Dynamic vacuum issues

1. Dynamic vacuum issues Sergio Calatroni For TE/VSC

2. Beam needs • Main beam linac requirements : p10-9 mbar (from D.Schulte) • Three separate issues: • Static vacuum • Dynamic vacuum due to breakdowns • Dynamic vacuum due to field emission • Static vacuum is in itself challenging, limited by conductance, at present it seems possible to achieve it only with NEG coatings or pumps (require bakeout) cf. Presentation by P. Chiggiato http://indico.cern.ch/getFile.py/access?contribId=3&resId=10&materialId=slides&confId=77234 Sergio Calatroni

3. Dynamic vacuum • Dynamic vacuum due to breakdowns • Addressed using outgassing data measured in DC spark testing + motecarlo simulations + past experiments in CTF3 (at 30 GHz) • Cf CLIC meeting 10 June 2005 • Conclusion: the outgassing during a breakdown can be re-pumped between two RF pulses. Only the first train is thus lost • Dynamic vacuum due to field emission • Fast outgassing due to field emitted electrons in absence of a breakdown. The outgassing itself is due to ESD • Timescale of the order of the RF pulse duration: outgassing at the beginning of the pulse may affect bunches within the same train. • Outgassed molecules can travel ~1 mm during 100 ns, thus the beam can see them • Very difficult to measure: we may have p>10-9 mbar locally during the 100 ns RF pulse (in all structures) in a small volume. When this is pumped through the conductance of the RF structure, it is not measureble Sergio Calatroni

4. How we are addressing dynamic vacuum due to F.E. • Two ways: simulation and experiments • Simulations will use e- trajectories calculated by SLAC advanced computation group (Zenghai Li) • These data normalised with e- currents measured in RF tests (Faraday cups) will be used to calculate outgassed molecules using standard ESD data • The molecules trajectories will be followed by Monte Carlo calculations (Cedric Garion) and instantaneous prerssures calculated. • Experiments with standard gauges (we have routinely 20 ms acquisition time constant) outside the structures seem out of reach, because of sensitivity • We are looking into laser techniques (cavity ringdownspectroscopy) to evaluate if they allow us to measure in our conditions Sergio Calatroni