Magnesium diboride superconducting RF resonant cavities for high energy particleacceleration

Abstract

Arguments in support of any particular superconducting coating must be framed in termsof its fundamental thermodynamic properties. The superconducting transition temperature,Tc, determines the surface resistance, and thus theQ of the cavity. This must remain sufficiently high that the system can be driven at the requiredfield gradients and frequencies without leading to excessive power loss. In this regard the 39 KTc ofMgB2 is advantageous. With an anticipated maximum accelerating field,EaccMAX, of77 MV m−1 and a BCS surfaceresistance, RsBCS (4 K,500 MHz), of 2.5 n Ω asdiscussed later, MgB2 represents an interesting possibility as a coating for SRF cavities. In addition, the higherHc2 ofMgB2 than Nb results in a slightly lower estimated trapped flux sensitivity. Recent measurements of anMgB2 film at the Los Alamos National Laboratory (LANL) have shown an RF surface resistancelower than that of Nb at 4 K, which is proof-of-principle evidence of the attractiveness ofMgB2. Our calculations are based conservatively on 4 K operation at 500 MHz. However, with aTc of39 K, MgB2-coated cavities should be less susceptible to thermal breakdown thanlow-Tc ones. Superconducting materials for use at GHz frequencies at voltage gradients>40 MV m−1, a recently cited target, will require both lowRs (highTc) and highHsh values. Witha Tc of 39 K,MgB2 clearly has thepotential to reduce RsBCS if the films are well prepared and free from defects and particles. Additionally, while theHc1 for MgB2 is relatively low, the superheating critical field,Hsh, is higher than that of Nb. Currently, there is some debate about the exact roles ofHc1 andHsh in the determinationof Eacc limits. However,the higher values of Hsh for MgB2 do suggest thepossibility of enhanced Eacc values. The exact roles of Hc1 and Hsh should be further investigated. Techniques exist that may enable cavity-like structures to be internally coatedwith a MgB2 film.