The Center for Accelerator Science at Old Dominion University has built a half-wave coaxial cavity (*) to measure the surface resistance of niobium as a function of frequency, temperature, rf field, preparation techniques, over a wide range of frequencies of interest for particle accelerators. The characteristics of the half-wave coaxial cavity provide these information on a same surface. The preliminary results showed clearly the frequency dependence of residual surface resistance (**). After establishing baseline, we have conducted a study of low temperature baking effect on the surface resistance under controlled environment. This paper will describe the details of the test procedure, results and we will explore underlying physics of the phenomenon.
* H. Park et al., MOPB003, Proc. SRF2015, http://jacow.org/** H. Park et al., THPB080, Proc. SRF2017, http://jacow.org

De Silva, Subashini

Delayen, Jean R.

Fu, Shinian, (Ed.)

Ho Chin, Yong, (Ed.)

Park, Hyekyoung

Pei, Guoxi, (Ed.)

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

Zhao, Ning, (Ed.)

English

Old Dominion University

Old Dominion University ODU Digital Commons Physics Faculty Publications Physics 2018 Investigation of the Surface Resistance of Niobium Between 325 MHz and 1300 MHz Using a Coaxial Half-wave Cavity Hyekyoung Park Old Dominion University, hpark@odu.edu Jean R. Delayen Old Dominion University, jdelayen@odu.edu Subashini De Silva Old Dominion University, pdesilva@odu.edu Guoxi Pei (Ed.) 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 Engineering Physics Commons Original Publication Citation Park, H., Delayen, J. R., & De Silva, S. U. (2018). Investigation of the surface resistance of niobium between 325 and 1300 MHz using a coaxial half-wave cavity. In G. Pei, V.R.W. Schaa, Y.H. Chin, S. Fu, & N. Zhao (Eds.) Proceedings of the 29th Linear Accelerator Conference (pp. 395-397). Joint Accelerator Conferences Website. https://doi.org/10.18429/JACoW-LINAC2018-TUPO031 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 Hyekyoung Park, Jean R. Delayen, Subashini De Silva, Guoxi Pei (Ed.), Volker R.W. Schaa (Ed.), Yong Ho Chin (Ed.), Shinian Fu (Ed.), and Ning Zhao (Ed.) This conference paper is available at ODU Digital Commons: https://digitalcommons.odu.edu/physics_fac_pubs/666 INVESTIGATION OF THE SURFACE RESISTANCE OF NIOBIUM BETWEEN 325 MHz AND 1300 MHz USING A COAXIAL HALF-WAVE CAVITY* H. Park1†, J. R. Delayen1, S. U. De Silva                                                                                        Center for Accelerator Science, Old Dominion University, Norfolk, VA 23529  1also at Jefferson Lab, Newport News, VA 23606, USA Abstract The surface resistance of superconductors depends on frequency, temperature, rf field strength, and surface preparation. In particular, a low temperature bake around 100 C has been shown to modify the high-field behavior of the surface resistance.  Using a coaxial half-wave reso-nator operating at 325, 650, 975 and 1300 MHz [1, 2] we have initiated a research program to better understand the role of low temperature heat treatment on the surface resistance of niobium. INTRODUCTION The well-known expressions for the surface resistance of superconductors in electromagnetic fields, and its de-pendence on frequency, temperature, and a few materials parameters, were obtained as a perturbation theory under the assumption that the magnitude of the electromagnetic field is much smaller than the critical field. This resulted in a surface resistance independent of the magnitude of the electromagnetic field. There have been several attempts at developing theories of the surface resistance at high rf field, up to the critical field [3-5], but, at present, there is no universally accepted consensus on the correct fully self-consistent theory. So far, the origin of residual resistance is believed to be a result of extrinsic mechanisms such as trapped vortices, metallic suboxide layers at the surface, non-superconducting