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Sparking of SPS electrostatic septa in presence of e-cloud

Sparking of SPS electrostatic septa in presence of e-cloud. K. Cornelis CERN. The SPS electrostatic septum (ZS) : a simple view. Anode. Cathode. -3kV. -6kV. -220kV. Ion-traps. Classical sparks. During extractions : Ionization due to beam losses. (Ion-traps)

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Sparking of SPS electrostatic septa in presence of e-cloud

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  1. Sparking of SPS electrostatic septa in presence of e-cloud K. Cornelis CERN

  2. The SPS electrostatic septum (ZS) : a simple view Anode Cathode -3kV -6kV -220kV Ion-traps

  3. Classical sparks • During extractions : Ionization due to beam losses. (Ion-traps) • Insulation fatigue (Protect insulation for radiation damage). • Synchrotron radiation : (Masks)

  4. A mysterious spark problem

  5. ANODE CURRENTS Lower intensity No extraction

  6. Other observations • Correlation of signals with bunch length • Increasing the ion-trap voltages helped. • Some changes in septum wiring gave bad results.

  7. Other observations

  8. Other observations

  9. Some years later • A strip detector was installed in the SPS in order to study the electron-cloud • problem with LHC-type beam (25 nsec. spacing) • With high intensity and short bunches we could also observe electron activity with the fixed target type of beam.( 5 nsec spacing)

  10. E-cloud observed when bunch density highest in cycle

  11. Comments • Although the e-cloud could be observed in the strip detector, the septum sparking stayed calm for a long time : the beam intensity was lower than in 1997 and we ran with systematic high ion traps. No indications of e-cloud in the septa. • In order to have e-cloud on the FT-beam in the strip detector the e-cloud had to be provoked by an LHC-beam in a neighboring cycle. • However, we think that the intensity in 1997 was high enough to provoke the cloud by itself and this in spite of the ion-traps. • We also think that the ion-traps (connected to the outside world by a very high impedance) can be influenced by beam induced fields, reducing their effectiveness. Once multi-pacting sets on, they lose completely their efficiency.

  12. New incidents • A septum was destroyed during a scrubbing run with LHC beam in 2002 while OFF. • During a high intensity test in 2004 sparking reoccurred in one septum.

  13. Conclusions • E-cloud has been observed in the SPS for high intensity FT beams. • Multi-pacting phenomena have been responsible for sparking and/or damaging electrostatic septa. • The ion-traps seem to lose their function as clearing electrodes under certain conditions.

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