Progress with Multipactor Testing of Carbon Coatings

1. Progress with Multipactor Testing of Carbon Coatings LIU-SPS-e-cloud meeting July,3rdMichael Holz

2. Content • Reminder: Goal of the test stand & experimental set-up • Summary of the results in 2011 (fully coated chamber) • Preliminary Results in 2012 (hollow cathode coating) Michael Holz BE-RF-BR

3. Goals of the Test Stand • Induce multipacting in order to measure e-cloud in stand-alone coated and uncoated beam pipes (SPS chambers) • Quality control for different carbon coatings • Find a method to perform all these measurements: - independent from the real beam- independent from the accelerator’s schedule- under laboratory conditions Michael Holz BE-RF-BR

4. Reminder: The Experimental Set-up • A tungsten wire was drawn along a SPS bending magnetinside the chamber (6.4 m) • The wire and the chamber form a coaxial transmission line (transversal electromagnetic waves) • The wire is terminated with a short circuit. Thus, the chamber becomes a resonator • We rely on a standing wave pattern to achieve the necessary E-Field strength (limited input power by the amplifier) Michael Holz BE-RF-BR

5. Reminder: The Experimental Set-up Michael Holz BE-RF-BR

6. Reminder: The Experimental Set-up Michael Holz BE-RF-BR

7. Summary of the Results 2011 Fully coated chamber (magnetron sputtering) Michael Holz BE-RF-BR

8. Characterization of the uncoated chamber • Characterisation of the uncoated StSt chamber • Reflected power for all magnetic fields with a pronounced multipacting at cyclotron resonance condition Michael Holz BE-RF-BR

9. Measurements on the fully coated chamber • Reflected power for the completely coated chamber • No multipacting except for one single event • After this event, pressure activity remained, but no additional reflected power was observed Michael Holz BE-RF-BR

10. Summary of Results 2011 • The fully coated chamber has shown a great multipacting suppression. • For all measurements below 3 kA, no reflected power nor outgassing could be observed, even under cyclotron resonance condition. • When using 3 kA through the magnet, a single rise in reflected power occurred. Afterwards, reflected power disappeared. However, pressure activity remained for all consecutive measurements. Michael Holz BE-RF-BR

11. Preliminary Results 2012 Hollow Cathode Coating Michael Holz BE-RF-BR

12. Characterization of the uncoated chamber • Reflected power measuring the uncoated chamber as received from chemical cleaning • Cyclotron resonance occurs at the expected value (in reflected power) • Outgassing does not show sharp correlations to the reflected power the system is relatively dirty and unconditioned Michael Holz BE-RF-BR

13. Low magnetic field @130.25 MHz, coated • Reflected power independent from the magnetic field Multipacting could happen outside the dipole section (e.g. pre-chamber, pumping ports) • Both, reflected power & pressure do not show a pronounced MP at cyclotron resonance Michael Holz BE-RF-BR

14. High magnetic field @130.1 MHz, coated • Reflected power shows basically the same amount as it does at low magnetic fields • Relatively little difference in outgassing at different magnetic fields Michael Holz BE-RF-BR

15. Low magnetic field @83.64 MHz, coated • We changed the injection frequency to a lower resonance • Reflected power shows significantly lower signals and almost no rise. • Also, there is no significant rise in outgassing Shifting the standing wave pattern to coated areas • No pronounced signals at cyclotron resonance Michael Holz BE-RF-BR

16. high magnetic field @83.64 MHz, coated No pressure rise was observed during these measurements except for 2000A and 2500A to the range of 2.6*10-6 mbar. This is just slightly visible in the reflected power. Michael Holz BE-RF-BR

17. Application of ceramic tubes • We put small ceramic tubes around the wire in the uncoated regions of the system (pre-chamber, pumping orts) • The results of these tests are still ambiguous • We have seen pressure rises that were not detectable by the VNA (see also results 2011) • For all frequencies (also 83 MHz), we got reasonable outgassing with zero field for the first few measurements • Ceramics have a relatively high SEY and could be responsible for the high outgassing  However: The fact that the MP reduces after a few measurements supports the idea, that MP is not in the ceramic insulated region. (otherwise, outgassing should remain due to the high SEY) Michael Holz BE-RF-BR

18. Summary of the hollow cathode coating • In total, we have seen just slight reflected power and pressure rises, which should still be sufficient for an improvement in the e-cloud issue • There is no real proof yet were the MP comes from • The possibility that MP occurs at the uncoated sides of thechamber still exists • We got a few hints that support the hypothesis of MP outside the dipole section • One single event in 2011 at high magnetic fields • Multipacting independent from magnetic field variation • Shifting the standing wave pattern to a coated area by changing the excited mode Michael Holz BE-RF-BR