IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 3500506 Imaging of the Surface Resistance of an SRF Cavity by Low-Temperature Laser Scanning Microscopy Gianluigi Ciovati, Steven M. Anlage, and Alexander V. Gurevich Abstract—Temperature mapping of the outer surface of a su- perconducting radio-frequency cavity is a technique that is often used to identify lossy areas on the cavity surface. In this contribu- tion, we present 2-D images of the superconducting state surface resistance R s of the inner surface of a superconducting radio- frequency (SRF) cavity obtained by low-temperature laser scan- ning microscopy. This technique, which is applied for the first time to study lossy regions in an operating SRF cavity, allows identify- ing “hotspots” with about one order of magnitude better spatial resolution (∼2 mm) than by thermometry. The R s -resolution is of the order of 1 μΩ at 3.3 GHz. Surface resistance maps with different laser power and optical images of the cavity surface are discussed in this contribution. It is also shown that the thermal gradient on the niobium surface created by the laser beam can move some of the hotspots, which are identified as locations of trapped bundle of fluxoids. The prospects for this microscope to identify defects that limit the performance of SRF cavities will also be discussed. Index Terms—Laser scanning microscopy, niobium, supercon- ducting accelerator cavities, surface resistance. I. I NTRODUCTION T EMPERATURE mapping of the outer surface of super- conducting radio-frequency (SRF) cavities immersed in either superfluid or subcooled liquid He has been the only technique to measure non-uniform RF losses occurring on the inner surface, where the superconducting current flows within the penetration depth (∼40 nm in Nb) [2], [3]. Some of the chal- lenges include the presence of ultra-high vacuum and strong electromagnetic fields inside the cavity, along with the needs to avoid particulate contamination of the inner surface and the fairly large surface areas involved, compared to coupon samples (the surface area of a single-cell 1.3 GHz cavity is ∼1 m 2 ). Low temperature laser scanning microscopy (LTLSM) is a well-known technique to characterize thin-film microwave res- onators made of high-temperature superconducting materials Manuscript received October 3, 2012; accepted December 6, 2012. Date of publication December 10, 2012; date of current version February 19, 2013. This manuscript has been authored by Jefferson Science Associates, LLC, under US DOE Contract DE-PS02-09ER09-05. The work at the University of Maryland was supported by DOE Grant DESC0004950 and ONR/AppEl Task D10, through Grant N000140911190. Additional support for this work was provided by the US Government Presidential Early Career Award for Scientists and Engineers. G. Ciovati is with Thomas Jefferson National Accelerator Facility, Newport News, VA 23606 USA (e-mail: gciovati@jlab.org). S. M. Anlage is with the Department of Physics, Center for Nanophysics and Advanced Materials, University of Maryland, College Park, MD 20742 USA (e-mail: anlage@umd.edu). A. V. Gurevich is with the Department of Physics, Old Dominion University, Norfolk, VA 23529 USA (e-mail: agurevic@odu.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2233253 [4], [5] and involves point-by-point raster scanning of a focused laser beam onto the surface of the sample, then measuring the photoresponse induced by the interaction of the light with the superconductor, as a function of the laser spot position (x, y). The same technique can be applied to map the surface resistance, R s , of a bulk cavity surface and an experimental apparatus was built at Jefferson Lab for such purpose. The apparatus is briefly described in Section II and in [1] in greater details. Hotspots have been located by conventional temper- ature mapping and 2D images of R s at these locations have been obtained by LTLSM for different parameters, such as laser power and modulation frequency. These results are shown in Section III. Magnetic flux due to residual Earth’s magnetic field inside the cryostat or to thermoelectric currents can be trapped in the niobium during cool-down below the critical temperature, T c . Trapped fluxoids are one among many possible causes of enhanced RF dissipation leading to hotspots [6]. The availabil- ity of a high-power laser on the LTLSM setup allows creation of a thermal gradient of intensity sufficient to de-pin fluxoids. Temperature maps taken before and after laser sweeping will be shown in Section IV. II. EXPERIMENTAL A. Experimental Setup The LTLSM is assembled on a vertical test stand, about 230 cm high and 81 cm in diameter, to be inserted in the cryostat. The optical components are mounted on an optical table bolted to the test stand’s top plate. The optical components include: a 10 W, 532 nm, continuous wave laser, a λ/2 wave- plate and a polarizer cube to adjust the laser power, a negative lens on a translational stage followed by a positive lens with long focal length to focus the beam at the cavity location, mirrors to keep the beam within the test stand perimeter and to bend the beam downward towards the cavity. By adjusting the position of the negative lens with respect to that of the fixed positive lens, the diameter of the laser beam at the cavity location can be adjusted between ∼0.9–3.0 mm. The laser beam travels inside a vacuum pipe, below the top plate, which is connected to a mirrors’ chamber, also under vac- uum (total pressure of about 2 × 10 −7 mbar), which houses two mini-stepper motors holding two flat laser mirrors positioned in such a way to allow x-y scanning of the beam. The SRF cavity is bolted at the bottom of the mirrors’ chamber and a UV-grade fused silica optical window separates the cavity volume from the mirrors’ chamber’s volume, to reduce particulate contamination of the cavity surface. 1051-8223/$31.00 © 2012 IEEE