----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 03.02.2006 present: FC, AG, WH, EM, TP, FR, GR, RT, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions -------------------------------------------- Moderated by FR, the team went through the open actions. Several items were found to be completed. Others were extended or re-assigned. After completing the literature search for 2-beam wake fields, new actions are required to reach a conclusion. => ACTION => AG will calculate the effect for cavity-like modes. => ACTION => FZ will derive formulae for the 2-beam resistive-wall wake => ACTION => Perform e-cloud simulations for the two SPS working points - FZ will check whether EB and/or Giuliano Franchetti have results; If not, the task will be re-assigned to GR and FZ. => ACTION => Review of predictions of incoherent electron-cloud effects in the LHC - reassigned from FZ and EB to FZ and GR. => ACTION => Prediction of beam lifetime and emittance growth as a function of density for the LHC and SPS based on HEADTAIL simulations for a dipole field - FZ will check whether EB has results; If not, the task will be re-assigned to GR and FZ. OLD ACTION => Compare collimator bench measurements with HFSS simulation AG reported that this "is always being delayed" due to lack of time and not high enough a priority. Given discussions on electron cloud in Chamonix@Divonne, FR recommended to write note describing our view of the situation. => ACTION => Organize a meeting with Laurent Tavian on cooling capacity for the development of scrubbing scenarios, also in the context of HHH. FR recalled that one of the HHH deliverables for summer 2006 is a structured list of intensity limits for CERN and GSI. This will be addressed in a bilateral GSI-CERN collaboration meeting at Darmstadt end of March (see http://care-hhh.web.cern.ch/CARE-HHH/workshops.htm). S.Y.Lee and S. Machida may attend as well. FR reported a presentation by Tom Kroyer in the APC, which covered the collimator bench measurements from half a year ago. => NEW ACTION => Produce a plot comparing the transverse impedance deduced from TK's and FC's measurement with the theoretical prediction taking into account the contributions from detuning and driving wakes (FZ). => ACTION => Compute longitudinal impedance for an IPM model; AG did the calculation, but he has not presented them at an RLC meeting. Measurements of a prototype are available. The design was changed 6 months ago following the recommendations by AG, FC and FR. Resistive coating of the ceramic and rf contacts are some of the changes now underway. The final device with all recommended modifications will also be measured. => ACTION => provide a proposal for LHC IR magnet requirements; evaluate chromatic performance of all IR solutions for the LHC upgrade - task reassigned to RT (2) Follow-up on the impedance of TDI, TCLIA, and TCTV (AG) ----------------------------------------------------------- AG first presented calculation results for TDI. His technical contact is Luca Bruno. Our earlier recommendations were implemented. A ferrite ring 4S60 is now installed for damping trapped modes on either side of the device, near the flanges. The already existing collimator block was cut to provide a tapering angle of 15 degree. Doing so increased the outer gap from 8 mm to 28 mm, which should reduce the broadband impedance. The TDI will be installed in April. FR asked for an estimate of the final TDI impedance => NEW ACTION => TDI final impedance (AG) FC would like to look for high Q modes, with wire measurements. Luca Bruno will be asked to give him access. => NEW ACTION => Access to FC for TDI wire measurements (AG via Luca Bruno, FR if needed) TCLIA consists of graphite and has a gap of 12 mm, whereas TCTV is made from tungsten, with a 3 mm gap. The former serves for protection, the latter for collimation. AG found a longitudinal mode near 1 GHz, with a total loss power of 170 W, and loss at one rf finger of 1.1 W. For 2 beams the losses could be higher, in the worst case by a factor of 4. The temperature increase of the rf finger is much larger than 1 degree per 0.1 W of power; the exact value is unknown. AG considered the addition of 4S60 ferrite near the rf finger. this reduces the total loss power from 170 W to 0.5 W, and the Q from 2550 to 26. Therefore, the design with ferrite is OK for the longitudinal modes. FC recommended to look out for 'magnetic vetoes' at all devices in which ferrites will be installed. A magnetic field in the vicinity of the ferrite could alter the permeability of the ferrite, which might reduce its damping properties. On the other hand, the magnetized ferrite could also perturb the magnetic field acting on the beam. To avoid the first effect, the ferrite should be at least 40 cm away from the next magnet (a residual field of 0.1 T is relevant). The perturbation on the beam is thought to be insignificant for the LHC. FC reassured the team that the ferrites can sustain a high radiation dose, similar to ceramics. A simulation for the transverse modes of TCTV/TLCIA is underway. (3) Single and coupled-bunch instabilities at LHC top energy vs Q' (EM) ----------------------------------------------------------------------- EM presented growth rates for 1-bunch and coupled-bunch instabilities at top energy for different values of chromaticity. He compared results with secondary and tertiary collimators made from graphite or copper, respectively. All collimator data were provided by Guillaume R-D on 11.10.2005. EM reminded the team that Q'=1 corresponds to a chromatic frequency shift of about 40 MHz. With graphite at Q'=0, 50% of the nominal intensity can be reached, whereas with copper it is 70%. However, for graphite a moderate increase in chromaticity to Q'~3-4 can stabilize the nominal intensity, while for copper the nominal beam is still unstable even with Q'=5. The copper calculations show a larger tune shift in the imaginary direction than for graphite. Therefore, replacing the secondary graphite collimators by copper ones does not necessarily improve the situation. EM suggested that his calculations be confirmed by a simulation code. (4) AOB ------- FC announced the observation of the effect of strong mechanical deformations of the beam screen with a period of 70 cm, attributed to the periodicity of the support rings holding the screen, and evidenced by an electric pattern at 4-8 GHz. The measurements were done with the microwave reflectometer. FC and EM mentioned discussions on TOTEM Roman Pots. => NEW ACTION => send email to Daniela Macina (FR) -> done! The message is that there should be no production before the rf design was reviewed by FC et al and possibly lab measurements were performed for the latest version. There have been no measurements of the transverse impedance. Posted on the web: Slides by AG and EM Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/