Summary of discussion on beam-beam compensation studies in RHIC
W. Fischer, J.-P. Koutchouk, F. Zimmermann, 10.12.2004
Motivation
Compensating the effect of LRBB collisions by a BBLR in RHIC can be considered a direct proof that this type of correction would work in the LHC. At the same time, it addresses one of the critical issues, and its possible cure, for the eRHIC upgrade.
Conditions
The 6 IPs in RHIC provide the possibility of simulating 6 long-range (LR) collisions by separation at the regular IPs. If the relative rf phase of the two beams is shifted, even two long-range collisions per IP, i.e., 12 LR collisions in total, can be realized. This is the maximum possible (every 3rd bucket filled, 108 ns spacing), limited by the injection kickers. The maximum intensity per bunch corresponds to 2e11 protons or about ½ this charge for Cu ions. Emittance (6) is 20 pi mm mrad normalized. Proton injection energy is 25 GeV and the protons are ramped to either 100 or 200 GeV. The separation range at the main IP is +/- 5 mm for the injection optics (10 m beta-star) which should be about +/- 4 sigma (check precise number) or a maximum separation between the two beams of the order of 8 or 9 sigma. The separation and plane of L collisions is preferentially vertical as in the horizontal plane the beams cross anyhow. (On the other hand, it may not be desirable to have both a vertical and a horizontal offset at the LR collisions points!) The physical aperture at injection is 7-7.5 sigma, restricted by the injection and/or dump kickers. Conducting the experiment at injection facilitates the procedure and allows for rapid re-injection. However, there is a concern that the phase advance between the foreseen location of the BBLR upstream of a final triplet and the LR collision points may be larger than 5 degrees, which is about the tolerance. The main diagnostics signal is the beam lifetime. Other possibilities could be experimental background, emittances, Schottky monitors, as available.
Preparations needed
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Check activation levels of BBLRs and flowmeter in SPS (Gerard?). I suspect the first original BBLR is the most likely item to be shipped, since this device has been working reliably for 3 years and the single wire of BBLR-1 can almost surely be placed in the shadow of the aperture. (Since the BBLR chamber is slightly larger than the RHIC beam pipe (12 cm diameter) we may be able shift the position of the wire by up to about +/- 10 mm at RHIC installation.)
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Check INB rules and procedures for shipping (US-CERN channel?). Special transport box may be required to protect ceramics from damage. Check export/import procedures and regulations for (possibly radioactive) item. Cost estimate for de-installation, packing, Swiss/US administration, shipping, and also the shipping duration (2 months?).
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Map machine apertures. BBLR should be in the shadow. Verify that beam orbit can be bumped over a reasonable range of sigma at the location of the BBLR.
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Check phase advances between BBLR location and anticipated LR collision point(s) closest to BBLR and all around the ring – can the latter be adjusted to multiples of pi? This study should be done for unsqueezed injection optics as well as for squeezed (and perhaps intermediate) optics.
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Exploration of water connection (with appropriate flow). Availability or acquisition of power supply (max. current 200-300 A, less than 1e-4 ripple for low-resistance BBLR load). Fabrication of spare beam pipe for rapid installation and replacement (e.g., in case of a vacuum leak). This may require vacuum valves in the vicinity (do they exist?) .
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Clarify how temperature and water-flow interlocks will be connected to the RHIC control system.
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Cost estimate of de-installation (free?), administrative procedures (INB, import/export regulations), packing, and shipping of 2 BBLR-1 units and flowmeter for CERN.
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Confirmation by RHIC management and implicated personal or groups (e.g,, vacuum) that the installation, connection and machine studies are likely possible in the foreseen time frame, and that adequate budget can be allocated for the above tasks of BNL.
Experiments
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A first experiment without BBLR can measure the beam lifetime as a function of the beam-beam distance. A weak-strong scenario with a single weak bunch interaction with a number of intense strong bunches would be one possibility, which would minimize the activation and could be directly compared with simulations. Tune and orbit should be corrected for each new transverse distance. Note that at the SPS a pure5th power law was found while compared with a 3rd power law at the Tevatron. The dependence of the power on the tune and of the coefficient with energy would be worthwhile topics of study in such an experiment. Only if this experiment demonstrates an effect, do we proceed with the BBLR shipping.
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Separating the beams at different IPs in different planes, various crossing schemes options can be compared experimentally (still without BBLR). This would address an important issue for the LHC (though probably too late to have consequences for the start-up, however useful for later).
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A BBLR compensation experiment with one or two compensators is the next step. Efficiency of the compensation should be demonstrated by tune and transverse-position scans, It would be ideal if all the other LR collisions were to happen a multiple of pi apart in betatron phase.
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Colliding the probe bunch with only 1 strong bunch per IP the phase advance between the BBLR and the LR-IP can be varied and the associated tolerance explored experimentally. It is thought that the tolerance is about 5 degrees.
Schedule
An exploratory long-range beam-beam experiment in RHIC should be performed as soon as possible (January/February/March?) to determine if an effect on the beam lifetime can be discerned. Installation of the BBLR at RHIC should happen in mid-June 2005 at the latest. The RHIC run would commence in fall 05. (Thus the BBLR(s) shipped to BNL will most probably not be available for the 2006 SPS MDs.)
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