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The High Luminosity LHC project (HL-LHC) foresees the construction and installation of important new equipment to increase the performance of the LHC machine. The Hollow Electron Lens (HEL) is a promising system to control the beam halo. It improves the beam collimation system of the HL-LHC and mitigates possible equipment damage in case of failure scenarios from halo losses. The halo can store up to 30 MJ energy. The specifications for this new device are quite demanding. The source, an electron gun with an annular shaped cathode, has to deliver a current up to 5 A. This is five times higher than the current in the existing electron lenses in Fermi and Brookhaven national laboratories. This note describes the programme carried out to design and test high-perveance guns equipped with two types of high-performance scandate cathodes. The size of the final gun is now considerably smaller than the one of the first prototype, allowing a reduction of diameter and cost of the superconducting magnet system used to steer the electron beam. The tests carried out at FNAL, BVERI and BJUT demonstrated that the developed cathodes fulfil the specifications and can supply a 5 A fully Space Charge Limited (SCL) current. INTRODUCTION: HOLLOW ELECTRON LENSES FOR HL-LHC The Large Hadron Collider (LHC), at CERN, is the largest and most powerful accelerator for high-energy physics [1]. This machine upgrade, the High Luminosity LHC (HL-LHC), will operate with extremely high beam power. The nominal stored beam energy will be 675 MJ, almost double with respect to the LHC design (nominal 362 MJ) [2]. In a particle accelerator, as the LHC, a number of processes may lead to beam losses. The manipulations needed to prepare the beams for collision and the collisions for physics entail unavoidable beam losses, which generally become greater as the beam current and the luminosity are increased. In addition, the LHC's superconducting environment demands an efficient beamloss cleaning to avoid magnet quenches from uncontrolled losses. The protection of the accelerator equipment from the consequences of uncontrolled release of energy is therefore essential. The circulating proton beams are made of 25-ns-spaced proton bunches. The cross section of a bunch has a highly populated central part called core and a periphery part named halo. Since the beam losses directly depend on the beam-halo behaviour, great attention must be paid to its formation, growth and interaction with the beam core. The collimation system has the primary role to clean away the beam halo and to maintain losses at sensitive locations below safe limits. Collimators are also used to minimize background in the experiments. Figure 1: HEL layout. In yellow the electron ring path, in red the HL-LHC protons path. Hollow electron lenses (HEL) are a promising method to improve the beam collimation system. HEL are expected to boost the performance of the LHC and of its HighLuminosity upgrade, through active control of halo particles’ diffusion speed and tail population [3]. Figure 1 shows the layout of the HL-LHC HEL. In the HEL a lowenergy, high-current hollow electron beam runs co-axially with the circulating hadron beam, over about three meters [4]. The electron beam acts on the halo particles at large transverse amplitudes from 3σ to 6σ (σ = 0.27 mm) without

The development programme of cathodes and electron guns for the Hollow Electron Lenses of the High Luminosity LHC project1