Compact Linear Collider

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 (>superconducting 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 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.

New CLIC Parameter Set ParametersSpecification Centre-of-mass Energy3 TeV Peak Luminosity7x10 4 /cm 2 /S Peak Luminosity (in 1% of Energy)2x10 4 /cm 2 /S Main Linac Frequency12 GHz Loaded Accelerating Gradient100 MV/m Repetition Rate50 Hz Beam Pulse Length200 ns Average Current in Pulse1 A Bunch Charge4x10 9 Hor./Vert. Normalised emittance660/20 nm rad Hor./Vert. IP beam size before pinch53/~1 mm Overall two Linac length41.7 km Total site length48.25 km Total power consumption390 MW

CLIC: Layout with train combination scheme after the damping rings and the modified drive beam production.

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.

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 <=0.082% 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 Torr in order to prevent ion-trapping instability.

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 4x10 9 particles each that are separated by 20 cm. The bunch length is σ z = 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 2π/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 Q=16 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.

TDS HDS

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.

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.

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 TM 01 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.