Joachim Tuckmantel

Conceptual design of the SPL II : A high-power superconducting

An analysis of the revised physics needs and recent progress in the technology of superconducting RF cavities have led to major changes in the specification and in the design for a Superconducting Proton Linac (SPL) at CERN. Compared with the first conceptual design report (CERN 2000–012) the beam energy is almost doubled (3.5 GeV instead of 2.2 GeV), while the length of the linac is reduced by 40% and the repetition rate is reduced to 50 Hz. The basic beam power is at a level of 4–5 MW and the approach chosen offers enough margins for upgrades. With this high beam power, the SPL can be the proton driver for an ISOL-type radioactive ion beam facility of the next generation (‘EURISOL’), and for a neutrino facility based on superbeam C beta-beam or on muon decay in a storage ring (‘neutrino factory’). The SPL can also replace the Linac2 and PS Booster in the low-energy part of the CERN proton accelerator complex, improving significantly the beam performance in terms of brightness and intensity for the benefit of all users including the LHC and its luminosity upgrade. Decommissioned LEP klystrons and RF equipment are used to provide RF power at a frequency of 352.2 MHz in the lowenergy part of the accelerator. Beyond 90 MeV, the RF frequency is doubled to take advantage of more compact normal-conducting accelerating structures up to an energy of 180 MeV. From there, state-ofthe-art, high-gradient, bulk-niobium superconducting cavities accelerate the beam up to its final energy of 3.5 GeV. The overall design approach is presented, together with the progress that has been achieved since the publication of the first conceptual design report.

H^-

Power couplers for the 352 MHz LEP2 superconducting RF cavities have been plagued by vacuum and electron outbursts which are attributed to multipacting. Processing of these couplers has been a lengthy operation which was often needed again after high power running even if only for a relatively short time. We report here on recent progress made in improved production methods of coupler parts and special treatment of surfaces, as well as practical tests and simulations of geometrical coupler modifications.

linac at CERN

Experience from the construction, assembly, and tests of two superconducting cavity modules for the Large Electron Positron colliding beam accelerator (LEP) are given. Each module consists of four individual four-cell 352 MHz Nb sputter-coated Cu cavities equipped with an RF power coupler, higher-order-mode (HOM,) dampers, and a frequency tuner, all housed in a single cryostat. The demountable HOM dampers of a new type designed for sputter-coated cavities allow Q/sub ext/ of 9000 for the HOMs with the largest (R/Q). Q values are higher (4.5 to 11*10/sup 9/) than those for similar Nb sheet cavities up to the maximum accelerating fields obtained (6 to 9.5 MV/m). The field limitation is electron loading and never thermal breakdown. Results on vertical tests of individual cavities are reported (Q value, maximum accelerating fields, residual resistance). They are complemented by results on horizontal tests of individual cavities, and on the fully equipped klystron-driven four-cavity module.<<ETX>>

Improvements to power couplers for the LEP2 superconducting cavities

The LEP2 superconducting RF system supplies at present about 2600 MV per turn to the electrons and positrons of the LEP beams, giving a beam energy largely above 90 GeV to create the desired massive W/sup /spl plusmn//-pairs for physics. It is by far the largest such system worldwide and a further upgrade is still under way to increase the W/sup /spl plusmn// production rate and cover new physics grounds. More than 10 MW RF power were routinely transmitted to the beam during last years smooth operation. The authors present the technical parts of this system: the cavities, couplers, tuners, cryostats, the cryogenics and the RF generation and distribution. They also report on operational experience and future plans.

Superconducting niobium sputter-coated copper cavity modules for the LEP energy upgrade

Longitudinal coupled bunch instabilities in the CERN PS represent a major limitation to the high brightness beam delivered for the LHC. To identify possible impedance sources for these instabilities, machine development studies have been carried out. The growth rates of coupled bunch modes have been measured, and modes have been identified using mountain range data. Growth rate estimations from coupled bunch mode theory are compared to these results. It is shown that the longitudinal impedance of the broad resonance curve of the main 10 MHz RF system can be identified as the most probable source. Several modes are driven simultaneously due to the large width of the resonance, which is considered for the analysis.

