Thin superconducting niobium- coatings for RF accelerator cavities J. LANGNER, M.J. SADOWSKI, R. MIROWSKI, P. STRZYŻEWSKI AND J. WITKOWSKI The Andrzej Soltan Institute for Nuclear Studies (IPJ), Otwock-Swierk, Poland L. CATANI, A.CIANCHI, J. LORKIEWICZ, R. RUSSO AND S. TAZZARI University Tor Vergata and INFN, Via della Ricerca Scientifica 1, Roma 2, Italy D. PROCH DESY, Hamburg, Germany International Congress on Optics and Optoelectronics 28 August – 2 September 2005, Warsaw, Poland

OUTLINE - Introduction - Introduction - UHV arc devices - UHV arc devices - Formation of niobium films - Formation of niobium films - Properties of the deposited films - Properties of the deposited films - Filtering of micro-droplets - Filtering of micro-droplets - Coating of Cu cavities - Coating of Cu cavities - Summary - Summary

OUR AIM D eposition of thin, high quality superconducting films of pure niobium on the inner surface of RF cavities for particle accelerators D eposition of thin, high quality superconducting films of pure niobium on the inner surface of RF cavities for particle accelerators

SUPERCONDUCTING RF CAVITIES - mainly based on Nb bulk technology - mainly based on Nb bulk technology - early 90s- copper RF cavities coating with thin niobium film ( LEP 2 ) - early 90s- copper RF cavities coating with thin niobium film ( LEP 2 ) - so far UHV cylindrical magnetron sputtering technology - so far UHV cylindrical magnetron sputtering technology - in 2000 new approach UHV arc deposition, Italian INFN grant – project „ARCO” - in 2000 new approach UHV arc deposition, Italian INFN grant – project „ARCO” - since 2004 FP6 program „CARE’, contract number RII3- CT since 2004 FP6 program „CARE’, contract number RII3- CT

CARE- JRA1-SRF WP4 - Thin film cavity production Two work packages: WP4.1 - Linear cathode arc coating J.Langner WP4.2 - Planar cathode arc coating J.Langner WP4.2 - Planar cathode arc coating S.Tazzari S.Tazzari coordinated by M.J. Sadowski

MERITS of Nb/Cu CAVITIES Nb/Cu cavities offer several advantages such as: Nb/Cu cavities offer several advantages such as: - better mechanical stability, - better mechanical stability, - insensitivity to external magnetic fields, - insensitivity to external magnetic fields, - better thermal stability, - better thermal stability, - easier conditioning on the machine, - easier conditioning on the machine, - easier connection to the cryostat - easier connection to the cryostat - lower cost - lower cost

WHY CATHODIC ARC ? - no working gas - no working gas - ionized niobium - ionized niobium - high ion energy - high ion energy - excellent adhesion - excellent adhesion - high purity - high purity - possible to apply bias and magnetic fields - possible to apply bias and magnetic fields

WHY UHV CONDITIONS ? Purity of a deposition process plays the crucial role during formation of thin superconducting niobium films. Reaching UHV standard (below Torr) can cause the practical elimination of impurities, like H 2 O, nitrogen, oxygen, hydrocarbides, CO 2 from the vacuum chamber. Reaching UHV standard (below Torr) can cause the practical elimination of impurities, like H 2 O, nitrogen, oxygen, hydrocarbides, CO 2 from the vacuum chamber.

HV and UHV conditions Torr Torr

FIRST UHV SET-UP WITH PLANAR ARC SOURCE (ROME)

UHV SET-UP WITH PLANAR ARC SOURCE

UHV SET-UP WITH LINEAR ARC (IPJ)

TRIGGERING SYSTEM FOR UHV ARC The triggering of arc discharges creates often many problems. At the UHV conditions these problems are multiplied. The triggering system for UHV arc - must be infallible - must be infallible - cannot produce any impurities - cannot produce any impurities

LASER TRIGGERING Two Nd YAG lasers: - energy - 60mJ and 100mJ - pulse duration -5 ns - repetition rate 20 Hz V booster applied After testing many known triggering methods from the point of view of operational reliability and cleanliness, we have finally decided to use a laser beam focused upon the cathode through a vacuum-tight glass window.

