A High-intensity H Linac at Cern Based on Lep-2 Cavities

In view of a possible evolution of the CERN accelerator complex towards higher proton intensities, a 2.2 GeV H linac with 4 MW beam power has been designed, for use in connection with an accumulator and compressor ring as proton driver of a muon-based Neutrino Factory. The high-energy part of this linac can use most of the RF equipment (superconducting cavities and klystrons) from the LEP collider after its decommissioning at the end of 2000. Recent results concerning low-beta superconducting cavities are presented, and the main characteristics of the linac design are described. The complete linac-based proton driver facility is outlined, and the impact on the linac design of the requirements specific to a Neutrino Factory is underlined. 1. THE LEP-2 RF SYSTEM The decommissioning of the CERN LEP e + e collider at the end of 2000 will pave the way to the construction of the Large Hadron Collider (LHC), but will also present the unprecedented challenge of the removal, storage or disposal, and possible recycling of the huge amount of valuable LEP equipment. A particularly valuable item is the 352.2 MHz superconducting RF system built for the Phase 2 of LEP, consisting of 288 four-cell cavities (Figure 1) operating at 4.5 °K and powered by 36 1.3 MW CW klystrons. It delivers a total accelerating voltage of about 3 GV to the electron beam. Eight more klystrons are used to power the normal-conducting RF system of LEP, for a total of 44 klystrons installed in the machine. Most of the superconducting cavities (272) were produced using the technique developed at CERN of sputtering a thin film of niobium onto copper [1]. The cavities were initially designed for a gradient of 6 MV/m, and during the 1999 run they achieved an average gradient of 7.5 MV/m, with up to 9 MV/m in some cavities [2]. In the basic LEP configuration, each klystron feeds 8 cavities via an array of magic tees, equipped with circulators and loads. Four cavities are grouped in a cryostat. The cavities and the cryostats are fully equipped with slow and fast tuners, power couplers matched for a beam current of 10 mA, high-order-mode couplers, superinsulation and insulation vacuum tanks. The present plans foresee to store most of the RF material for possible future use. The cryogenic system of LEP will be used for the LHC magnets. Figure 1: The LEP-2 accelerating cavity 2. A LINAC BASED ON LEP CAVITIES Some proposals for re-using this expensive hardware have been made, such as for a Free Electron Laser [3] or to build the ELFE machine on the CERN site, a recirculating electron linac for nuclear physics [4]. An early proposal already opened the perspective of using the LEP cavities in a high beam power superconducting linac driving a hybrid reactor [5-7]. The main limitation for using these cavities in proton linacs comes from the fact that they are designed for β=1, their transit time factor drastically decreasing for a proton beam at low beta. Figure 2 shows a calculation of the effective cavity gradient as function of energy that can be reached by LEP cavities operating at a nominal gradient of 7.5 MV/m. While in principle they can be used for proton acceleration from about 500 MeV, they become efficient and economically justified only from about 1 GeV, i.e. in an energy range beyond the usual requirements of high-power linacs for spallation sources, transmutation or hybrid reactors. Figure 2: Effective gradient of the LEP-2 cavities as a function of energy XX International Linac Conference, Monterey, California