Superconducting RF Cavities for Particle Accelerators An 超导射频腔的粒子加速器

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1、Superconducting RF Cavities for Particle Accelerators: An IntroductionIlan Ben-ZviBrookhaven National LaboratoryIn a word: Superconducting RF (SRF) provides efficient, high-gradient accelerators at high duty-factor. SRF accelerator cavities are a success story. Large variety of SRF cavities, dependi

2、ng on: Type of accelerator Particle velocity Current and Duty factor Gradient Acceleration or deflecting modeWhat is a resonant cavity and how do we accelerate beams? A resonant cavity is the high-frequency analog of a LCR resonant circuit. RF power at resonance builds up high electric fields used t

3、o accelerate charged particles. Energy is stored in the electric & magnetic fields.QffQPill-box cavity20212)405. 2(261. 2EJdaUaQ=G/RsG=257Rs is the surface resistivity.Some important figures of merit U=PQ/ A cavity is characterized by its quality factor Q and the geometric factors R/Q and G Diss

4、ipated power per cavity depends on voltage, surface resistivity and geometry factors.V2=PQR/QFor a pillbox cavity R/Q=196Per cavity: P = V2 Rs 1/G Q/ROther quantities of interest for a pillbox cavity:Epeak /Eacceleration =1.6 (2 in elliptical)Hpeak /Eacceleration = 30.5 Gauss / MV/m(40 in elliptical

5、 cavities)RF Superconductivity Superconducting electrons are paired in a coherent quantum state, for DC resistivity disappears bellow the critical field. In RF, there is the BCS resistivity, arising from the unpaired electrons.Hc(T)=Hc(0)1-(T/Tc)2For copper = 5.8107 -1 m-1 so at 1.5 GHz, Rs = 10 m f

6、RsTGHzfTRBCS67.17exp5 . 110224For superconducting niobiumRs = RBCS + Rresidul and at 1.8K, 1.5 GHz, RBCS = 6 n Rresidual 1 to 10 nVarious SRF materials only one practical and commonly usedMaterialTc (K)Hc1 (kGauss)H c2(kGauss)Lead7.70.8 0.8Niobium9.21.74Nb3Sn180.5300MgB2400.335“Superheating” field f

7、or niobium at 0 K is 2.4 kGaussDesign Considerations Residual resistivity: RactualRBCS+Rresidual Dependence on field shape, material, preparation “Q slope” Electropolishing, baking Field emission- cleanliness, chemical processing Thermal conductivity, thermal breakdown High RRR Multipacting cavity s

8、hape, cleanliness, processing Higher Order Modes loss factor, couplers Mechanical modes stiffening, isolation, feedback Measure of performance:The Q vs. accelerating field plotMagnetic fields of 1.7 kGauss (multi-cell) to 1.9 kGauss (single cell)Can be achieved, and recently 2.09 kGauss achieved at

9、Cornell.Limit on fields Field emission clean assembly Magnetic field breakdown (ultimate limit) - good welds, reduce surface fields Thermal conductivity high RRR material Local heating due to defectsFields of 20 to 25 MV/m at Q of over 1010 is routineChoice of material and preparation High “RRR” mat