Charrier, J P

Congretel, G

Visentin, B

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________________________________ # bvisentin@cea.fr   CHANGE  OF  RF  SUPERCONDUCTIVITY  PARAMETERS  INDUCED  BY  HEAT  TREATMENT  ON  NIOBIUM  CAVITIES.   B. Visentin#, J.P. Charrier, G. Congretel CEA-Saclay, DSM/DAPNIA/SEA, 91191Gif/Yvette Cedex - France  Abstract  We have investigated the thermal quench evolution of a Niobium cavity, chemically polished, during ultra high vacuum annealings from 100 to 800°C.  Ten cavities have been tested before heat treatment (HT) and the results show a good agreement with the BCS theory for a diffuse reflection of the conduction electrons on the metal surface. All along the HT we have clearly identified at first the oxygen diffusion from the surface, followed by a cleaning phase of the RF surface and finally, above 800°C, the appearance of a new contamination probably due to the furnace residual gases. All this events involve consequences for the quench position through the thermal conductivity.   1. INTRODUCTION The quest for high gradients in accelerating structures is a important economical challenge for the collider project TESLA. The "Q0 slope" vanishing, observed after the cavity in-situ baking [1,2,3,4] and due to the reduction of residual losses by Interface Tunnel Exchange [5], allows to push away the maximum of accelerator field achieved in Niobium cavities. So the ultimate performances of the cavity is now due to the thermal breakdown limitation rather than to the "RF power limitation. Several experimental observations [2] seem to indicate that this limit is not intrinsic to the cavity:  • the electropolishing (EP) treatment, applied to clean the cavity surface, gives a better result (maximum accelerator field value Equench) than the buffer chemical polishing (BCP) with roughly 40 MV/m for a single-cell cavity (10 MV/m more than BCP).  • performances with BCP are increased by using a fresh chemical bath. • the in-situ baking at 150°C reduces Equench. These observations let think of a possible surface pollution by chemical wastes.  At this time, the EP process is better than the BCP one. Nevertheless it requires a more advanced equipment and the resulting surface is more sensitive in particular when the cavity is subject to the action of processed emitters.   For all these reasons and also to avoid a too early rejection of the BCP process, it is important to understand the reason of this low thermal breakdown value for a BCP cavity.     2.  THERMAL BREAKDOWN A surface defect in a RF cavity generates losses and consequently the temperature increases locally. If it exceeds the critical temperature TC (9.2K), Nb around the defect becomes normal conducting and a thermal instability appears: this is the breakdown, also named quench.  A simple model [6-7] allows to express the maximum of the accelerator field versus the defect radius r, the normal conducting resistance of the material Rnc , the thermal conductivity κ and temperature Tbath to cool the cavity ( in our case 1.6K ): ( )ncbathcquench RrTTE−∝κ4 According to this formula the Equench fluctuations in our observations could be due to a modification of the defect size and, or to a change in the Nb thermal conductivity near the surface, all around the defect. In other words if we want to improve the quench limit of the cavity it is necessary to reduce r and increase κ. A possible way, through a diffusion process, is a furnace treatment after the chemical etching of the cavity.  3.  PARAMETERS MEASUREMENT  In order to understand what happens in the cavity when it is heated, we have systematically measured different RF superconductivity parameters (, TC , RBCS , ∆/k, Rres, Equench). they are determined through the surface resistance RS by measuring the intrinsic quality factor Q0 of the cavity: RS=G/Q0 where the constant G is the geometrical factor. These measurements are made during the cooling down of the cavity from 10K to 1.6K. Above 4.2K, the measurement of the loaded quality factor QL is enough to determine Q0 .  3.1 Electron Mean Free Path  in Normal Conducting State. In the normal conducting state (T>9.2K), the RF fields penetrate a depth δ (), from 300 to 1200 nm, and a simple relationship exists between RS and , if the condition  <δ  is satisfied (normal skin effect): 1∝= δρSR  where  ρ  is the electrical resistivity. In the anomalous skin effect (>δ) an empirical formula [7-8] allows to calculate  : 0-7803-7191-7/01/$10.00 ©2001 IEEE. 1056Proceedings of the 2001 Particle Accelerator Conference, Chicago    ( ) ( ) ( )2757.0157.11 −+∞= SSS RR α  where ( ) 30 143 ρωµα =S .  