Technical Challenges for the Muon Collider

Technical Challenges for the Muon Collider.

The MC is most easily thought of as 4 separate sections: 1. Muon Source consisting of a proton driver delivering 4 MW at roughly 8 GeV (optimal) or higher and target; 2. Muon capture and cooling of ~250 MeV/c muons; 3. Muon acceleration to TeV-scale energies; 4. Collider ring. Each of these stages is a major accelerator system and has significant challenges that are large extrapolations from the present technical state-of-the-art. A comparison of the challenges of the Muon Collider accelerator systems and those required for CLIC or an X-band linear collider are listed in Table 1 and more detail is provide below.

Each of these accelerator systems will likely require a large-scale systems demonstration before the construction of the multi-billion $ Muon Collider could be started. One possible scenario is to demonstrate the Muon Source using an enhanced version of Project-X – see Fig 1 (Palmer, 2008). The Muon Capture and Cooling and the Muon Acceleration could be demonstrated as part of a Neutrino Factory before the final Muon Collider would be constructed.

1. The target source must deliver ~1014 /s (5 orders-of-magnitude higher than the best previously achieved at PSI of 109 /s). o The proton driver must deliver four times the proton beam power of the SNS linac however the real challenge is in the time structure. To generate the appropriate muon bunches, the protons must be compressed into very intense bunches with roughly 300x1012 protons in 1~2 ns. This compares with macro pulses in the SNS of roughly 150x1012 in 700 ns. o The target will have to operate with 4 MW of incident power. The MERIT experiment at CERN demonstrated that a liquid mercury target in 15 T field could be viable with 30x1012 protons (10x lower than the 8 GeV charge per pulse) however there may be issues related to the intense beam power, damage due to cavitation in the mercury jet, radiation activation, and survivability of accelerator components in the environment. Single pulse energy density into mercury is 20~40 times higher than SNS. o Major development effort to develop and test target systems requiring intense proton beam. Might be based on an enhanced version of Project- X. 2. Muon capture and cooling must capture muons at ~100 MeV/c, phase rotate the longitudinal phase space and then cool 6-D phase space by factor of 105 while dealing with significant muon losses of many 10’s % per stage. This system has the largest technical challenges. o Cooling will be done with LiH or LH2 – little concern about the ionization cooling mechanism but cooling requires ~10 stages with 100’s of absorbers in complicated geometries, different magnetic fields and different rf frequencies – see Fig. 2 (Palmer, 2009).

1 o MICE experiment at RAL will demonstrate a single cooling stage with ~10% cooling however small alignment, field or rf errors between stages will rapidly dilute effect of each stage. Will need multi-stage demonstration requiring intense muon source. o Cooling must be done in as short distance as possible because muons decay. High gradient acceleration is difficult in magnetic fields – see Fig. 3 (Palmer, 2009). o Complicated magnetic lattices may be error prone and will be engineering challenge due to large forces in complicated geometry – see Fig. 4 (Palmer, 2009). o Later cooling stages require 50~60 T solenoid fields to effectively cool muons. Challenging magnets and matching between stages. NHFML demonstrated 45 T Bitter dc magnet. Requires 5 or 6 stages. See Fig. 5 (Palmer, 2009 and NHFML web site). 3. Muon Acceleration must accelerate the muons from the ~200 MeV/c level to TeV-scale. Likely based on high gradient SC recirculating linacs (RLAs). o Energy gain per RLA limited to 4~10 requiring multiple linac systems o Long bunches and intense charge leads to more severe collective effects than in ILC likely requiring significant cavity and coupler modifications compared to ILC technology. o Arcs require high field magnets to limit path length and muon decays during acceleration. Both arcs and linacs must deal with intense radiation from muon decays. 4. Collider Ring must collider muon bunches. Require high field SC magnets to limit ring size and maximize luminosity before muons decay. Detector must be protected from decay background sources. o Muons continually decay into neutrino and electron or positron which is deflected by high magnetic fields generating intense synchrotron radiation and showering in vacuum chambers. Need to protect superconducting magnets from beam heating and radiation damage and must design beamline components to limit activation. o Decay products must be prevented from generating large backgrounds in IP. Studies from 1996 suggest background levels similar to LHC. Will require complicated masking around IP and collimation around the ring. May limit detector solid angle – see Fig. 6 (Palmer, 2008). o Final focus system and near-isochronous ring optics (need to preserve bunch length) require complicated control of nonlinearies to prevent beam loss. Satisfactory optics designs are still to be developed. o Ring has no damping and trajectory is not necessarily periodic. Feedback to maintain trajectory collisions during 1000 turn will be needed.

2 Table 1. Comparing Muon Collider and CLIC/X-band Linear Collider Accelerator Muon Collider CLIC or X-band LC System Particle source 1014 /s generated with ~4 1014 e+/s and polarized e-/s; MW proton beam on target; polarized e- demonstrated at Previously, 109 /s generated SLAC E-158; e+ requires 200 at PSI. kW e- or photon beam on target; few 1012 e+/s generated at SLAC SLC. Cooling 6-D phase space cooled by 6-D phase space cooled by 1012 105 using ionization cooling. using synchrotron radiation in MICE experiment will storage ring. Well demonstrated demonstrate 10% cooling in technology. Extracted 6-D ~2012. Requires technical phase space within 10 of KEK development of 50T ATF and PSI SLS. solenoids, high gradient rf in solenoidal fields, cooling channels with complex magnetic fields. Acceleration SCRF RLA’s with ~25 Klystron or Two Beam MV/m gradient and using Acceleration (TBA) X-band ~4x circulation. Beam linac with ~100 MV/m gradient. dynamics significantly more Klystron-based rf unit challenging than ILC demonstrated at NLCTA. TBA requiring hardware initial demonstration at CTF3 modifications. Isochronous but may need 2nd generation arcs that can survive muon demonstration. Tight tolerances decay radiation. for acceleration systems. Collider ring / BDS Near-isochronous collider Beam Delivery System ring with high field SC demonstrated at SLAC FFTB magnets. BDS based on LC- and being demonstrated at KEK style FFS but more ATF2. Complicated collimation complicated masking, and accelerator protection due collimation, and accelerator to intense beams. Challenging protector due to muon decay component jitter and beam radiation and intense beams. feedback tolerances to maintain Challenge to maintain collisions. trajectory and collisions over 1000 turns. Unclear physics reach due to backgrounds.

3 Fig. 1. Possible concept for development towards a Muon Collider. Multiple large scale demonstrations will be needed along development path (Palmer, 2008).

Fig. 2. Required cooling for the Muon Collider. There would be ~10 different cooling stages with different geometry, magnetic fields, rf frequencies and absorber materials (Palmer, 2009).

4 Fig. 3. Gradient vs magnetic field for different rf frequencies; desired operating points indicated with circles (Palmer, 2009).

Fig. 4. 5-turn Guggenheim 6-D cooling channel using LH2 absorbers and 201 MHz rf. One stage of roughly 10 cooling channels. Roughly 150 m in length with ~3 T solenoidal field. Other ideas similar in concept but trying to develop more efficient systems (Palmer, 2009).

5 Fig. 5. SC + NC 45 T magnet demonstrated at NHMFL uses 33 MW power and would have an unknown lifetime in a 24/7 accelerator environment due to stress and radiation damage (Palmer, 2009 and NHFML web site).

6 Fig. 6. Masking in detector at ~300 mrad to shield against muon decay products (Palmer, 2008).