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1
Status of the HeLiCal Contribution to the
Polarised Positron Source for the International
Linear Collider
Abstract The baseline design of the positron
source for the International Linear Collider
(ILC) incorporates a helical undulator and
pair-production target in order to generate the
unprecedented quantities of positrons required to
sustain the intended ILC physics programme. This
configuration is challenging but readily
achievable by using novel adaptations of existing
technologies to avoid problems inherent in
conventional positron sources in which the
stresses in the target(s) and activation of the
target station are both serious problems. In
addition, a highly polarized positron beam,
essential for realizing the full potential of the
ILC, can be produced by a simple upgrade to the
baseline design. A major contribution to the
international design effort is being led by the
UK-based HeLiCal collaboration. The collaboration
takes responsibility for the design and
prototyping of the helical undulator itself,
which is a short period device with a small
aperture, and also leads development of the start
to end simulations of the polarized particles to
ensure that high levels of polarization are
maintained from the sources, through the beam
transport systems and up to the interaction
point(s). Members of the collaboration are also
involved in the EUROTeV-funded research programme
to produce a design for a pair-production target
which can operate reliably in the high photon
flux of the undulator. This paper will provide an
update on the work of the collaboration, focusing
on the design, construction and testing of
components of the polarized positron source, and
will also discuss future plans.
Positron Source Overview
The ILC positron source will have to produce of
order 1014 positrons per second, with a nominal
bunch structure of 2820 bunches per pulse and 5
pulses per second, and a bunch duration of 1 ps.
In the current design the ILC electron beam is
passed through a helical undulator of length
approximately 100 m (see panel below left)
producing synchrotron radiation with a first
harmonic energy of 10 MeV which interacts with a
pair-production target (see panel below right).
Positrons produced from the target are captured
by a tapered magnetic field before being
accelerated to 5 GeV and passing through a
damping ring.

Proposed ILC layout.
Pair-Production Target
Role of Polarised Positrons at ILC

• The photon beam is incident on the rim of the
  target wheel. Simulations show that 10 of the
  beam energy will be absorbed by the target (lt 30
  kW).
• A rotating target design has been adopted to
  reduce the photon beam power density. The rate at
  which the target can be cooled determines the
  required angular velocity to the target rim.
• The diameter of the wheel is then determined by
  the characteristics of the drive motor, the rate
  of radiation damage to the target, and the
  required target lifetime.

The undulator-based positron source has been
chosen as the baseline technology for the
positron source because it offers the lowest-risk
alternative for producing the required number of
positrons for the ILC. In addition, it has the
strong advantage of producing the positrons in a
longitudinally polarized state. Higher levels of
polarization can be achieved by lengthening the
undulator and collimating the synchrotron
radiation. Having both the electron and positron
beams polarized is essential for maximising the
physics reach of the ILC an example of the role
of polarized positrons for determining the
quantum numbers of supersymmetric particles is
shown on the right. More details of the physics
case for polarized positrons and all aspects of
the positron source can be found at
http//www.ippp.dur.ac.uk/gudrid/source/
Helical Undulator Insertion Device

• Superconducting technology has been selected for
  the ILC positron source as it offers high field
  quality and easily tunable field strength.
• Four short superconducting undulator prototypes
  with a length of 300 mm have already been built.
• All prototypes have successfully demonstrated
  their full design field levels. One more short
  prototype will be built this year.
• Full scale prototype will have a 4 m cryostat
  containing two 2 m undulators.
• After magnetic testing the full scale prototype
  will undergo electron beam transport tests.

• Magnets or current elements are used to generate
  a (spatially) rotating magnetic dipole field
  along the major axis of the undulator.
• Charged particles entering the undulator describe
  helical trajectories in the field.
• This leads to the emission of intense
  circularly-polarised synchrotron radiation on
  axis.

