| LIFETRAC: Beam-Beam Simulation
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| Beam-Beam phenomena in Tevatron are summarized in detail in V.Shiltsev's talk [1]. Our project is aimed mainly at creating a credible model of luminosity evolution when the beams are put into collision at 980 GeV. This implies first, taking into account all diffusion mechanisms and their interference with beam-beam interaction and second, realistic representation of the machine optics. | |
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| take place on Fridays, at 3 pm in the Penthouse,
WBT.
If you are interested to be notified of the upcoming meetings and their agenda, send email to Valery Lebedev. Past Meetings 5/21/2004 Sasha: continued with scallops.
Varied initial emittance for the antiproton bunch number 2. The blow up
changes with emittance but the amplitude of change is approx. twice
smaller than observed between bunches 2-11. Changing machine
chromaticity in order to adjust the difference between bunches due to
beam-beam did not have any effect on the situation. There was very
strong dependence on the emittance of the proton bunch, but the
simulation should be repeated conserving Np rather than xi. We propose
to test a possibility of changing the working point and moving it
closer to half-integer resonance. Namely, Valeri produced a Tevatron
optics with the tunes 0.53, 0.52. The tune pace in this region is free
from strong resonances. The main problem encountered are chromatic
aberrations of the beta function which limit available dynamical
aperture (longit.)
Alvin: Analysis of longitudinal oscillation spectra.
5/14/2004 Sasha: simulation of scallops
with lifetrac. For some working
points a scallop-like distribution of emittances is seen after first
10,000 turns. However, theory predicts only bunches #1 and #12 to be
different because of different tunes. Idea of explanation of difference
between bunches 2-11: a) they have different emittances. b) different
chromaticities.
Plan:
5/7/2004 Yuri: Resonance crossing driven
by synchrotron phase
fluctuations caused by RF noise. The conclusion: RF noise of the order
of 1E-10 results in huge (~0.4 rad) betatron phase fluctuation which
randomizes the resonance completely.
Yuri: Coherent oscillations
excited by dipole (local) can be
significant because of different phase advances for protons and
antiprotons between excitation location and IP. Conclusion: excited
coupled oscillations with large pbar amplitudes and small proton.
Valeri: after optics correction
faster decay of particles with large
dp/p is observed. In future, beam-beam will limit sigma dp/p and hence
the life time. A strong call for improvement of beam-beam effect to
cure this. Proposals: 1. consider the CERN working point. 2. Change
bunch pattern to eliminate the nearest parasitic IP's.
Sasha: Investigation of
scallops. First stage - analysis of
experimental data. The emittance measurements in the beginning of store
are done first before and second after! scraping. This leads to
stronger 'shaving' of first bunches in a train (those with larger
emittances). Will try fitting with some simple model.
Plan:
4/30/2004 VS: parametrization of
luminosity lifetime
Sasha: short description of simulation being in progress (xi=0) 4/23/2004 Alvin about his progress in
simulating diffusion mechanism.
Panagiotis on Strong-Strong status: first efforts were successful, the
new code runs. Now ready to specify realistic physics parameters.
Sasha: implemented calculation of beam size fitted with a Gaussian.
The plan:
4/16/2004 Yuri on noise levels: the
additional simulations were finished and they confirm previous
conclusions. Sasha about results of the week: testing of 12 and 19
longitudinal slices at the main IP's coincide well with predictions of
analytical calculation. Calculation for 1st and 6ths bunch (will we see
the 'scallops') is underway.
We decided to use a different
characteristic instead of the rms emittance to display beam properties,
since large-amplitude particles contribute strongly to this value. For
example, a gaussian fit can be used. The plan:
4/09/2004 Elliott McCrory on Fitting
the Luminosity Decay. Sasha on results of the past week. Yuri on
phase averaging for different slicing schemes (12,15,23). He evaluated
the resonance driving terms:
Plan for the week:
4/02/2004 Alvin presented analysis of SBD
data of the longitudinal
phase space density evolution during store #3261. He compared the
experimental data and an analytical model involving solution of the
diffusion equation. There is a discrepancy in the results: the model
suggests that the tails should be more populated than it is observed.
Alex showed simulation results for the past week (see the results section).
Plan for the next week: survey what number of slices is required (with 55 cm longitudinal beam size) - 12, 15 or 23. 3/26/2004
3/19/2004
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| last modified 5/24/2004 A. Valishev |
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