dc magnetometry of niobium thin film superconductors deposited using high power impulse magnetron sputtering

We performed a systematic investigation of the dc magnetic properties of superconducting niobium thin films deposited by high power impulse magnetron sputtering (HiPIMS) as a function of the main deposition parameters: the temperature, T, of the heated substrate and the applied dc bias voltage, V, during the sputtering process. The measured dc magnetization curves between 0 and 1000 mT were used to calculate the relative volume of each sample into which the applied magnetic field had penetrated, ðΔV=VÞM. The sample deposited at 700°C with −80 V biased substrate exhibited the least penetration by the magnetic field. ðΔV=VÞM appeared to be highly dependent on the bias voltage at both room temperature and 500°C; however, a broad range of bias voltages showed comparatively similar results at increased temperatures of 700°C. Samples deposited at 700°C exhibit smaller upper critical fields, HC2, than samples deposited at room temperature and 500°C, with the lower temperatures exhibiting a greater dependency on the applied bias. The films deposited at 700°C also display a more stable magnetization curve suggesting that an enhanced flux pinning was achieved when compared to lower temperatures. Consequently, films with stable pinning were found to have the most repeatable dc magnetic behavior. Our results are particularly relevant to the superconducting radio-frequency accelerator scientific community where thin films have been suggested as a technology which may ultimately surpass the performance of bulk niobium. They are also relevant to the fundamental area of superconducting thin films and any applied area where thin films produced by HiPIMS are used, such as superconducting electronics

Reduction of secondary electron yield for E-cloud mitigation laser ablation surface engineering

Developing a surface with low Secondary Electron Yield (SEY) is one of the main ways of mitigating electron cloud and beam-induced electron multipacting in high-energy charged particle accelerators. In our previous publications, a low SEY < 0.9 for as-received metal surfaces modified by a nanosecond pulsed laser was reported. In this paper, the SEY of laser-treated blackened copper has been investigated as a function of different laser irradiation parameters. We explore and study the influence of micro- and nano-structures induced by laser surface treatment in air of copper samples as a function of various laser irradiation parameters such as peak power, laser wavelength (λ = 355 nm and 1064 nm), number of pulses per point (scan speed and repetition rate) and fluence, on the SEY. The surface chemical composition was determined by x-ray photoelectron spectroscopy (XPS) which revealed that heating resulted in diffusion of oxygen into the bulk and induced the transformation of CuO to sub-stoichiometric oxide. The surface topography was examined with high resolution scanning electron microscopy (HRSEM) which showed that the laser-treated surfaces are dominated by microstructure grooves and nanostructure features

DC magnetism of Niobium thin films

Niobium thin films were deposited onto a-plane sapphire with varying kinetic energy and varying substrate temperature. There were no consistent trends which related the particle energy or substrate temperature to RRR. The sample which displayed the largest RRR of 229 was then compared to both a thin film deposited with similar conditions onto copper substrate and to bulk niobium. DC magnetometry measurements suggest that the mechanism of flux entry into thin film niobium and bulk niobium may vary due to differences in the volumes of both defects and impurities located within the grains. Results also suggest that magnetic flux may penetrate thin films at small fields due to the sample geometry

Physical vapour deposition of thin films for use in superconducting RF cavities

The production of superconducting coatings for radio frequency cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. Optimizing superconducting properties of Nb thin-films is therefore essential. Nb films were deposited by magnetron sputtering in pulsed DC mode onto Si (100) and MgO (100) substrates and also by high impulse magnetron sputtering (HiPIMS) onto Si (100), MgO (100) and polycrystalline Cu. The films were characterised using scanning electron microscopy, x-ray diffraction and DC SQUID magnetometry

Low secondary electron yield of laser treated surfaces of copper, aluminium and stainless steel

Reduction of SEY was achieved by surface engineering through laser ablation with a laser operating at • = 355 nm. It was shown that the SEY can be reduced to near or below 1 on copper, aluminium and 316LN stainless steel. The laser treated surfaces show an increased surface resistance, with a wide variation in resistance found de-pending on the exact treatment details. However, a treated copper surface with similar surface resistance to aluminium was produced

