Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film
Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film Magnetron sputtering of Superconducting Multilayer Nb3Sn Thin Film
electron higher than 200GeV.The linear accelerators utilize a linear array of RF cavities and becomeincreasingly long, with lengths up to the tens of kilometer range, to achieve particleenergies in the GeV to TeV energy range that is of present High Energy Physics (HEP)interest. Such systems will require on the order of 20,000 RF cavities. Improving theaccelerating voltage per unit length will result in tremendous cost savings, since therequired length to achieve a given beam energy in a linear accelerator is inverselyproportional to the electric field that can be generated per unit length. In addition,power requirements are significant, ranging from 100 to 250 MW for a 500 GeVlinear accelerator.The main figures of merit for RF cavities are the quality factor Q 0 (defined by theratio between the energy stored in the cavity and the energy loss in one RF period)and its average accelerating field E acc . Q 0 values as high as 10 11 have been achieved insuperconducting cavities. Superconducting cavities exhibit approximately a factor 10 6higher Q 0 than normal conducting cavities due to the reduced microwave surfaceresistance. Even when accounting for cooling penalties the required input power isstill about a factor 10 3 lower when using superconducting cavities instead of normalconducting cavities, saving drastically in operating costs. The InternationalTechnology Recommendation Panel of the International Committee for FutureAccelerators therefore, amongst other considerations, selected superconducting RFcavities above normal conducting cavities as the preferred technology for future linearaccelerators [1] .1.2 The performance of the needed superconductor cavityThe efficiency of a RF cavity is usually depicted by plotting Q 0 as a function ofE acc . For an ideal superconducting cavity, Q 0 remains constant with increasing E accand collapses when a maximum E acc is reached. This point is determined by themagnetic components of the RF standing wave. Once the magnetic componentreaches a certain threshold value, vortices penetrate the superconductor and theMeissner state is lost. Vortices that move inside the superconductor dissipate energyand cause the cavity to quench. Even though vortices can be elastically pinned bymaterial imperfections, their oscillations in an RF field will still cause dissipations [2] .Hence, vortex penetration has to be prevented to retain a high Q 0 and a cavity has tooperate in the Meissner state.4
In Type-II superconductors, vortex penetration becomes energetically favorableat the bulk lower critical magnetic field H c1 . However, in magnetic fields which areparallel to the surface, vortices have to overcome the Bean-Livingston positivesurface energy barrier to enter the superconductor [3] . The Meissner state can thereforepersist metastably beyond H c1 , up to the so-called superheating field H sh , at which thesurface barrier disappears. Note that in practice H sh is not necessarily reached, due toearlier vortex penetration at surface irregularities [4] and demagnetization effects oftransverse magnetic field components [5] .So, one of the fundamental limitations of increasing the accelerating gradient ofthe superconductor cavity is the critical magnetic field H c1 of the RF field, which isalso called as H rf c [6] . Other limitations include the electron field emission, the thermalbreakdown, the surface resistance and T c .The electron field emission means that the electron emits from the peak of thecavity surface with high electrical field, which can decrease the Q value and inducelocal heat. The worst case of the electron field emission is the electrical breakdown.The electron field emission can be improved by optimizing the cavity shape to reducethe electrical field on the surface and improving the roughness of the surface.The thermal breakdown is also a kind of local effect. If some particle withoutsuperconductivity adhere or embed in the surface, the heat will be produced when theelectron pass through them. If the heat is high enough, it will heat thesuperconductivity atom near the particle to case the superconductivity, and theninduce more atoms to case the superconductivity, at last, the whole cavity may casethe superconductivity. One method to avoid the thermal breakdown is that thetemperature of the superconductor cavity operating is great lower than T c . The T c isthe critical temperature of superconductor. When the temperature of the conductordecreases lower than the T c , the conductor changes from the normal state to thesuperconductivity state.The resistance of the superconductor can be neglected at the condition of DCcurrent. For the RF field, although the resistance of the superconductor is very small,it can not be neglected.For the normal conductor, RF currents flow only on the surface of conductorswithout passing through the internal conductor bodies. The penetration depth of RFcurrent in conductor is1δ = (1.1)πfμσ5
- Page 1: UNIVERSITÀ DEGLISTUDI DI PADOVAFac
- Page 7 and 8: IndexINDEX ........................
- Page 9 and 10: IntroductionThe superconductor acce
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- Page 31 and 32: Fig.2.8 turbomolecular pumpFig. 2.9
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- Page 35 and 36: Fig. 2.17 the gasket and the magnet
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- Page 39 and 40: Fig. 2.26 The interface of the surf
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- Page 43 and 44: Fig. 2.34 the performance of the th
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In Type-II superconductors, vortex penetration becomes energetically favorableat the bulk lower critical magnetic field H c1 . However, in magnetic fields which areparallel to the surface, vortices have to overcome the Bean-Livingston positivesurface energy barrier to enter the superconductor [3] . The Meissner state can thereforepersist metastably beyond H c1 , up to the so-called superheating field H sh , at which thesurface barrier disappears. Note that in practice H sh is not necessarily reached, due toearlier vortex penetration at surface irregularities [4] and demagnetization effects <strong>of</strong>transverse magnetic field components [5] .So, one <strong>of</strong> the fundamental limitations <strong>of</strong> increasing the accelerating gradient <strong>of</strong>the superconductor cavity is the critical magnetic field H c1 <strong>of</strong> the RF field, which isalso called as H rf c [6] . Other limitations include the electron field emission, the thermalbreakdown, the surface resistance and T c .The electron field emission means that the electron emits from the peak <strong>of</strong> thecavity surface with high electrical field, which can decrease the Q value and inducelocal heat. The worst case <strong>of</strong> the electron field emission is the electrical breakdown.The electron field emission can be improved by optimizing the cavity shape to reducethe electrical field on the surface and improving the roughness <strong>of</strong> the surface.The thermal breakdown is also a kind <strong>of</strong> local effect. If some particle withoutsuperconductivity adhere or embed in the surface, the heat will be produced when theelectron pass through them. If the heat is high enough, it will heat thesuperconductivity atom near the particle to case the superconductivity, and theninduce more atoms to case the superconductivity, at last, the whole cavity may casethe superconductivity. One method to avoid the thermal breakdown is that thetemperature <strong>of</strong> the superconductor cavity operating is great lower than T c . The T c isthe critical temperature <strong>of</strong> superconductor. When the temperature <strong>of</strong> the conductordecreases lower than the T c , the conductor changes from the normal state to thesuperconductivity state.The resistance <strong>of</strong> the superconductor can be neglected at the condition <strong>of</strong> DCcurrent. For the RF field, although the resistance <strong>of</strong> the superconductor is very small,it can not be neglected.For the normal conductor, RF currents flow only on the surface <strong>of</strong> conductorswithout passing through the internal conductor bodies. The penetration depth <strong>of</strong> RFcurrent in conductor is1δ = (1.1)πfμσ5
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