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During 2002 SPS running with the high intensity LHC type beam the
breaking of several of the carbon wires in the wire scanners has been
observed. This damage occurred with the scanners in their parking
position. The observation of large changes in the wire resistivity and
thermionic electron emission indicated clearly a strong RF beam induced
heating and its bunch length dependence. A subsequent analysis in the
laboratory, simulating the beam by a RF-powered wire, showed two main
problems. The housing of the wire scanner acts as a cavity with a mode
spectrum starting around 350 MHz and high impedance values around 700
MHz. The carbon wire used appears to be an excellent RF absorber and thus
dissipates a significant part of the beam-induced power. The classical
cavity mode technique is used to determine the complex permittivity and
permeability of different samples. As a resonator, a rectangular TE01N
type device is used. Different materials such as silicon carbide (SiC),
carbon and quartz fibres as well as other samples were measured, since no
data for these materials was available. In particular SiC properties are
of interest, since SiC bulk material is often used as a microwave
absorber. As a result, the carbon wire will be replaced by a SiC wire,
which shows much less RF losses. Placing ferrite tiles on the inner wall
of the wire scanner housing considerably reduces the impedance of the
cavity modes. The reduction of the Q values of these modes is confirmed
by laboratory measurements.
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