Friedhelm Caspers

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^-

The coaxial wire method is a useful and efficient tool for (simulated) beam-coupling impedance and loss-factor measurements. Usually one obtains reliable results for a single, localized impedance with Z < ZL. This does not mean that in other cases (several discontinuities Z ⩾ ZL) the results are incorrect, but caution and cross checking are required. It should be kept in mind that the wire permits coupling (= energy exchange) between two or more discontinuities, which would not be coupled in the case of an empty beam pipe. In general, transmission rather than reflection measurements are better. The effect of multiple reflections due to mismatch in adapter pieces can be eliminated by time filtering (= gating).

linac at CERN

A critical review of resistive losses in the LHC beam screen, taking into account anomalous skin effect and surface roughness, has triggered a programme of surface resistance measurements at different temperatures, frequencies and magnetic field intensities. The aim is to establish a realistic heating budget for the LHC cryogenic system and to optimize the fabrication process for the copper coating of the beam screen. Preliminary results at cryogenic temperatures (without magnetic field) indicate a surface resistance about a factor two larger than previously estimated: an absolute measurement precision of a few per cent is reached by comparing the quality factors of even and odd TEM modes in a cylindrical structure with two inner conductors.

Bench methods for bean-coupling impedance measurement

We discuss the results of recent impedance measurements for an LHC dump kicker prototype, performed at CERN using the coaxial wire method. The kicker design includes a vacuum barrier consisting of a ceramic chamber internally coated with a thin metallic layer having good electric contact with the external beam pipe. For the bench test the coated ceramic tube was replaced by a kapton foil with a 0.2 /spl mu/m copper layer having the same DC resistance of 0.7 /spl Omega/. The measurements show that this resistive coating provides a very effective RF screening down to frequencies below 1 MHz, where the skin depth is two orders of magnitude larger than the layer thickness and one could expect full penetration of the electromagnetic fields. We also present simulation results and analytic considerations in agreement with the measurements, showing that the return currents almost entirely flow through the copper layer down to frequencies where the reactive impedance of the kicker elements located behind it becomes comparable to the layer resistance. Finally we discuss the relevance of such coaxial wire measurements to the RF shielding by thin metallic layers in the presence of a highly relativistic proton beam.

Surface resistance measurements for the LHC beam screen

In the framework of the CERN program on the electron cloud effects in existing and future accelerators, a coaxial multipacting test stand was built. It consists of a 100 mm diameter vacuum chamber forming the outer conductor and 6 wires cageaerial-type as the inner conductor. In order to simulate the bunched beam, this test stand is submitted to short RF pulses. The available field strength in a travelling wave mode allows to trigger electron multipacting in as received or baked stainless steel surfaces, but not in chambers treated to reduce the secondary emission yield. Thus a number of upgrades in the bench set-up have been pursued, mainly in two directions. The first one is a general improvement on mismatches and losses. Second, instead of dumping the pulsed power into a load, it is re-circulated in a multiple frequency ring resonator. For this purpose, we designed a directional coupler with several kV DC isolation, very low transmission losses and a four octave bandwidth. In this paper, we give an overview of the present status of the set-up highlighting the latest improvements.

RF screening by thin resistive layers

The two rings of the LHC beam vacuum system have a total length of about 54 km of which almost 48 km will be at 1.9 K, the temperature of the superconducting magnets. The total synchrotron radiation power emitted by the two beams is 0.41 Wm/sup -1/. A a so-called beam screen, maintained at a temperature between 5 K and 20 K by gaseous helium flow, is inserted in the magnet cold bore to intercept this power. We discuss the beam screen, magnetic permeability/vapour pressure aspects, beam screen vacuum behaviour, photon-induced gas desorption, intermagnet connection, pressure measurement, warm sections and the insulation vacuum.

RF test benches for electron cloud studies

A major achievement in particle accelerator physics has been the invention of stochastic cooling, a method which increases the density of beams of rare particles, like antiprotons, by several orders of magnitude. The beam circulates in a storage ring where it is sampled by electromagnetic devices which detect and correct the statistical fluctuations in position and energy. The efficiency is related to the sampling resolution which is itself associated with the system frequency bandwidth, a few gigahertz in practice. The coupling structures are made of electrode arrays connected by combiner or splitter networks. The dynamic range may exceed 150 dB yet fulfilling stringent linear characteristics. At the detection stage, the thermal noise is reduced using cryo-electronic techniques. At the other end of the amplification chain, solid state amplifiers delivering up to 100 watts CW power have been preferred to travelling wave tubes for reasons of phase linearity, lifetime and economy. The performances and technological aspects of the microwave systems are discussed by the example of the CERN antiproton project, ACOL.

The Large Hadron Collider vacuum system

The PS Multi-Turn Extraction Study Group M. J. Barnes*, O. E. Berrig, A. Beuret, J. Borburgh, P. Bourquin, R. Brown, J.-P. Burnet, F. Caspers, J.-M. Cravero, T. Dobers, T. Fowler, S. Gilardoni, M. Giovannozzi (Study Group Leader), M. Hourican, W. Kalbreier, T. Kroyer, F. di Maio, M. Martini, V. Mertens, E. Métral, K.-D. Metzmacher, C. Rossi, J.-P. Royer, L. Sermeus, R. Steerenberg, G. Villiger, T. Zickler