SPS scrubbing experience: electron cloud observables

L. Mether on behalf of the LIU-SPS e-cloud team

LIU SPS scrubbing review, September 8, 2015

Outline

• Outline

• Electron cloud • Electron cloud monitors• Electron cloud evolution during cycle• Electron cloud evolution over scrubbing

• Arc pressures• Pressure gauges• Pressure evolution during cycle• Pressure evolution over scrubbing

• Summary

Electron cloud monitors• Large aperture dipole magnets installed in a

straight section• Vertical magnetic fields, set to B = 0.12 T, field

in SPS bends at injection

• Each magnet is equipped with a liner…• Stainless steel MBA/MBB type vacuum

chamber, with holes drilled along the side

• …with a strip detector on the outside• 48 channels

• Signal integrated over multipleturns (20 – 200 ms)

• Provide transverse profile of electron distribution

• Help to understand the e-cloud evolution and scrubbing process in SPS bends

Electron cloud during SPS cycle• Nominal SPS 25 ns cycle

• 4 batches of 72 bunches, 1.2x1011 p+/bunch

Electron cloud during SPS cycle• Electron cloud signal along nominal 25 ns cycle

• E-cloud signal low, but still visible• Large enhancement during ramp to flat top, mainly due to bunch shortening

Ramp

Nominal intensityMBBMBA

Electron cloud during SPS cycle• Horizontal profile of electron cloud signal

MBA

MBB

Electron cloud with high-intensity beam• During the four scrubbing runs this year, the machine was run mainly

with a high-intensity beam, with around 2x1011 p+/bunch

• For this purpose a scrubbing cycle with up to 4 batches of 72 bunches, at flat bottom was used

• E-cloud signal in MBA and MBB monitors along scrubbing cycle

Ramp

• Electron cloud twice as strong as for nominal intensity beam

• Significantly more electron cloud in MBB chamber than in MBA

High intensity

Nominal intensity

Electron cloud with high-intensity beam

MBBMBA

MBBMBA

Electron cloud with high-intensity beam• Horizontal profile of electron cloud signal

• Electron cloud extends beyond region scrubbed with nominal intensity beam

High intensity

Nominal intensity

MBA MBB

Electron cloud with high-intensity beam• Horizontal profile of electron cloud signal

• This region has been effectively scrubbed!

High intensityW50

Nominal intensity

MBA MBB

High intensityW25

Electron cloud evolution during scrubbing• Study the evolution of signal in electron cloud monitors over scrubbing period

• MBA and MBB chambers• Two observation points:

− Signal after first injection− Maximum signal along cycle

MBBMBA

Electron cloud evolution during scrubbing• E-cloud signal in MBA monitor after injection of first batch

W16 W17 W23 W25

Electron cloud evolution during scrubbing• Maximum e-cloud signal in MBA monitor

First batch

Maximumsignal

W16 W17 W23 W25

Electron cloud evolution during scrubbing• E-cloud signal in MBB monitor after injection of first batch

W16 W17 W23 W25

Electron cloud evolution during scrubbing• Maximum e-cloud signal in MBB monitor

First batch

Maximumsignal

W16 W17 W23 W25

Outline

• Outline

• Electron cloud • Electron cloud monitors• Evolution during cycle• Evolution during scrubbing

• Arc pressures• Pressure gauges• Evolution during cycle• Evolution during scrubbing

• Summary

Arc pressures• Pressure gauges located between dipole magnets in each arc

• We follow two families of gauges:

X2940

MBA MBB

X0660

MBB

MBB

• Located between an MBA and an MBB dipole

• Located between 2 MBB dipoles

Arc pressures along cycle• Pressures during nominal intensity cycle with ramp

and high-intensity flat-bottom scrubbing cycle• Gauges X0660 (MBB-MBB)

High intensityNominal intensity

Ramp

Arc pressure evolution during scrubbing• Pressure rise over scrubbing cycle at gauge 40660

• Located between two MBB dipoles

W16 W17 W23 W25

Arc pressure evolution during scrubbing• Pressure rise over scrubbing cycle at gauge 42940

• Located between an MBA and an MBB dipole

W16 W17 W23 W25

Summary & conclusion• Electron cloud with high-intensity beam

• Electron cloud is more than doubled with high intensity vs nominal intensity at flat bottom− MBB type chamber more prone to e-cloud than MBA type chamber

• Scrubbing efficiency • Based on data from electron cloud monitors and pressures:

− Conditioning seen mainly in the first week of high-intensity beam− Full suppression not seen over four batches

• E-cloud monitors provide information also along the cycle:− Significant improvement seen after the injection of one batch:

full suppression in MBA, significant reduction in MBB

• Conclusion • SEY lowered during scrubbing, but not sufficiently to fully suppress e-cloud • Effect of scrubbing seems to level off

− If further scrubbing can be done, required timescales much longer than days/a few weeks

Thank you for your attention!