CLIC est un des projets actuels de construction d'un collisionneur lineaire de haute energie. La taille verticale des faisceaux de 0,7nm lors de la collision et le mouvement rapide du sol de quelques nanometres imposent une stabilisation active des doublets finaux au cinquieme de nanometre au-dessus de 4Hz. La majorite de mon travail a porte sur l'etude des vibrations et de la stabilisation active de poutres en porte-a-faux et elancees afin de representer les doublets finaux de CLIC. Dans une premiere partie, les performances mesurees de differents types de capteurs de vibration associes a une instrumentation appropriee ont montre que des mesures precises du mouvement du sol sont possibles de 0,1Hz jusqu'a 2000Hz sur un site calme. Egalement, des capteurs electrochimiques repondant a priori au cahier des charges de CLIC peuvent etre incorpores dans la stabilisation active au cinquieme de nanometre. Dans une deuxieme partie, une etude experimentale et numerique des vibrations d'une poutre a permis de valider l'efficacite de la prediction numerique incorporee par la suite dans la simulation de la stabilisation active. Egalement, une etude de l'impact du mouvement du sol et du bruit acoustique sur les vibrations d'une poutre a montre qu'une stabilisation active est necessaire jusqu'a au moins 1000Hz. Dans une troisieme partie, les resultats sur la stabilisation active d'une poutre a ses deux premieres resonances sont montres jusqu'a des amplitudes d'un dixieme de nanometre au-dessus de 4Hz en utilisant en parallele un systeme commercial realisant une stabilisation passive et active de l'encastrement. La derniere partie a porte sur une etude d'un support pour les doublets finaux d'un prototype d'un collisionneur lineaire en phase de finalisation, le prototype ATF2. Ce travail a montre que le mouvement relatif entre ce support et le sol est en-dessous des tolerances imposees (6nm au-dessus de 0,1Hz) avec des conditions aux limites appropriees.

Etude des vibrations et de la stabilisation à l'échelle sous-nanométrique des doublets finaux d'un collisionneur linéaire

In order to study the principle of the Compact Linear Collider (CLIC) based on the Two Beam Acceleration (TBA) scheme at high frequency, a CLIC Test Facility (CTF) has been set-up at CERN. After four years of successful running, the experimental programme is now fully completed and all its objectives reached, particularly the generation of a high intensity drive beam with short bunches by a photo-injector, the production of 30 GHz RF power and the acceleration of a probe beam by 30 GHz structures. A summary of the CTF results and their impact on linear collider design is given. This covers 30 GHz high power testing, study of intense, short single bunches; as well as RF-Gun, photocathode and beam diagnostic developments. A second phase of the test facility (CTF2) is presently being installed to demonstrate the feasibility of the TBA scheme by constructing a fully engineered, 10 m long, test section very similar to the CLIC drive and main linacs, producing up to 480 MW of peak RF power at 30 GHz and accelerating the beam up to 320 MeV. The present status of CTF2 is reported. 1. HISTORY AND OVERVIEW The construction of the CTF accelerator experiment was launched in 1988 aiming to study CLIC [1] drive beam generation by means of a RF photo-injectors, to act as a source of 30 GHz RF pulses of 60 MW for CLIC prototype testing and to have a test bench for beam diagnostic instruments of various types. After first beam was obtained at the end of 1990, the CTF was gradually improved and reached its design performance in 1994. While during the first years of operation the work on CTF was focused on achieving the design performance in terms of accelerated charge and RF power the 1994/95 programme emphasized testing the high power behaviour of various 30 GHz CLIC prototype components, beam monitors of various kinds, bunch compression experiments and photocathode work. The CTF experimental programme finished at the end of 1995 and the CTF was completely dismantled to make space for the successor experiment CTF2. The progress of CTF is documented in numerous conference reports [2-7]. Figure 1 shows a scheme of the CTF installation as it was in 1995. For simplicity magnetic focusing elements and beam diagnostic elements are omitted. An S-band RF-gun [8] produces single bunches or trains of up to 48 bunches from a photocathode illuminated by a short pulse, high power laser system [9]. A magnetic chicane, together with a longitudinal correlation introduced by the RF-gun phasing, is used for longitudinal bunch compression [10]. After compression a 1 m long S-band section boosts the electron energy to 65 MeV. This section is borrowed from LAL/Orsay where it was built and used for high gradient experiments [11]. The RF-gun and the structure are powered by two klystrons, one of them equipped with RF pulse compression. With typical power levels of 18 MW for the first and 27 MW for the second klystron we run well below their maximum power of 35 MW each. A drift space between the S-band section and a magnetic spectrometer is used to test beam-monitor and 30 GHz RF structure prototypes with beam. The sections CAS1 and CAS2 are identical prototypes of CLIC 30 GHz accelerating structures. While CAS1 is used to produce 30 GHz power due to the beamloading from the electron beam, CAS2 reaccelerates the electrons. The achieved accelerating gradients in CAS2 can be readily computed from the RF gun high field TW section CAS1 CAS2 30 GHz power bunch compressor beam monitor &

