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pros and cons of 75-ns period of operation

pros and cons of 75-ns period of operation. Gianluigi Arduini, Frank Zimmermann. pros of 75 ns vs. 25 ns operation. reduced long-range beam-beam effect, scaling as (constant L )

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pros and cons of 75-ns period of operation

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  1. pros and cons of 75-ns period of operation Gianluigi Arduini, Frank Zimmermann G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  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: Nb=9x1010, b*=2 m, gex,y=1.9mm→qmin= 90 mrad 25 ns: Nb=4x1010, b*=2 m, gex,y=3.75mm→qmin=180 mrad • instrumentation: no parasitic crossings for Q1 BPMs; 75-ns might also help in some timing set up G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  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 G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  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 G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  5. illustrating material G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  6. crossing angle, IBS rise time, event pile up: Table from Francesco Ruggiero, Chamonix 2003 G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  7. LHC IBS growth rates without crossing-angle & separation bumps for Nb=9x1010 as a function of transverse and longitudinal emittance nominal 75 ns G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  8. long-range beam-beam: diffusion rate vs. amplitude from WSDIFF simulation (2002) G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  9. e- cloud: simulated heat load vs. bunch spacing M. Furman & V. Chaplin, 2005 2004 simulation heat load at 25 ns & Nb=4x1010 is >10 times larger than at 75 ns spacing andNb=9x1010 G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

  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 @ 30 K • Multipacting is still present with 75 ns bunch spacing but at a much lower level G. Arduini, F. Zimmermann, LHCCWG 22.02.2006

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