Nb3Sn Targets Synthesis via Liquid Tin Diffusion for Thin Films Depositions

The deposition of superconducting Nb3 Sn on copper accelerating cavities is interesting for the higher thermal conductivity of copper compared to common Nb substrates. The better heat exchange would allow the use of cryocoolers reducing cryogenic costs and the risk of thermal quench [1]. The magnetron sputtering technology allows the deposition of Nb 3 Sn on substrates different than Nb, however the coating of substrates with complex geometry (such as elliptical cavities) may require target with non-planar shape, which are difficult to realize with classic powder sintering techniques. In this work, the possibility of using the Liquid Tin Diffusion (LTD) technique to produce sputtering targets is explored. The LTD technique is a wire fabrication technology, already developed in the past at LNL for superconducting radio frequency (SRF) applications [2], that allows the deposition of very thick and uniform coating on Nb substrates even with complex geometries [3]. Improvements in LTD process, proof of concept of a single use LTD target production, and characterization of the Nb 3 Sn film coated by DC magnetron sputtering with these innovative targets are reported in this work

Plasma Electrolytic Polishing and Vibrotumbling as innovative surface treatments for Superconducting Radiofrequency cavities

Superconducting radio frequency (SRF) cavities performances strongly depend on the surface preparation. For that reason, surface treatments represent an important aspect in the development of future accelerators. The state of art of the Nb cavity production relies on 60-years’ experience on superconducting radiofrequency (SRF) cavities field dealing with this material. Nb is a type II superconductor and high critical temperature natural material. A copper substrate may be used for deposition of a superconducting (SC) layer to reduce costs and improve mechanical stability comparing to the bulk niobium technology. The conventional protocol of substrates surface preparation includes chemical and electrochemical polishing techniques, and may include as well mechanical processing, such as chemical barrier polishing (CBP) and grinding. Harsh and corrosive solutions are typically used for chemical preparation: concentrated HF and H2SO4 acids for Nb, and H3PO4 with Butanol mixtures for Cu. Vibro-tumbling (VT) technique is a variation of the vibratory and mass-finishing process. Usually, the systems are composed of an eccentric motor that provides the vibration motion at a certain frequency. The rotation of the cavity with the abrasive media and vibration allows uniform abrasion of the internal surface given by a step motor. A VT system has been refurbished and updated. A series of experiments were conducted on both Cu and Nb prototype cavities varying frequency, abrasive, media, volume. A study was done on the two types of substrates: elliptical 6 GHz cavities and coupons, placed in the dummy 6 GHz cavities. A two-step protocol was developed, that includes erosion step with the peak rate of 24 μm/min and a finishing step with coconut shells abrasive. Surface morphology and roughness, erosion rates and other parameters were studied, and a final protocol was implemented into the continuous workflow of the National Legnaro Laboratories Cu 6 GHz cavity production. In this dissertation, an innovative treatment – Plasma Electrolytic Polishing (PEP) has been studied to substitute the conventional treatments and possibly eliminate the need of mechanical preparation. Plasma electrolytic polishing is an evolution of electropolishing. The PEP uses diluted water-based salt solutions, able to achieve low roughness values of ≤100 nm. The smoothing of the surface is superior comparing to the standard treatments at the parity of the removal thickness. This is possible due to the different working regime, that involves high voltage DC anodic polarisation, high temperature of the bath and the formation of uniform stable vapor-gas layer (VGL) over the anode surface, allowing the ignition of the plasma. The stable VGL and plasma discharges allow the levelling of the surface. There is no common theory on the mechanism of the PEP technique. In this research work, 4 unique solutions were developed for Nb and Cu PEP processing. The process parameters behaviour was characterised by parameters such as voltage, temperature, and solution composition. Surface morphology and roughness, erosion rates, current efficiency were obtained and discussed. Experiments were conducted on the planar samples, and more complex geometries like dummy 6 GHz cavities, Quadrupole resonator (QPR) samples. 