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).
o MICE experiment at RAL will demonstrate a single cooling stage with
~10% cooling however small alignment, field or rf errors between stages
1
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
System
Muon Collider
CLIC or X-band LC
Particle source
1014 /s generated with ~4
MW proton beam on target;
Previously, 109 /s generated
at PSI.
Cooling
6-D phase space cooled by
105 using ionization cooling.
MICE experiment will
demonstrate 10% cooling in
~2012. Requires technical
development of 50T
solenoids, high gradient rf in
solenoidal fields, cooling
channels with complex
magnetic fields.
SCRF RLA’s with ~25
MV/m gradient and using
~4x circulation. Beam
dynamics significantly more
challenging than ILC
requiring hardware
modifications. Isochronous
arcs that can survive muon
decay radiation.
Near-isochronous collider
ring with high field SC
magnets. BDS based on LCstyle FFS but more
complicated masking,
collimation, and accelerator
protector due to muon decay
radiation and intense beams.
Challenge to maintain
trajectory and collisions over
1000 turns. Unclear physics
reach due to backgrounds.
1014 e+/s and polarized e-/s;
polarized e- demonstrated at
SLAC E-158; e+ requires 200
kW e- or photon beam on
target; few 1012 e+/s generated
at SLAC SLC.
6-D phase space cooled by 1012
using synchrotron radiation in
storage ring. Well
demonstrated technology.
Extracted 6-D phase space
within 10 of KEK ATF and PSI
SLS.
Acceleration
Collider ring / BDS
3
Klystron or Two Beam
Acceleration (TBA) X-band
linac with ~100 MV/m gradient.
Klystron-based rf unit
demonstrated at NLCTA. TBA
initial demonstration at CTF3
but may need 2nd generation
demonstration. Tight tolerances
for acceleration systems.
Beam Delivery System
demonstrated at SLAC FFTB
and being demonstrated at KEK
ATF2. Complicated
collimation and accelerator
protection due to intense beams.
Challenging component jitter
and beam feedback tolerances to
maintain collisions.
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).
Fig. 6. Masking in detector at ~300 mrad to shield against muon decay products (Palmer,
2008).
6