SUPERCONDUCTING CAVITIES — BASICS

The basic properties and design criteria of superconducting cavities for electron accelerators are discussed with special emphasis on the following topics: technical motivation for the use of superconducting cavities; design and production; surface resistance; the critical field of superconductors; anomalous losses; materials other than niobium; technological achievements for accelerating cavities. 1 . INTRODUCTION High-energy physics has prospered for the last few decades thanks largely to progress in particle accelerator techniques. The quest was always for larger centre-of-mass energies and larger luminosities. The most recent accomplishment is the operation at centre-of-mass energies between 91 GeV and 2 TeV of LEP at CERN, SLC at SLAC, HERA at DESY and the Tevatron at Fermilab. An increasingly demanding problem is how to reduce investment and operation costs of such large accelerators. For this reason the benefits of superconductivity are exploited for magnets (at Tevatron and HERA) and RF cavities (at LEP and HERA). To combine larger energies with affordable costs is different for proton and electron accelerators. Circular accelerators offer a cost-effective way to obtain high energies because the particles traverse the accelerating sections many times. The energy limit is determined by the emission of synchrotron radiation, which increases with the fourth power of γ, the ratio of the particle energy over its rest mass. Accelerators for protons, a relatively massive particle, are still far away from this limit. Therefore future proton accelerators such as the LHC at CERN will be circular machines. On the other hand, circular accelerators for electrons, relatively light particles, emit large amounts of synchrotron radiation. An estimation shows [1] that future accelerators for electrons will be linear colliders like the SLC at Stanford, the pioneering one. The reason is that the investment costs are proportional to the length (for linear colliders) or the circumference (for circular colliders). For a beam energy E and an accelerating gradient g, they are proportional to E/g and to E2/√g, respectively. Hence, above a certain energy a linear collider is cheaper. Since particles will traverse the accelerating section only once, it is essential to have high accelerating gradients. The other demanding need is to keep the mains power as low as possible for a given luminosity and particle energy; in other words, the efficiency of transforming mains power to beam power should be at maximum. In principle, radio-frequency (RF) super-conductivity is supposed to provide a technical solution for both constraints, that is a high accelerating gradient and high mains-to-beam-power conversion efficiency. 2 . TECHNICAL MOTIVATION FOR THE USE OF SUPERCONDUCTING CAVITIES 2 . 1 The Mains-to-Beam-Power Conversion Efficiency Electrical power from the mains to the beam is transferred via the following chain: highvoltage power supply — klystron — waveguide — RF cavity — beam. For the time being the largest power loss has to be tolerated between the RF cavity and the beam. In order to establish the accelerating voltage V in the cavity, RF currents are induced at its surface generating the heat Pc. A shunt impedance R = V2/(2Pc) is defined to account for these RF losses. R should be as large as possible. RF accelerating cavities manufactured from high-conductivity copper have shunt impedances (per unit length) of typically 10–15 MΩ/m. With the beam current Ib (d.c. component), the beam power Pb = VIb (for the particle riding on the crest of the RF wave), the conversion efficiency is defined as η = Pb Pb + Pc = 1 1 + Pc Pb = 1 1 + V 2RIb .