3d printed coupons were studied as well, and polished with PEP. The planar samples were a key tool for preliminary study of the PEP process and later for solutions optimisation and characterisation of final surface quality (linear profilometer, optical and electron microscopy). Dummy 6 GHz cavities substrates were used to simulate the elliptical geometry of the cavities during the PEP process and to analyse the possibility of PEP application in cavity preparation. QPR sample is a test instrument for characterisation of surface resistance with accuracy in the nano-ohm range. Thus, the polishing of QPR samples by PEP were done for future comparison with conventional treatments.Superconducting radio frequency (SRF) cavities performances strongly depend on the surface preparation. For that reason, surface treatments represent an important aspect in the development of future accelerators. The state of art of the Nb cavity production relies on 60-years’ experience on superconducting radiofrequency (SRF) cavities field dealing with this material. Nb is a type II superconductor and high critical temperature natural material. A copper substrate may be used for deposition of a superconducting (SC) layer to reduce costs and improve mechanical stability comparing to the bulk niobium technology. The conventional protocol of substrates surface preparation includes chemical and electrochemical polishing techniques, and may include as well mechanical processing, such as chemical barrier polishing (CBP) and grinding. Harsh and corrosive solutions are typically used for chemical preparation: concentrated HF and H2SO4 acids for Nb, and H3PO4 with Butanol mixtures for Cu. Vibro-tumbling (VT) technique is a variation of the vibratory and mass-finishing process. Usually, the systems are composed of an eccentric motor that provides the vibration motion at a certain frequency. The rotation of the cavity with the abrasive media and vibration allows uniform abrasion of the internal surface given by a step motor. A VT system has been refurbished and updated. A series of experiments were conducted on both Cu and Nb prototype cavities varying frequency, abrasive, media, volume. A study was done on the two types of substrates: elliptical 6 GHz cavities and coupons, placed in the dummy 6 GHz cavities. A two-step protocol was developed, that includes erosion step with the peak rate of 24 μm/min and a finishing step with coconut shells abrasive. Surface morphology and roughness, erosion rates and other parameters were studied, and a final protocol was implemented into the continuous workflow of the National Legnaro Laboratories Cu 6 GHz cavity production. In this dissertation, an innovative treatment – Plasma Electrolytic Polishing (PEP) has been studied to substitute the conventional treatments and possibly eliminate the need of mechanical preparation. Plasma electrolytic polishing is an evolution of electropolishing. The PEP uses diluted water-based salt solutions, able to achieve low roughness values of ≤100 nm. The smoothing of the surface is superior comparing to the standard treatments at the parity of the removal thickness. This is possible due to the different working regime, that involves high voltage DC anodic polarisation, high temperature of the bath and the formation of uniform stable vapor-gas layer (VGL) over the anode surface, allowing the ignition of the plasma. The stable VGL and plasma discharges allow the levelling of the surface. There is no common theory on the mechanism of the PEP technique. In this research work, 4 unique solutions were developed for Nb and Cu PEP processing. The process parameters behaviour was characterised by parameters such as voltage, temperature, and solution composition. Surface morphology and roughness, erosion rates, current efficiency were obtained and discussed. Experiments were conducted on the planar samples, and more complex geometries like dummy 6 GHz cavities, Quadrupole resonator (QPR) samples. 3d printed coupons were studied as well, and polished with PEP. The planar samples were a key tool for preliminary study of the PEP process and later for solutions optimisation and characterisation of final surface quality (linear profilometer, optical and electron microscopy). Dummy 6 GHz cavities substrates were used to simulate the elliptical geometry of the cavities during the PEP process and to analyse the possibility of PEP application in cavity preparation. QPR sample is a test instrument for characterisation of surface resistance with accuracy in the nano-ohm range. Thus, the polishing of QPR samples by PEP were done for future comparison with conventional treatments

