----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 15.07.2005 present: FC, UD, AG, TP, FR, FZ, Federico Roncarolo, Miguel Furman web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, announcements ----------------------------------------------------------- The list of actions is posted at http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm . There were no comments to the minutes of the previous meeting. (2) LHC Ionization Profile Monitor: Impedance Measurements (FRo) ---------------------------------------------------------------- FRo explained the functioning principle of the ionization profile monitor (IPM). The IPM is the only monitor which can measure the beam size at full intensity and high energy, in addition to the synchrotron light monitor. The standard version gives an average beam profile every 20 ms, a fast gated version will provide the beam size of individual bunches. An IPM is installed in the SPS since 2002 for test purposes. The LHC will have two of these monitors per ring. FC pointed out that the monitor can be viewed as a stripline pickup. The analysis of current-dependent betatron phase advances from turn-by-turn BPM readings in the SPS, which was presented by FZ to the APC in May, hinted at a large impedance source near quad 507, not far from the IPM (517). The effective impedance was comparable in magnitude to that of the kicker magnets. FRo presented the results of lab bench measurements of the IPM longitudinal impedance performed with FC. The IPM was in its vacuum tank during these measurements. The first measurements, of the S21 with a network analyzer, revealed that rf power is stored in the IPM, with resonances visible at 140 and 220 MHz. Some of the resonances could be damped by loading one or several of the 8 outside connectors with 50-Ohm terminations. Other resonances are only marginally affected by the loaded connectors. FC stressed that the insertion of ferrite absorbers is problematic, since the IPM is located inside a dipole magnet. Therefore, damping could not easily be added. The main concern is a possible effect on the IPM performance for the longitudinal impedance and the effect on the beam for the transverse impedance. FC reported that a possible modification, replacing the metallic high-voltage plates by ('invisible') resistive-coated ceramics is under investigation. He estimated the impedance of the present device to be in the kOhm range. FR suggested that the installation of the present IPM could be vetoed, and that the alternative invisible scheme be pursued. The motivation for the IPM should also be discussed, in view of so many alternative calibration methods via wire scans, synchrotron light monitors and collimator scans. FR mentioned that an explicit value of the IPM impedance would be valuable. FC thought that this value is more easily obtained in numerical simulations than in the lab measurements. => ACTION => Compute longitudinal impedance for an IPM model (AG) (3) Transverse Collimator Impedance from FC's Bench Measurement (FZ) -------------------------------------------------------------------- In the previous meeting FC presented measurements of the longitudinal impedance as a function of transverse position at different rf frequencies. FZ used FC's data to extract the real part of the transverse impedance as a function of frequency. The transverse impedance is obtained by a parabolic fit, followed by an application of the Panofsky-Wenzel theorem. The transverse impedance between 100 MHz and 1.5 GHz is of the of order 50-100 kOhm, and it shows an oscillatory pattern. This impedance value is comparable to trapped-mode impedance computed by AG around 1 GHz and also agrees, within a factor of ~2, with the expected effect of the resistive-wall impedance. During the meeting, FC interpreted the oscillatory pattern as an in-situ measurement of the contact resistance. After the meeting, FC found a different explanation of the strange pattern without the need for invoking a contact resistance. He assumes that the length of graphite equals 1.2 meter and the total length of the wire between end flanges of the tank about 2.6 meter. Since the collimator with tank resembles a coaxial resonator where only half of the length is "high loss"[graphite] and the other half is low loss and taking into account that only n* lambda/2 resonances - with a maximum of the current always in the middle of the graphite bar (where the minimum of the electric field is) - are seen, a possible explanation that would reasonably fit is [60 cm=lambda/4 and 3/4 lambda etc]. (4) AOB ---------------------------------------------------------- AG was contacted by Vincent on the TDI design, specifically on whether 50-g stress was needed for the sliding contact. Following AG's proposal, the sliding contact was replaced by a fixed contact. FR and FC reported a decision was taken to build the LHC Wall Current Monitors, APWL, of Thomas Bohl without a 100 um Cu layer. Posted on the web: Slides by FRo, FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/