Beta-srf-a new facility to characterize srf materials near fundamental limits

Demands of CW high-power LINAC require SRF cavities operating at the frontier of high accelerating gradient and low RF power dissipation, i.e. high quality factor (Q0). This requirement poses a challenge for standard surface treatment recipes of SRF cavities. In a recent breakthrough, elliptical SRF cavities doped with Nitrogen have been shown to improve Q0 by a factor of 3, close to the fundamental SRF limit. The fundamental mechanisms at microscopic level and optimum doping recipe, however, have still not fully been understood. Materials other than Nb have also been proposed for SRF cavities to overcome the fundamental limit already reached with Nitrogen doping, e.g. Nb3Sn, MgB2, and Nb-SIS multilayer. At TRIUMF, a unique experimental facility is currently being developed to address these issues. This facility will be able to probe local surface magnetic field in the order of the London Penetration Depth (several tens of nm) via β decay detection of a lowenergy radioactive ion-beam. This allows depth-resolution and layer-by-layer measurement of magnetic field shielding effectiveness of different SRF materials at high-parallel field (up to 200 mT). Design and current development of this facility will be presented here, as well as commissioning and future measurements strategies for new SRF materials. BACKGROUND AND MOTIVATION SRF (Superconducting RF) cavities are the backbone of modern LINACs (Linear Accelerators) due to the low power dissipation and high accelerating gradient. The base performance of SRF cavities can be characterized by theQ0 (Quality Factor) vs Eacc (accelerating gradient) curve. Higher Q0 reduces the cryogenic power consumption while larger Eacc (in the unit of MV/m) reduces the overall length of the LINACs. For CW (continuous wave) LINACs, Q0 plays a more important role while for pulsed high-gradient LINACs, Eacc is more crucial. Pushing the performance of SRF cavities has been made possible through decades of intensive research into the fabrication, assembly, surface processing, and metallurgy for over five decades [1]. Most of SRF cavities have been made with bulk Niobium (Nb). Recently the discoveries of Nitrogen-doping [2] and Nitrogen-infusion [3] of Nb SRF cavities have pushed its performance on both Q0 and Eacc close to its fundamental limit. Understanding the ∗ ethoeng@phas.ubc.ca 1 This is true in the case of N-infusion, while N-doped SRF cavities have higher Q0 at lower Eacc . mechanism of this doping effect require microscopic studies of the Niobium composition and superconducting properties (such as critical magnetic field of the Meissner state) at the nanometer scale. Active research in alternative materials such as Nb3Sn and MgB2 has also been pursused since the bulk Nb SRF cavities have almost reached the maximum performance. These alternative materials are based on thinfilms due to the fact that only several nm of the inner RF surface layer determine the performance of the SRF cavities. TRIUMF SRF group conducts both SRF cavities design & fabrication, and fundamental SRF studies. Currently there are two SRF LINACs, ISAC-II and ARIEL e-LINAC (electron LINAC), operating. TRIUMF SRF group also collaborates in the fabrication of both cavities and cryomodules for international projects such as RISP (South Korea) and VECC (India) rare-isotope projects. In the field of fundamental SRF, previous studies of N-doped Nb and novel thin film SRF materials have been done with μ-SR (muon spin resonance) technique [4] to measure the field of first magnetic flux entry. Nb, which is a type-II superconductor, is operated in the Meissner superconducting state in SRF cavities. Local magnetic probe techniques such as μ-SR can measure more accurately the fraction volume inside the superconductor which is not in the Meissner state. Therefore this technique is a powerful method to characterize the critical Meissner field that can be achieved. The higher the critical field of the Meissner state, the higher accelerating gradient that the SRF cavity can achieve. Recently, the TRIUMF SRF group has commissioned a RF induction furnace for doping and heat-treatment studies of both sample and test cavities. Two multi-mode coaxial test resonators (a quarter-wave and a half-wave resonator) have also been fabricated to study the doping and heat-treatment effects at lower frequency (low beta) cavities [5]. The purpose of this proceeding is to describe a new facility, β−SRF for the specific purpose of investigating thin-films and multi-layer SRF materials.