Cavity mode related wire breaking of the SPS Wire Scanners and loss measurements of wire materials

During the SPS high intensity run 2002 with LHC type beam, the breaking of several of the carbon wires in the wire scanners has been observed in their parking position. The observation of large changes in the wire resistivity and thermionic electron emission clearly indicated strong RF heating that was depending on the bunch length. A subsequent analysis in the laboratory, simulating the beam by two probe antennas or by a powered stretched wire, showed two main problems: i) 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; ii) the carbon wire used so far appears to be an excellent RF absorber and thus dissipates a significant part of the beaminduced power. Different wire materials are compared with the classical cavity mode technique for the determination of the complex permittivity in the range of 2-4 GHz. As a resonator a rectangular TE01n type device is utilized. 1 WIRES HEATING IN THE SPS TUNNEL During the two last Machine Development periods in the SPS 2002 run, several wires were found broken. Such breaking can be typically related to the wire heating due to some energy deposition by the traversing protons on the wire. Dedicated electronics has been installed in order to have an indication of the wires heating during the LHC type beam injection and ramp in the SPS. In particular a constant current was supplied to the wire and the voltage drop across it was fed to a digital scope together with the difference between the input and output currents. The differential current (Iout−Iin) grow up is due to the wire heating and consequent emission of electrons for thermionic effect. Fig. 1 shows such voltage and differential current evolutions during the SPS cycle with LHC type beam. No scans were performed along this cycle. It is thus evident that the wire heating does not depend on the direct wire-beam interaction. In particular it is possible to relate the wire heating to the beam intensity (two batches of 72 bunches with 1.1 · 10 p/bunch injected in this case) and to the bunch length which is decreasing along the beam ramp to 450 GeV. The measurements described in the previous section clearly revealed that the bunch length shortening causes a larger wire heating than the beam intensity. Such observations lead the study of possible RF coupling effects between the Time [s] 0 5 10 15 20 25 S ig n al [ V ]