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EuCARD >> News >> Newsletters >> Issue 8 >> Article 3

ILC dogleg prepares

electron beam for success

The International Linear Collider (ILC) is the proposed particle physics machine which will be the successor to the Large Hadron Collider (LHC).  It will operate at energies of up to 1 TeV centre of mass and will be able to explore physics beyond the reach of the LHC.  The ILC will collide beams of electrons and positrons together to achieve new results in particle physics.

The ILC is hoping to make new discoveries in particle physics including contributing to the search for the elusive Higgs Boson: Image courtesy of DESY. Thumbnail image on main page courtesy of SLAC.

Ongoing changes to ILC design

The Reference Design Report (RDR) for the ILC was produced in 2007 but design work has been ongoing since then, and has resulted in the production of the Strawman Baseline 2009 proposal. This proposal is a modification to the RDR, which takes into account both a reduction in cost and a more complete and robust design approach.

Beam Delivery System

The Beam Delivery System (BDS) describes the section of the ILC after the beam has been accelerated to its full energy but before it reaches the Interaction Point (IP), where collisions occur. The purpose of this section is to modify the properties of the full energy beam so that the best results can be achieved when the beam reaches the IP. The beam passes through a series of magnets so that properties such as beam size and energy spread can be measured and modified. The beam will also pass through a series of collimators so that particles which are outside the acceptable range of positions and energies are removed.

Prior to entering the BDS, electrons are accelerated in the main linac and then pass through an undulator (a series of magnets which makes the electron beam oscillate from side to side).This produces a very high energy photon beam which impacts on a target 400m downstream, and produces positrons for later collisions. At the end of the undulator the electron beam needs to be displaced transversely from the photon beam so that they cannot interact with each other and so there is enough space for the target. This displacement is achieved by designing a ‘dogleg’ section of the BDS.

The undulator is located immediately after the main linac in this new Strawman Baseline 2009 design, a major change from the RDR. Consequently components have to be placed at the end of the main linac to protect the undulator, which has a very narrow gap, from mis-steered electron beams. These components include sacrificial collimators which will absorb any stray electrons and an energy chicane to detect if the beam is the wrong energy. If an off energy beam is detected the electron beam will be deflected along the fast abort line and into a beam dump.

Designing the dogleg

Layout of the electron side of the beam delivery system, IP is the interaction point. Click on image for larger version. Image courtesy of J. Jones, STFC Daresbury Laboratory.

 

The dogleg section of the BDS needs to transversely displace the electron beam by a minimum of 1.5m over a longitudinal distance of only 400m. This is done by using dipole magnets to steer the electron beam along a curved path. However, when electrons pass through a dipole, synchrotron radiation is produced. This can lead to an increase of the emittance of the electron beam. Emittance is a measure of both the position and angular divergence of the electrons in a beam and it contributes to the overall beam size. The emittance growth in the dogleg section of the BDS must be minimised to ensure that the beam’s properties can be modified suitably in the remainder of the BDS before reaching the IP, and so maintain the collision rate.

A dogleg section has been designed with a Theoretical Minimum Emittance lattice, a design of magnet layout designed to minimise the emittance growth. In this case the emittance growth is about 3.8%. The combination of the spatial and emittance constraints of the dogleg section mean that the dipoles used can only have bend angles of up to 1.1 mrad. This in turn leads to constraints on the focussing magnets used and results in a very strong focussing lattice. Three different designs were simulated which used different strength dipole magnets, quadrupole (focussing) magnets and spatial layouts. One of these designs was shown to satisfy all the necessary requirements for the beam and was physically feasible.

When simulating the passage of electrons through these designs certain assumptions have to be made about the properties of the incoming beam. In reality the beam properties will vary so the design has to tolerate these variations. Calculations have been carried out which show that the tolerance of the design to varying incoming beam properties is quite low. This in inevitable though due to the strong focussing required and the spatial constraints.

The ILC Strawman Baseline 2009 dogleg design has been compared to different design of doglegs used in earlier versions of the ILC. The emittance growth is much lower than in these designs so this work shows a marked improvement in this area of research. While emittance growth is minimised, further work needs to be done to assess if the tolerances can be relaxed further.

This research is part of Work Package 9 of the EuCARD project.

The original article, submitted by J. Jones and D. Angal-Kalinin to IPAC’10, can be found at: http://cdsweb.cern.ch/record/1299166/files/EuCARD-CON-2010-032.pdf

- Naomi Wyles, CERN, EuCARD-DCO (WP2)

 

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