Vacuum arc-based devices investigated in Rome and Swierk Device Cathode FilteringVacuumFilmLabTask PAUHV-2Planar-UHVNb, WRomesample deposition FPAUHV-1Planar+UHVNbRomesample deposition CCLAUHV -2 Linear-UHVNbSwierkCu cavity cell coating CCPAUHV -1 Planar-UHVNbRomeCu cavity cell coating TSFPAUH V Planar+UHVNb, Pb, Mg Swierkdroplet filtering tests TSFLAHVLinear+HVNbSwierk droplet filtering tests CPAUHVPlanar-UHV+N 2 NbN, TiNbN Romesample deposition

UHV LABORATORY IN ROME

UHV LABORATORY AT SWIERK

FORMATION OF NIOBIUM SUPERCONDUCTING FILMS substrats: sapphire and Cu substrats: sapphire and Cu -base pressure hPa -arc current A -bias 20 – 100 V -temperature C -deposition rate (planar arc) 10 nm/s

VACUUM CONDITIONS Behavior of the partial pressure (in ion-current units) of residual gases during the arc discharge. The downward slope is due to the pumping effect of a freshly deposited film

PROPERTIES OF THE DEPOSITED FILMS RRR The Nb Residual Resistivity Ratio (RRR defined as the Resistivity at room temperature divided by the Resistivity at 10K) is very sensitive to impurities. The Nb Residual Resistivity Ratio (RRR defined as the Resistivity at room temperature divided by the Resistivity at 10K) is very sensitive to impurities. Typical RRR values for Nb films deposited by sputtering at room temperature range from 2 to 10. RRR of arc deposited Nb films range from 20 to 50, while heating the substrate to ≈150 o C resulted in a record value of ≈80. Typical RRR values for Nb films deposited by sputtering at room temperature range from 2 to 10. RRR of arc deposited Nb films range from 20 to 50, while heating the substrate to ≈150 o C resulted in a record value of ≈80. We believe such a high value of RRR is obtained thanks to the absence of contaminating auxiliary gases and to the atomic-scale heating due to the high kinetic and potential energy of Nb ions impinging on the film surface which results in a local temperature higher than the average temperature reached by the substrate and recorded by thermometry. We believe such a high value of RRR is obtained thanks to the absence of contaminating auxiliary gases and to the atomic-scale heating due to the high kinetic and potential energy of Nb ions impinging on the film surface which results in a local temperature higher than the average temperature reached by the substrate and recorded by thermometry.

RESIDUAL RESISTIVITY RATIO RRR = 80 The transition to superconducting state for the thin niobium film deposited on sapphire

PROPERTIES OF THE DEPOSITED FILMS T C The critical temperature, Tc, of the deposited material is very sensitive to impurities. The critical temperature, Tc, of the deposited material is very sensitive to impurities. Tc, transition width ΔTc and surface current density (Jc) values of our best film samples have shown values identical, within the measurement error, to those of bulk Nb, i.e. Tc = ( ) K, ΔTc ≈ 0.01K and Jc = 3x10 7 A/cm2. The narrow transition width (<0.01K) is a strong indicator of uniform and clean film while the absolute Tc value indicates that stresses in arc-deposited films are lower than in magnetron sputtered ones.

CRITICAL TEMPERATURE (2002) Measured transition temperature curves of various arc deposited Nb film samples in 2002 and 2004

RF MEASURMENTS Q values as function of temperature

High Field RF measurements(6 GHz) High field RF measurements, at 6 GHz, were performed in Cornell on the four filtered, large Nb coated Cu substrates. The quality factor (Q) of the best sample was comparable within the errors to the present limit value of the host cavity of ≈3*10 8

SURFACE MORFOLOGY ( AFM ) Magnetron sputtering Arc deposition AFM pictures of niobium films deposited on supphire substrate. Nb grains in arc deposited film are visible and their avarege dimension is 200nm

SAMPLE MORFOLOGY SEM images of niobium samples deposited on sapphire substrate without and with the magnetic filter

MICRO-DROPLETS Micro-droplets distribution for 4 samples deposited at the same conditions for different times. The main difference is the amount of particles with a radius larger than the film thickness.

MAGNETIC FILTER FOR PLANAR CATHODE (2003)

MAGNETIC FILTER FOR PLANAR CATHODE (2005) I Aksenov-type magnetic filter

MAGNETIC FILTER FOR PLANAR CATHODE (2005) II T-type magnetic filter

MAGNETIC FILTER FOR CYLINDRICAL CATHODE (2005) General view of the cylindrical magnetic filter before installation. The cylindrical magnetic filter fixed upon the linear (cylindrical) cathode.

COATING OF CU CAVITIES (SWIERK) First coating of this single cell has just been performed without any micro- droplet filtering. Single-cavity has been equipped with appropriate UHV flanges, and after that it has been installed within the prepared facility. The coated single-cell has been cut along its symmetry axis in order to perform an analysis of inner surfaces.

COATING OF CU CAVITIES (ROME) Simulated distribution of magnetic field lines in „cusp” configuration used for plasma transport for deposition Set-up in „cusp” configuration

SUMMARY -very high RRR (T<50C), 80 (T≈200C) -less stressed and more homogeneous than standard Nb films produced by sputtering -larger grains (200 nm) -critical temperature,Tc = 9.25 K ±0.03 K -very narrow transitions widths ( ~ 0.01 K ) -RF properties ( Tc, Q ) are comparable with niobium bulk