With a growing effort in research and development of an alternative material to bulk Nb for a superconducting radiofrequency (SRF) cavity, it is important to have a cost effective method to benchmark new materials of choice. At Old Dominion University\u27s Center for Accelerator Science, a cryogenic probe station (CPS) will be used to measure the response of superconductor samples under RF fields. The setup consists of a closed-cycle refrigerator for cooling a sample wafer to a cryogenic temperature, a superconducting magnet providing a field parallel to the sample, and DC probes in addition to RF probes. The RF probes will extract a quality factor from a sample patterned in a coplanar waveguide resonator structure on a 2\u27 wafer. From the measured quality factor, the surface resistance and the penetration depth as a function of temperature and magnetic field will be calculated. This paper will discuss the design and measurement procedures of the current CPS setup

Ciovati, Gianluigi

Delayen, Jean R.

Gurevich, Alex

He, Yuan, (Ed.)

Li, Lu, (Ed.)

Makita, Junki

Schaa, Volker R.W., (Ed.)

Zhao, Ning, (Ed.)

English

Old Dominion University

Old Dominion University ODU Digital Commons Physics Faculty Publications Physics 2018 Cryogenic Probe Station at Old Dominion University Center for Accelerator Science Junki Makita Old Dominion University, jmaki002@odu.edu Jean R. Delayen Old Dominion University, jdelayen@odu.edu Alex Gurevich Old Dominion University, agurevic@odu.edu Gianluigi Ciovati Jefferson Lab, gciovati@odu.edu Volker R.W. Schaa (Ed.) See next page for additional authors Follow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubs  Part of the Electrical and Electronics Commons, and the Engineering Physics Commons Original Publication Citation Makita, J., Delayen, J., Gurevich, A., & Ciovati, G. (2018). Cryogenic probe station at Old Dominion University Center for Accelerator Science. In V.R.W. Schaa, Y. He, L. Li, N. Zhao (Eds.), Proceedings of the 18th International Conference on RF Superconductivity (128-130). Joint Accelerator Conferences Website. https://doi.org/10.18429/JACoW-SRF2017-MOPB035 This Conference Paper is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion in Physics Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact digitalcommons@odu.edu. Authors Junki Makita, Jean R. Delayen, Alex Gurevich, Gianluigi Ciovati, Volker R.W. Schaa (Ed.), Yuan He (Ed.), Lu Li (Ed.), and Ning Zhao (Ed.) This conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/638 CRYOGENIC PROBE STATION AT OLD DOMINION UNIVERSITYCENTER FOR ACCELERATOR SCIENCE∗J. Makita†, J. R. Delayen, A. Gurevich, Old Dominion University, Norfolk, USAG. Ciovati, Thomas Jefferson National Accelerator Facility, Newport News, USAAbstractWith a growing effort in research and development ofan alternative material to bulk Nb for a superconductingradiofrequency (SRF) cavity, it is important to have a costeffective method to benchmark new materials of choice. AtOld Dominion University’s Center for Accelerator Science,a cryogenic probe station (CPS) will be used to measurethe response of superconductor samples under RF fields.The setup consists of a closed-cycle refrigerator for coolinga sample wafer to the cryogenic temperature, a supercon-ducting magnet providing a field parallel to the sample, andDC probes in addition to RF probes. The RF probes willextract a quality factor from a sample patterned in a copla-nar waveguide resonator structure on a 2in wafer. Fromthe measured quality factor, the surface resistance and thepenetration depth as a function of temperature and magneticfield will be calculated. This paper will discuss the designand measurement procedures of the current CPS setup.INTRODUCTIONOver the decades of developmental efforts, there havebeen reported instances of Nb cavities operating at theirtheoretical limits. In order to meet the increasing demandsof future accelerator performance and cost requirements,significant efforts have been made into research and devel-opment of new materials alternative to a bulk Nb. Someof the new innovations include nitrogen doped and infusedniobium cavities [1–3], multi-layer coatings consisting