Crystal experiments on efficient beam extraction

Silicon crystal was channeling and extracting 70-GeV protons from the U-70 accelerator with efficiency of 85.3±2.8% as measured for a beam of ∼10 12 protons directed towards crystals of ∼2 mm length in spills of ∼2 s duration. The experimental data follow very well the prediction of Monte Carlo simulations. This success is important to devise a more efficient use of the U-70 accelerator in Protvino and provides a crucial support for implementation of crystal-assisted collimation of gold ion beam in RHIC and slow extraction from AGS onto E952, now in preparation at Brookhaven Nat'l Lab. Future applications, spanning in the energy from sub-GeV (medical) to order of 1 GeV (scraping in the SNS, extraction from COSY) to order of 1 TeV and beyond (scraping in the Tevatron, LHC, VLHC), can benefit from these studies. The technique of bent crystal channeling to steer particle beams, with applications in extraction from accelerator and in beam collimation, has progressed rapidly thanks to efforts at IHEP Protvino[1], CERN[3], and FNAL[4]. Beams of up to 10 6 proton/s were extracted from CERN SPS and Tevatron by Si crystals of just 4 cm in length, with typical efficiencies on the order of 10-20%. It was predicted[5] that efficiency of crystal channeling extraction can be boosted to much higher values by multiple particle encounters with a shorter crystal installed in a circulating beam. To clarify this mechanism a new experiment was started at IHEP at the end of 1997, with intention to test very short crystals and achieve very high efficiencies of extraction[6, 7]. The benefits of a crystal-assisted extraction are fourfold. In hadron colliders this mode of extraction can in general be made compatible with the colliding mode of operation. The time structure of the extracted beam is practically flat, since the extraction mechanism is resonance-free. The size of the extracted beam is smaller and more round than in a resonant extraction. Finally polarized beams can be extracted without detrimental effects on the polarization.