Influence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities
IInfluence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities
Influence of Furnace Baking on Q-E Behavior of SuperconductingAccelerating Cavities
H. Ito, a) H. Araki, K. Takahashi, and K. Umemori
1, 2 High Energy Accelerator Research Organization (KEK), 305-0801 Tsukuba, Ibaraki,Japan The Graduate University for Advanced Studies, SOKENDAI, 305-0801 Tsukuba, Ibaraki,Japan (Dated: 29 January 2021)
The performance of superconducting radio-frequency (SRF) cavities depends on the niobium surface condition. Re-cently, various heat-treatment methods have been investigated to achieve unprecedented high quality factor (Q) andhigh accelerating field (E). We report the influence of a new baking process called furnace baking on the Q-E behaviorof 1.3 GHz SRF cavities. Furnace baking is performed as the final step of the cavity surface treatment; the cavitiesare heated in a vacuum furnace for 3 h, followed by high-pressure rinsing and radio-frequency measurement. Thismethod is simpler and potentially more reliable than previously reported heat-treatment methods, and it is therefore,easier to apply to the SRF cavities. We find that the quality factor is increased after furnace baking at temperaturesranging from 300 ◦ C to 400 ◦ C, while strong decreasing the quality factor at high accelerating field is observed afterfurnace baking at temperatures ranging from 600 ◦ C to 800 ◦ C. We find significant differences in the surface resistancefor various processing temperatures.A superconducting radio-frequency (SRF) cavity is a keycomponent of particle accelerators used to generate chargedparticle beams. An SRF cavity exhibits a lower energy dissi-pation and a lower surface resistance ( R s ) under a radio fre-quency (RF) field, compared to a normal-conducting accel-erating cavity, which enables continuous-wave operation at ahigh accelerating field ( E acc ). Owing to decades of researchfocused on the improvement of SRF cavities , various sur-face treatment techniques have been established; thus SRFcavities with superior performance in terms of the quality fac-tor ( Q ) and E acc have been developed .In recent years, further surface treatment techniques, suchas nitrogen doping , nitrogen infusion , and two-stepbaking , have been investigated to increase the Q and E acc .The nitrogen-doped cavities have an extremely high Q andshow increasing the Q as a function of the E acc which is re-ferred to as the anti-Q slope. However, the maximum E acc ob-tained with nitrogen doping is lower than that of the conven-tional surface-treated cavity. Moreover, the nitrogen-dopedcavities are highly sensitive to trapped magnetic flux com-pared with standard treated cavities . The nitrogen dopingprocess has been applied in the Linac Coherent Light Source(LCLS − II) cavity fabrication process because it is highlyreproducible and has resulted in high Q and anti-Q slope inseveral studies . In the nitrogen infusion technique, the Q-Ebehavior does not change significantly, and both Q and E acc are improved compared with those values obtained using thestandard surface treatment methods ; however, the repro-ducibility has been limited to a few laboratories. The two-stepbaking process developed at Fermi National Accelerator Lab-oratory (FNAL) can produce cavities with a maximum E acc ofapproximately 50 MV/m , and other laboratories are cur-rently verifying the effectiveness of two-step baking. a) Electronic mail: [email protected]
In the typical surface treatment planned for the Interna-tional Linear Collider (ILC), the following procedure is im-plemented: after fabricating the SRF cavity, a 100 µ m layerof the cavity inner surface is removed by bulk electropolish-ing; this results in the elimination of the surface layer dam-aged during cavity fabrication. After electropolishing, the sur-face is thoroughly rinsed with ultrapure water, and ultrasoniccleaning is performed by filling ultrapure water with a surfac-tant, followed by high-pressure ultrapure water rinsing (HPR).Then, annealing is performed in a vacuum furnace at approxi-mately 800 ◦ C to desorb the hydrogen that was absorbed on theniobium surface during electropolishing. Subsequently, lightelectropolishing is used to remove a layer of approximately 20 µ m of the cavity inner surface to eliminate dirt from the innersurface, followed by sufficient water rinsing, ultrasonic clean-ing, HPR, and assembly in a cleanroom. Next, as the final stepin the surface treatment process, the cavity is vacuumed andheat-treated at 120 ◦ C for 48 h.In this study, a new heat-treatment