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Liverpool Accelerator Physics Group
International Linear Collider (ILC) R&D
Beam Delivery System and ATF Test Beam Running
The ILC (left) is a design proposal for a TeV energy-scale ‘next generation’ linear collider, colliding electrons and positrons. Providing high-accuracy measurements of particle interactions, the ILC physics programme
is complementary to those being followed by hadron colliders of similar energies such as the LHC at CERN, Geneva. The Beam Delivery System (BDS) is the region after the main linacs (linear accelerators),
leading up to the Interaction Point(s) (IP) and detector(s). It consists of a large number of specialised sections for measuring and correcting beam properties. The beam is transported and manipulated using a series
of magnets, known as the optics. Institutions involved in the design include SLAC, KEK and Daresbury laboratories, and the universities of Cambridge, Lancaster, Manchester, Oxford, Royal Holloway, UCL and
Liverpool.
The numbers shown here represent the current baseline design and are all subject to change!
Possible ILC layout.
The ATF damping ring is part of the accelerator complex at KEK in
Tsukuba, Japan. As a fully operational accelerator, it is being used as
a test bed for new accelerator technologies, particularly in the area of
beam diagnostics equipment. Shown to the right is the entire damping
ring where the beam is stabilised and to the far right is the extraction
line where damped beam bunches are passed through any
diagnostics equipment.
 p 
e   D
 p 


A new tool, known as Laserwire, is being developed to measure the size
of the beam. Laser light is directed on to the beam and is then forward
scattered by the beam bunches (see below left). A beam intensity
camera shows the laser light being brought towards the beam (below)
and into collision with it (below right). Laserwire allows the sizes of high
energy beams to be measured to the micron level.
2
In general, it is desirable for the beam size to be as
small as possible to maximise collision luminosity. The
beam size is given by the formula shown to the left,
where e and p are the beam emittances and momenta,
and D is the dispersion of the beam. The resolution of
the laserwire deteriorates below 10 mm so larger beam
sizes are required at the point where the beam is to be
measured. This is achieved by varying the extraction
line magnet optics. Various optics designs have been
proposed, such as the one shown left which has been
developed at Liverpool. Optics testing on a real
machine is an invaluable precursor to the designing of
the final ILC optical layout.
The conventional method of measuring the beam
size is to use wire scanners which pass a wire
through the beam and build up an intensity profile
(right). ILC beams will be of too high energy to
employ this method as they will melt the wires!
The heLiCal collaboration has members from
Daresbury Laboratory, Rutherford Appleton
Laboratory, DESY, Durham and Liverpool.
One of the key components of the ILC design
is the positron source. It will have to produce of
order 1014 positrons per second, with the
nominal ILC bunch structure of 2820 bunches
per pulse and 5 pulses per second. As a part of
the heLiCal collaboration and the EUROTeV
project, Liverpool contributes to the R&D for an
undulator-based positron source.
Schematic of undulator-based positron source.
Spin Transport
The Positron Source
In this 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 typical
energy of approximately 10 MeV which collides
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.
The resulting positron beam is injected into the
main accelerator where it is accelerated to the
required energy (nominally 250 GeV) before
passing through the beam delivery system and
finally being brought into collision with the
opposing electron beam at the interaction point.
Possible ILC layout.
Polarised beams allow the structure of particle interactions to be probe
more precisely than possible with unpolarised beams. The ILC baselin
design specifies that the electron beam should be at least 80% spin
polarised. There is also a strong physics case favouring the use of a spin
polarised positron beam with a polaristion of approximately 60%. Thi
degree of polarisation can be achieved by the helical undulator positro
source described in the panel to the right.
As part of the PPARC-funded LC-ABD (Accelerator Beam Delivery
project, Liverpool heads a group developing computer simulations tha
track the evolution of polarised beams as they travel through the ILC from
the sources to the beam dumps.
