Summary notes of the LCU meeting on 03/06/2008

Present: IA, MA, A, GB, CB, RB, HB, RC, MG, WH, JJ, DK, PL, MM, AM, JN, TP, VP, TR, FR, RT, TW, SW, SY, FZ

Report from meetings

• ICC -> MG
  Sector 5-6 retraining of dipoles progresses but somewhat slowly, gaining about 30 Amps from one quench to the next, so far never on the same magnet.
  Powering tests in 7-8 started.
  All LHC sectors are cold or started cool down.
  The TI8 re-commissioning started well on May 24 but got then interrupted by problems with a 18 kV cable. The TI2 tests are scheduled for June 14-15.
• SLM -> MG
  Nothing particular to report. Last meeting was moved to Wednesday because of an internal PS2 review on Thursday.

Combined betatron and momentum collimation in IR3 -> RA (pdf)

The issue is possible radiation damage of not sufficiently radiation hard electronics installed too close to IR7. Even though all the elements to draw conclusions were available since quite some time, it was the recent CNGS accident that triggered a study on the radiation conditions in the LHC underground areas. This in view of  assessing whether dangerous single even upsets to electronics could be anticipated. The positive outcome of the study lead to a crash programme to define a strategy for a long term solution: unfortunately, such a definitive solution will take time to be implemented.
The situation happens to be much less critical in IR3 were electronics is further away from the beam.
A special effort to study if temporarily also the betatron collimation could be done in IR3 in combination with the momentum collimation was launched recently. Simulation results confirm that this should be possible. The loss in collimation efficiency should be tolerable for the first year(s) of operation, while implementing a long-term solution for the electronic devices.
Consequences for ion collimation were also studied - even if currently no ion run is planned for the first year: it should be ok with intensities ten times below nominal ion operation, which should be adequate for a first ion run.
It is planned to study the optimization of betatron and momentum cleaning in IR3 by re-matching the optics, namely reducing by about 30% the dispersion at the primary momentum collimators.

Effect of the magnetic hysteresis on the LHC crossing angle and separation bumps -> SW (pdf)
SW reported on studies done with magnet experts (Nicholas Sammut, Walter Venturini Delsolaro) on the effects of the magnetic hysteresis of the insertion correctors used for separation and crossing angle bumps. The hysteresis is modeled using Fidel and based on measurements. The effects on optics were studied with MAD-X. The work was started to control systematic effects in luminosity scans and results will be described in an EPAC contribution. It was found that the effects produced by the MCBC magnets which are relevant for luminosity scans, will be rather small, about 0.04 sigma in separation.
Full separation and nominal operation with both separation and crossing angle will also require the MCBX magnets. Effects will then be larger, up to 1 sigma in separation, 10% in crossing angle in IP5 and residual rms orbits of 0.07 mm in x and 0.06 mm y, and require rather accurate simulation.
From the simulations, the required level of accuracy of the magnetic model is of the order of 0.2%: at this level the orbit perturbation will be at the same level as that induced by beam-beam effects.
Further studies with simulation of the full magnetic cycles are planned.

Results of beam-beam tracking studies in 2007 -> DK (pdf)
DK apologised that some plots he wanted to show were missing today due to technical problems with his computer.
He showed results for the dynamic aperture (DA) and the onset of chaos for head-on collisions at 450 GeV at beta* = 11m. The studies included measured magnetic errors. b1 and a1 errors generates some seed to fail due to unstable Twiss (closed orbit not found). Hence, the resulting DA results were based on a smaller number of seed than 60. JJ suggested that the issue with closed orbit may be circumvented by turning sextupoles off. Dynamic apertures were found to be about 12 sigma. Tune scans around the injection tune, showed that the over a wide tune range (0.01) DA is essentially constant. Non perfect closed orbit correction were used in order to assess the impact on the DA (feed down effects): a sizeable effect is observed in case the max closed orbit exceeds 6 mm (which is in any case not compatible with the aperture specifications). The effect of magic phase advances between IP1 &5 with pi/2 in x and 3/2 pi in y were also studied and found to increase the chaos threshold.
Studies at 7 TeV confirmed that DA at 11m beta* is very comfortable. For beta*=2 m a reduction is observed, but for some values of the phase space angle.

AOB
None

Last update: 06-June-08

MG & HB

Back to LCU Minutes