Hays, T

Padamsee, H

CERN Document Server

Particle Accelerators, Vol. 60, pp. 183-191Reprints available directly from the publisherPhotocopying permitted by license only© 1998 OPA (Overseas Publishers Association) N.V.Published by license underthe Gordon and Breach SciencePublishers imprint.Printed in India.MEASURING THE RF CRITICAL FIELDOF Pb, Nb, AND Nb3Sn*THOMAS HAYSt and HASAN PADAMSEELaboratory of Nuclear Studies, Cornell University, Ithaca, NY 14853, USA(Received in final form 17 March 1998)High peak pulsed power is used to raise the cavity fields well above the cw limits. Byraising the fields fast enough, high magnetic fields are reached at the superconductingsurface before local defects create large normal conducting regions. In this way thefundamental rf critical magnetic field is measured. By measuring the Qo of the cavityduring the pulse, one is sure that the cavity is still superconducting at a given rf fieldlevel. The rf critical magnetic field is measured as a function of temperature up to Tcfor 1.3 GHz cavities of lead on copper, niobium, and Nb3Sn on niobium. A 3 GHzmeasurement of Nb3Sn on niobium is also presented. N"iobium and lead measurementswere consistent with the superheating critical field model whereas the, Nb3Sn results fallshort of that prediction.Keywords: Critical magnetic field; Niobium; Niobium-tin; Lead; Superheating field;RF superconductivity1 INTRODUCTIONPresently the achievable accelerating gradients reached in super-conducting cavities are limited either by electron field emission orthermal breakdown. As these barriers are rapidly being pushed backthrough improved cavity production and assembly techniques, amore fundamental limit will be realized - the rf critical magneticfield, H~f. This fact suggests two questions. What is H~f for Pb andNb, the popular superconductors used for accelerators? As we*Work supported by the NSF with supplementary support from the US-JapanCooperative Agreement.t Corresponding author. Fax: 6072544552. E-mail: trh@lns598.lns.comel1.edu.183184 T. HAYS AND H. PADAMSEEapproach this limit in cw operation, what alternate materials (withhigher H~f) could be used in the future to push back this H~f barrier?The measurements presented here attempt to answer these two ques-tions. H~f values for Nb and Pb are measured to determine the presentlimits. H~f for Nb3Sn is measured to see if that material presents itselfas an alternate material for very high gradient accelerators of the future.2 SUPERHEATING CRITICAL FIELDFor Type I superconductors, He is the magnetic field above whichsuperconductivity will stop because it is energetically more favorablefor the fields to penetrate the superconductor (and quench the super-conductivity) than to persist in the Meissner state excluding the field.According to the theory of the superheating critical field, the Meissnerstate is metastable above He. Like a supersaturated solution or asuperheated liquid, the superconductor might stay in the Meissnerstate if no nucleation of the normal conducting region occurs. Thefield above which this metastability disappears is called the super-heating critical field, H sh . For strong Type II superconductors, H sh islower than He. The following relations l ,5 are predictions for Hsh madeby solving the one dimensional Ginzburg-Landau equation.{0.89 H for /'1; // 1.J"fi e ...................... ,Hsh ~ 1 2H r ,-....,; 1.. e lor /'1;,-....,; ,0.75He for /'1;» 1.(1)Here /'1; is the Ginzburg-Landau parameter. More thorough discus-sions of the superheating critical field and its relationship to rf fieldscan be found in Miiller2 and Padamsee et al. 3Reaching the superheating critical field in dc is possible but verydifficult since it requires the absence of nucleation sites. If the nuclea-tion of flux penetration sites takes much longer than an rf cycle, thesuperheating critical field should be easy to achieve in rf.3 MEASURING THE RF CRITICAL FIELDIf one is working with a thermal-defect-free and field-emission-free cavity, measuring H~f can be as easy as turning up the incidentRF CRITICAL FIELDS 185power and observing the field at which the superconducting cavityquenches. Depending upon the surface resistance of the cavity and theheat conduction path away from the rf surface, the temperature of therf surface may be elevated much above the cooling bath temperature.The observed quench field would then be an indirect measure of thetemperature dependent rf critical magnetic field.To circumvent the problem of localized defects, H~f is measured ina pulsed mode instead of continuous wave. By coupling in power verystrongly, the surface fields in the cavity are raised much faster thanthe characteristic