AMORPHUS CARBON COATINGS FOR MITIGATION OF ELECTRON CLOUD IN THE CERN SPS

Amorphous carbon thin films have been applied to the liners in the electron cloud monitors and to vacuum chambers of three dipole magnets in the SPS. The electron cloud is completely suppressed for LHC type beams in the liners even after 3 months of air venting and no performance deterioration is observed after more than one year of SPS operation. In stainless steel (StSt) liners upon variation of the magnetic field in the monitors the electron cloud current maintains its intensity down to weak fields of some 40 Gauss. This is in agreement with previous findings and also with dark traces observed on the RF shields made of StSt, which are located between dipoles and quadrupoles. The dynamic pressure rise has been used to monitor the behavior of the dipole magnets. It is about the same for coated and uncoated magnets, apart from a weak improvement in the carbon coated ones under conditions of intense electron cloud. SPS EXPERIMENTAL SET-UP Electron-cloud is one of the main limitations for the LHC beam in the SPS [1]. The goal of this work is to investigate one of the most promising methods to eliminate the e-cloud in order to make the SPS able to deliver the ultimate beam to LHC and reach maximum luminosity for the machine. Thin film coatings produced by d.c magnetron sputtering with carbon have been tested on the existing SPS vacuum chambers. To observe electron cloud we used the same type of monitors as in previous tests [1] [2]. The Electron Cloud Monitors (ECM) equipped with StSt liners with and without coatings are installed in special dipoles. Unless otherwise specified, during all the experiments the field was kept at 1.2 kGauss (the SPS injection value). Four ECMs can be used at the same time. An ECM with a StSt liner has been used as reference. The StSt liner has been exchanged before certain Machine Development (MD) runs in order to have a non conditioned surface as reference (see different liners in Table 1). Various amorphous carbon (a-C) coatings have been tested in several MD runs, as well as a NEG (TiZrV), as listed in Table 1. After successful tests with a-C coated liners in 2008 (see [2]), three dipole magnets of B-type (MBB) have been coated with a thin film of the same material and installed in the SPS in March 2009 (Fig 1). The total air exposure time of the coating before installation (on the ground and in the tunnel) was around one week. Pressure measure∗ christina.yin.vallgren@cern.ch ments (1 Hz sampling rate) were performed with Penning gauges installed on the pumping port between two uncoated dipoles used as reference and between a-C coated dipoles. In addition a gauge was placed between a coated and an uncoated dipole. The inter-magnet pumping ports with RF shields are of bare StSt. In MD 8, the RF shield between the two carbon coated magnets was also coated. Table 1: The list of liners in the SPS experiments. CKr: coating of a-C with Kr as discharge gas. CNe: coating of a-C with Ne as discharge gas. C-strip: longitudinal strip with CNe of 40 mm width. C/Zr: CNe coating on top of rough Zr coating. SPS experiment Date Liner Scrubbing run 10 12 June, 2008 CKr4 NEG StSt1 MD1 8 July, 2008 CNe8 NEG StSt2 MD2 12 August, 2008 CNe13 NEG StSt3 MD3 6 8 October, 2008 CNe13 NEG StSt3 MD4 15 18 June, 2009 CNe13 MD5 15 16 July, 2009 C-strip MD6 12 August, 2009 C/Zr MD7 15 September, 2009 StSt4 MD8 3 November, 2009 Figure 1: The position of the coated and uncoated magnets in the SPS. QD: quadrupole. P: Penning gauge. RESULTS AND DISCUSSIONS The Secondary Electron Yield (SEY) of a witness sample coated simultaneously with each liner was measured in the laboratory. The corresponding curves are reported in Fig 2. Proceedings of IPAC’10, Kyoto, Japan TUPD048 05 Beam Dynamics and Electromagnetic Fields D05 Instabilities Processes, Impedances, Countermeasures 2033 From Fig 2, one can see very low SEY obtained for the a-C liners, compared to the threshold value (δmax = 1.3) of the SPS with nominal LHC beam [3] [4]. More details about the SEY measurements are given in [5]. 0 200 400 600 80