found also prevents the typical deformation induced by the welding and makes the production pre-alignment easier. Shields will be welded through small aluminum fingers that reduce the resistance section of the joint, while increasing the elasticity, unloading the welded joints and preventing possible damage during the cooldown.

The European XFEL will use a superconducting third harmonic section to achieve the necessary beam quality for the FEL process. The concept has been successfully proven at the FLASH linac at DESY, with a 4 cavity superconducting module built by FNAL. The design of the third harmonic system at the XFEL injector is being finalized and prototypes of the components (cavities and couplers) have been fabricated and are currently in the testing stage. The paper will provide a status of the activities.

The Tera-eV Energy Superconducting Linear Accelerator (TESLA), a 32 km long superconducting linear electron/positron collider of 500 GeV (upgradeable to 800 GeV) centre of mass energy, presently in the planning phase at DESY, will consist of about 21000 superconducting RF 9-cell cavities of pure Niobium. Each cavity of about 1 m length has to be cooled in a 2.0 K helium bath and operated at 1.3 GHz with 5 Hz beam repetition rate. The cavities, equipped with their individual helium vessels made from titanium, RF input couplers , tuners and magnetic shielding, will be assembled in groups of 12 in about 16 m long cryostats (cryomodules) with thermal radiation shielding at 5-8 K and 40-80 K. Part of the cryomodules will also be equipped with superconducting quadrupoles and steering coils. About 10% of the cryomodules will contain cryogenic valves and instrumentation required for cooling purposes. The paper presents design, construction and assembly details of the cryomodules, which are based on the experience with somewhat shorter cryomodules, already in use at the TESLA Test Facility (TTF) at DESY.