timescales of heat propagation. A growing normalconducting region in the vicinity of a defect does not have time toenvelop the cavity. The cavity quenches due to its intrinsic H~f whilethe normal zone is still small. The size of the normal zone is estimatedby measuring the Qo of the cavity.There have been three general methods used to pulse super-conducting cavities to measure H~f. These differ in the way that theymeasure the cavity Qo.3.1 Method #1: Estimating Qo after the PulseTo have the best chance of beating the growth of the normal region,Campisi4 used a very short (~2 J.ls) pulse of high power rf. To evalu-ate whether the cavity quenched during the pulse, they turned up theincident power on successive pulses and observed when the cavitydeviated from being perfectly superconducting. At the end of thepulse, the power emitted from the cavity was integrated to measurethe cavity's maximum stored energy. When the stored energy devi-ated from that expected for the incident power level, the peak surfacefield had exceeded H~f.Figure 1 illustrates conceptually what was being measured. Thestored energy as a function of time for a cavity experiencing a quenchdeviates from the expected value. The point at which the cavity devi-ates is the value taken as H~f in this method.3.2 Method #2: Estimating Qo during the PulseYogi5 measured H~f with a longer pulse length and a lower powerlevel than used in Method #1. During the incident power pulse, thereflected power (Pr ), incident power (PD, and cavity stored energy (U)186 T. HAYS AND H. PADAMSEETimeFIGURE 1 Comparing the behavior of a cavity that quenches with an idealizedcavity with no quench. The point where the two diverge is the H~f.were measured as a function of time. By conservation of energy,Pc == Pi - Pr - d-Ujdt, (2)the power dissipated in the cavity's walls, Pc, is measured. The Qo ofthe cavity as a function oftime can then be found fromQ ( ) = wU(t)o t Pc(t) . (3)One difficulty of this method is that it requires the simultaneousprecision measurement of three time dependent signals.3.3 Method #3: Estimating Qo during the PulseIn this work, we measure Qo during the pulse by a technique similarto Yogi's. For a high Q resonator, the reflected power can be predictedfrom incident power and the cavity's stored energy by the relation(4)where Qe is the "external" Q of the input coupler. This expressionsubstituted into (2) allows one to solve for the cavity's Qo:1Qo2(VPjW/Qe - dv'V/dt)wv'V (5)The advantage of this method over Method #2 is that it requires onlytwo simultaneous precision measurements of time dependent signalsinstead of three.RF CRITICAL FIELDS 1871500 108Nb 0-H5.6 Kelvin .... -Pf1070<> <> QO1000 .. • • • . -0- • . . . ..Gi' <>Q. <> 106 Q:J: 0 00000 •500 o<><>o~000 00 1050 1040 50 100 150 200 250Time [microsec.]FIGURE 2 An example pulse illustrating the use of the Qo extraction technique tointerpret cavity behavior during a quench.An example of using this technique is shown in Figure 2. A 1MWpeak power pulse was used to drive the cavity. The Qo can beobserved to drop until it reaches the normal conducting value of2 x 105 . The peak magnetic field during the pulse occurred when thecavity was almost completely normal. By extracting the Qo, a mag-netic field can be selected when 90% of the cavity is still super-conducting, i.e., when Qo == 2 X 106.If all of the signals were measured with arbitrary precision, onecould obtain a Qo versus E curve from one of these pulses. The Qowould be measured as the cavity is filling to avoid the additionallosses due to the quench. Unfortunately, with the strong externalcoupling needed to ramp the cavity fields in these short timescales, thecavity Qo is not measurable until it drops closer toward Qe'4 APPARATUSMost of the measurements were made using a 1.3 GHz klystron capa-ble of producing a peak power of 2 MW for 300 J.lS. For one sample a3 GHz klystron was used to provide a peak power of 150 kW.A uniform cavity temperature was achieved either by immersing thecavity in liquid helium or flowing helium gas past the cavity depend-ing upon the temperature range desired. To show that gas cooling is188 T. HAYS AND H. PADAMSEEadequate for this type of pulsed measurement, it was verified thatliquid cooling at 4.2 K and gas cooling at 4.2 K yield the same results.Thermometers were located above and below the cavity to record anytemperature gradients. Usually the gradient was kept well below 0.1 K.5 MEASUREMENTSMethod #3 described above was used to measure the rf critical fieldfor three superconductors: niobium, Nb3Sn on niobium, and lead oncopper. The lead on copper cavity started out as a 1.3 GHz coppersingle-cell cavity made at Cornell. A pure lead coating (nominally2 J.lm) was electroplated onto the cavity by John Noe's group atStony Brook. Measurements of the rf critical magnetic field areshown in Figure 3. For this Type I superconductor, He is clearlyexceeded. Like those of Yogi, 5 these measurements are not quite ashigh as the superheating critical field would suggest.The bulk niobium 1.3 GHz cavity was made at Cornell fromRussian high-RRR material and post-purified by solid state getteringto 1000 RRR. The results from this measurement are presented in900.0 ..