This experimental value corresponds to a measurement on a depth δ, so we will write it δ.  3.2 Surface Resistance in Superconducting State  Around 9.2K Niobium becomes superconducting and the sudden RS drop down allows to measure TC.  When T < TC/2 the surface resistance becomes the sum of two terms. The first term RBCS decreases exponentially with the temperature and is well describe by the BCS theory, the second term Rres is independent of the temperature. ( ) reskTFLS ReTATlR +=∆−2,,),(ωξλ   where λL, ξF and ∆ are respectively the London penetration depth, the coherence length and half the energy of the Coopers electrons pair.  In the superconducting case the magnetic field penetration depth λ(,T) is lower than δ, few tens of nanometers. To avoid confusion and by analogy with the normal conduction we will write λ  the electron mean free path which appears in the formula above (λ  and δ  could be different if the material purity is or becomes non uniform in depth). Experimentally, RS has the same behaviour described by the theoretical formula, so it is easy to determine RBCS(,T), Rres and the slope of the exponential decrease ∆/k.  3.3 Maximum Accelerator Field at 1.6K When the cavity is at 1.6K we increase the injected RF power until Equench is found.  4. RBCS ( l ) BEHAVIOUR  The experimental plot, RBCS(T=4.2K) vs δ  (Fig.1), can be compared to the calculated one resulting from the BCS theory using Halbritters code [9] where we use the Nb material data λL=31nm, ξF=62nm and the ∆ value that we have measured (1.46 meV). Among the ten 1.3 GHz cavities that we have tested at Saclay, five have been purified at 1300°C with titanium and three others at only 800°C (without Ti) to remove hydrogen. Before the test, a clean RF surface is achieved by either a BCP or an EP process, followed by a high pressure water rinsing (HPR). At last cavities are assembled in a class 100 clean room.  The Fig.1 plot calls for some comments: • A good agreement appears between experimental points RBCS(δ) and the theoretical curve RBCS(λ) for a diffuse reflection of the conduction electrons on the metal surface (continuous line) rather than a specular reflection (dotted line). This demonstrates the homogeneity of the niobium impurities in the cavity wall since  δ ≈ λ ( ≈ bulk for the C114, see table 1 ). • As expected, Ti treatment improves the RRR of the cavity. On the contrary, the strong degradation of cavities treated at 800°C, is amazing: it probably results from a contamination by the residual gas in the furnace.  Figure 1: BCS resistance at T=4.2K vs electron mean free path. Theoretical curves and experimental points. Color refers to Table 1.  Table 1: Characteristics of experimental data.  5. HEAT TREATMENT OF THE CAVITY Until 150°C, the C118 cavity is baked in-situ on the test bench [1] during 60 hours to ensure a stabilisation of the diffusion process [3]. For the other temperatures, a furnace HT is achieved each time after a BCP (20 µm) and before a HPR. In these conditions we start with the same initial layer on the surface. The furnace, where a turbomolecular pump (1000 l/s) provides a clean vacuum (p=1.10-7 mbar at 1000°C) is refilled, after the cavity heating, to the atmospheric pressure by introduction of Ar, O2 or N2 gas.  The experimental results, shown on Table 2, require some comments:    40050060070080090010001 10 100 1000 10000 ( nm )RBCS( nΩ )Cavity Nb manufacturer Purifying bulk Chemical RBCS δ ( nm )RRR ( initial - final ) °C nm Etching nΩ ab/normal  skin effectD101 Wah Ch.  ( 300 - ? ) 1400 + Ti  ? BCP 815 1320C105 Heraeus   ( 280 - ? ) 1200 + Ti  ? BCP 825 875C118 Tokyo D. ( 200 - ? ) 1300 + Ti  ? BCP 805 795C114 Tokyo D. ( 190 - 350 ) 1300 + Ti 800 EP KEK 755 750C117 Tokyo D. ( 200 - ? ) 1300 + Ti  ? BCP 755 710C103 Heraeus   ( 280 ) no 640 EP KEK 750 605P101 Tokyo D. ( 250 ) no 570 BCP 740 475D123 Wah Ch.  ( 300 ) 800 DESY 690 EP CERN 640 360C116 Wah Ch.  ( 280 ) 800 640 EP CERN 620 290D121 Wah Ch.  ( 300 ) 800 690 EP CERN 605 2651057Proceedings of the 2001 Particle Accelerator Conference, Chicago    Table2: RF superconductivity parameters change versus the heating temperature of  BCP cavities.  • At low temperature (T≤150°C), as expected, RBCS decreases and suggests, through the λ decrease, a diffusion process  in the superconducting layer [1-3]. On the contrary, the normal conducting layer is still not affected (δ almost unchanged). Between 150 and 250°C, the diffusion coefficient increases, the diffusion depth grows and δ decreases.  • The critical temperature TC has the same evolution than δ. This implies that the oxygen is the diffusing element in niobium [10]. The oxygen, at the beginning, comes from NbOx , the metallic interface oxyde and then from Nb2O5 , the dielectric oxyde of the surface which is totally decomposed at 250°C [11].  • Above 250°C the avalaible oxygen quantity decreases and the diffusion is so deep that the δ  trend is inverted. • After the heat treatment of the cavity, the furnace is refilled with gas to the atmospheric pressure. If the injected gas is not reactive with Nb ( e.g. Ar or N2 ), it acts as an impurity for RF superconducting layer so λ decreases and RBCS becomes anomalously low. Only refilling with O2 gives a RBCS value such as the data point RBCS(δ) is restored on the theoretical curve of Fig.1. • Around 800°C a new diffusion appears which induces a new decrease of δ . The HT at 1000°C on C117 cavity confirms this fact. It is also the same phenomenum involved in the three cavities, treated at 800°C to remove H2 , and that we have mentionned earlier in the Fig.1 comments. This new contamination is probably due to the residual gas of the furnace. The residual gas analysis shows CO as the main component.  • The last test on C118, without heating, shows a bulk pollution since we do not find the initial results. However, the initial characteristics have been restored on C118 and C117 cavities by a HT at 1300°C with Titanium as a getter. • About Equench , as for TC , a correlation with δ is found. This can be explained by the dependence between   and the thermal conductivity κ . During the experiment we have been confronted with some electron emission, perhaps due to the activation of electron emitters with the temperature as mentionned in [12]. • All along the cavity HT, the ∆/k value (16.96 K-1) which is the exponential slope of RS(1/T) did not change.  6. CONCLUSION These preliminary results concerning the heat treatment of BCP cavity demonstrate: • The good agreement between experimental data and BCS theory, • The clear identification of oxygen diffusion in the superconducting layer even at low temperature, • The importance for RBCS of the gas choice to the furnace refilling, • The cavities pollution for a HT from 800°C. Unfortunately the residual gas contamination screens the Equench  evolution above 800°C and a possible improvement beyond the starting value. So, in the next future, we plan to continue the HT experiment at 800 and 1000°C by using a titanium box to protect the cavity from the furnace vacuum.   7. ACKNOWLEDGEMENT We would like to thank our colleagues A.Aspart, B.Coadou, Y. Gasser, M. Jablonka, G. Monnereau, and J.P Poupeau for their contribution to this work.  8. REFERENCES [1]-B.Visentin et al., "Improvements of Superconducting Cavity Performances", 6th EPAC, Vol.III p. 1885, Stockholm SWEDEN (1998). [2]-B.Visentin et al., "Cavity Baking : a Cure for the High Accelerator Field Q0 Drop", 9th Workshop on RF Superconductivity, Santa Fe - New Mexico - USA (1999), Vol.I, p.198. [3]-P.Kneisel, "Peliminary Experience with In-Situ Baking of Niobium Cavities", ibid. [2], Vol. I, p. 328. [4]-K.Saito, P.Kneisel, "Temperature dependence of the Surface Resistance of Nb at 1300MHz", ibid. [2], Vol. I, p. 277. [5]-J.Halbritter et al.,  "Electric Surface Resistance of Nb/Nb2O5-y Interfaces and Q-Drop of Superconducting Nb Cavities", IEEE Trans. Appl. Superconductivity  11, 1864 (2001).  [6]-W.Weingarten "Superconducting Cavities", CERN Accelerator School 92-03, V2, 318 (1991). [7]-H.Padamsee, J.Knobloch, T.Hays "RF Superconductivity for Accelerators", Wiley Series, Wiley-Interscience Publication ( 1998 ). [8]-Y.Bruynseraede et al. "Surface Resistance Measurements", Physica 54, 137 (1971). [9]- J.Halbritter, "Comparison between measured and calculated RF losses in the superconducting state" Z.Phys. 238, 466 (1970). [10]-J.K.Hulm et al., "Superconductivity in TiO and NbO systems" J.Low.Temp.Phys. 7, n°3/4, 291 (1972). [11]-F.Palmer et al., "Oxide overlayers and the superconducting rf properties of yttrium-processed high purity Nb" NIM A297, 321 (1990). [12]-Ph.Niedermann et al., "Field emission from broad-area Nb cathodes: effects of high-temperature treatment" J.Appl.Phys. 59, 892 (1986).  Cavity Atm. T TC RBCS δ ( nm ) Equench RresRefill. °C K nΩ ab/normal MV/m nΩskin effectC118  - 30 9,23 805 795 power limit 4,2vacuum 100  - 600 695 29 6,2vacuum 120  - 465 675 26 9,4vacuum 150 9,23 410 600 25 12,1Ar 190 9,11 460 130 22 8Ar 250 8,95 455 45 electrons 47,8N2 320 9,00 460 90 electrons 9,4O2 400  - 525 120 electrons 53,1N2 490 9,04 470 145 18 2,4N2 650 9,08 455 280 electrons 14,5N2 800 9,12 415 255 21 2,6 - 30 9,08 630 605 21 1,6C117  - 30  - 735 575 24 1,2Ar 1000  - 420 135 22 3,81058Proceedings of the 2001 Particle Accelerator Conference, Chicago

Change of RF superconductivity parameters induced by heat treatment on Niobium cavities

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