SPS performance with PS2

Abstract

The upgrade of the PS to the PS2 would allow injection into the SPS at higher energy (up to 70 GeV/c). Possible advantages deriving from a higher injection energy into the SPS include the improvement of space charge at flat bottom, absence of transition crossing for all proton beams and a higher threshold for the horizontal electron cloud coupled bunch instability. Transverse Mode Coupling Instability (TMCI) and vertical Electron Cloud Instability (ECI) thresholds are studied in greater detail. Their dependence on energy is defined in simulations with the HEADTAIL code and the results of this study are presented. INTRODUCTION AND MOTIVATIONS Studies for the LHC performance upgrade include the improvement of the existing LHC injectors and/or the design of possible new rings in the injector chain [1]. Several scenarios, aimed at overcoming the existing bottlenecks, are presently being taken into consideration. The crucial point of the most promising option consists in raising the injection energy into the existing SPS from the present 26 GeV/c to 50–70 GeV/c. This could allow first to better cope with some of the existing limitations and secondly, a future upgrade of the SPS to a higher extraction energy ring (1 TeV). This scenario would require the corresponding upgrade of the present SPS injector, the PS ring, to PS2 or PS2+ [2]. The first part of this paper will be devoted to the description of the present SPS intensity limitations, and how a higher injection energy could (or would not) help in this regard. A specific question, which requires a deeper analysis and is addressed in the second part of this paper, is how raising the SPS injection energy would affect the transverse single bunch instability thresholds. In particular, TMCI is expected to be a potential danger in the SPS with the enhancement of the broad-band impedance due to the installation of 9 new MKE kickers in the ring since 2003 [3]. In addition, the vertical electron cloud single bunch instability (to some extent also TMCI type) is at present a limiting factor for the vertical emittance of the nominal LHC beam (the instability can be overcome by operating the ring with a rather high vertical chromaticity after a scrubbing run) [4]. The energy dependence of the threshold for the onset of these instabilities is therefore the subject of the second part of this paper. The study of the effect of higher injection energy into the SPS is carried out following the steps outlined below. We will consider an LHC-type beam interacting with a broad-band resonator impedance or an elec∗Giovanni.Rumolo@cern.ch tron cloud. The essential parameters are listed in Table 1. Even if the LHC-type beam is injected at 26 GeV/c, energy is actually scanned between 14 and 450 GeV/c in order to cover a broader range for the energy dependence study. Table 1: Parameters used in our study Name Symbol Unit Value Momentum p0 GeV/c 14–450 Norm. transv. emitt. x,y μm 2.8 Long. emitt. (2σ) z eVs 0.35 Bunch length σz m 0.3 Bunch population N 1.15× 10 Vertical tune Qy 26.13 Momentum comp. α 0.00192 Shunt impedance RT MΩ/m 20 Quality factor Q 1 Resonance frequency ωr/2π GHz 1.3 E-cloud density ρe m−3 1× 10 Taking the reference at 26 GeV/c (which corresponds to γ = 27.7), the main assumptions of our study are: • The longitudinal emittance and the bunch length are kept constant at the values of 0.35 eVs and 0.3 m (having assumed the present beam production scheme in the PS). The momentum spread ∆p/p0 is scaled by 1/γ when changing the energy and the matched voltage is re-adjusted according to |η|/γ (η is the slip factor) from the matched value of 700 kV at 26 GeV/c (see Fig. 1). • The normalised transverse emittances are constant, 2.8 μm. Consequently the transverse beam sizes are re-scaled by √ 1/γ when changing the energy. SPS INTENSITY LIMITATIONS AND EFFECTS OF A HIGHER INJECTION ENERGY Limitations to the maximum LHC beam intensity that can be accelerated in the SPS can come from single-bunch or multi-bunch effects. In particular, the total beam current is obviously limited by coupled bunch phenomena, but then this current cannot be arbitrarily distributed in the machine, because the bunch intensity is also limited by single-bunch collective mechanisms. Single bunch intensity limitations in the SPS are determined by: LHC-LUMI-06 PROCEEDINGS