--........,---..,------.,----r---..,..---.,..----.Pbon Cu800.0700.0i 600.0:::Im~ 500.0:I: 400.0300.0200.0100.00.00.0 1.0 2.0 3.0 4.0T (K]5.0He• meas.6.0 7.0FIGURE 3 Measuring the H~f of lead by pulsing a lead coated copper 1.3 GHzcavity.RF CRITICAL FIELDS 189Figure 4. On the same plot is shown the theoretical value for H shfrom (2). These measurements show that He is exceeded and are consis-tent with the existence of a superheating critical field.There were two Nb3Sn on niobium single-cell cavities made andtested, one at 3 GHz and one at 1.3 GHz. The cavities were made atCornell and the Nb3Sn coating was done by Cryoelectra. Figure 5summarizes all the measurements made on these two cavities. The1.3 GHz cavity was tested three times. The first two times, the cavityreceived no surface treatment except for high pressure water rinsingafter the Nb3Sn coating process. Measurements were made at differ-ent external Q values to see if the cavity fill rate had a large effect onthe measured H~f. It turned out that a lower Qe yielded a slightlyhigher H~f. The test on the 3 GHz cavity and the final test of the1.3 GHz cavity were made after removing 0.1 J.lm from the surfacethrough annodization followed by a dip in HF. This removalappeared to have no significant effect on the H~f.Figure 6 shows where the measurements of niobium and Nb3Sn fallin relation to the theoretical predictions of (2). Niobium shows itselfto be close to the theoretical values but Nb3Sn is considerably lower.Measurements by Campisi4 were slightly higher than those presentedhere but were still considerably lower than H sh .2000.0 r----~---....,.----r----___r---__NiobiumHsh = 1.2*Hc• measured1500.0'i'oi' 1000.0 -500.00.00.0 0.2 0.4 0.6(TfTc)"20.8 1.0FIGURE 4 Measuring the H~'f of niobium by pulsing a 1.3 GHz bulk niobiulTI cavityof high RRR.190 T. HAYS AND H. PADAMSEE3GHz0.1 urn chern.1.3 GHzQe=7.3E5Qe=1.4E50.1 urn chern.Qe=1.15E51000(j) 800Q.J:600400200oo 2 4 6 8 10 12 14 16 18 20Temperature [K]FIGURE 5 Measuring the H~f of Nb3Sn by pulsing a Nb3Sn coated niobium1.3 GHz cavity and a Nb3Sn coated niobium 3 GHz cavity. Multiple measurementswere made on the 1.3 GHz cavity with different coupling and surface treatment.0.80.4 0.62(TlTc >0.2oo5001000~ 1500J:2500 r--r--r--r--.,.---r----r----r--"<"".,..--........-........-..--...,.......-...,.....--...,.....--...,.....--....--...,.....--~.,..__..,"", ,!V6" '1~2000 1\16 f.t. 'oFIGURE 6 Comparing the niobium and Nb3Sn measurements against the super-heating critical field predictions.Why is niobium close to the theoretical H sh and Nb3Sn so faraway? The Nb3Sn tested here differs from niobium in three significantways. First, it has 20 times the /<1; of niobium. Are the superheatingcritical field calculations we are using valid for such strong Type IImaterials? Second, the Nb3Sn is highly granular. Third, it is a thinRF CRITICAL FIELDS 191film instead of a bulk superconductor. It remains to be seen if any orall of these factors are playing a significant role in explaining the lowH sh values measured for Nb3Sn.AcknowledgementsWe are grateful to John Noe and his team for all their hard work inpreparing the lead coating on the copper cavity.References[1] J. Matricon and D. Saint-James, Phys. Lett. A, 24, 241 (1967).[2] G. Miiller, in Proc. of the 3rd Workshop on RF Superconductivity, edited by K.W.Shephard, Argonne Natl. Lab., Argonne, IL, p. 331 (1988). ANL-PHY-88-1.[3] H.S. Padamsee et aI., RF Superconductivity for Accelerators, Wiley and Sons, Inc.(1998). To be published.[4] I.E. Campisi. On The Limiting RF Fields in Superconducting Acclerator Cavities.Technical Report SLACjAP-58, Stanford Linear Accelerator Center, 1987.[5] T. Yogi. Radio Frequency Studies of Surface Resistance and Critical MagneticField of Type I and Type II Superconductors, PhD thesis, California Institute ofTechnology, 1977.

Measuring the RF critical field of Pb, Nb, and Nb ${3}$ Sn

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Measuring the RF critical field of Pb, Nb, and Nb3{3}3Sn

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Measuring the RF critical field of Pb, Nb, and Nb ${3}$ Sn