1. Impedance results of SLAC RC MD N. Biancacci, E.Mètral, B.Salvant, A.Valimaa OP, & Collimation Team

2. MD overview Purpose: characterize the SLAC RC from the impedance point of view. Tune shift Vs collimator gap variation -> Im(Zt) 18-08-2015: Measurement at 55 GeV 19-08-2015: Measurement at 270 GeV

3. Analytical estimations Dipolar impedance of 2D infinitely extended parallel copper plates Vs gap. 3e-6

4. 55 GeV Fill #1Fill #2 No visible tune shift when closing the RC. Large losses induced by scraping the beam when the RC half gap approaches 5mm

5. 55 GeV Fill #3: Measurement sensitive to the machine drift. Fill #4: Again no visible tune shift. Fill #3Fill #4

6. Instability when scraping to full gap of 10mm at 55 GeV 0.127 Reminder: Qs @ 55GeV ~ 0.01 -> Mode +1 observed When closing the RC we provoked scraping of the beam when approaching 5mm half gap. Qx Qx + Qs 0.1356

7. 270 GeV Tune drift We repeated the measurements at 270 GeV to have smaller emittance and approach closer RC gaps (even if the tune shifts is lower). Fill #5: Tune shift below the measurement accuracy. Fill #6: Tune drift (the gap was kept constant) Fill #5Fill #6

8. 270 GeV Tune drift (starts before gap moving) Fill #7 Fill #7: Tune shift below the measurement accuracy. Fill #6: Tune drift (the gap was kept constant)

9. Conclusions The expected tune shift is ~3e-6 at 55GeV. Measurements at 55GeV do not show gap- induced tune shift (under machine tune noise) Measurements at 270 GeV are also quite flat (1 st scan). Other measurements affected by tune drift (orbit effect? tbc). Instability observed when deep scraping at 55GeV (+1 mode, Qs~0.01) -> loss of Landau damping due to emittance reduction? tbc.

10. MD proves that resistive wall impedance is small (< measurement capabilities) What about HOM impedance? Do we need another MD..? Outlook