Lambertson Septum Magnet Design for the LCLS-II Beam Spreader at SLAC John W. Amann, Magnet Engineer1

1. SLAC National Accelerator Laboratory, 2575 Sand Hill Rd. MS 52, Menlo Park, CA 94025

Presented at the Magnet Technology 25th Conference, 2017 Aug 27 – Sept. 1 Session: Resistive Accelerator Magnets; Program I.D. number: Tue-Af-Po2.02 [130]

Background

The LCLS-II is a revolutionary 2nd generation x-ray free electron laser (FEL) currently under construction at the

SLAC National Accelerator Laboratory in Menlo Park, CA. The laboratory is operated by Stanford University

under contract to the United States Department of Energy. The LCLS-II project consists of two different linacs,

the original normal conducting 15GeV S-Band linac and the new 4GeV L-Band superconducting linac, which feed

the hard and soft x-ray undulator beamlines. In order to rapidly switch electron beams from the normal

conducting and superconducting linacs to the hard and soft x-ray undulator beamlines, a system of magnetic

optics consisting of multiple fast transmission line type kicker magnets and DC septa magnets is employed. This

poster depicts the elements of design, from physics requirements to drawings for manufacturing.

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Design Features

Field Free Region (NiB plated)

Dipole Field Region (NiB plated)

Electrical Safety Cover

Septum Steel Thickness = 3mm

Tungsten Spoiler BlockAlignment Fiducials

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Beam Stay Clear

To verify the correct alignment of the septum magnet field free and dipole field regions w.r.t. the electron beam, a 3D CAD model of the septum magnet and electron beam trajectory is created. Cross sections show the size and location of the kicked and un-kicked electron beams at the entrance, midpoint, and exit of the septum magnet.

Program I.D. number: Tue-Af-Po2.02 [130]

Beam Spreader

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Magnetic Field Requirements

The magnetic field requirements for the beam spreader septum magnet are communicated via Physics Requirements Documents (PRD) under revision control of the LCLS-II Project. The magnet engineer then creates an engineering specification document (ESD) which serves as record of how the design requirements have been satisfied.

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Magnet Design Parameters

The ESD includes details of the mechanical, electrical and thermal design and performance for the septum magnet. Of note the coil cross-section is oversized for this design due to limitations in the existing low conductivity water system (LCW). The trim coil is designed such that the current can be doubled if additional strength is needed for matching.

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Magnetic Modelling

The results of the magnetic field analysis for harmonic errors are summarized in the table below.

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Drawings for Manufacturing

Some views from the drawings for manufacturing the septum magnet are shown below. Of note is the core pinning sub-drawing in which the septum magnet steel core is first assembled and the gap tolerances are checked. Once satisfied the core meets mechanical tolerances, the assembly is matched drilled and pinned to ensure the core can be disassembled and re-assembled to the required tolerances. The un-allowed harmonics in a dipole magnet are due to assembly tolerances of the dipole gap.

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Magnetic Modelling

A plot of the B field along the path of the un-kicked beam in the field free region, B(T) vs. Z(mm), is shown in the first figure below. The limit for the maximum integrated dipole strength in the field free region is < 0.02 kGm. Results from the magnetic model analysis predict this value to be 0.003 kGm. The large spikes seen at both ends of the plots correspond to the fringe field region at the ends of the septum magnet core.

Field Free Requirement:

Int. Strength < 0.02 kGm

OPERA = 0.003 kGm

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Septum Magnet C1006 Low Carbon Steel Core Manufacturing

Manufacturing of the septum magnet steel core, from C1006 low carbon steel plates, is currently in progress at the SLAC MFD machine shop. The core blanks are cut from the mill finish plates by water-jet process. In comparison to flame or plasma cutting, water-jet does not create a heat affected zone (HAZ) in the vicinity of the cutting region. The pole shape is first milled from the core blank to oversize dimensions before the core blank is sent to be annealed in a dry hydrogen furnace.

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Magnetic Optical Lattice Design Team

The most important part of designing a complicated magnet such as the LCLS-II beam spreader Lambertson septum is communication amongst the team members responsible for the design of the LCLS-II Electron Systems Magnetic Optical Lattice. The team members work together from the early conceptual design stage to the final in tunnel checkout to ensure the rapid turn on and commissioning of the LCLS-II electron systems.

John Amann – LCLS-II DC Magnet Engineer

Tor Raubenheimer – SLAC Accelerator Physicist

Paul Emma – LCLS-II Lead Accelerator Physicist

Yuri Nosochkov – LCLS-II Accelerator Physicist

Mark Woodley – LCLS-II Accelerator Physicist

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The LCLS-II Collaboration

The LCLS-II project at SLAC is made possible by a collaboration of the US DOE Laboratories SLAC, LBNL, FNAL, ANL, JLAB, and Cornell University.

This work performed [in part] under DOE Contract DE-AC02-76SF00515

Integrated Vacuum Chamber

The water-jet cutting machine uses a highpressure jet of water with an abrasive medium.

Due to the large size of the core blanks, the pole is milled from the blank on a large horizontal turret mill.

Core blanks rough cut from the mill finish plates by water-jet process.

Core blank with pole rough machined.