Recent workshops on crystal channeling held at CERN established the present situation of crystal channeling in all its aspects and demonstrated the essential role of an experiment at CERN SPS to access the feasibility of using crystal channeling for a successful LHC experimental program up to maximum luminosity.
Bent-crystal techniques are well established as a means for extracting high-energy beams from accelerators. Following the pioneering proposals [1,2], experiments in Protvino [3], at the CERN SPS [4] and at the Tevatron [5] have observed extraction efficiencies in good agreement with simulations, up to beam energies of 900 GeV. Highest efficiencies of 50-80% were achieved in the Protvino experiments using a thin (3-5 mm) Si crystal with 0.5-1.5 mrad bending radius.
In 1991 it was proposed to apply crystals for halo collimation in the SSC [6]. The underlying idea was that a bent crystal as primary collimator should extract incident halo particles onto a subsequent secondary collimator. Indeed, simulations with the STRUCT/MARS codes showed that the addition of bent crystals reduced beam losses by about three orders of magnitude. More recently, based on realistic modeling it was decided to implement a bent-crystal collimation system in the Tevatron, which is expected to reduce beam loss in the superconducting magnets by a factor of 4 and the detector background by up to an order of magnitude [7]. Commissioning of the bent-crystal collimation system at the Tevatron started in 2005. The crystal used at the Tevatron was previously employed in a crystal-collimation attempt at RHIC. The RHIC experiment [8] was unsuccessful, which has been ascribed to the absence of a secondary collimator behind the crystal and to the poor quality of the crystal, the extraction properties of which have not been characterized with beam.
An efficient and robust collimation system is mandatory for any superconducting hadron collider, in particular for the LHC, which will store a beam of unprecedented high intensity and energy. The present LHC baseline collimation system is known to fail at above half the design intensity, where its impedance is predicted to drive the LHC beam unstable. Highly efficient and short primary bent-crystal collimators offer the prospect of retracting the secondary collimators, so as to reach nominal and ultimate intensity. The LHC experiments have suggested other interesting applications of bent crystals notably their use in diffractive proton proton interactions in conjunction with Roman pots to significantly extend the angular sensitivity to diffractive Higgs events [9]. Yet a further possible role of crystals LHC would be the use as a last resort for safely extracting the LHC beam in the unlikely event of a complete dump kicker failure. In addition, with an optimized crystal-based collimation system at the LHC, one can expect a substantial reduction of beam loss rates and accelerator-related beam background in the detectors [7].
Over the last years, groups in Russia (St. Petersburg) [10] and Italy (Ferrara) [11] have independently developed novel crystal-production methods, which are reported to considerably improve the crystal quality as compared to the crystals used in the 1990s for previous experiments at the SPS and in the Tevatron.
In view of the imminent crystal-collimation experiments at the Tevatron, the potential improvement compared with the phase-1 LHC collimation system, and the recent progress in crystal technology, we propose to start as soon as possible a series of crystal experiments in the SPS, which could address a number of objectives:
1) qualification of the new crystals to be used in the Tevatron collimation experiment (e.g., measuring their channeling efficiency) and confirmation of the recode extraction efficiencies measured in Protvino
2) measuring the channeling efficiency of long crystals with 1 mrad and/or 8 mrad bending angle
3) modeling the crystal set up for performing diffractive physics (this requires a 2-stage crystal system and Roman pots)
4) extraction of the full SPS beam with crystals (for use as LHC protection device in the rare case of dump kicker failure)
5) comparison of loss patterns around the ring for a crystal with one for amorphous material, e.g., for benchmarking simulations of crystal-collimation efficiency
From the scientific arguments presented above, we perceive a high probability to improve the LHC halo collimation with bent-crystal scrapers. This is an important issue because of the extreme requirements posed by the LHC machine, especially for its nominal and ultimate intensity. It is not much time left to develop a detailed technical design of crystal scrapers for the LHC, and we consider it not as wise to wait for further scientific justification from outside CERN. We note that the SPS is an ideal machine for this type of experiments, since its properties are well understood, and since it will not be under the same pressure as the Tevatron and later the LHC. Any additional data from Fermilab, Protvino, and other laboratories would be very welcome, and will strengthen the case. However, in our opinion, CERN should now aim to take a leadership role of the development, since CERN is solely responsible for the success of the LHC program.
[1] E.N. Tsyganov, Some Aspects of the Mechanism of a Charged Particle Penetration Through a Monocrystal, TM-682 (1976).
[2] A.F.Elishev, E.N. Tsyganov, et al., Steering of Charged Particle Trajectories by a Bent Crystal, Physics Letters 88B, no. 3,4 (1979)
[3] Y. Fedotov, Extraction of the Proton Beam from 70 GeV IHEP Accelerator, CARE-HHH workshop CC-2005, CERN, 7-8 March 2005.
[4] The RD22 Collaboration, Status Report of RD22: Crystal Extraction at the SPS, CERN/DRDC 92-51
[5] R. Carrigan, et al., Beam extraction studies at 900 GeV using a channeling crystal, Phys. Rev. AB, vol. 5, E043501 (2002).
[6] M.A. Maslov, N.V. Mokhov, I.A. Yazynin, The SSC Beam Scraper System, SSCL-484 (1991).
[7] N.V. Mokhov Crystal Collimation at SSC and Tevatron, CARE-HHH workshop CC-2005, CERN, 7-8 March 2005.
[8] R.P. Fliller III, The Crystal Collimation System of the Relativistic Heavy Ion Collider, PhD thesis (2004).
[9] K. Eggert, Can Micro Channeling Improve the TOTEM Experiment? , CARE-HHH workshop CC-2005, CERN, 7-8 March 2005.
[10] Y. Ivanov, Crystal Technology in St. Petersburg, CARE-HHH workshop CC-2005, CERN, 7-8 March 2005.
[11] V. Guidi, Crystal Technology in Ferrara, CARE-HHH workshop CC-2005, CERN, 7-8 March 2005.
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