ofthin dielectric layers sandwiched between superconductinglayers [4], a thin film niobium over copper [5], and cavitiesmade with other materials such as MgB2 and Nb3Sn [6, 7].This paper will discuss a closed-cycle cryogenic probestation at Old Dominion University that will be used as partof an R&D effort for benchmarking such new SRF materials.A sample will be fabricated into a planar transmission lineresonator, and the CPS will be used to extract superconduct-ing surface resistance and penetration depth under an RFfield with varying DC parallel magnetic field. The obtainedinformation would give insight into how the SRF cavitieswould perform using those new materials.EXPERIMENTAL SETUPSampleA sample that will be prepared for the measurementsare a series of superconducting half-wavelength coplanarwaveguide (CPW) resonators designed on top of a dielectric∗ Work supported by NSF Award PHY-1416051† jmaki002@odu.edusubstrate as shown in Fig 1. Because it is only necessaryto deposit the superconducting film on one side of the sub-strate, the CPW structure provides an affordable method fora sample fabrication compared to other transmission linestructures which requires a ground plane and a conductingstrip on both sides of the substrate. The CPW resonatorsare patterned by photolithography after a film is depositedonto a dielectric substrate. A single 2in wafer can containmultiple resonators given that they are spread enough so thatany cross coupling of power between adjacent resonators isnegligible.(a)(b)Figure 1: (a) A schematic diagram showing the top viewof half-wave CPW resonator capacitively coupled to theRF input and output and (b) the side view with the centerconductor width w, gap s, and a substrate with a dielectricconstant εr and thickness h.The design of the CPW resonator is chosen to achievedesirable resonant frequencies and also to minimize losses.Main components of the CPW resonator consist of a centerconductor, ground planes separated by a gap S, coupling ca-pacitors and contact pads where the RF probesmake contacts.The ratio between the center conductor width W and the gapS are determined such that the characteristic impedance ofthe transmission line is approximately 50Ω for maximumpower transfer. A first sample that will be measured herewill contain Nb resonators with two different center width:W = 10 µm and 15 µm and the gaps S = 6.15 µm and 8.77 µmin order to experiment with different geometric factors. Thethickness of the film will be 200 nm or approximately fivetimes the penetration depth λ. The lengths of the straightresonators are l = 17 mm, 20 mm, and 24 mm correspond-ing to the resonant frequencies of 2.5GHz, 3.0GHz, and3.5GHz respectively. Due to the space limitation, the lowestfrequency achievable using a straight resonator is 2.5 GHz;18th International Conference on RF Superconductivity SRF2017, Lanzhou, China JACoW PublishingISBN: 978-3-95450-191-5 doi:10.18429/JACoW-SRF2017-MOPB035MOPB035128ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Projects/FacilitiesProgressl = >./ 2 w s ~-I I _ Er -I -I _I t: however, it is possible to construct a meander-shaped res-onators to test at lower resonant frequencies, for example ataround 1.0 GHz to 1.5 GHz, relevant to SRF cavities. TheRF power is delivered to the CPW from the network analyzerby touching the Ground-Signal-Ground probes to the contactpads.It is essential to minimize all sources of dissipation inorder to isolate internal quality factor from the measuredloaded quality factor. Other than the conductor loss, twomain sources of dissipation come from dielectric and radia-tion. Reduction of dielectric loss can be readily achievableby using a dielectric substrate with low loss tangent. Forexample, using Al2O3 with εr = 11.5, the dielectric lossquality factor Qd ≈ 105 can be attained. As for the radiationloss, it may be suppressed by using a micro-scale featuresize of the CPW. Any radiation produced from a currentflowing in the center conductor is nearly canceled by theradiation from the current of opposite polarity in the groundplanes [8]. Therefore, minimizing the geometries of theCPW to the extent allowed by a current photolithographytechnique helps keep power