Development of thin films for superconducting RF cavities

Superconducting coatings for superconducting radio frequency (SRF) cavities is an intensively developing field that should ultimately lead to acceleration gradients better than those obtained by bulk Nb RF cavities. ASTeC has built and developed experimental systems for superconducting thin-film deposition, surface analysis and measurement of Residual Resistivity Ratio (RRR). Nb thin-films were deposited by magnetron sputtering in DC or pulsed DC mode (100 to 350 kHz with 50% duty cycle) with powers ranging from 100 to 600 W at various temperatures ranging from room temperature to 800 °C on Si (100) substrates. The first results gave RRR in the range from 2 to 22 with a critical temperature Tc ≈ 9.5 K. Scanning electron microscopy (SEM), x-ray diffraction (XRD), electron back scattering diffraction (EBSD) and DC SQUID magnetometry revealed significant correlations between the film structure, morphology and superconducting properties

High power impulse magnetron sputtering of thin films for superconducting RF cavities

The production of superconducting coatings for radio frequency cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. The use of thin films made from superconductors with thermodynamic critical field, Hc>Hn/cb, allows the possibility of multilayer superconductor – insulator – superconductor (SIS) films and also accelerators that could operate at temperatures above the 2 K typically used. SIS films theoretically allow increased acceleration gradient due to magnetic shielding of underlying superconducting layers [1] and higher operating temperature can reduce cost [2]. High impulse magnetron sputtering (HiPIMS) and pulsed DC magnetron sputtering processes were used to deposit NbN and NbTiN thin films onto Si(100) substrate. The films were characterised using scanning electron microscopy (SEM), x-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and a four point probe

Physical vapour deposition of NbTiN thin films for superconducting RF cavities.

The production of superconducting coatings for radio frequency (RF) cavities is a rapidly developing field that should ultimately lead to acceleration gradients greater than those obtained by bulk Nb RF cavities. The use of thin films made from superconductors with thermodynamic critical field, Hc > HC(Nb), allows the possibility of multilayer superconductor – insulator – superconductor (SIS) films and accelerators that could operate at temperatures above 2 K. SIS films theoretically allow increased acceleration gradient due to magnetic shielding of underlying superconducting layers [1] and higher operating temperature can reduce cost [2]. High impulse magnetron sputtering (HiPIMS) and pulsed DC magnetron sputtering processes were used to deposit NbTiN thin films onto Si(100) substrate. The films were characterised using scanning electron microscopy (SEM), x-ray diffraction (XRD), Rutherford back-scattering spectroscopy (RBS) and a four-point probe

Test particle Monte Carlo simulation of NEG coated narrow tubular samples

The pumping properties of the NEG coated vacuum chambers play an important role in the efficiency of vacuum system of accelerators. The sticking probability of the NEG films is one the most important parameters to characterise the pumping properties of the NEG coated vacuum chambers. In order to investigate the NEG film sticking probability, Test Particle Monte-Carlo (TPMC) models were used. The models were based on the design of the installed experimental setup in ASTeC Vacuum Science group laboratory at Daresbury Laboratory (DL). The results of the simulations have been used for inter-preting the results of the measurements in the experimental setup.</p

Single metal Zirconium non-evaporable getter coating

Non-evaporable getter (NEG) coating has been used for years in many particle accelerator facilities due to its evenly distributed pumping speed, low thermal outgassing, and low photon and electron stimulated desorption yields. In our earlier work, it has been demonstrated that quaternary Ti–Zr-Hf-V coating deposited from an alloy target in a form of a rod has the lowest desorption yields, the highest sticking probability and sorption capacity. In this paper, a single element Zr target has been explored. This type of target is widely available and produced in a form of a wire that is easy to apply for a uniform coating of various shapes of vacuum chambers. Two samples coated with Zr film of dense and columnar structure were analysed and results of the pumping properties and electron stimulated desorption results are reported. The results show that pure Zr coating could be an economic solution, despite not being as effective as can be achieved with a quaternary NEG film. It shows that columnar Zr coating can be activated and reaches full pumping capacity at 160 °C. This is close to the activation temperature of Ti–Zr-Hf-V film and lower than that for the widely used ternary Ti–Zr–V film. Single metal Zr coatings demonstrated that it could be a more efficient and inexpensive way of producing the NEG-coated vacuum chambers.<br