Progress with Multipactor Testing of Carbon Coatings

Progress with Multipactor Testing of Carbon Coatings

LIU-SPS-e-cloud meeting July,3rd

Michael Holz

Michael Holz BE-RF-BR2

Content

03/07/2012 SPS-LIU e-cloud meeting

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)

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Goals of the Test Stand


Michael Holz BE-RF-BR

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

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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)

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Summary of the Results 2011

Fully coated chamber (magnetron sputtering)



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


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


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.


Preliminary Results 2012

Hollow Cathode Coating



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


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


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


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


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.


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)


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 section1. One single event in 2011 at high magnetic fields2. Multipacting independent from magnetic field variation3. Shifting the standing wave pattern to a coated area by

changing the excited mode

Documents

Progress with Multipactor Testing of Carbon Coatings