Compact Linear Collider

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Compact Linear Collider
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Overview
The aim of the CLIC study is to investigate the
feasibility of a high luminosity linear e-/e
collider with a centre of mass energy reach of
3TeV. It is based on normal conducting
accelerating structures operating at a very high
gradient (gtsuperconducting structures) in order
to minimise the total length. A recent
optimisation study has lead to a major parameter
revision for CLIC. The main modification are the
change of the main linac RF freq. from 30 GHz to
12 GHz and the decrease of accelerating gradient
from 150 MV/m to 100 MV/m. This brings the total
length including beam delivery system to 48.25 km
for 3TeV. CLIC is based on Two-Beam Acceleration
(TBA) method in which the RF power for sections
of the amin linac is extracted from a secondary,
low-energy, high intensity electron beam running
to the main linac. The power is extracted from
the beam by special Power Extraction and Transfer
Structure (PETS). For a 3 TeV collider, there are
22 such drive-beams, each of which provides
enough power to accelerate the main beam by 70
GeV.
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New CLIC Parameter Set
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CLIC Layout with train combination scheme after
the damping rings and the modified drive beam
production.
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Overall Injection Complex

• Electron Production System The laser system and
  the photocathode RF electron gun generate a 10
  MeV, low-charge beam. Pre-injector linac provides
  an energy gain of 190 MeV and an electron beam
  energy at the exit of 200 MeV. The injector linac
  accelerates the beam by 1.78 GeV, giving a final
  energy of 1.98 GeV. This linac accelerates
  alternately the train of electrons and positrons.
  A DC dipole magnet separates the e- beam from e
  beam. Then, there are, successively the damping
  ring for e-, the first stage of the bunch
  compressor working at 3GHz and 1.98 GeV, the
  booster linac accelerating alternately e- and e
  beams up to 9 GeV, the transfer line, and finally
  the second stage of the bunch compressor working
  at 30GHz and 9 GeV at the entrance of the main
  linac.
• Positron Production System The electron primary
  linac sends a 2 GeV beam on to a e target.
  Following the conventional e source, which
  receives primary e- beam, the e pre-injector
  linac accelerates e up to 200 MeV and rest
  acceleration up to 9 GeV is same as described
  above in electron production system.

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Damping ring and Bunch Compressor

• Damping rings The CLIC damping ring is composed
  of two long FODO-cell straight sections with
  wigglers, two TME-cell arcs and four dispersion
  suppressors connecting the arcs and the
  straights, forming a race track shape. The
  damping ring provides e- and e bunch trains at a
  repetition period of 10ms with a normalised
  emittance of 430 nm.rad in the horizontal and 3
  nm.rad in the vertical plane.
• Then Electron and Positron Damping Rings (EDR
  PDR) are assumed to have same ring, cell and
  wiggler geometry.
• Bunch Compressor and Transfer lines The damping
  is designed to deliver a beam at the energy of
  1.98 GeV, bunched at the RF frequency of 3GHz, of
  relative r.m.s. Energy spread of lt0.082 and
  r.m.s. Bunch length of 3mm. The required bunch
  length in the main linac should be 30µm in order
  to reduce the dilution effect of transverse
  Wakefield on the vertical emittance. The
  corresponding compression ratio is 100 which
  cannot be obtained by a single compression stage.
  Thus, the two stages of compression are
  proposed one at 1.98 GeV and one at 9 GeV.
• Electron and Positron beams at 9 GeV are
  transported through transfer lines to the
  entrance of second bunch compressor, before
  injection in the main linac. These two transfer
  lines are consist of regular FODO cells, with
  sufficiently low vacuum of the order of 10-10
  Torr in order to prevent ion-trapping instability.

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Main Linac

• The Main Linac Lattice The main Linac
  accelerates the beam from 9Gev up to 3TeV. Each
  main Linac module contains four long accelerating
  structures. In between these structures
  quadrupoles are used to provide necessary
  focussing. A Beam Position monitor (BPM) is
  placed at the head of each girder. The beam line
  consist of twelve sectors, each containing FODO
  cells of equal length and phase advance.
• The beam consist of a train of 154 bunches with a
  charge of 4x109 particles each that are separated
  by 20 cm. The bunch length is sz 30µm.
  
