Beam-beam issues for LHC upgrade phases 1 and 2

Abstract

While long-range beam-beam interaction will not be the limiting effect in the first years after LHC start-up, it will definitely become one in the upgrade scenarios. Upgrade phase 1 will include an exchange of the triplet magnets allowing for a β∗ = 25 cm optics. Phase 2 is an even more ambitious upgrade that will include a modification of the detectors. Currently two phase-2 upgrade scenarios are proposed: the “Dipole Zero” (D0) and the “Large Piwinski Angle” (LPA) option. After some general notes and a brief description of the applied simulation model, the upgrade phase 1 issues and optics will be discussed with regard to beam-beam performance. The following two sections will deal with upgrade phase 2. GENERAL BBTrack [5], a weak-strong 6D tracking code, was used to track (linear transfer matrices between nonlinear elements, interaction points (IPs) 1 & 5 only) particle distributions (initial energy offset δp/p = 2.7× 10−4) for 300,000 turns in LHC at top energy (7 TeV) and determine the particle stability with help of the Lyapunov exponent. The dynamical aperture (DA) is defined as the amplitude at which 40% of the particles in a radial range of width δr = 0.2σ are chaotic. For comparison, the main beam-beam parameters of the nominal LHC are: 15 LR collisions at each side of the IP (β∗ = 0.55) with a full crossing angle θ = 284μrad (average separation d ≈ 9.5σ) at 1.15 p/bunch. This crossing angle was chosen to obtain an acceptatble long-range beam-beam effect [4]. Namely with this crossing angle a dynamic aperture (DA) of 5.4σ is expected that could be improved to DA = 7.2σ by a wire compensation [6]. LHC UPGRADE PHASE 1 By 2013 the whole triplet will need to be exchanged and a new interaction region (IR) scheme with β∗ = 25 cm will be implemented in order to boost the luminosity. In the following, 3 different optics “low β max”, “modular” and “compact” as proposed by R. de Maria et al in [7] are briefly discussed. A forth option, similar to the low β max one, called “symmetric” was proposed by J.P Koutchouk, E Todesco et al in [2]. In order to keep an average beam-beam separation of d ≈ 9.5σ the crossing angle in all three options is increased with respect to the nominal LHC (from θ = 284μrad to 450μrad). Given the same magnet technology, the stronger focussing requires a longer triplet and hence it introduces more long-range beam-beam encounters (LRBBIs). The number of long range beam beam encounters (LRBBIs) and other important parameters are summarized in Table 1 and Fig. 1. In order to cope with these additional LRBB encounters and potentially also with a higher beam current or simply to improve the nominal beam-beam performance a wire compensator (BBLR) is foreseen. A wire compensation does not interfere with the IR design as it only requires a) that the wire be placed at a position with equal β-function in both transverse planes, b) a reasonably large β to allow accommodating a wire compensator with a practical wire diameter and c) a small phase advance between the wire and the LRBBIs. Suitable positions can be found in all scenarios. Simulations showed that the simple criterion variable nominal low β Compact modular max β∗ [m] 0.55 0.25 0.25 0.25 #LRBBIs 16 19 22 23 wire @ [m] 104 136 170 160 βwire [m] 1780 3299 2272 3000 σdsep 1.6 3.6 2.2 X Table 1: Comparison of three proposed phase 1 upgrade optics with respect to their long-range beam-beam (LRBB) performance. 1324