The 731-kilogramme stabilisation table at Annecy, with its active feet.

Group photo of some participants of the Annecy ATF2 meeting on 10-12 November 2006.

At the ILC, the beam size is aimed to be unprecedentedly small – about only 6 nanometres in the vertical plane. The project requires important R&D. The ATF2 international project at KEK (See also 8 June 2006 issue of NewsLine) will help demonstrate the feasibility of this goal. It will provide an almost exact scaled-down copy of the final focusing system planned at the ILC. In France, a team of ILC scientists is supported by the new French-Japanese International Associated laboratory and has also just obtained funds from the newly created French research funding agency ANR (National Agency for Research) to join the ATF2 collaboration.

Three IN2P3 French laboratories, LAPP at Annecy, LAL at Orsay and LLR at Palaiseau will contribute to beam stabilisation at the final focus, development of optical corrections and optimisation of background simulation tools, in direct partnership with teams working on these issues at KEK. The starting French contribution was recently reviewed during a dedicated ATF2 workshop, which took place in Annecy, France, on 10-12 November 2006. Attended by 20 participants from Japan, France, CERN, Spain, UK and the US, this meeting was a rare occasion to discuss several of the key issues in detail.

One key element for the measurement of tiny beam dimensions is to insure the control of the beam position. This implies a nearly perfect stabilisation of the mechanical structure at the so-called final doublet, just before the interaction point. In the ATF2 hall, the ground movement magnitude is expected to be at the 10 nm level and therefore , a much smaller -- on the order of 1nm -- level will be required. The LAPP team is working on a stabilisation table which was lent by CERN and was previously used for CLIC studies. It consists of a 731-kilogramme-honeycomb block, aimed to damp the vibrations. The table feet - the active part of the stabilisation- are made of geophones and piezoelectric sensors. In Annecy so far, they reached approximately a stabilisation of 0.5 to 1 nm for frequencies above 4 Hz. "R&D should now continue in the range of very low frequencies, where the stabilisation will be critical at ATF2", said Andrea Jeremie, a scientist leading this project at LAPP. The Annecy laboratory will soon receive from SLAC one magnet, and four magnet supports, which came from the former FFTB beam line, for further stabilisations tests in more realistic conditions.

Acting like an optical lens on a beam of light, the magnets will curve the beam envelope and eventually focus it to a small transverse size in a given plane. At the ILC, high order chromatic and geometrical aberrations must be corrected. At ATF2, and therefore also at the ILC, most of the correction is done locally, near the final doublet, right where the main aberrations are produced. This is new and unconventional but will help reduce the size of the correction system and, most of all, it allows the system to scale with energy, enabling to extrapolate its key features from ATF2 (where the beam energy is 1.3 GeV) to ILC. At LAL, a team led by Philip Bambade will study these corrections, evaluating new constraints which were not considered during the designing phase, in close collaboration with other scientists at KEK, CERN, SLAC and in the UK. The resulting know-how which will be developed is expected to be directly applicable to ILC, given the high degree of similarity with ATF2. They will also help qualifying the properties of the beam injected into ATF2, and make sure that the instrumentation planned there – beam position monitors and laser wires – is adequate (see also 26 January 2006 and 24 August 2006 issues of NewsLine).

Another ATF2 R&D involves adequate using, developing and optimising the possibilities of GEANT4, the widely used simulation toolkit in particle physics. The goal is to simulate the production and propagation of secondary particles from beam losses on collimators and elsewhere in the beam line, which can produce backgrounds in several of the sensitive instruments used to measure the beam. At LLR, Marc Verderi and his team, together with LAL and Royal Holloway, University of London (RHUL), will participate in the development of an adaptative simulation programme, which will give more weight to some prominent and important configurations. These studies will be of great importance once a full simulation of the whole 100-metre beam line of ATF2 line will be implemented, in order to reduce the computing time. "If implementing such 'event biasing' methods at ATF2 can be shown to produce reliable results, it will surely be quite useful at the ILC where the beam lines are much longer and where predicting secondary particle production accurately is even more challenging statistically," said Bambade. "This could end up saving an enormous amount of computing time."

-- Perrine Royole-Degieux