Copper RF Cavity Research Articles

Niobium Coating of Cavities Using Cathodic Arc

Niobium thin film coated copper RF cavities are an interesting alternative to niobium bulk cavities for the development of high performance superconducting accelerators. The main limiting factor in their use is the degradation of the quality factor Q with increasing accelerating field (the ldquoQ-Sloperdquo). To try and overcome this limitation, we have developed an alternative coating technique based on a Cathodic Arc system working under UHV conditions (UHVCA). High quality Nb samples have been synthesized under different deposition angles and their characteristics are presented. The UHVCA technique has been used to deposit 1.3 GHz TESLA-type single cell cavities. To further improve cavity performance the first critical field has to be enhanced. The use of multilayers consisting of alternating insulating and superconducting layers may produce the desired enhancement providing that the superconducting layer thickness is smaller than the London penetration depth. To this aim, we present also the experimental characterization of the superconducting properties (TC, BC1) of Nb/AlOx/Nb multilayers.

Quality measurement of niobium thin films for Nb/Cu superconducting RF cavities

Niobium-coated copper resonator cavities are an interesting alternative to bulk niobium ones for superconducting RF (SCRF) applications to particle accelerators. Magnetron sputtering is the technology developed at CERN for depositing niobium films and applied over the past 20 years. Unfortunately, the observed degradation of the quality factor with increasing cavity voltage, not completely understood, prevents their use in future large accelerators designed to work at gradients higher than 30 MV m−1 with quality factors in the 1010 (or higher) region. At the beginning of the new millennium, new deposition techniques were proposed to overcome the difficulties encountered with magnetron sputtering. This paper presents the main analysis applied to the niobium films to investigate their quality and the obtained results. The correlations between the niobium film properties and the RF performances of accelerating cavities are summarized. The properties of niobium films obtained by magnetron sputtering and by cathodic arc deposition in ultra-high vacuum (UHVCA) are compared. The UHVCA-produced Nb films have structural and transport properties closer to the bulk ones, providing a promising alternative for niobium coated, high voltage, high Q, copper RF cavities with respect to the standard magnetron sputtering technique.

A summary of high power cryogenic experiments with engineering model rf structures

We have recently completed a series of experiments on two high power cw cryogenic copper rf cavities, a drift tube linac (DTL), and a radio-frequency quadrapole (RFQ). The DTL was conditioned to a cw field level 160% of design, with a peak field of 46 MV/m. We believe that this is a record for cryo-copper cw performance. As a result of this experiment we conclude that routine cryo-copper cw DTL operation at 40 MV/m is achievable. The engineering model RFQ was conditioned to a peak field of 30 MV/m cw at cryogenic temperatures. The cavity had not been previously conditioned at room temperature and had seen a minimum of pulsed operation before cw conditioning. This result is extremely significant because it verifies that the room temperature portion of conditioning for cavities of this type may be bypassed. Frequency and quality factor were measured as a function of temperature during the experiments. The Q enhancement factor (QEF) (defined as Q (room temp.) Q (operating temp.) at an operating temperature of 31 K was found to be 3.46 for the RFQ and 5.1 for the DTL.

Profiling of carbon, oxygen and argon contamination in sputter-deposited films using nuclear backscattering and nuclear reaction spectroscopy

Carbon, oxygen and argon contaminations are known to play an important role in modifying the properties of surface coating films. We have profiled such elements using quantitative and non-destructive nuclear analysis techniques. Carbon and oxygen profiling is performed using nuclear backscattering of high-energy α-particles. Measurements of carbon and oxygen contamination on niobium-coated copper RF cavities used in high-energy accelerator technology are carried out. Oxygen and carbon sensitivity limits are respectively 10 15 and 5 × 10 15 atoms/cm 2 while surface depth resolutions are 30 and 25 μg/cm 2. Argon contamination profiling is performed using the 40Ar(p, γ) 41K resonant nuclear reaction at a proton incident energy of 1101.8 keV. Argon profiling in niobium films is studied: the depth resolution is around 5 μg/cm 2 while the sensitivity limit is 10 14 atoms/cm 2 within each depth resolution step.