Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film

More documents

Recommendations

Info

Where, f is the frequency, μ is the permeability and σ is conductivity of theconductor.In fact, after only a few penetration depths, in conductor internal body the RFcurrent becomes negligible. The surface resistance of a conductor isπfμR n S= 1 =(1.2)δσ σIt is obvious that the penetration depth will increase and the surface resistancewill decrease when the frequency decreases. At GHz range, only a few microns of thecopper surface gives a contribution to the RF current, while the rest major part servedas substrate.The RF losses of the accelerator cavity, in the absence of vortices, are mainlydetermined by the RF surface resistance (R S ), which is usually represented as asummation of the Bardeen-Cooper-Schrieffer (BCS) surface resistance R BCS and aresidual resistance term (R res ) as shown in Eq.1.3. R res is usually on the order of a fewnΩ. Provided that the surface is clean and properly manufactured, R S is usuallydominated by R BCS .R = R + R = R + R(1.3)SWhere,phR phresBCSresis the phonons resistance. The phonons resistance comes from thecrystal lattice and is same with the BCS resistance. Among the factors that define theR res , there are extrinsic causes e.g. trapped magnetic flux, can be avoided. Othercauses are intrinsic and due to the structural imperfections of the material. Likeinhomogeneities, impurities, grain boundaries or surface serrations. Materials with alarge coherent length will be insensitive to large defects without an appreciableincrease of the R res . This is quite desirable for applications of the superconductorcavity, since the superconducting surfaces are exposed to RF field, and are difficult toprepare completely 'defect-free'. Therefore it is important to minimize the residualsurface resistance. The R BCS can be defined as:RBCSBTc−TA= σnω3λ 2 e(1.4)TWhere, A and B are two constants weakly depend on material, ω is the RFangular frequency, σ n is the normal state conductivity of the material,λ is theeffective penetration depth, and T c is the critical temperature.Because the surface resistance of superconductor in the RF field is non-zero, atiny RF loss can heat the superconductor surface. So the more cryogenic power is6
needed for preventing the superconductivity ceasing as shown in Table 1.1 [7] . In Table1.1, Dynamic means to put the RF power into the superconductor cavity. At 2K, theratio of the installed power over heat load is about 1000, but at 8K, the ratio is about250. This means that the cryogenic power per unit of heat load at 2K is about fourtimes of that at 8K. This conclusion also can be obtained by the Carnot cycle:PPcoolsourceT − TT2 1≥ (1.5)1Where, T 2 is the room temperature, T 1 is the cryogenic temperature. BecauseT 2 >>T 1 , the ratio should be inverse proportion to T 1 .Table 1.1 RF unit cryogenic heat loads and installed AC cryogenic plant power to remove the heat.In order to decrease the cryogenic power, two parameters should be taken intoaccount. The first parameter is the surface resistance, and the second is the cryogenicoperation temperature. According to the Eq.(1.3), we should decrease R res and R BCS .To decrease R res , we should increase the residual resistance radio (RRR).RRRR(300K)Rphonons= = +(1.6)res1RR(300K)resWhere, R phonons (300K) is a constant.According to the Eq.(1.4), to decrease R BCS , we should increase the criticaltemperature T c and decrease the cryogenic operation temperature T. However,according to the Eq.(1.5), to decrease the cryogenic power, we should increase thecryogenic operation temperature T. So, the best way is to increase T c .1.3 The necessary of Nb 3 Sn superconductor thin filmAmong the pure metal, Niobium have the highest T c , which is about 9.2K.However, in order to obtain the better performances, such as Q value, acceleratinggradients, thermal stability, and so on, the Nb cavity should only operate at thetemperatures below 2.1 K, which is under the very expensive superfluid He bathcooling conditions. Fortunately, some of metal compounds have the Tc higher than7
Page 1: UNIVERSITÀ DEGLISTUDI DI PADOVAFac
Page 7 and 8: IndexINDEX ........................
Page 9 and 10: IntroductionThe superconductor acce
Page 11 and 12: Fig. 1.1 Disc-loaded traveling-wave
Page 13: In Type-II superconductors, vortex
Page 17 and 18: The B1 crystal structure has a cubi
Page 19 and 20: NbN cavities literature, is often h
Page 21 and 22: superconducting magnet construction
Page 23 and 24: 775°C. From the same diagram, it i
Page 25 and 26: two types of electrons at the Fermi
Page 27 and 28: Of course, there are some other kin
Page 29 and 30: as shown in Fig.2.3. The four opera
Page 31 and 32: Fig.2.8 turbomolecular pumpFig. 2.9
Page 33 and 34: Fig. 2.14 The chamber covered with
Page 35 and 36: Fig. 2.17 the gasket and the magnet
Page 37 and 38: shown in Fig. 2.21 and Fig. 2.22. T
Page 39 and 40: Fig. 2.26 The interface of the surf
Page 41 and 42: Fig. 2.30 put the sample into the T
Page 43 and 44: Fig. 2.34 the performance of the th
Page 45 and 46: 1008038.5°{110}69.6°{211}sample1s
Page 47 and 48: 2dsinθ= nλ (2.3)Fig.2.44 the diff
Page 49 and 50: Chapter 3 Multilayer deposition of
Page 51 and 52: 1 minute. The time of depositing mu
Page 53 and 54: chamber 1 and is connected to the s
Page 55 and 56: 3.2.2.2 Analysis resultⅠ The thic
Page 57 and 58: In order to detect the component of
Page 59 and 60: measure the RRR. So the resistance
Page 61 and 62: 110100903480relative Intensity70605
Page 63 and 64: frequency of the cavity which is in
Page 65 and 66:
Fig. 3.28 the inductor coilDuring t
Page 67 and 68:
Fig. 3.32 fixing the sample with Nb
Page 69 and 70:
second transition temperature is th
Page 71 and 72:
source is the bulk Nb sample. Altho
Page 73 and 74:
of 1A in the Fig. 3.37 and Fig. 3.4
Page 75 and 76:
Table 3.10 the description of the p
Page 77 and 78:
1101001.0A10'-IND3 45 6 7 9 1011 12
Page 79 and 80:
1101009080341.8A10'INDrelative inte
Page 81 and 82:
Reference1. A. Godeke, Nb3Sn FOR RA
show all

Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?