Damping ring simulations
Photons(≈ 10 MeV )
Helical
Undulator
(≈ 200 m)
Electrons
(150 GeV to
250 GeV)
Photon
Collimator
Conversion
Target (0.4X0 Ti)
After production, the electrons and positrons pass through damping
rings containing wiggler magnets which act to radiatively ‘cool’ the
beams.
Polarised
Positrons
(≈ 5 MeV)
Helical Undulator Insertion Device
 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.
Pair-Production Target
Liverpool heads the EUROTeV-funded project to develop a pair-production target as
part of a high-intensity polarised positron source, and works in collaboration with the
Stanford Linear Accelerator Centre (SLAC) and Lawrence Livermore National
Laboratory (LLNL) in the US on the development of a water-cooled rotating wheel
target design. The wheel consists of a titanium alloy (Ti-6%Al-4%V) disc 0.4
radiation lengths thick and with a radius of 1 m which rotates at approximately
1000rpm. A conceptual design for the target is shown below.
The heLiCal collaboration has developed two undulator prototype modules using
different technologies: superconducting and permanent magnet.
The superconducting module
prototype.
The simulation to the le
is an example of
calculation
used
t
estimate how much of th
beam polarisation is los
through radiative spi
diffusion as the beam
circulates in a ring.
Capture Optics
The permanent magnet module
prototype (built at Liverpool) shown in
two halves.
Positron beam pipe
Target wheel
Photon
beam pipe
The
superconducting
undulator
module consists of an aluminium
former into which has been machined
two interleaved helical grooves with
a period of 14mm. Superconducting
(NbTi) wires ribbon are wound into
the grooves and current is passed in
opposite directions along the two
helices to give a design field of 0.8T
on axis. The results of on-axis Hall
probe field measurements are shown
below.
Further
prototypes
are
currently under construction.
Field profile
1
0.8
Radial field, T
-0.2
0
50
100
150
200
-0.4
-0.6
-0.8
-1
Z position, mm
250
300
350
TESLA
parameters
Before Interaction
The permanent magnet undulator
module consists of trapezoids of
NdFeB magnets arranged to form
rings with a dipole field on axis
(illustrated below). Successive rings
forming the undulator were rotated
with respect to each other to give the
necessary field. Field measurements
for this prototype are ongoing.
Spread in Polarisation
PINIT=1.0
During Interaction
After Interaction
During Interaction
After Interaction
Vacuum feedthrough
Photons incident on the target (e.g. from the
synchrotron radiation produced in an undulator)
produce electromagnetic showers of electrons,
positrons and photons. The simulation on the left
shows a square of the target with photons incident
from the left side. The green lines show photons
which have passed through the target. The
(relatively few) red and blue lines show electrons
and positrons. If the incident photons are circularlypolarised then the outgoing positrons will tend to
be longitudinally spin-polarised.
0.4
0
As the bunches of electrons and positrons approach each other in the
interaction region, their Coulomb fields perturb the spin orientation of the
individual electrons and positrons. This depolarising effect is shown
below for two different set of possible ILC beam parameters. In each
case a bunch of electrons starts with 100% spin-polarisation which then
evolves as the electrons approach a bunch of positrons.
Motor
0.6
0.2
Bunch-bunch depolarisation
Approximately 20kW of heat is expected to be deposited in the target during
operation at the ILC. The heat will be dissipated by the water-cooling system whilst
the rotation of the wheel will prevent any one spot on the target from over-heating.
Studies of heating, radiation damage, neutron activation and remote-handling
systems are all on-going, and Liverpool will shortly begin constructing target
prototypes.
low Q
parameters
PINIT=1.0
Before Interaction
The Cockcroft Institute
The University of Liverpool is the lead organisation in the
newly formed national centre for accelerator science - the
Cockcroft Institute. Liverpool’s partners in the Cockroft Institut
are the universities of Lancaster and Manchester, Daresbury
laboratory (CCLRC) and the North West Development Agency
(NWDA).