Heiko Damerau

Performance potential of the injectors after LS1

The main upgrades of the injector chain in the framework of the LIU Project will only be implemented in the second long shutdown (LS2), in particular the increase of the PSB-PS transfer energy to 2GeV or the implementation of cures/solutions against instabilities/e-cloud effects etc. in the SPS. On the other hand, Linac4 will become available by the end of 2014. Until the end of 2015 it may replace Linac2 at short notice, taking 50MeV protons into the PSB via the existing injection system but with reduced performance. Afterwards, the H− injection equipment will be ready and Linac4 could be connected for 160MeV H− injection into the PSB during a prolonged winter shutdown before LS2. The anticipated beam performance of the LHC injectors after LS1 in these different cases is presented. Space charge on the PS flat-bottom will remain a limitation because the PSB-PS transfer energy will stay at 1.4GeV. As a mitigation measure new RF manipulations are presented which can improve brightness for 25 ns bunch spacing, allowing for more than nominal luminosity in the LHC.

LIU: EXPLORING ALTERNATIVE IDEAS

The baseline upgrade scenarios for the injector complex cover the connection of Linac4 to the PSB, the increase of the PSB-PS transfer energy from 1.4 GeV to2 GeV and the major SPS RF upgrade during LS2. The achievable beam characteristics will nonetheless remain below the expectation of the HL-LHC project. Therefore, alternative or additional options like, e.g., special bunch distributions, the use of injection optics optimized for high space charge or extra RF systems will be discussed. The expected beam parameters, possible implementation and impact on beam availability for these more exotic options will be analysed and compared to the LIU baseline plan. Moreover, the potential interest of further batch compression schemes will be evaluated.

Can the proton injectors meet the HL-LHC requirements after LS2?

The LIU project has as mandate the upgrade of the LHC injector chain to match the requirements of HLLHC. The present planning assumes that the upgrade work will be completed in LS2, for commissioning in the following operational year. The known limitations in the different injectors are described, together with the various upgrades planned to improve the performance. The expected performance reach after the upgrade with 25 and 50 ns beams is examined. The project planning is discussed in view of the present LS1 and LS2 planning. The main unresolved questions and associated decision points are presented, and the key issues to be addressed by the end of 2012 are detailed in the context of the machine development programs and hardware construction activities. HL-LHC REQUIREMENTS AFTER LS2 The stated performance objective of HL-LHC is to accumulate 3000 fb of integrated p-p luminosity at 14 TeV centre of mass collision energy [1]. In order to achieve this, an annual figure of 250-300 fb has been posited, requiring instantaneous luminosity capability of around 7–8×10 cms, levelling to 5×10 cms and high machine efficiency [2]. The present paper covers the first of these challenging requirements: how to deliver the beam from the injector complex for these luminosities almost an order of magnitude above LHC design. The HL-LHC project has previously outlined possible parameter sets for 25 and 50 ns spacing which give the required luminosity, summarised in Tab. 1, adapted from [2]. Strictly speaking the HL-LHC needs the specified beams from the SPS after LS3, when the major work for the HL-LHC project is planned. The LIU work will take place largely in LS2, so that the period LS2 to LS3 will be an important one in terms of achieving the maximum performance from the injector chain. The figures quoted are for beams at the start of the collision process at 7 TeV – any beam loss or emittance dilution after extraction from the SPS is not included. The assumptions on the beam loss and emittance dilution for all machines are given in Tab. 2, where it can be seen that the total assumed beamloss -ΔI/I0 is 27%, and the emittance growth Δε/ε0 is 33%, corresponding to a brightness which is reduced to 55% of the original value. Table 1: Parameters and requirements from HL-LHC Parameter Nom. HL 25 ns HL 50 ns N [e11 p+] 1.15 2.0 3.3

Lower-Harmonic RF System in the CERN SPS

Significant beam losses increasing with intensity are observed at capture and along the SPS flat bottom for the LHCtype proton beams. The intensity should be doubled for High-Luminosity LHC and high losses may be a major performance limitation. Bunches extracted from the PS, the SPS injector, are produced in a 40 MHz RF system applying a bunch rotation at the end of the cycle and therefore cannot be perfectly matched to the 200 MHz SPS RF bucket. The possibility of using a lower-harmonic, additional RF capture system in the SPS was already proposed after the LEP era in preparation for transfer of the nominal LHC beam but the bunch rotation was the preferred solution, since the induced voltage in the SPS 200 MHz RF system would be too large to ensure stability in a low harmonic system without mitigation measures. However, the use of the upgraded one-turn delay feedback and the 200 MHz RF system as a Landau cavity could help to improve beam stability. The feasibility of this scenario to reduce capture losses in the SPS is analysed and presented in this paper. The beam transfer to the main 200 MHz RF system is simulated using a realistic bunch distribution.

Machine development studies for PSB extraction at 160 MeV and PSB to PS beam transfer

This paper collects the machine development (MD) activities for the beam transfer studies in 2016 concerning the PSB extraction and the PSB-to-PS transfer. Many topics are covered: from the 160 MeV extraction from the PSB, useful for the future commissioning activities after the connection with Linac4, to new methodologies for measuring the magnetic waveforms of kickers and dispersion reduction schemes at PS injection, which are of great interest for the LHC Injectors Upgrade (LIU) [1] project.

