Acknowledgements F. Caspers, H. Damerau, M. Hourican, S.Gilardoni, M. Giovannozzi, E. Métral, M. Migliorati, B. Salvant Dummy septum impedance measurements S. Persichelli, O. Berrig, J. Kuczerowski, J. Herbst Dummy septum meeting

Remarks from last impedance meeting 1)In the unlikely event of unexpected failure or damage of the dummy septum after installation and shielding, repairing action will not be possible. This fact justifies the interest in finding preventive measures, for example reducing the impact of a mode that, from simulation results, is not predicted to be an issue for the stability of the beam. A trapped mode at 118 MHz was localized mainly in the gap between the screen and the support table and in the gap between the screen and the blade

Remarks from last impedance meeting 2)As an outcome of impedance studies, lasted two years and ended now with the bench measurements, the decision has been taken to install the sliding contacts between the RF beam screen and the support table With sliding contacts the mode at 118 MHz in cancelled and the main trapped modes is now at 270 MHz, localized in the 3mm gap between the blade and the screen.

Measurement plan We did measurements with the wire technique in three days:  7/11/2013 (Serena, Olav, Joseph, Johannes) Dummy septum with BTV in out position, blade in nominal position, three different positions for the wire/beam tested.  4/12/2013 (Olav, Joseph): After an oil leakages the septum was open, cleaned and re-assembled. Measurements without BTV, blade in unknown position, only extraction position tested.  8/1/2014 (Serena, Joseph): After broken fingers substitutions and the insertion of a new support table. BTV in out position, blade in nominal position, two different positions for the wire/beam tested.

Sliding contacts: mode cancelling Dummy Septum: impedance measurement 5 Sliding fingers allow contact between the screen and the support table, avoiding the generation of some low frequency trapped modes

Measurement setup Dummy Septum: impedance measurement... 6

Design differences Dummy Septum: impedance measurement 7 First measurement 7/11/2013 Third measurement 8/1/ mm BTV in OUT position

Measurement setup differences Dummy Septum: impedance measurement 8 First measurement 7/11/2013 Sucobox length = 27mm Sucobox with cylinder inside Metal film resistance R s = 261 Ω ± 1% 27 mm 16 mm Sucobox length = 16mm Sucobox without cylinder inside Carbon resistances R s1 = 276 Ω R s2 = 281 Ω Third measurement 8/1/2014

Differences between the two measurement setup Dummy Septum: impedance measurement 9 x2 10 dB attenuators have been used for both measurements

Measurement results  7/11/2013 (Serena, Olav, Joseph, Johannes) Dummy septum with BTV in out position, blade in nominal position, three different positions for the wire/beam tested.  4/12/2013 (Olav, Joseph): After an oil leakages the septum was open, cleaned and re-assembled. Measurements without BTV, blade in unknown position, only extraction position tested.  8/1/2014 (Serena, Joseph): After broken fingers substitutions and the insertion of a new support table. BTV in out position, blade in nominal position, two different positions for the wire/beam tested.

Low frequency S 21 Dummy Septum: impedance measurement 7/11/13 11 It seems that the wire cannot excite trapped modes below 100 MHz!

Low frequency S 21 Dummy Septum: impedance measurement 12 It seems that the wire cannot excite trapped modes below 100 MHz!

Wire in different positions Dummy Septum: impedance measurement 7/11/13 The 270 MHz mode is generated by resonances due to the space (3mm) between screen and blade. Measurements confirm what observed in simulations, that during extraction increases the amplitude of this mode. 13

Comparison measurement and simulations Dummy Septum: impedance measurement 7/11/13 14 Measurements have been compared with simulations in time/frequency domain. Both measurement and simulation agree on the first trapped mode at the frequency of 270 MHz. The fingers are working well on low frequency mode cancelling.

Measurement results  7/11/2013 (Serena, Olav, Joseph, Johannes) Dummy septum with BTV in out position, blade in nominal position, three different positions for the wire/beam tested.  4/12/2013 (Olav, Joseph): After an oil leakages the septum was open, cleaned and re-assembled. Measurements without BTV, blade in unknown position, only extraction position tested.  8/1/2014 (Serena, Joseph): After broken fingers substitutions and the insertion of a new support table. BTV in out position, blade in nominal position, two different positions for the wire/beam tested.

Low frequency S 21 Dummy Septum: impedance measurement 8/01/14 16 This time also, it seems that the wire cannot excite trapped modes below 100 MHz!

Dummy Septum: impedance measurement 8/01/14 17 Extraction Change in the baseline of 0.5 dB due to the difference in the measurement setup (sucobox length, coupling resistances….) Detuning of the second impedance peak: probably due to the different position of the BTV

Orbiting Dummy Septum: impedance measurement 8/01/14 18 Same remarks of last time: in orbit position the amplitude of the first peak is lower than at extraction, while the amplitude of the second peak is constant

Measurement results  7/11/2013 (Serena, Olav, Joseph, Johannes) Dummy septum with BTV in out position, blade in nominal position, three different positions for the wire/beam tested.  4/12/2013 (Olav, Joseph): After an oil leakages the septum was open, cleaned and re-assembled. Measurements without BTV, blade in unknown position, only extraction position tested.  8/1/2014 (Serena, Joseph): After broken fingers substitutions and the insertion of a new support table. BTV in out position, blade in nominal position, two different positions for the wire/beam tested.

Comparison between the three measurements Dummy Septum: impedance measurement 20 Mode at 360 MHz: normally is damped by the BTV For measurement nº 2 difference in the baseline of 20 dB: due to the absence of the x2 10 dB attenuators Detuning: due to the absence of the BTV

21 Conclusions Wire measurements for the dummy septum with sliding contacts confirmed that the first trapped mode in the dummy septum has a frequency of 270 MHz, that is predicted to be too high to be a source of coupled bunch instability in the PS. Measurements are in agreement with simulation performed with CST both in time and in frequency domain for the first mode, while the broad peak at 300 MHz is not visible in simulations. Sliding contacts work well on mode cancelling: we believed that the insertion of ferrite is not necessary. The presence of the BTV seems to have an impact on the high frequency eigenmode arrangement: it provokes damping and coupling effects. The dummy septum is currently in the PS tunnel and installation is starting in the next weeks. NB. The aim of the measurements was to confirm the results from simulation and exclude the generation of low frequency (<200 MHz) trapped modes