dissipation small. By making ananalogy with field patterns of a slot antenna, Zmuidzinas [8]estimated the radiation quality factor to beQrad ≈ 5 × 10−3(λ0W)2(1)where λ0 is the wavelength at the operating frequency inthe free space and W is the center conductor width. For astructure with W=10 µm at fr = 2.5 GHz and 3.5GHz, theradiation quality factor would be roughly Qrad ≈ 7.2 × 105and 4.0 × 105 according to Eq. (1). By carefully choosingthe dielectric substrate, minimizing the CPW structure, andoperating at low frequencies, potential sources of dissipationmay be reduced.Cyogenic Probe StationThe CPS is housed in a vacuum chamber rated at10−5mbar at a room temperature. To dampen vibration ofthe system to less than 1 µm, it is stationed on top of airsuspension legs. The precise placement of the RF probesonto the CPW resonators are achieved using probe armswith translation resolution better than 10 µm, and by inspect-ing through an optical microscope placed on top of a glasswindow above the sample. The system has two additionalprobe arms perpendicular to the RF probes for applying DCcurrent to the sample. Shown in Fig. 2 is the CPS vacuumchamber with its lid off.The sample is cooled to cryogenic temperature using atwo-stage cryocooler (RDK-415D2, Sumitomo Heavy In-dustries, Ltd.) for conductive cooling of the sample. Thetemperature of the first stage is 39.60K with 45.0W of heatload, and the second stage temperature is as low as 3.99Kwith 1.50Wof the applied heat load. The first stage is respon-sible for cooling a thermal shield surrounding the sample,and the second stage is connected to a sample holder anda superconducting magnet via copper braids extending in-side the shield. The sample holder is made of a gold plated,Figure 2: A CPS vacuum chamber with four probe arms.Two of the probes are used for RF probing and the other twoare for sending in DC current. Inside the chamber is a shieldhousing a sample and a superconducting magnet. Threepairs of Helmholtz coils are wrapped around the shield.oxygen-free coper and can hold a sample as large as 2 inchesin diameter, and the superconducting magnet can producefield up to one Tesla parallel to the sample. The shield com-ponent is supported by a six-servomotor system which canbe operated to fine adjust the field orientation to within 10 de-grees in all three directions. Finally, surrounding the shieldare three pairs of Helmholtz coils to suppress Earth’s straymagnetic fields from affecting the sample. It is essentialto eliminate any fields during the cool down to avoid anyvortex trappings inside the superconductor.MEASUREMENT ANALYSISThe network analyzer will measure the complex trans-mission coefficient S21 of the CPW resonator sample, fromwhich a loaded quality factor is extracted. The measuredquality factor takes the form1QL=1Q0+1Qext+1Qd+1Qrad, (2)whereQ0 is the internal quality factor of the superconductingresonator, and Qext is the coupling quality factor. In generalwe need Qd and Qrad to be at least an order of magnitudegreater than the Q0 so that the effect of such dissipation maybe neglected. From Q0, the surface resistance Rs and λ areextracted using a numerical technique developed for a striptransmission line by Sheen [9] and later applied to a CPWresonator by Porch [10].In this method, a cross-section of the CPW geometry isdivided into N smaller patches, with higher concentrationof patches near edges of the conductors where the currentvaries rapidly, as shown in Fig. 3. On each patch, currentsare assumed to be constant, and the sum of all the currents,including the ones on the ground planes, are set to zero. Thepatches can then be treated as a system of parallel N-coupledtransmission line. The current distributions are calculated18th International Conference on RF Superconductivity SRF2017, Lanzhou, China JACoW PublishingISBN: 978-3-95450-191-5 doi:10.18429/JACoW-SRF2017-MOPB035Projects/FacilitiesProgressMOPB035129ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 3: A cross section of a CPW divided into N patches.Smaller patches are located along the edges where the cur-rent varies over the penetration depth.by solving the system of transmission line equations:−∂vn∂z=N∑m=1(rmn + jωlmn) im (3)where vn and im are the voltage and current on each patch.The resistance matrix rmn is given byrmn = Re(1σA0+ δmn1σAn)(4)with An the cross-sectional area of the nth patch and σ =σ1− jσ2 the complex conductivity of a superconductor. Theinductance matrix lmn is the sum of an internal and externalinductance. The internal inductance arises due to kineticenergy of Meissner currents and is given bylmn(internal) = Im1ω(1σA0+ δmn1σAn)(5)An expression for the external inductance can be obtainedby calculating the mutual inductance between each pair oftransmission lines and is only a function of the geometry.Therefore, given two unknowns, σ1 and σ2, the current dis-tribution within the CPW is solved by taking an inverse ofEq. (3). Once the current distribution is calculated, the totalresistance R and inductance L of the system is solved byR =1I2tot∑ni2nRn (6)L =1I2tot∑mniminLn (7)The internal quality factor can then be predicted to beQ0 = ωLR. (8)By comparing this analytical value with the measured Q0’sfrom two CPW resonators differing only in geometries, σ1and σ2 can be extracted. Finally, the surface resistance ofthe superconductor is calculated byRs =12µ20ω20σ1λ3L (9)where λL is extracted from1σ2= µ0λ2Lω0. (10)CONCLUSIONThe closed-cycle cryogenic probe station will be used atOld Dominion University to benchmark new materials forSRF cavities. The superconducting CPW resonators willbe measured for extracting surface resistance under an RFfield, and the penetration depth as a function of appliedDC parallel magnetic field will be analyzed. The systemwill provide a low cost way to study new superconductingmaterials and understand their performance in SRF cavities.REFERENCES[1] A. Grassellino et al., "Nitrogen and argon doping of niobiumfor superconducting radio frequency cavities: a pathway tohighly efficient accelerating structures," Supercond. Sci. Tech-nol., vol. 26, no. 10, 102001, Jul. 2013.[2] P. Dhakal, G. Ciovati, P. Kneisel, G. R. Myneni, "Enhance-ment in quality factor of SRF niobium cavities by materialdiffusion," IEEE Trans. Appl. Supercond., vol. 25, 3500104,Jun. 2015.[3] A. Grassellino et al., "Unprecedented Quality Factors at Ac-celerating Gradients up to 45 MV/m in Niobium Supercon-ducting Resonators via Low Temperature Nitrogen Infusion,"Supercond. Sci. Technol., to be published.[4] A. Gurevich, "Enhancement of rf breakdown field of super-conductors by multi-layer coating," Appl. Phys. Lett. vol. 88,012511, 2006.[5] S. Calatroni, "20 Years of experience with the Nb/Cu technol-ogy for superconducting cavities and perspectives for futuredevelopments," Physica C: Superconductivity, vol. 44, 95,2006.[6] E. Collings, M. Sumption, T. Tajima, "Magnesium diboridesuperconducting RF resonant cavities for high energy particleacceleration," Supercond. Sci. Technol., vol. 16, no. 9, S595,2004[7] S. Posen, D. Hall, "Nb3Sn superconducting radiofrequencycavities: fabrication, results, properties, and prospects," Su-percond. Sci. Techno., vol. 30, no. 3, 033004, 2017[8] J. Zmuidzinas, superconducting Microresonators: Physicsand Applications," Annual Review of Condensed MatterPhysics, vol. 3, pg. 169-214, 2012.[9] D. M. Sheen, S. M. Ali, D. E. Oates, R. S. Withers, andJ. A. Kong, "Current distribution, resistance and strip induc-tance for superconducting strip transmission lines," IEEETrans. on Applied Superconductivity, vol. 1 no. 2, pg. 108,1991.[10] A. Porch, M. J. Lancaster, and R. G. Humphreys, "TheCoplanar Resonator Technique for Determining the SurfaceImpedance of YBa2Cu3O7−δ Thin Films," IEEE Trans. Mi-crow. Theory Techn., vol. 43, no. 2, pg. 306, 1995.18th International Conference on RF Superconductivity SRF2017, Lanzhou, China JACoW PublishingISBN: 978-3-95450-191-5 doi:10.18429/JACoW-SRF2017-MOPB035MOPB035130ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2017).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Projects/FacilitiesProgress

Cryogenic Probe Station at Old Dominion University Center for Accelerator Science

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Cryogenic Probe Station at Old Dominion University Center for Accelerator Science

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