method, which is simplerand more reliable than the surface treatment method describedabove, is investigated from the viewpoint of oxygen diffusionof a niobium oxide layer, and the effects on the properties of Q , the Bardeen-Cooper-Schrieffer (BCS) resistance ( R BCS ),and the residual resistance ( R res ) for each E acc are studied. Inthe 1980s and 1990s, SRF cavities that were heat-treated invacuum in the range of 250 to 300 ◦ C and 1100 to 1400 ◦ C wereinvestigated to understand the effect of an oxide layer on theSRF cavity performance . It was revealed that the heat-treatment at 250 ◦ C dissolves the oxide layer and decreases R BCS . Therefore, it is expected that the heat treatment in thisstudy can be performed in the same temperature range to cre-ate a cavity with high Q . We used several 1.3 GHz TESLAand STF (TESLA-like) single-cell cavities that had undergonevarious surface treatments. As a first step, a 10 or 20 µ mlayer of the cavity inner surface was removed by the lightelectropolishing to reset the surface conditions in the cavity,followed by HPR to eliminate any remaining impurities on a r X i v : . [ phy s i c s . acc - ph ] J a n nfluence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities 2the surface. Subsequently, the cavities were placed in a largevacuum furnace. The inner diameter of the furnace chamberis φ
950 mm, and the length is 2080 mm . This vacuum fur-nace can be depressurized to 1 × − Pa at room temperatureusing a cryopump. Due to the insufficient cooling capacity ofthe cryopump, heat treatment at 800 ◦ C increases the temper-ature of the cryopump. In such a case, we switch from thecryopump to a turbo molecular pump for achieving the de-sired level of vacuum . The Quadrupole Mass Spectrometer(Q-mass) was equipped with a vacuum furnace to monitor thepartial pressure of each element during heat treatment. Thecavities were baked in a temperature range of 200 to 800 ◦ Cfor 3 h in this vacuum furnace. This baking process is referredto as “furnace baking”, which is different from the “medium-temperature bake” (mid-T bake) that is performed at FNALand Institute of High Energy Physics (IHEP) . The mid-Tbake process requires special heat treatment equipment to per-form the RF measurement without exposing the inner surfaceof the cavity to the air after heat treatment at 250 to 400 ◦ C,whereas the furnace baking process is a simple method thatcan be performed with existing cavity treatment systems be-cause the heat treatment is performed in a vacuum furnace. Inthe first step, the baking time was fixed at 3 h to investigate theoptimum temperature and achieve high Q . To perform thefurnace baking, the cavity temperature was ramped up fromroom temperature at a ramp rate of 200 ◦ C/h to a target tem-perature of 200 to 800 ◦ C. The vacuum was 1-2 × − Pa atroom temperature, and it was maintained at the order of 10 − Pa even during baking. After the furnace baking process andcooling down to below 50 ◦ C, the furnace was purged with N gas, and the cavity was packed and placed on the HPR standfor final rinsing before assembly. A new niobium oxide layergrows on the inner surface during this step because of expo-sure to air and water; however, this is not expected to affectthe oxygen diffusion region formed by furnace baking. Af-ter assembly, no further baking was performed, and the RFmeasurements were conducted.The cavity was then cooled down by depressurizing liquidhelium to 1.5 K, which is the lowest temperature that can beachieved in the cryostat available at High Energy AcceleratorResearch Organization (KEK). Then, the Q-E curve at eachtemperature was obtained by calculating the input, reflected,and transmitted RF powers for 0.1 K temperature increments.Finally, the RF measurement was performed up to the quenchfield at 2 K. To minimize the magnetic flux trapping duringthe cooling process, the magnetic field around the cavity wasreduced to less than ∼ . Figure 1 shows the Q-E curve for the cavitythat was furnace-baked at 350 ◦ C. The cavity was quenchedonce during the measurement at 1.5 K, which caused it to trapthe magnetic flux and subsequently decrease the Q . How-ever, higher Q and anti-Q slope were still clearly observed inthe 2 K measurement results compared to the standard treatedcavity (see Fig. 3). R s at each temperature and E acc is calculated using R s = G / Q , where G is the geometric factor that is independent [MV/m] acc E Q · · · · · · · · · S v ] m X -r a y [ - Radiation at 2 K