• In order to stabilize the beam, the so-called
  BNS(Balakin-Novokhatsky-Smirnov)damping is used.

The main Accelerating structure 1) Tapered
Damped Structure(TDS) It is an old structure
designed to operate at 2p/3 travelling wave mode
with 150 cells, 500 mm long. Long range wakefield
are suppressed through a combination of strong
damping and detuning. The damping is accomplished
by coupling to each cell of the structure four
individually terminated wave-guides. This results
in a Q16 for lowest dipole mode. A taper in the
iris diameter from 4.5 mm to 3.5 mm provides a
detuning spread of 2GHz. But this structure would
not be able to operate with high accelerating
gradient of 150 MV/m. 2)Hybrid Damped
Structure(HDS) It introduces iris slot in
addition to damping wave-guides in order to
improve suppression of long range wakefields with
little increase of the pulsed surface heating.
The lowest dipole mode coupled mainly to the slot
than to the waveguide, and the weak dependence of
this coupling on the damping waveguide aperture
size, the surface of the cell outer wall can be
increased compared to TDS.
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TDS
HDS
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Schematic layout of CLIC RF power source

• The CLIC study focuses on high gradient, high
  frequency acceleration for multi-TeV linear
  colliders. Short RF pulses of high peak power are
  typically required in high frequency linear
  colliders. For CLIC, 130 ns long pulses at about
  230 MW per accelerating structure are needed, but
  no conventional RF source at 12GHz can provide
  such pulses. This leads naturally to the
  exploration of two beam acceleration technique,
  in which electron beam (the drive beam) is
  accelerated using standard, low frequency RF
  sources and then used to produce RF power at high
  frequency.

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Drive Beam and RF Power Source

• The drive beam generation complex is located in
  the centre of the linear collider complex, near
  the final focus system. The energy of the RF
  production is initially stored in a 92µs long
  electron beam pulse which is accelerated to about
  1.2 GeV by normal conducting low frequency (937
  MHz) TW linac.
• The beam pulse is composed of 32x22 sub-pulses
  each 130 ns long, in each sub-pulse electron
  bunches occupy alternately even and odd buckets
  of drive beam accelerator fundamental freq. 937
  MHz
• As the long pulse leaves drive beam accelerator,
  it passes through a delay line combiner where odd
  and even pulses are separated by a transverse
  deflector at the freq. Of 468.5MHz. The net
  effect is to convert a long pulse in to a
  periodic sequence of drive beam pulses with gaps
  in between. After the recombination (two-by-two)
  the pulse is composed of 16x22 subpulses.
• Same recombination of the subpulses takes place
  in the first combiner ring, 78m long and in
  second combiner ring, 312m long (four-by-four)
  and obtaining the final 22 trains required for
  the main linac. At this time point, each train is
  39m long and consist of 1952 bunches with charge
  of 16 nC/bunch and an energy of 1.18 GeV.
• Such drive beam pulses are distributed down the
  main linac via a common transport line, in a
  direction opposite to the direction of main beam.
  Pulsed magnets deflect each beam at the
  appropriate time in to a turn-around. After a
  turn-around each pulse is decelerated in a 624m
  long sequence of low impedance Power Extraction
  Transfer Structures (PETS) down to a minimum
  energy close to 0.12 GeV, and the resulting
  output power is transferred to accelerate the
  high energy beam in the main linac.

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PETS
The PETS is a passive microwave device in which
the bunches of the drive beam interact with the
impedance of periodically loaded waveguides and
excite preferentially the synchronous hybrid TM01
mode. In the process, the beam kinetic energy is
converted in to electromagnetic energy at the
mode frequency, which travels along the structure
with the mode group velocity. The microwave power
produced is collected at the downstream end of
the structure by means of the couplers and
conveyed to the main linac accelerating
structures by means of rectangular wave
guides. The PETS design, eight HOM damping
slots are placed symmetrically around the
circumference, splitting the whole structure into
8 identical pieces. In this configuration the
600mm PETS active length is sufficient to extract
642 MW of RF power from the 181A, 15 GHz drive
beam.