Current Status of the ALPI Linac Upgrade for the SPES Facilities at INFN LNL

The SPES project is based at INFN LNL and covers basic research in nuclear physics, radionuclide production, materials science research, nuclear technology and medicine. The Radioactive Ion Beam (RIB) produced by SPES will be accelerated by ALPI, which is a linear accelerator, equipped with superconducting quarter wave resonators (QWRs) and operating at LNL since 1990. For RIB acceleration it is mandatory to perform an upgrade of ALPI which consists of the implementation of two additional cryostats, containing 4 accelerating cavities each, in the high-ß section. The QWRs production technology is well established. The production technology of Nb/Cu QWRs should be adjusted for high-ß cavities production. In the framework of the upgrade, several vacuum systems were refurbished, optimal parameters of the biased sputtering processes of copper QWR cavities and plates were defined. The process of mechanical and chemical preparation, sputtering and cryogenic measurement of the high-ß Nb/Cu QWR cavities were adjusted. Several QWR cavities were already produced and measured. Currently, the production of the Nb/Cu sputtered QWR cavities and plates is ongoing

Smoothening of the down-skin regions of copper components produced via Laser Powder Bed Fusion technology

Additive manufacturing (AM) is revolutionizing the industrial scenario. Four copper samples have been printed via Laser Powder Bed Fusion (LPBF) at DIAM Laboratory (INFN—Sezione di Padova, Padova, Italy). Samples had different geometrical characteristics, to test the feasibility of the AM as a productive technique for the creation of unsupported copper structures that are characterized by surfaces with a very small inclination angle, where supports cannot be placed. Parts have been printed successfully even in case of 18° of inclination of unsupported walls with respect to the horizontal plane, and on the same samples, surface finishing treatments (performed by Rösler Italiana S.r.l. and INFN-LNL) have been performed to reduce the roughness of the down-facing surfaces. Indeed, the down-skin regions are the most critical areas of AM parts. Several surface treatments are under investigation: mass-finishing treatments (mechanical and chemically assisted mechanical processes), chemical polishing, and electropolishing, and for some of them, the results are extremely positive: from an initial roughness (Ra) of 30–35 µm, the treatments allowed us to achieve a Ra value lower than 1 µm. The study here exposed presents a good way to rapidly reduce the roughness of 3D-printed parts, reaching a mirror-like aspect

Smoothening of the down-skin regions of copper components produced via Laser Powder Bed Fusion technology

Additive manufacturing (AM) is revolutionizing the industrial scenario. Four copper samples have been printed via Laser Powder Bed Fusion (LPBF) at DIAM Laboratory (INFN - Sezione di Padova, Padova, Italy). Samples had different geometrical characteristics, to test the feasibility of the AM as a productive technique for the creation of unsupported copper structures that are characterized by surfaces with a very small inclination angle, where supports cannot be placed. Parts have been printed successfully even in case of 18° of inclination of unsupported walls with respect to the horizontal plane, and on the same samples, surface finishing treatments (performed by Rösler Italiana S.r.l. and INFN-LNL) have been performed to reduce the roughness of the down-facing surfaces. Indeed, the down-skin regions are the most critical areas of AM parts. Several surface treatments are under investigation: mass-finishing treatments (mechanical and chemically assisted mechanical processes), chemical polishing and electropolishing, and for some of them, the results are extremely positive: from an initial roughness (Ra) of 30-35 μm, the treatments allowed us to achieve a Ra value lower than 1 μm. The study here exposed presents a good way to rapidly reduce the roughness of 3D-printed parts, reaching a mirror-like aspect

Nb3Sn Targets Synthesis via Liquid Tin Diffusion for Thin Films Depositions

The deposition of superconducting Nb3 Sn on copper accelerating cavities is interesting for the higher thermal conductivity of copper compared to common Nb substrates. The better heat exchange would allow the use of cryocoolers reducing cryogenic costs and the risk of thermal quench [1]. The magnetron sputtering technology allows the deposition of Nb 3 Sn on substrates different than Nb, however the coating of substrates with complex geometry (such as elliptical cavities) may require target with non-planar shape, which are difficult to realize with classic powder sintering techniques. In this work, the possibility of using the Liquid Tin Diffusion (LTD) technique to produce sputtering targets is explored. The LTD technique is a wire fabrication technology, already developed in the past at LNL for superconducting radio frequency (SRF) applications [2], that allows the deposition of very thick and uniform coating on Nb substrates even with complex geometries [3]. Improvements in LTD process, proof of concept of a single use LTD target production, and characterization of the Nb 3 Sn film coated by DC magnetron sputtering with these innovative targets are reported in this work

