Switching to 50/75 ns bunch trains G. Arduini with input from R. Assmann, P. Baudrenghien, K. Cornelis, B. Goddard, W. Herr, W. Höfle, G. Rumolo, R. Steerenberg, J. Wenninger
Why? • Early look at potential problems for next year operation: • More insight in present and potential vacuum issues total number of bunches comparable with those achieved with 150 ns • beam-beam at injection and flat-top (scaling of observations with 150 ns to shorter spacings) • e-cloud effects • capture efficiency with shorter spacings (capture losses could become more pronounced due to finite filling time of SPS RF cavities) issue for injection losses • behaviour of Beam Instrumentation and RF/damper with tighter spacing • exploration of luminosity production with tighter spacings (backgrounds, lifetimes, etc.)
Status in the Injectors • 50 ns: • Ready in the PSB/PS (single batch transfer). Taken during an MD in the SPS (up to 144 bunches). Well above the requirements of the LHC this year. Margin in bunch intensity above nominal for nominal transverse emittances. • Need a few (~3) shifts to check beam parameters in PSB/PS • Need a few (~3) shifts to check beam parameters and conditions in the SPS and set-up extraction (to first TED) • 75 ns: • No set-up this year with this bunch spacing • Need ~1 week to set-up the cycle in PSB/PS + few shifts (5) in the SPS for setting-up of the cycle and extraction
LHC • Starting assumption: • no change in optics (in particular we remain with the present squeeze to 3.5 m), crossing/separation schemes, orbit • Injection/Injection protection: • 2 shifts required to set-up the transfer (steering and TCDI alignment if needed) • No impact on collimation set-up provided that the starting assumption is satisfied and that we do not change bunch intensity. • Closer bunch spacing will increase the amount of beam transmitted in the case of an asynch dump, and hence the risk of damage in case of any protection failure. But there is no sharp limit, and the increase in risk factor on this basis is maybe 2-3
LHC • Transverse feedback: 2 shifts might be required to verify transient behaviour (75 ns should be easier) • RF: no dedicated time required mostly observation (capture efficiency, RF cavity behaviour w.r.t. cavity field control, performance of the controlled longitudinal blow-up) • Start with bunch trains of 8 (12) bunches and collisions with 3x8 bunch/beam (1 fill) • Which intensity ramp-up policy? • Go to a full PS batch (24/36 bunches) after this initial test? • Ramp up the total intensity for physics fills in steps of ~100 bunches/fill provided we do not see issues and until we reach the intensity achieved with 150 ns?
Summary • 50 ns beam can be ready in the injectors in a few days. 75 ns beam would take longer (> 1 week). We should identify our preferred flavour • No significant difference in commissioning time between 50 and 75 ns beam in the LHC: 4 to 5 shifts (beam time) • Switching back and forth from 50(75) ns to 150 ns might take time (1 shift?) if we need to set-up again the transfer line collimators • Would go with the potentially more demanding case (50 ns)