precipitates (hydrides, etc). The full theory should include residual resistance. Extracting fre-quency, temperature, rf field dependence will provide significant insight to a better understanding. Because of the statistical nature of multiple materials defect, techno-logical contributions to surface resistance and the lack of reproducibility, it is difficult to extract accurate frequency dependence of the surface resistance when different cavi-ties of different frequencies are tested. Coaxial half wave resonator provides the frequencies reasonably separated and the same location where the high surface magnetic field is distributed. FREQUENCY DEPENDENCE In the low-field limit the BCS surface resistance of su-perconductors is given by  2 2 30 1ln expn BBCSB BC k TRk T k Tµ ω λ σω⎡ ⎤Δ Δ⎡ ⎤≅ −⎢ ⎥⎢ ⎥⎣ ⎦ ⎣ ⎦h  (1)  In real materials, however, the non-isotropy of the gap and the Fermi surface, and the smearing of the gap can produce deviations from Eq. (1).   In the past the frequency dependence of the surface re-sistance was usually obtained by measurements in differ-ent frequencies and, over the range of interest for particle accelerators was of the form 1.7 2 .ω →   Not included in Eq. (1) also is the so-called residual surface resistance which includes all the sources of power dissipation that do not vanish as 0.T →   Figure 1 shows the temperature dependence of the low-field surface resistance measured in the four lowest TEM modes of the half-wave cavity.   Figure 1: Temperature dependence of the low-field sur-face resistance of the four lowest TEM modes.  The results of Fig. 1 were obtained after the following surface preparation and treatments.  1. Post fabrication cleaning. Cleaning followed typical UHV cleaning procedure. The cavity was cleaned in a heated ultrasonic tank and triple rinsed with deion-ized ultra pure water. 2. Bulk BCP. Damaged niobium layer was removed by the chemical composition of HF (48%):HNO3 (60%):H3PO4 (85%) = 1:1:2 inside the clean room. It was aimed to remove 150 microns but the thickness  ____________________________________________  * Work supported by NSF Award No. PHY-1416051 † hkpark@jlab.org 29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW PublishingISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-TUPO031TechnologySuperconducting RFTUPO031395ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.• 1300 1-- '"-..._r•..I..• t--+-+--t--H • 975 R. 1--ill_>-+-,~<-+-"----+---+---+---+---< e 650 (nD) • ~ C , '•~.._ 100 e 325 -- ..... ... 10 I 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 l fT (K1) measurement showed the actual removal 125 micron on average. After the first test additional 50 microns were removed. Figure 2 is showing how it was set up.    Figure 2: BCP set up in the clean room.  3. Heat treatment. High temperature heat treatment was performed at 800 deg C for 2 hours. During the heat treatment elements (especially hydrogen) were moni-tored.  4. Light BCP. Ten micron removal performed following same set up of bulk BCP. 5. High pressure rinse.  6. Clean room assembly. Rinsed cavity was dried over-night and it was assembled with ancillary parts. The additional parts were input and pick up couplers, vacuum valve and burst disc for safety. Other ports were blanked off as shown in Fig. 3.    Figure 3: Assembled cavity taken out from clean room. 7. Cool down. The cavity was loaded in a dewar of 28 inch diameter. The initial volume of 4.3 K helium liquid was 590 liters.   The data of Fig. 1 can be fitted to the formula  ( )( ) exps resAR T RT kTω Δ⎡ ⎤= − +⎢ ⎥⎣ ⎦  (2) where ( ), / ,A kω Δ  and resR  are fitting parameters.  The frequency dependence of the three fitting parameters are shown in Figs. 4, 5, and 6.   Figure 4: Fitting parameter ( ).A ω   In this frequency range and after the surface prepara-tion described above the surface resistance shows a power frequency dependence ~1.8.  