Open Questions about Phase A.10:

A. Systematic uncertainties which have an impact on the luminosity:

1. Bunch to bunch current variation:

  • The bunches ( 43, 54, 108, 156, > 2000) will be spread in intensity and emittance.

  • What is acceptable? Aim for < 10% in intensity and 20% in emittance. Discussed with W. Herr, to be studied further.

  • Seems to match about what is feasible from injectors (G. Arduini)

  • If not? Anything foreseen to equalize the bunches?; check with scrapers.

  • Measurements: for optimizing collisions and total integrated lumi it is sufficient to take the sum from individual bunches.

  • For a full analysis and optimization of lifetime, background and stability, measurements should be able to distinguish between bunches, for quantities like: current, beam size (emittance), tune and lumi.

2.  Non-Gaussian distribution (or truncated)

3.  Beams not colliding head-on: When the beams don't collide head on, there is a loss in luminosity. For Gaussian beams the remaining luminosity fraction is given by the expression:

£/£0 = exp[-(δx/2σx)2 - (δy/2σy)2],

where δx and δy are the horizontal and vertical separation between the two beams and σx, σy the r.m.s. beam sizes. Numerical values are listed in Table 1.

δx (σ)

δy (σ)

£/£0

0

0

1.0000

0.1

0

0.9975

0.2

0

0.9901

0.3

0

0.9778

0.4

0

0.9608

0.5

0

0.9394

0.5

0.5

0.8825

1

0

0.7788

1

1

0.6065

2

0

0.3679

2

2

0.1353

Table 1: Remaining luminosity fraction from 0 to 2 σ separation (Gaussian beams).

4.  Beam-beam effect (in principle should be small)

5. Longitudinal charge distribution: un-bunched particles or extra bunches in one beam would be counted in the intensity as measured with a DC-BCT but would not contribute to the luminosity. This will be observable with several instruments: the gap monitor, comparison of fast and DC BCT, RF pick-ups, and to some extend using BLMs. The transverse damper can be used to eliminate such unwanted beam components.

6.  Hour glass effect: For collisions of long bunches the luminosity decreases because of the increase in beam sizes around the IP. This is known as the hour glass effect. It is significant if the beta* function and the bunch length are comparable. Nevertheless, this effect is negligible in LHC.

B.  Background rejection

C.  Extended halo: halo scraping

D.  Solenoid compensation (small effect at 7 TeV)

E.  Need to check corrector transfer functions/hysteresis

F. Going back to ADJUST from STABLE BEAMS once "end of physics run" declared:

  •  HERA experience (by B. Holzer, HERA machine coordinator):  

  1. If there were more to do than simple things, HERA always dumped both beams, introduced a special time slot for MD (~ 1 or 2 hours) and proceeded with a train of pilot bunches ( 1 or 10 bunches of limited intensity) to perform the study. The reason was MPS driven; one single quench or beam loss that triggers the MPS system was much more severe and time consuming than a dedicated well prepared study. During luminosity operation the machine was running at the beam-beam effects limit, and therefore, emittance growth were expected. HERA started with a typical beam emittance (2 sigma normalized) at the beginning of the run of 15π mm mrad, and at the end of the run the emittance grew easily to 25 - 30. This meant that at the end there was not much room for beam gymnastics. The beam was filling nearly the aperture defined by the collimators, and even steering a bit the angle at the interaction regions, this led very quickly to beam scraping, losses at the low beta quads, etc.

  2. For simple things like orbit correction in one beam, steering to find better vertex position, etc, they dumped one beam and go on with the other for a while. But this was rarely done.

  3. HERA could unsqueeze both beams to guarantee more aperture for the studies (in the case of the electrons they could even decelerate them).

  • TEVATRON experience (by J. Annala, Tevatron machine coordinator):

  1. At Tevatron they go to MD mode often after Physics is over. The most common studies are fairly benign, but the experiments turn off most of their sensitive equipment. They often do things like crystal collimator studies, separation scans, collimator alignment, etc.

  2. They have unsqueeze beams and also decelerated protons, but this is not very commonly used. Their biggest problem is to have both protons and anti-protons in the same beam pipe.