Measurements of the CERN PS Longitudinal Resistive Coupling Impedance

The longitudinal coupling impedance of the CERN PS has been studied in the past years in order to better understand collective effects which could produce beam intensity limitations for the LHC Injectors Upgrade project. By measuring the incoherent quadrupole synchrotron frequency vs beam intensity, the inductive impedance was evaluated and compared with the impedance model obtained by taking into account the contribution of the most important machine devices. In this paper, we present the results of the measurements performed during a dedicated campaign, of the real part of the longitudinal coupling impedance by means of the synchronous phase shift vs beam intensity. The phase shift has been measured by using two different techniques: in one case, we injected in the machine two bunches, one used as a reference with constant intensity, and the second one changing its intensity; in the second case, more conventional, we measured the bunch position with respect to the RF signal of the 40 MHz cavities. The obtained dependence of the synchrotron phase with intensity is then related to the loss factor and the resistive coupling impedance, which is compared to the real part of the PS impedance model.

Study of the beam-cavity interaction in the PS 10 MHz RF system

The eleven main accelerating cavities of the Proton Synchrotron (PS) at CERN consist of two ferrite-loaded coaxial lambda/4 resonators each. Both resonators oscillate in phase, as their gaps are electrically connected by short bars. They are in addition magnetically coupled via the bias loop used for cavity tuning. The cavities are equippedwith a wide-band feedback system, limiting the beam loading, and a further reduction of the beam induced voltage is achieved by relays which short-circuit each half-resonator gap when the cavity is not in use. Asymmetries of the beam induced voltage observed in the two half-cavities indicate that the coupling between the two resonators is not as tight as expected. The total cavity impedance coupling to the beam may be affected differently by the contributions of both resonators. A dedicated measurement campaign with high-intensity proton beam and numerical simulation have been performed to investigate the beam-cavity interaction. This paper reports the result of the study and the work aiming at the development of a model of the system, including the wide-band feedback, which reproduces this interaction. INTRODUCTION The PS is equipped with different cavities covering on a wide frequency range (2.8-10, 20, 40, 80 and 200MHz) for acceleration, RF gymnastics and longitudinal emittance blow-up. Among them the 10MHzRF system plays a crucial role for the beam dynamics, since it is the one responsible for beam acceleration and also performs beam manipulations, such as bunch splitting and rotation. It has been shown that it represents the main source of longitudinal beam instabilities in the PS, namely coupled bunch instabilities [1]. The parameter relevant to the beam-cavity interaction can be modeled as an impedance excited by the beam current crossing the cavity. The reduction of this impedance is already achieved by a wide-band feedback system [2]. However, with the increasing intensity expected in the PS in the framework of the Injectors Upgrade (LIU), a further reduction of the equivalent impedance is wished to improve longitudinal beam stability. BEAM LOADING AND LONGITUDINAL IMPEDANCE A beam travelling in a synchrotron loses part of its energy at each turn, generating an electromagnetic (EM) wake field that acts back on the beam itself. Depending on the intensity ∗ giorgia.favia@cern.ch this may cause significant modification in the dynamics of the particles motion [3]. A cavity can be modeled as a complex impedance Z . When a bunch of current Ib travels across the cavity, a beam induced voltage Vb develops at the cavity gap [4]: Vb = −Z · Ib, (1) where the minus sign indicates that the induced voltage leads to an energy loss. 10MHz RF System Each 10MHz cavity is driven by a tetrode based amplifier, which provides the necessary current to reach up to 10 kVp per gap. In addition the system is equipped with a direct wide-band feedback, which reduces the cavity impedance [5]. The evaluation of the cavity impedance has to take into account the effect of the electronics of the amplifier driving the cavity as well as the reduction due to the wide-band feedback. Figure 1 presents a simplified model of the 10MHz system. A detailed description of the RF amplifier and its upgrade

The PS 10 MHz High Level RF System Upgrade

In view of the upgrade of the injectors for the High Luminosity LHC, significantly higher bunch intensity is required for LHC-type beams. In this context an upgrade of the main accelerating RF system of the Proton Synchrotron (PS) is necessary, aiming at reducing the cavity impedance which is the source of longitudinal coupled-bunch oscillations. These instabilities pose as a major limitation for the increase of the beam intensity as planned after LS2. The 10 MHz RF system consists in 11 ferrite loaded cavities, driven by tubebased power amplifiers for reasons of radiation hardness. The cavity-amplifier system is equipped with a wide-band feedback that reduces the beam induced voltage. A further reduction of the beam loading is foreseen by upgrading the feedback system, which can be reasonably achieved by increasing the loop gain of the existing amplification chain. This paper describes the progress of the design of the upgraded feedback system and shows the results of the tests on the new amplifier prototype, installed in the PS during the 2015-16 technical stop. It also reports the first results of its performance with beam, observed in the beginning of the 2016 run.