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Results from the CLIC test facility

The luminosity that can be achieved in a linear collider strongly depends on the vertical emittance of the beams at the interaction point. One of the most important sources of emittance growth is the accelerating part of the main linac especially in the case of the compact linear collider, CLIC[1], where the wakefields are strong due to the high frequency of the accelerating structures (30GHz). Possible lattices for the main linacs for centre-of-mass energies of 1TeV and 3TeV are presented which allow the use of BNS-damping for emittance preservation. A new ballistic beam-based alignment technique to keep the emittance growth below the required limit is investigated. The influence of different prealignment and field errors on the correction efficiency is investigated.

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Emittance Preservation in the Main LINAC of CLIC

The second phase of the Compact Linear Collider Test Facility (CTF II) has demonstrated the feasibility of two key ingredients of the Compact Linear Collider scheme (CLIC) [1], namely the acceleration with a 30 GHz normal conducting linac and the 30 GHz RF power production by a tightly-bunched, high-charge drive beam running parallel to the main beam. This beam is produced and accelerated with a 3 GHz linac using an RF-photoinjector and two travelling-wave sections, all specially developed for handling very high charges. A magnetic chicane compresses the micro-bunches to their nominal length. A mm-wave spectrometer, coupled to the beam pipe, allows non-destructive measurements of bunch length. So far a total acceleration of 60 MeV has been obtained using a string of five accelerating structures with a total active length of 1.4 m. The corresponding drivebeam deceleration is 6 MeV. The flexibility and extensive beam instrumentation allows a variety of other experiments, such as measurements of emittance growth and energy loss in bunch compressors due to coherent synchrotron radiation, high-gradient tests in single-cell 30 GHz cavities, high-power tests of a planar 30 GHz RF structure and tests of beam position monitor prototypes.

Experimental results and technical research and development at ctf ii

In this paper, we will give a brief review of some important beam physics in circular and linear electron-positron collider designs, covering beam-beam tune limits, longitudinal and transverse single bunch collective effects, electron cloud and space charge effects, dynamic aperture estimations, etc. The main feature of this review is that the corresponding beam physics treatments are coming from author's previous research works which are scattered in different scientific publications both for circular and linear colliders. With the progresses of future linear colliders, such as ILC,(1) and future circular electron-positron colliders, such as CEPC,(2) it is high time to review the key beam physics issues in the optimization designs of these two kinds of machines.