Nb3Sn Targets Synthesis via Liquid Tin Diffusion for Thin Films Depositions

The deposition of superconducting Nb3 Sn on copper accelerating cavities is interesting for the higher thermal conductivity of copper compared to common Nb substrates. The better heat exchange would allow the use of cryocoolers reducing cryogenic costs and the risk of thermal quench [1]. The magnetron sputtering technology allows the deposition of Nb 3 Sn on substrates different than Nb, however the coating of substrates with complex geometry (such as elliptical cavities) may require target with non-planar shape, which are difficult to realize with classic powder sintering techniques. In this work, the possibility of using the Liquid Tin Diffusion (LTD) technique to produce sputtering targets is explored. The LTD technique is a wire fabrication technology, already developed in the past at LNL for superconducting radio frequency (SRF) applications [2], that allows the deposition of very thick and uniform coating on Nb substrates even with complex geometries [3]. Improvements in LTD process, proof of concept of a single use LTD target production, and characterization of the Nb 3 Sn film coated by DC magnetron sputtering with these innovative targets are reported in this work

RF Characterisation of Bulk Niobium and Thin Film Coated Planar Samples at 7.8 GHz

International audienceResearch is ongoing into the use of superconducting thin films to replace bulk niobium for future radio frequency (RF) cavities. A key part of this research requires measuring the RF properties of candidate films. However, coating and testing thin films on full-sized cavities is both costly and time-consuming. Instead, films are typically deposited on small, flat samples and characterised using a test cavity. A cost-effective facility for testing such samples has recently been built and commissioned at Daresbury Laboratory. The facility allows for low power surface resistance measurements at a resonant frequency of 7.8 GHz, temperatures down to 4 K and sample surface magnetic fields up to 1 mT. A brief overview of this facility as well as recent results from measurements of both bulk Nb and thin film coated samples will be presented

Thick Film Morphology and SC Characterizations of 6 GHz Nb/Cu Cavities

Thick films deposited in long pulse DCMS mode onto 6 GHz copper cavities have demonstrated the mitigation of the Q-slope at low accelerating fields. The Nb thick films (~40 microns) show the possibility to reproduce the bulk niobium superconducting properties and morpholo-gy characterizations exhibited dense and void-free films that are encouraging for the scaling of the process to 1.3 GHz cavities. In this work a full characterization of thick films by DC magnetometry, computer tomography, SEM and RF characterizations are presented

Magnetic Field Penetration of Niobium Thin Films Produced by the ARIES Collaboration

Superconducting (SC) thin film coatings on Cu substrates are already widely used as an alternative to bulk Nb SRF structures. Using Cu allows improved thermal stability compared to Nb due to having a greater thermal conductivity. Niobium thin film coatings also reduce the amount of Nb required to produce a cavity. The performance of thin film Nb cavities is not as good as bulk Nb cavities. The H2020 ARIES WP15 collaboration studied the impact of substrate polishing and the effect produced on Nb thin film depositions. Multiple samples were produced from Cu and polished with various techniques. The polished Cu substrates were then coated with a Nb film at partner institutions. These samples were characterised with surface characterisation techniques for film morphology and structure. The SC properties were studied with 2 DC techniques, a vibrating sample magnetometer (VSM) and a magnetic field penetration (MFP) facility. The results conclude that both chemical polishing and electropolishing produce the best DC properties in the MFP facility. A comparison between the VSM and the MFP facility can be made for 10 $\mu$m thick samples, but not for 3 $\mu$m thick samples