Beam Coupling Impedance Measurement and Mitigation for a Totem Roman Pot

The longitudinal and transverse beam coupling impedance of the first final TOTEM Roman Pot unit has been measured in the laboratory with the wire method. For the evaluation of transverse impedance the wire position has been kept constant, and the insertions of the RP were moved asymmetrically. With the original configuration of the RP, resonances with fairly high Q values were observed. In order to mitigate this problem, RF-absorbing ferrite plates were mounted in appropriate locations. As a result, all resonances were sufficiently damped to meet the stringent LHC beam coupling impedance requirements. THE TOTEM ROMAN POTS The LHC experiment TOTEM [1] is designed for measuring the elastic pp scattering cross-section, the total pp cross-section and diffractive processes. These physics objectives require the detection of leading protons with scattering angles of a few μrad, which is accomplished using a Roman Pot (“RP”) system with stations at 147 m and 220 m from the interaction point 5 where also CMS will be located. Each station is composed of two RP units separated by a few metres depending on beam equipment integration constraints. Each RP unit consists of a vacuum chamber equipped with two vertical insertions (top and bottom) and a horizontal one (Fig. 1). Each insertion (“pot”) contains a package of 10 silicon detectors in a secondary vacuum. The pots can be moved into the primary vacuum of the machine through vacuum bellows. In order to minimise the distance of the detectors from the beam, and to minimise multiple scattering, the wall thickness of the pot is locally reduced to a thin window foil. The low impedance budget of the LHC machine (broadband longitudinal impedance limit Z/n ≈ 0.1Ω) imposes a tight limit on the RPs’ beam coupling impedance. Figure 1: Left: the vacuum chambers of a RP unit accomodating the horizontal and the vertical pots and a Beam Position Monitor. Right: the pot with the thin window and a Ferrite collar (black). IMPEDANCE MEASUREMENT WITH THE WIRE METHOD Longitudinal Impedance The beam coupling impedance measurement was performed with the wire method like with the first RP prototype in 2004 [2]. After pulling a 0.3 mm thick wire through the RP along its beam axis, a vector network analyser was used to measure the complex transmission coefficient S21(f, dx, dy) between the two ends of the RP (Fig. 2) as a function of the frequency f and of the horizontal and vertical pot distances (dx, dy) from the wire.