Design of high-performance guns for the HL-LHC HEL

Abstract

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, must 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 for the HL-LHC lenses 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.

Induction-Coil Measurement System for Normal- and Superconducting Solenoids

“…2, the first set of coils (D i ) consists of five nested circular disks, while the second set consists of 4×4 arcs of 90 degrees opening angle (Q q,j ) where r i are the radii of the outermost wire turn of the disks, while r j are the barycenters of the arcs. The disks are numbered from inside out (i ∈ [1,5]), while the quarter arcs are numbered with the first index (q) denoting the quadrant (clockwise) and the second (j) the radius from inside out. When the measurement system is displaced in the longitudinal direction in the magnetic bore, along the z-axis, the induced voltages across the coils are acquired "on the fly".…”

“…The alignment of the solenoid assembly is demanding and requires dedicated tools and iterative processes. Projects such as the Hollow Electron Lens (HEL) [4], [5] and Electron Cooler [6] required us to develop a different approach for expressing the local fluxdensity distribution for measuring the axis and aligning the solenoids by the transverse field components at the fringefield zones. Following previous experience [7], we propose a method to locate the magnetic axis of solenoids, employing a translating induction-coil magnetometer that is displaced in the bore of a normal-conducting magnet, or within an anticryostat in the case of a superconducting magnet tested at crygoenic temperatures.…”

Electron Cooling with Space-Charge Dominated Proton Beams at Iota

“…Figure 2 depicts a model of the electron cooler setup showing the thermionic source, collector and all the solenoids which keep the electron beam magnetized throughout the transport line. The structure of the thermionic source will be similar to others developed at Fermilab, including for the hollow electron lens at the HL-LHC [16]. While the strengths of the source and the main solenoid may be adjusted to provide a beam size expansion factor of 2, the baseline design assumes that both the source and the main solenoid are set to 0.1 T. Diagnostics for the electron beam include a Faraday cup to measure current and a scintillating screen to image the transverse profile at the entrance to the collector.…”

Hollow electron lenses for beam collimation at the High-Luminosity Large Hadron Collider (HL-LHC)

“…The development of high currentdensity cathodes is important for the HL-LHC HEL in order to keep reasonably moderate magnetic compression factors while aiming at small electron beam sizes. Details of the gun developments are discussed in a companion paper in this newsletter [37]. Initial designs for the HL-LHC HELs gun started from the state-of-the-art gun developed at Fermilab for collimation studies, under the scope of US-LARP and relied on a cathode with a 6.75 mm inner radius that was tested extensively [38].…”

“…Figure11. Cross section of the HL-LHC thermionic gun, optimized for the small cathode design with outer radius of 8.05 mm, from[37].…”