CERN: LEP in the Alps

Full text: In January, when CERN's LEP electron-positron collider is enjoying a well-earned break, it has now become traditional for the hard pressed LEP team to have no respite. Instead they pack their bags and depart for Chamonix in the nearby French Alps to review the past year's experience and plan for the future. In the cold January 1993 light of Chamonix, 1992 (January/February, page 4) was deemed to have been a good year for LEP operations, with the switch to 90° betatron phase operation having paid off. The 65% improvement in integrated luminosity over 1991 was attributed to longer beam lifetimes, faster filling and improved overall efficiency. The commissioning of the eight-bunch 'pretzel' scheme was facilitated with the new optics, and break-even quickly achieved, so that physics could benefit from more bunches in the machine. During 1992, the injection chain was fully tested with eight bunches, and when this comes into routine operation this year, the pretzel scheme will benefit. Pretzel running also opens the possibility of still higher luminosity, up to 2x10³¹ per sq cm per s, doubling the present level. However the finishing touches to high luminosity running are still more an art ('haute cuisine') than a science. Continuing studies of the inter correlation of different LEP conditions will help make this more systematic. The main factors affecting performance at 45 GeV are transverse mode coupling instabilities. The present working point gives good results, but there are still potentially interesting regions which need to be checked out. Beam lifetime and background are both limited by beam size and aperture. Background was reduced by improved focusing, while beam size is dominated by beam-beam effects. 90° operation proved its worth in 1992, but the inability to produce polarized beams was a disappointment, and a combined 90°/60° horizontal/vertical combination looked like offering the best of both worlds. Although some realignments, etc, could yet bring polarization with 90° operation, this is by no means certain. With polarization providing precision fixes on the beam energy for physics, the 90760° combination was voted clear favourite for 1993 running. 45 GeV running continues to be LEP's bread-and-butter, but behind the scenes (and sometimes even in front of them) preparations are pushing ahead for boosting LEP beam energy towards 100 GeV (LEP2). Chamonix sessions reviewed LEP2 progress (January/February, page 1) and looked at the expected performance. Producing an integrated physics luminosity of 500 inverse picobarns (over several years) will call for some ingenuity but is not ruled out. A major LEP2 milestone this year will be the installation of a complete niobium-coated four-cavity superconducting radiofrequency module, supplied by industry