Review of some important beam physics issues in electron-positron collider designs

In this paper we discuss a new approach to two-beam acceleration. The energy for RF production is initially stored in a long-pulse electron beam which is efficiently accelerated to about 1.2 GeV by a fully loaded, conventional, low frequency (∼1 GHz) linac. The beam pulse length is twice the length of the high-gradient linac. Segments of this long pulse beam are compressed using combiner rings to create a sequence of higher peak power drive beams with gaps in between. This train of drive beams is distributed from the end of the linac against the main beam direction down a common transport line so that each drive beam can power a section of the main linac. After a 180-degree turn, each high-current, low-energy drive beam is decelerated in low-impedance decelerator structures, and the resulting power is used to accelerate the low-current, high-energy beam in the main linac. The method discussed here seems relatively inexpensive, is very flexible and can be used to accelerate beams for linear colliders over the entire frequency and energy range.

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A new method for RF power generation for two-beam linear colliders

We discuss the RF system, the drive linac, drive beam generation, the isochronous ring drive beam scheme, the main linac injector system, machine parameters, beam dynamics and final focus studies and the alignment test facility and beam monitor test results.

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CLIC-a compact and efficient high energy linear collider

Linac-on-ring schemes for high-luminosity colliders are based on collisions between an intense positron beam in a conventional storage ring and relatively low-intensity bunches of electrons from a high-repetition rate superconducting linac. The electrons are highly disrupted as they pass through the positron bunch and the electron bunch envelope suffers a longitudinal pinch that varies in form according to the chosen beam parameters. This pinch must be incorporated into any beam-beam simulation program that is used to study the stability of the positron beam. The synchrobetatron resonances due to collision point oscillations are particularly sensitive to the form of the pinch and also to any longitudinal jitter on the electron bunches. >

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The incoherent beam-beam effect in linear-on-ring colliders

The CERN Linear Collider (CLIC) study explores the technologies that will be needed to build a two-beam linear collider in the centre-of-mass energy range from 500 GeV to 2 TeV. The relatively low energy drive linac (a few GeV) delivers a 12 ns burst of electron bunchlets, of total charge 7 pC, to a 30 GHz transfer structure from which microwave power is drawn to feed the main linac. The duration of the burst equals the filling time of the main accelerating cavities and it is split into four or more trains of bunchlets separated by the RF period (350 MHz). The burst repetition rate is 1.7 kHz. To generate and handle these intense bunchlets several solutions are being studied. The source can be a conventional cathode or a laser photocathode. Preacceleration options range from induction accelerators to boosted S-band RF guns. One way to create the required 30 GHz time structure of the drive beam is to assemble many individual bunchlets from multiple sources, each compressed in length to 1 mm, into trains of bunchlets spaced by 10 mm. Sequential combination in momentum space is the main tool for this purpose, but a variety of optional hardware solutions exist, including the use of an isochronous collector ring. Alternatively, the 30 GHz time structure may be generated by the bunching process inherent to the operation of a Free Electron Maser, FEM. This attractive “in line” technique is now studied experimentally. Other studies include the generation of high charge from laser photocathodes, harmonic RF acceleration with compensation of beam loading and control of extraneous modes, and the specification of beam quality and stability.

CLIC drive beam generation : a feasibility study

The Compact Linear Collider (CLIC) study of an e+/e- linear collider in the TeV energy range is based on Two-Beam Acceleration (TBA) in which the overall RF power needed to accelerate the beam is extracted from high intensity relativistic electron beams, the so-called drive beams. Due to the high beam power, acceleration and transport of the drive beams in an efficient and reliable way is specially challenging. An overview of a potentially effective scheme is presented. It is based on the generation of trains of short bunches, accelerated in low frequency c.w. superconducting cavities, stored in an isochronous ring and combined at high energy by funneling before injection by sectors into the drive linac. The various systems of the complex are discussed as well as the beam dynamics all along the process. An original method has been specially developed to stabilize such an intense beam during deceleration and RF power production in the drive linac.

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C D Johnson

Papers

A new method for RF power generation for two-beam linear colliders

CLIC-a compact and efficient high energy linear collider

The incoherent beam-beam effect in linear-on-ring colliders

CLIC drive beam generation : a feasibility study

A multi-drive beam scheme for two-beam acceleration in a TeV linear collider