pros and cons of 75-ns period of operation

1
pros and cons of 75-ns period of operation

• Gianluigi Arduini, Frank Zimmermann

2
pros of 75 ns vs. 25 ns operation

• reduced long-range beam-beam effect, scaling as
  
• (constant L)
• 2 times lower initial transverse emittance ?
  much more tolerant to emittance blow-up
• for same luminosity b can be larger
• reduced long-range effect, smaller emittance,
  larger b ? smaller crossing angle acceptable
• 75 ns Nb9x1010, b2 m, gex,y1.9mm?qmin 90
  mrad
• 25 ns Nb4x1010, b2 m, gex,y3.75mm?qmin180
  mrad
• instrumentation no parasitic crossings for Q1
  BPMs 75-ns might also help in some timing set up

3
more pros of 75 ns vs. 25 ns operation

• mitigates electron-cloud related problems
  (pressure rise, poor lifetime, instabilities,
  heat load)
• scrubbing performed at beam-screen region
  relevant for nominal LHC conditions
• highest possible luminosity if beam current is
  limited
• 25-ns bunch spacing for same luminosity might
  have implications for collimation
• more than 10 bunches at 25-ns spacing with
  nominal bunch intensity would require
  installation of all dilution kickers bunches at
  75-ns spacing do not

4
cons of 75 ns vs. 25 ns operation

• larger number of pile-up events
  (constant L), at 1033 cm-2s-1 luminosity 6
  vs 2 events / crossing
• shorter luminosity lifetime
• more charge per bunch might lead to single bunch
  instabilities
• more emittance growth from IBS
• if there is an electron-cloud problem at 25-ns
  spacing, we will discover it later

5
illustrating material
6
crossing angle, IBS rise time, event pile up
Table from Francesco Ruggiero, Chamonix 2003
7
LHC IBS growth rates without crossing-angle
separation bumps for Nb9x1010 as a function of
transverse and longitudinal emittance
nominal
75 ns
8
long-range beam-beam
diffusion rate vs. amplitude from WSDIFF
simulation (2002)
9
e- cloud simulated heat load vs. bunch spacing
M. Furman V. Chaplin, 2005
2004 simulation
heat load at 25 ns Nb4x1010 is gt10 times
larger than at 75 ns spacing and Nb9x1010
10
e- cloud 25-ns 75-ns spacing in the SPS
(Miguel Jimenez)

• Comparison between 25 and 75 ns bunch spacing in
• dipole field regions (2003 SPS run)
• Smaller pressure rises ? factor 4
• Smaller electron flux to the walls ? factor 20
  measured in a dipole field _at_ 30 K
• Multipacting is still present with 75 ns bunch
  spacing but at a much lower level