This is different from the quadratic frequency dependence often assumed but con-sistent with numerical calculations. As expected the fitting parameter / kΔ  is frequency independent. The component of the surface resistance that remains finite at T=0 displays a power frequency dependence ~1.5.  This is quite different from what would be expected from trapped flux or a normal inclusion (~0.5).  y = 18.194x-0.018R² = 0.428116171819200.1 1Δ/kFrequency [GHz] Figure 5: Fitting parameter / .kΔ    29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW PublishingISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-TUPO031TUPO031396ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.TechnologySuperconducting RFl.E+06 A(w) (nOK) l.E+05 l.E+04 0.1 y = 123396x1.794S R'= 0.9967 I ... •········· ······ ... ·•··· Frequency (GHz) • .... , ·····················•··········•·······• y = 12.133x1.5244R² = 0.95251101000.1 1Rres(nΩ) Frequency [GHz] Figure 6: Fitting parameter .resR   FIELD DEPENDENCE  Figure 7 shows a Q-curve as function of the peak sur-face magnetic field obtained at 4.35 K in the 325 MHz mode.   Figure 7: Q-curve at 325 MHz and 4.35 K. From the Q-curve and the geometrical factor of 59 Ω, an average surface resistance can be obtained and shown as the blue dots in Fig. 8.  Following the method de-scribed in [6], the actual field-dependent surface re-sistance ( )sR B  can be derived and is shown as the red curve.  Figure 8: Average surface resistance from Fig. 5 (blue dots and purple curve) and actual surface resistance (red curve).  ON-GOING WORK AND PLANS The results presented here are part of an investigation of the low-temperature baking (~120 C) of niobium cavi-ties that has shown that it could reduce the high-field surface, at least at high frequency.  The underlying phys-ics is still poorly understood and by looking at the fre-quency dependence one may be able to shed some light on the underlying processes.  The complete program involves looking at the evolution of the frequency de-pendence of the three fitting parameters and the field-dependence of the surface resistance after a low-temperature baking schedule of: no baking, 6 hours, 6+6 hours, 6+6+12 hours, and if warranted 6+6+12+24 hours. After the completion of this series of tests, our plans are to repeat them following nitrogen infusion in the niobium cavity.  REFERENCES [1] H. Park, S. U. De Silva, and J. R. Delayen, “Superconduct-ing cavity for the measurements of frequency, temperature, rf field dependence of the surface resistance”, in Proc. SRF2015, Whistler, Canada, September 2015, pp. 70-73, doi:10.18429/JACoW-SRF2015-MOPB003 [2] H. Park, S. U. De Silva, and J. R. Delayen, “Measurement of frequency, temperature, rf field dependent surface re-sistance using superconducting half-wave cavity”, in Proc. SRF2017, Lanzhou, China, July 2017, pp. 925-927, doi:10.18429/JACoW-SRF2017-THPB080  [3] B.Xiao, C. Reece, and M. Kelly, Physica C: Superconduc-tivity 490, 26 (2013). [4] A. Gurevich, Phys. Rev. Lett. 113, 087001 (2014). [5] A. Gurevich, Superconductor Science and Technology 30, 034004 (2017). [6] J. R. Delayen, H. Park, S. U. De Silva, “Determination of the field dependence of the surface resistance of supercon-ductors from cavity tests”, presented at LINAC2018, Bei-jing, China, September 2018 Paper TUPO035, this confer-ence. 29th Linear Accelerator Conf. LINAC2018, Beijing, China JACoW PublishingISBN: 978-3-95450-194-6 ISSN: 2226-0366 doi:10.18429/JACoW-LINAC2018-TUPO031TechnologySuperconducting RFTUPO031397ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.l.E+09 Q -~-'·· ••••• ••• ••• I ••• ••• 1.E+08 ~ -~----------------~ 0 10 20 30 40 50 60 70 80 90 100 600 500 400 R,. (nO) 300 -200 100 0 BP (mT) 0 10 20 30 40 50 60 70 80 90 100 B (mT) 

Investigation of the Surface Resistance of Niobium Between 325 MHz and 1300 MHz Using a Coaxial Half-wave Cavity

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Investigation of the Surface Resistance of Niobium Between 325 MHz and 1300 MHz Using a Coaxial Half-wave Cavity

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