Target and Collection Optimization for Muon Colliders

To achieve adequate luminosity in a muon collider it is necessary to produce and collect large numbers of muons. The basic method used in this paper follows closely a proposed scheme which starts with a proton beam impinging on a thick target ( ,,~ one interaction length) followed by a long solenoid which collects muons resulting mainly from pion decay. Production and collection of pions and their decay muons must be optimized while keeping in mind limitations of target integrity and of the technology of magnets and cavities. Results of extensive simulations for 8 GeV protons on various targets and with various collection schemes are reported. Besides muon yields results include energy deposition in target and solenoid to address cooling requirements for these systems. Target composition, diameter, and length are varied in this study as well as the configuration and field strengths of the solenoid channnel. A curved solenoid field is introduced to separate positive and negative pions within a few meters of the target. This permits each to be placed in separate RF buckets for acceleration which effectively doubles the number of muons per bunch available for collisions and increases the luminosity fourfold. INTRODUCTION Interest in a muon collider for future high energy physics experiments has greatly increased recently [1]. Muons suffer far less synchrotron radiation than electrons providing hope that well-known circular machine technology can be extended to much higher energies than presently available---or even contemplated--for lepton colliders. The short muon lifetime and the difficulty and expense of producing large numbers of them makes a useful muon collider luminosity hard to achieve. Because of their short lifetime muons must be generated by a single proton pulse for each new acceleration cycle. Techniques for efficient production and collection of an adequate number of muons are thus needed to make a muon collider viable. Earlier estimates of muon yield, based on conventional lithium lens and quadrupole magnet collection methods, indicate that roughly 1000 protons are needed for *Work supported by the U. S. Department of Energy under contract No. DE-AC02-76CH03000. © 1996 American Institute of Physics 61 Downloaded 22 Aug 2009 to 128.112.85.160. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp every muon delivered to the collider rings [2]. This results from inherent limitations in the momentum acceptance of these systems (typically less than 4-5 percent) which causes most (potential) muons produced to be wasted. Motivated by neutrino beamline experience, a solenoid collection scheme for pions has been suggested [3]. Cursory simulations indicate significant improvement in muon yields for proton energies below 100 GeV while above this a collection system with two lithium lenses could surpass a solenoid. However the power required for a 15 to 30 Hz rapid-cycling proton synchrotron with 1014 protons per pulse becomes expensive above 30 GeV. Along with considerations on space charge limits and pion yields this suggests a kinetic energy of the proton driver between 3 and 30 GeV. Because of interest at Fermilab in upgrading its 15 Hz Booster to higher intensity (5 x 1013 protons per pulse) for the hadron program, a proton beam kinetic energy of 8 GeV is assumed in this study. This choice also might enable experimental verification of results presented here. Actual numbers reported here, such as yields and energy densities, may well depend considerably on incident energy. But intercomparisons and conclusions derived from them, such as in the optimization of target size or solenoid field with respect to muon yield, are expected to be much less sensitive to incident energy. 30 25 £ E -1 ~ 2o r r c--