The LEP2 superconducting RF system

A proton beam with the basic structure defined by the LHC requirements was first available for injection into the SPS in 1998. At the end of 2002, following a significant beam-studies and RF hardware upgrade programme, a beam having both the nominal LHC intensity and the correct longitudinal parameters was obtained at top energy for the first time. This beam, characterized by high local density, must satisfy strict requirements on bunch length, longitudinal emittance and bunch to bunch phase modulation for extraction to the LHC, where only very limited particle losses are acceptable. The problems to be solved came mainly from the high beam loading and microwave and coupled bunch instabilities which led both to beam losses and to unacceptably large longitudinal emittance on the flat top. In this paper the steps taken to arrive at these nominal beam parameters are presented.

Longitudinal coupled-bunch instabilities in the CERN PS

Several authors proposed the construction of superconducting proton linacs using the LEP2 cavities once LEP will be decommissioned. However only a fraction (about half) of these cavities can be used as they are for the high-energy part (/spl beta//spl sim/1) of such a linac, the low energy part requiring the development of accelerating structures optimized for lower values of the particle velocity. At CERN an R&D programme on reduced-/spl beta/ single-cell cavities started in 1996 in order to study and explore the limits of the technology successfully used for the production of LEP2 cavities (copper cavities niobium-plated using the magnetron sputtering technique). Four different geometries were extensively investigated, each representing part of a multicell structure optimized for particles having /spl beta/=0.48, /spl beta/=0.625, /spl beta/=0.66 and /spl beta/=0.8 respectively. The results were encouraging for the last two types and therefore a new phase of R&D aimed at the production of multicell cavities for /spl beta/=0.66 and /spl beta/=0.8 was started. The goal is to demonstrate simultaneously the feasibility of such cavities and the possibility of producing them by low-cost modification of LEP cavities. In the paper, after a brief review of the results obtained on the single-cell cavities, we present in more detail the procedure for the transformation of the LEP cavities, which should allow a realistic estimate of the costs of such operation.

Nominal longitudinal parameters for the LHC beam in the CERN SPS

The technology of sputter coating of Cu cavities with Nb has been developed at CERN. The advantages of this technique have led CERN to order 168 of such cavities in industry. After an initial phase of technology transfer and of prototype development, the series production has been started in fall 92 by the three contractors. The results of the bare cavity tests are reported. Fixed and movable 120 kW power couplers (MC) have been designed, manufactured and put into operation. Various models of higher order mode (HOM) couplers have been developed to cope with foreseen increase of the beam intensity. Special care is given to the conditioning of power couplers and of HOM couplers before installation in the machine.<<ETX>>

Technical developments on reduced-/spl beta/ superconducting cavities at CERN

Using a superconducting LEP (Large Electron Positron Collider) prototype cavity, an e/sup +/ beam was accelerated at E/sub a/=5.5 MV/m. The SPS (Super Proton Synchrotron) proton accelerator/proton-antiproton collider/LEP injector was chosen for the implementation of the cavity. After 8000 h at 4.5 K E/sub a//sup max/ remained unchanged (7.1 MV/m); the Q-value (5 MV/m) was 2/3*10/sup 9/. No major system failure occurred. RF feedback guaranteed very low fluctuations of the cavity field amplitude and phase.<<ETX>>

Superconducting cavities for the LEP energy upgrade

At CERN a 5 cell 500 MHz superconducting accelerating cavity has been constructed. At present the cavity is installed in the PETRA storage ring at DESY for a test run which will start middle of March. The cavity is equipped with a high power coupler and with 5 higher order mode couplers, two of the loop type and three of the antenna type. Frequency tuning and regulation are achieved by varying the length of the two end-cells. In a test of the complete cavity-cryostat system at CERN, acceleration fields of 2.8 MV/m and Q0-values above 109 have been reached. The cryostat losses remained below 10 W.