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Collimator Wire Measurements

Collimator Wire Measurements. F. Caspers, T. Kroyer. Measurement Strategy. 0.5 mm diameter copper wire placed in center of collimator Mainly the highly sensitive resonator method was applied

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Collimator Wire Measurements

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  1. Collimator Wire Measurements F. Caspers, T. Kroyer

  2. Measurement Strategy • 0.5 mm diameter copper wire placed in center of collimator • Mainly the highly sensitive resonator method was applied • Semi-automized routine implemented on network analyser to speed up tedious repetitive measurement tasks • Time domain transmission measurements used to find out the line impedance Z0 that is needed for evaluation of the resonator data • Measurements were performed for many jaw positions

  3. Time Domain Step Mode Measurement • S11 measured in step mode • At each position S11 can be read and the line impedance Z0 calculated from the relation Z0 = 50W*(1+S11)/(1-S11) • Values for Z0 determined for the region of the collimator jaws and for the transition pieces S11 in low pass step mode • Collimator open: flat response • Collimator closed: big drop • Slope in jaw region mainly due to attenuation and not to a real change in line impedance • Z0 of transition pieces differs significantly from Z0 of jaws • 1200 mm jaw length used, 100 mm long transitions on either extremity neglected Transition Transition Jaws

  4. Line Impedance for Different Jaw Positions • Line impedance at the positions of the jaws measured using S11 and S22, averaged data in red curve • Theoretical value calculated for wire between two infinitely extended perfectly conducting planes in blue curve • Theoretical data systematically lower than measured data => WHY??? skin depth should not play so big a role

  5. Effective Jaw Aperture • Using the measured line impedance, the effective aperture was calculated by inverting the formula on the previous page • The effective half aperture is roughly 1.5 mm bigger than the mechanical aperture • This corresponds to the skin depth at 1 MHz using the DC graphite resistivity of 10 mW*m of the graphite [1] • A linear frequency sweep between 1 MHz and 2 GHz was used for the measurement [1] Assmann, R. et al., LHC Collimation: Design And Results From Prototyping And Beam Tests

  6. Resonator Measurement • The TEM transmission line composed of the collimator jaws and the wire can be used as a TEM resonator • Capacitive coupling with low coupling coefficient k chosen, k = S21/(1-S21) • Loaded quality factor QL measured for each peak as well as S21 • Line attenuationa can be easily calculated from • with Q0 = QL * (1+k) • For too strong coupling, the unloaded Q factor can be obtained by correcting the measured Q factor knowing S21 • For too weak coupling the Q measurement gets impacted by noise • An appropriate coupling coefficient between this two extremes was chosen

  7. Resonance Pattern • The typical resonance pattern for open and closed collimator is plotted below • Equidistant peaks when the collimator is open, variable peak spacing when it is closed

  8. Typical Resonator Results • Q factor measured at 20 resonance peaks below 1500 MHz • Total attenuation only slightly above the values calculated for the wire used • Wire attenuation removed using with the wire conductivity r, wire diameter d and the outer conductor diameter D; ln(D/d) can be found from the measured line impedance Z0; separate correction necessary for the transition pieces, since they have got a Z0 different from the jaws Collimator fully open

  9. Determination of Impedance • The DC resistance R0 of a wire of length L, resistivity r and diameter d is given by • For skin depths d small compared to d the high frequency resistance R = R0 * d/(4d) • This resistance is proportional to the line attenuation • Comparing the measured collimator attenuation to the calculated wire attenuation the real part of the collimator impedance can be found Detail - Collimator fully open All Data - Collimator fully open

  10. Impedance for Different Apertures • The jaws were moved in symmetrically • In a simple DC resistance measurement, the wire was found to be ~0.3 mm offset from the center Aperture Aperture All Data Detail

  11. Wire offset Transverse Sweep (1/3) • Aperture kept at 10 mm • Jaws moved together from the center to the side; a positive offset means that the wire is closer to the right jaw

  12. Transverse Sweep (2/3) • Wire offset swept in 0.5 mm steps from -4.2 to +4.8 mm

  13. Full Transverse Sweep (3/3) • The resonator peaks below 500 MHz as a function of the wire offset • First sweep in the range from +0.3 to +4.8 mm, second sweep from +0.3 mm to -4.2 mm. Discontinuity in the center of the traces as a consequence of long time drift

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