(Numbers refer to LLNL study of earlier
solid-disc design with 220 kW photon beam.)
The figure on the left shows a possible variant
of the target wheel design with five spokes for
mechanical rigidity. Part of the rim has been
removed to reveal the internal water-cooling
channel. The thin rim is used in preference to a
solid disc target in order to reduce eddy
currents in the target wheel induced by the
magnetic field of the adjacent capture optics.
The heLiCal collaboration has developed short
undulator prototype modules for the ILC using two
different technologies superconducting and
permanent magnet.
Superconducting module prototype.
Permanent magnet module prototype.
The four short superconducting prototype modules

• Simulations of the activation of the target from
  neutron production predict dose rates 1 m from
  the target at a depth of 10 mm of soft tissue to
  be 150 mSv / h after 1 week of shutdown
  compared with 4000 mSv /h for an equivalent
  conventional positron source
• (A. Ushakov, Proceedings of EPAC 2006
  conference.)
• EU exposure limit for radiation workers is 20 mSv
  / year.
• A full remote-handling system is required. A
  concept with two hot cells is shown below.

The first superconducting undulator module
consists of an aluminium former into which has
been machined two interleaved helical grooves
with a period of 14 mm. Superconducting (NbTi)
wire ribbons are wound into the grooves and
current is passed in opposite directions along
the two helices to give a design field of 0.8 T
on axis. The results of on-axis Hall probe field
measurements are shown in the figure on the
right.
The permanent magnet undulator module consists of
trapezoids of NdFeB magnets arranged to form
rings with a dipole field on axis. Successive
rings forming the undulator were rotated with
respect to each other to give the necessary field
configuration. The photograph above shows the
undulator in two halves.
In the sketch shown on the left, targets can be
moved into the hot cells and repaired using
manipulator arms. Other concepts are also being
considered.
A. Birch, J.A. Clarke, O.B. Malyshev, D.J.
Scott CCLRC ASTeC Daresbury Laboratory,
Daresbury, Warrington, Cheshire WA4 4AD, UK E.
Baynham, T. Bradshaw, A. Brummitt, S. Carr, Y.
Ivanyushenkov, A. Lintern, J. Rochford CCLRC
Rutherford Appleton Laboratory, Chilton, Didcot,
Oxfordshire OX11 0QX, UK I.R. Bailey, P.
Cooke, J.B. Dainton, L. Jenner, L.
Malysheva Department of Physics, University of
Liverpool, Oxford St., Liverpool, L69 7ZE, UK
D.P. Barber, P. Schmid DESY-Hamburg,
Notkestraße 85, 22607 Hamburg, Germany G.A.
Moortgat-Pick Institute of Particle Physics
Phenomenology, University of Durham, Durham DH1
3LE, UK, and CERN, CH-1211 Genève 23,
Switzerland Cockcroft Institute, Daresbury
Laboratory, Daresbury, Warrington, Cheshire WA4
4AD, UK

• The University of Liverpool is planning on
  leading the prototyping of the target, with a
  series of prototypes being assembled and tested
  at Daresbury Laboratory in the period 2007-2010.

The HeLiCal Collaboration
I.R. Bailey1,3 ,V. Bharadwaj4, D. Clarke5, P.
Cooke3, J.B. Dainton1,3, J. Gronberg2, N. Krumpa5
, D. Mayhall2, T. Piggott2, D.J. Scott1,3, J.
Sheppard4, W. Stein2, J. Strachan5, P.
Sutcliffe3 1 Cockcroft Institute, Daresbury
Laboratory, Warrington, Cheshire, WA4 4AD, UK. 2
Lawrence Livermore National Laboratory,
Livermore, CA 94551, USA. 3 Department of
Physics, University of Liverpool, Liverpool, L69
7ZE, UK. 4 Stanford Linear Accelerator Center, PO
Box 20450, Stanford, CA 94309, USA. 5 CCLRC
Daresbury Laboratory, Warrington, Cheshire, WA4
4AD, UK.
Accelerator Science and Technology Centre
www.astec.ac.uk