Impedance RF Measurement Coffee Meeting Minutes
Present: Fritz Caspers, Elias Métral, Federico Roncarolo, Benoit Salvant.
Practical issues:
prototype collimator not available for measurement before two weeks
PS reference magnet room has to be cleared by the end of the month
PIMS:
Measurements in a frequency range of 100 Hz to 2 MHz show a decrease of the peak value of the transverse impedance at low frequency (<20 kHz) when contact fingers are isolated, associated with asubstantial increase of the mean value at higher frequencies (between 20 kHz and 2 MHz). See PIM measurements.
an intermediate measurement (8 adjacent fingers removed out of 30) show a similar pattern, but much less pronounced. See PIM measurements.
What to do next:
do measurements with more intermediate number of fingers removed (all but 1, half)
put a metal sheet (copper or steel) between the inner tube and the fingers to simulate a bad contact due to rugosity.
Dampers gain is lower at high frequencies (up to 20 MHz). Therefore, it would be interesting to get to this range. The current coil (14 turns, 1.6 cm thick) has a resonance at 4 MHz. We should build another one with less turns, and/or a reduced width. The range of frequencies reachable by a coil can not exceed these 20 MHz, as quasi static approximation do not hold there. We have to be careful. Only 2 wire technique over this value. See PIM measurements for follow-up.
0.5 mm fiberglass capacitance is negligible up to 100 MHz. It was used to isolate the fingers from the inner tube.
longitudinal impedance -> need of contact DC resistance, and we can trust the simulations. Signal is probably to small with a single wire. Hence, no measurement foreseen in the immediate future.
Fritz's normalization coefficient was experimentally found to be around Z0. (sentence of the day...)
Collimators:
Measurements under way.
Author: Benoit Salvant CERN
AB/ABP-LIS