1. General Introduction

1.2 Overview of the linear collider

A high-energy (0.5-5 TeV centre-of-mass), high-luminosity (10³⁴-10³⁵cm-2s-1) e⁺e^- Compact LInear Collider (CLIC) is being studied at CERN [1.3], [1.4] as a possible new high-energy physics facility for the post-LHC era [1.5]. The general parameters and an overall layout are given in the Appendix. The maximum energy of 5 TeV is well above those presently being proposed by other linear collider studies. The physics experiments require a luminosity of at least 10³⁴  cm ^-2 s^-1 at 1 TeV c.m. and this luminosity should increase at higher energies [1.4]. Although the design study has been optimized for 3 TeV centre of mass, the collider could start operation at a lower energy and then be upgraded in stages. The design is such that these upgrades can be made without major modifications. CLIC is based on the Two-Beam Acceleration (TBA) method in which the RF power for sections of the main linac is extracted from a secondary, low-energy, high-intensity electron beam running parallel to the main linac. The power is extracted from the beam by special Power Extraction and Transfer Structures (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. All the drive-beams are generated in a centrally located facility. The only difference between the drive-beam generation schemes for high and low colliding beam energies is the length of the modulator pulse (the installed hardware is exactly the same). This means that the entire drive-beam generation system has to be installed in the first stage. The overall layout of CLIC is sketched in Fig.1.1 . Two interaction points (IPs) are foreseen, one for e⁺ -e ^- and one for g-g . For a c.m. energy of 3 TeV and an accelerating gradient of 150 MV/m, and allowing ~10 km for the Beam Delivery (BD) area, CLIC would cover a total length of approximately 38 km. The generation, acceleration and delivery of the main beams (see the upper half of Fig.1.1 ) and the micro-alignment system are described in Section 2 . The drive-beam complex, and the production of 30 GHz RF power (see the lower half of Fig.1.1 ) is presented in Section 3. Sections 4 and 5 are devoted, respectively, to machine protection questions, and to the CLIC test facilities which are intended to demonstrate the feasibility of the TBA scheme.

Fig. 1.1 : Overall layout of CLIC for a centre-of-mass energy of 3 TeV

A big advantage of the TBA scheme is that, since there are no active components such as modulators or klystrons, both linacs can be housed in a single small-diameter tunnel. This results in a very simple, cost-effective and easily extendable arrangement (Fig. 1.2 ). The tunnel cross-section shows the two linacs installed on a common concrete base. The various transfer lines fixed to the roof carry the main- and drive-beams from the generation complexes to their respective injection points (Fig. 1.1).

Fig. 1.2 : CLIC tunnel cross-section

Among the different tehcnological ways of building a linear collider, the CLIC study explores the technical feasibility of two-beam acceleration using high-frequency, room-temperature, travelling-wave structures. A high RF frequency has been chosen in order to be able to operate at a high accelerating gradient which reduces the length, and in consequence, the cost of the linacs. The choice of 30 GHz is considered to be close to the limit beyond which standard technology for the fabrication of accelerating structures can no longer be used. The main drawback of operating at a high RF frequency is that the accelerator iris aperture is very small (~4 mm), leading to the generation of strong wakefields and the related dilution of the transverse emittance. The effects of these strong wakefields can be minimized by a judicious choice of beam and linac parameters. These parameters have been optimized using general scaling laws derived from linear collider studies covering more than a factor 10 in frequency [1.6]. The optimization uses a figure of merit M, defined at a given energy as the luminosity normalized by the beamstrahlung parameter d _B and the wall-plug power consumption P _AC . In the low-beamstrahlung regime ( U<< 1, Y being the critical photon energy normalized to the beam energy), the factor M depends only on the mains-to-beam power transfer efficiency  and the vertical normalized emittance  at the IP. In the high-beamstrahlung regime ( U>> 1), the factor M depends, in addition, on the (r.m.s.) bunch length s _z . To obtain stable beam operation with an infinite number of bunches, and to minimize the energy spread at the end of the linac, the charge per bunch and the bunch length must be scaled with RF frequency w/2p , mean-loaded gradient G _a and initial normalized emittance e _ny0 (before blow-up) like [1.6].

   and   

For CLIC at 30 GHz this gives N _b  = 4  ¥  10 ⁹ and s _z  = 30  m m. After fixing these two quantities, the RF-to-beam transfer efficiency  is optimized (assuming a large number of bunches) by adjusting the field attenuation constant t via the accelerating structure length L _st , according to the following law and its corollary

which makes  nearly independent of the accelerating gradient and of the RF frequency. This optimum corresponds to t _opt  = 0.675 and L _st  = 0.5 m. In spite of the reduced charge per bunch and of the high gradient, the efficiency remains high because the time between bunches is small and the shunt impedance of the structure is high. In addition, the effects of the wakefields and the consequent beam emittance blow-up are effectively made independent of the frequency with the chosen values of N _b , s _z and t. At higher frequencies this requires stronger focusing, tighter alignment tolerances of the cavities and position-monitors, and a sufficiently large momentum spread for BNS damping* [1.7].

At 3 TeV, in the high-beamstrahlung regime ( U>> 1) which, if the other parameters are kept constant, gives a larger luminosity, then both the factor of merit and the total luminosity increase with w but are independent of G _a , according to Ref. [1.6].

where U _f is the final energy of the main beam, L the luminosity and  the vertical betatron amplitude at the IP.

Hence, limiting the power consumption at 3 TeV requires operating CLIC with U>> 1 and with an extremely low vertical (normalized) beam-emittance which implies a very small growth of this emittance during acceleration. The other important beam parameters are deduced from constraints in the interaction region. The horizontal beam size at the IP is adjusted to get a good trade-off between the total luminosity and the fractional luminosity near the nominal c.m. energy. The vertical beam size is limited by the synchrotron radiation and the chromaticity generated in the IP quadrupole doublets. These conditions on the spot size and beam optics considerations determine the betatron-function amplitudes at the crossing point and in consequence the precise values of the emittances. With such modest bunch charges, the only way to obtain the specified high luminosity is to operate in a multibunch mode and at a repetition rate of the order of 100 Hz. The resulting main-beam and linac parameters corresponding to CLIC at 3 TeV c.m. are listed in Table 1.3.