----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 17.09.04 present: EM, WH, FR, EV, FZ excused: EB, DS special guests: F. Caspers, A. Grudiev, S. Redaelli ----------------------------------------------------------------------- (1) RF Measurements of Trapped Modes in the LHC Collimator Prototype (SR) ---------------------------------------------- SR performed measurements on the prototype collimator installed in the SPS. The prototype contains carbon jaws of 25 mm thickness. The collimator gap can be varied between 60.9 mm and about 1 mm. Three antennas are mounted on the two ends of the collimators, outside of the tapered region. Two antennas are on the top (on either end) and one antenna is on the side (at the downstream end). The measurements were done with a network analyzer in a frequency range between 0 and 3 GHz. The transmission measurements from the top on one end to the side antenna on the other end appeared to give the most sensitive signal. Reflection measurements were also conducted. For these preliminary measurements the minimum gap was set to 20 mm. With a gap of 52.9 mm a high-Q resonance is visible in the reflection measurement from the side. Reflection measurements from the top give a different signal and do not show the same resonance. The transmission measurement exhibits little signal when the jaws are fully open, but already at 52.9 mm gap size two regions with a number of high-Q resonances appear. The frequency of these resonances moves up by 100-200 MHz as the jaws are closed further. Reflection measurements were done for 0.8-2.0 GHz and transmission measurements from 1 to 3 GHz. FR considered the dependence on the gap size as rather weak, FC as strong and dramatic compared to the 40 MHz LHC bunch frequency. Similarly, the resonance Q of about 1e3 is considered either low (for the beam - FR) or high (for such a carbon structure - FC). The rf antenna can also be used to detect beam-induced signals. So far measurements with beam were done only with jaws open, and no sign of trapped modes was detected in this case. FC bets a bottle of champagne that a beam-induced trapped-mode signal will be seen when the jaws are closed, at least 20 dB above the noise. FC expects that the R/Q of the resonances can be crudely estimated from the beam-induced signals taking into account the coupling to the antenna and the attenuation by about 300 m cable. FC mentioned that he has performed rf measurements on the SPS roman pot as well on the same day. FZ asked whether the temperature of the rf fingers is monitored for HOM heating. This is not the case. FC thinks there is a nonzero risk that the fingers may melt for a certain gap. He proposes to install ferrite to damp HOMs in the LHC collimator tanks, if heating is a problem. Comparison with simulations (DS and AG) is underway. AG commented that the correct model for the sliding contacts is needed. The person with this information is Oliver Aberle. FR asked whether we can draw conclusions on the localization of the modes. This is not easy. Probably parts of the modes reside inside the tank and not on the jaw, in view of their fairly high Q. AG suggested to connect an rf probe to the tank. FC proposed to do parasitic MDs as soon as possible, where the collimator gap is closed by 10%, for which he already expects a significant signal. FR presented this idea to the APC in the afternoon. FR will inform Ian Wilson about simulation work by AG. (2) New Definition of the Montague-Resonance Stopband for Intensity-Dependent Emittance Transfer Based on Envelope Equations, Etc. (EM) ------------------------------------------------------- EM reported that the SPS reached an intensity of 4.4e13 protons in the last high-intensity MD, which is close to the record 4.8e13 from several years ago and consistent with the CNGS target value. Concerning the Montague resonance, EM presented a new and old definition of the stopband, based on his solution to the envelope equations. The new definition has the advantage that the stopband width goes to zero in the limit of a round beam. It characterizes the increase in the vertical emittance after the resonance crossing. EM pointed out a joint paper at EPAC04, which for the first time presented the result of a dynamic simulation of the resonance crossing rather than static simulations with constant tunes. Various static simulations (GSI code and ORBIT) predict an asymmetric situation for the static case, which is not observed in experiments. The dynamic experiments seem to be roughly consistent with the dynamic simulations which include synchrotron motion. The synchrotron motion has a strong effect. FZ asked whether the variation of the lattice functions around the ring would have an influence on the result and whether it is included in some of these simulations. EM will investigate whether the lattice is modelled by the GSI code. (3) Actions from Last Meeting (FR) ----------------------------------- FR reported that there was a round-table discussion on ongoing activities. One outstanding action item is the unification of the ECLOUD code. FZ responded that he sent 4 versions of the code (including two from Giulia Bellodi) to DS, who has merged them with his own version of ECLOUD. After the meeting DS reported an agreement of the new code with earlier simulations. Some more tests may be required to validate the version. The interfacing of the different options should also still be agreed on. FR mentioned that he submitted a request for two new staff positions, one for "Impedance Estimates and Simulation of Collective Effects for an Upgrade of the LHC Accelerator Complex" and the other for "Study of LHC Beam Intensity Limitations and Cures, Contribution to Machine Experiments and Participation in the LHC Commissioning". In total there may be ~36 posts for the AB department. (4) Measurements of Kicker Effect & Damping (EV) ------------------------------------------------- EV continued studies on the damping of kicker-induced betatron oscillations for high energy when ejecting the first CNGS batch. SPS measurements were done at injection. The last bunches are strongly affected by the kicker pulse. The feedback damping depends on the bunch position within the gap, with faster damping near the center, due to limited bandwidth. Some measurements were also taken at transition. There was a discussion whether the strong effect on the last bunches can be avoided by proper tuning of the kicker or not. (5) Collimator Tests (EM, FR) ----------------------------- Collimator tests will be done in weeks 42 to 44 (1 day per week). For impedance studies, two 6 hr periods are foreseen. Based on APC discussions, the measurements proposed by GA are tune shift measurements and detection of the orbit kick induced by an asymmetric aperture. The week 43 overlaps with the ICFA workshop. In week 41, there is the set up of TOTEM and LHC beams. Other impedance measurement schemes could be tried, if time allows, such as feedback based techniques, or recording of coherent growth/damping rates. Attached: Slides by EM, SR