Progress with Multipactor Testing of Carbon Coatings LIU-SPS-e-cloud meeting July,3 rd Michael Holz

Content 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR2 1. Reminder: Goal of the test stand & experimental set-up 2. Summary of the results in 2011 (fully coated chamber) 3. Preliminary Results in 2012 (hollow cathode coating)

Goals of the Test Stand 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR3  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

Reminder: The Experimental Set-up 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR4  A tungsten wire was drawn along a SPS bending magnet inside 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)

Reminder: The Experimental Set-up 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR5

Reminder: The Experimental Set-up 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR6

Summary of the Results 2011 Fully coated chamber (magnetron sputtering) 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR7

Characterization of the uncoated chamber 03/07/2012 SPS-LIU e-cloud meeting  Characterisation of the uncoated StSt chamber  Reflected power for all magnetic fields with a pronounced multipacting at cyclotron resonance condition Michael Holz BE-RF-BR8

Measurements on the fully coated chamber 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR9  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

Summary of Results /07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR10  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.

Preliminary Results 2012 Hollow Cathode Coating 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR11

Characterization of the uncoated chamber 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR12  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

Low magnetic MHz, coated 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR13  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

High magnetic MHz, coated 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR14  Reflected power shows basically the same amount as it does at low magnetic fields  Relatively little difference in outgassing at different magnetic fields

Low magnetic MHz, coated 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR15  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

high magnetic MHz, coated 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR16 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.

Application of ceramic tubes 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR17  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)

Summary of the hollow cathode coating 03/07/2012 SPS-LIU e-cloud meeting Michael Holz BE-RF-BR18  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 the chamber still exists  We got a few hints that support the hypothesis of MP outside the dipole section 1. One single event in 2011 at high magnetic fields 2. Multipacting independent from magnetic field variation 3. Shifting the standing wave pattern to a coated area by changing the excited mode