FIG. 1. Q-E curve at each temperature for the cavity that wasfurnace-baked at 350 ◦ C. Colored circles show Q-E curves at eachtemperature, and purple squares show radiation levels at 2 K mea-surement. of material properties . R s can be expressed as the sum of R BCS , which decreases exponentially with temperature, and R res , which is a weak temperature-dependent or temperature-independent term that cannot be accounted for in R BCS . R s isdecomposed into R BCS and R res at each E acc using the data setat the same E acc from the Q-E curve. The decomposition isperformed using the following fitting equation: R s ( T ) = A ω T e − ( ∆ / kT ) + R res , (1)where A is a fitting constant that depends on superconductingproperties, T is the temperature, k is the Boltzmann constant,2 ∆ is the energy gap of the superconductor, which is treated asa fitting parameter, and ω is the frequency of the cavity. Thefirst term in this equation corresponds to R BCS . The coloredcurves in the upper figure of Fig. 2 show fitting parametersfor R s ( T ) at each E acc . The red closed circles in the lower fig-ure of Fig. 2 illustrate the behavior of R res for E acc . The blueclosed circles depict the behavior of R BCS , which decreasessharply as E acc increases in the case of 350 ◦ C furnace bak-ing. This behavior is considerably different from the behaviorof standard treated cavities (120 ◦ C and 48 h baked cavities),and it is similar to the behavior observed in a nitrogen-dopedcavity . Red open circles depict the estimated behavior of R res before the flux trapping; R res is smaller than that of the stan-dard treated cavities for 350 ◦ C furnace baking.Figure 3 shows a comparison of the Q-E curves measuredat 2 K for cavities that were furnace-baked at various tempera-tures (200 to 800 ◦ C) and a standard treated cavity (120 ◦ C and48 h baking under vacuum directly followed by RF measure-ment, no exposure to air or water). The Q-E behavior of the200 ◦ C and 3 h furnace-baked cavity (purple points) is similarto that of the standard treated cavity (black points). This in-nfluence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities 3 ] W [ s R - · - · - · - · - · - · - · - · - - · ) + p2T1 exp(-p1 T1 = p0 s R at 2 MV/mat 4 MV/mat 6 MV/mat 8 MV/mat 10 MV/mat 12 MV/mat 14 MV/mat 16 MV/m [MV/m] acc E ] W [ n r e s R and B C S R res R Before trap res
R at 2.0 K
BCS R FIG. 2. Temperature dependence of R s for each E acc (upper figure)and behavior of R BCS and R res for E acc (lower figure). R res before thetrap was obtained by subtracting R BCS obtained after the trap fromthe R s at 1.5 K before the trap. dicates that the conventional performance can be achieved byreplacing the 120 ◦ C and 48 h baking with 200 ◦ C and 3 h fur-nace baking, which may be significantly effective for massproduction of cavities. Four furnace-baked cavities, bakedat 300 to 400 ◦ C, have high Q and anti-Q slope, but a low E acc compared with the standard treated cavity. This behav-ior is typically associated with nitrogen-doped cavities . Inparticular, 300 ◦ C furnace baking produces an extremely highQ cavity, with a Q of over 5 × at 16 MV/m. Further-more, 300 ◦ C furnace baking has the same effect for two dif-ferent cavities, indicating good reproducibility. These resultsare in good agreement with those obtained for single-cell cav-ities that are furnace-baked in the temperature range of 250 to400 ◦ C at IHEP . The high-temperature furnace-baked cav-ities baked at 600 ◦ C and 800 ◦ C did not reach high Q , and the Q values were comparable to those of the standard treatedcavity. A phenomenon called high field Q slope (HFQS), inwhich the Q decreases significantly at high E acc28 , was ob-served in these cavities. This HFQS is considered to be re-lated to the diffusion of oxygen and hydrogen on the innersurface of the cavity , and the effect of suppressing theHFQS diminished due to oxygen diffusion at high tempera-tures. These results indicate that diverse Q-E behaviors wereobtained when the furnace baking temperature was changedfrom 200 to 800 ◦ C. In particular, furnace baking at 300 to400 ◦ C resulted in high Q and anti-Q slope. The low E acc issimilar to that of the nitrogen-doped cavity and may be re-lated to the low superheating field at the dirty limit . Theo-retical considerations suggest that the Q varies depending onthe cavity surface condition . [MV/m] acc E Q · · · · · · · · S v / h ] m X -r a y [ - FIG. 3. Comparison of Q-E behavior measured at 2 K for cavitiesthat were furnace-baked at various temperatures (200 to 800 ◦ C) anda standard treated cavity (120 ◦ C and 48 h baking). Colored closedpoints show the Q-E curve for each furnace-baked cavity, and the col-ored opened points show the radiation levels corresponding to eachcolored closed point.
The upper panel of Fig. 4 shows the relationship between R BCS at 2 K and E acc for each furnace-baked and standardtreated cavity. R BCS behavior is classified into three types:one that increases with increasing E acc , one that decreaseswith increasing E acc , and one that does not increase as muchas the first type. The 200 ◦ C furnace-baked cavity and thestandard treated cavity correspond to the first type mentionedabove, and the slope of R BCS is steep compared with thoseobtained for other cavities. The 300 to 400 ◦ C furnace-bakedcavities correspond to the second type. In these cavities, R BCS decreases as E acc increases, which is the origin of the anti-Q slope. This behavior is the most pronounced in 300 ◦ Cfurnace-baked cavities. The 600 ◦ C and 800 ◦ C furnace-bakedcavities correspond to the third type. These cavities alreadyhave a high R BCS at low E acc ; however, the slope is less steepcompared with the first type. From these results, it was foundnfluence of Furnace Baking on Q-E Behavior of Superconducting Accelerating Cavities 4that R BCS behavior varies significantly with differences in thebaking temperature, resulting in the variation of Q-E behavior.Further, it was suggested that there is an inflection point be-tween 200 ◦ C and 300 ◦ C, where the behavior of R BCS changessignificantly, and that a similar inflection point exists in theregion between 400 ◦ C and 600 ◦ C. The lower panel of Fig. 4shows the relationship between R res and E acc for each furnace-baked cavity and standard treated cavity. It was found that R res is lower for all the furnace-baked cavities compared withthat of the standard treated cavity, and R res behavior changeswith differences in baking temperature but not as drasticallyas R BCS behavior. Notably, the 600 ◦ C furnace-baked cavityhas an extremely low R res of 0.2 n Ω , which corresponds to a Q of over 1 × . Because R res dominates R s at temper-atures of approximately 1 K, this 600 ◦ C furnace baking hasthe potential to be a useful processing method for supercon-ducting devices, such as those used at cryogenic temperaturesin the mK region rather than the SRF accelerator applicationoperating at 2 K.The sensitivity of the mid-T (300 to 400 ◦ C) furnace-bakedcavity was estimated by measuring the Q-E curve after cool-ing slowly in a 20 mG field. (Slow cooling allowed the mag-netic field of 95% to be trapped in the cavity.) The sensitivity S describes the amount of increase in R s per unit of trappedfield B trap and can be expressed as S = ∆ R s B trap . (2)Figure 5 shows the measurement results of the sensitivity ofthe mid-T furnace-baked cavity and the comparison to a stan-dard treated cavity. The mid-T furnace-baked cavities have ahigh sensitivity compared with the standard treated cavity andresemble the nitrogen-doped cavity . The sensitivity of the300 ◦ C furnace-baked cavity is higher than that of the nitrogen-doped cavity. When such a high-sensitivity cavity is installedas an accelerator component, it is necessary to consider theeffect of the magnetic field trapping due to the ambient mag-netic field more severely than in the standard treated cavitybecause it is difficult to realize the ideal magnetic field shield-ing in real accelerator components as in the RF measurementof this study.In this study, a new heat-treatment process called furnacebaking has been investigated at several baking temperaturesranging from 200 to 800 ◦ C and a baking time of 3 h. Thebehavior of Q-E is found to be sensitive to the baking temper-ature. Furthermore, the quench field changes with the bakingtemperature. The mid-T (300 to 400 ◦ C) furnace baking pro-duces a cavity with high Q and anti-Q slope. In particular, the300 ◦ C furnace-baked cavity has an extremely high Q of over5 × at 16 MV/m and 2 K. The quench field is 20 to 25MV/m for the mid-T furnace-baked cavities, which is lowerthan the quench field of the standard treated cavity. Althoughthe achievable E acc is low, its high Q is very impressive, andcombined with the simplicity of the furnace baking procedure,it is clear that the mid-T furnace baking can be successfullyadapted to various SRF applications in the future. R res is lowerfor all the furnace-baked cavities compared with that of thestandard treated cavity. For the 600 ◦ C furnace-baked cavity, [MV/m] acc E ] W [ n B C S R [MV/m] acc E ] W [ n r e s R FIG. 4. R BCS behavior at 2 K for E acc for each furnace-baked cavityand standard treated cavity (upper panel). Relationship between R res and E acc for each furnace-baked cavity and standard treated cavity(lower panel). an extremely low R res of 0.2 n Ω is obtained. The sensitiv-ity of the mid-T furnace-baked cavity is higher than that ofthe standard treated cavity and resembles that of the nitrogen-doped cavity. 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