Prospects for an Energy-Frontier Muon Collider

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Prospects for an Energy-Frontier Muon Collider

• Tom Roberts
• Muons, Inc.
• Illinois Institute of Technology

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Outline

• Background
• Why muons?
• The major challenges
• Surmounting the challenges
• Recent innovations that have improved the
  prospects for success
• Viewgraph-level design of a Muon Collider
• Current RD Efforts
• Summary

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Background Reminders

• Historically, every significant increase in
  energyhas taught us something completely new.
• Every new type of particle beam has alsotaught
  us something completely new.
• The LHC is turning on later this year, so the
  energy frontier is above 14 TeV for protons,or
  above 1.5 TeV for leptons.

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The Livingston Plot
X
5 TeV MC
Constituent Center-of-Mass Energy
X
ILC
2025
Panofsky and Breidenbach, Rev. Mod. Phys. 71,
s121-s132 (1999)
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Why Muons?

• Electrons have problems at the energy frontier
• At the TeV scale, radiative processes limit both
  energy and luminosity for electrons
• Synchrotron radiation losses ? linear, large, and
  very expensive
• Beamstrahlung E2, approaches the beam energy in
  one crossing? low luminosity at peak energy,
  huge beam energy spread
• Remember those beautiful, narrow peaks for the
  J/?? They wont happen again because
• The beam energy spread is very large
• Resonances above 2MW will have large weak-decay
  widths
• Protons have problems at the energy frontier
• Without some tremendous breakthrough in
  high-field magnets, the machine must be truly
  enormous (expensive)
• As composite particles, beam energy must be
  considerably higher than for leptons

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Muons

• Clearly a whole new window into electroweak
  processes
• A path to the energy frontier
• Radiative processes are far from limiting (as for
  electrons)
• Circular machine is possible, as are
  recirculating linacs
• Lepton, so beam energy and machine size are
  significantly lower than for protons
• For S-channel Higgs production, cross-section
  m2 40,000 times larger than for ee-.

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Muons
A 5 TeV muon collider could fit on the
existingFermilab site.
Ankenbrandt et al., PRST-AB 2, 081001 (1999)
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The Major Challenges

• Muons decay in 2.2 microseconds
• Muons are created with a very large emittance,
  too large for conventional accelerators, too
  large to give reasonable luminosity
• Muon production from 8-40 GeV protons scales
  roughly as proton beam power, independent of
  energy
• A 1 to 4 Megawatt proton beam is required
• The production target is also a challenge
• Muons decay into an electron plus neutrinos
• Electron backgrounds in detector
• Neutrino radiation problem (!)

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Reducing the Phase Space Cooling

• Loosely the muons produced occupy the size of a
  beach ball (60 cm), the ILC accelerating cavities
  can accept a BB (4 mm)
• take advantage of ILC RD and optimization.
• overall reduction in phase space 106.
• Luminosity N2e--2 so lower transverse
  emittance permits a reduction in N (which reduces
  other problems).
• Must select a process that avoids Liouvilles
  theorem.
• Must select a method consistent with the muon
  lifetime (2.2 µsec).
• Desirable to select a method consistent with the
  peak momentum of the produced muons (300 MeV/c).

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Muon Ionization Cooling
p- reduced, p unchanged
Absorber dp/dz -p
RF Cavity dp/dz z

• Alternate absorbers and RF cavities
• RF cavities restore the energy lost in the
  absorbers
• A factor of 1/e reduction in transverse phase
  space occurs when the total energy lost in
  absorbers equals the beam energy (both planes)
• Optimal energy corresponds to a momentum of
  100-250 MeV/c
• Works only for muons (electrons shower, hadrons
  interact)
• Transverse cooling only (small longitudinal
  heating due to straggling)

(Skrinsky Parkhomchuk, 1981)
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Muon Ionization Cooling
Transverse Emittance change per unit length in
the absorber
Heating term (multiple scattering)
Cooling term (energy loss)

• Want
• Lower ß- (stronger focusing at the absorber)
• Minimize multiple scattering
• Maximize energy loss

Lattice design
Absorber Material
Here ? is the normalized emittance, Eµ is the
muon energy, dEµ/ds and X0 are the energy loss
and radiation length of the absorber material, ??
is the transverse beta-function of the magnetic
channel, and ? is the particle velocity.
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Absorber Materials
Fcool (Energy Loss) / (Multiple Scattering)
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Emittance Exchange
Ionization cooling is only transverse. To get
longitudinal cooling,use emittance exchange.
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Innovation Helical Cooling Channel
These coils just surround the beam region. All
coils are normal to the Z axis their centers are
offset in X and Y to form the helix. The helical
solenoid is filled with a continuous absorber,
and perhaps with RF cavities.
Beam Follows Helix

• Cools in all 6 dimensions higher-energy
  particles have longer path length in the absorber
• A remarkable thing occurs for specific values of
  the geometry, the solenoid, helical dipole, and
  helical quadrupole fields are all correct.
• With absorber and RF, parameters remain constant
  with absorber only, parameters decrease with
  momentum.
• Acceptance is quite large compared to most
  accelerator structures.

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HCC Simulation

• Four sequential HCCs with decreasing diameter and
  period, increasing field (8 T max)
• Emittance reduction is 50,000 over 160 m(15
  decay)
• In the analogy of starting with a beach ball and
  needing a BB, this is a small marble (1 cm dia.)

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Related Innovation Guggenheim Cooling Channel

• Helix with radius gtgt period
• Also capable of emittance exchange
• More like a ring cooler that has been stretched
  vertically

Figure is mine concept is Palmer et al, BNL
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Innovation High Pressure Gas RF Cavities

• High-pressure hydrogen reduces breakdown via the
  Paschen effect
• No decrease in maximum gradient with magnetic
  field
• Need beam tests to show HPRF actually works for
  this application.

805MHz
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Innovation High Pressure Gas RF Cavities

• Copper plated, stainless-steel, 805 MHz test cell
• H2 gas to 1600 psi and 77 K
• Paschen curve verified (at Fermilabs Lab G and
  MuCool Test Area)
• Maximum gradient limited by breakdown of metal
• Fast conditioning seen
• Unlike vacuum cavities, theres no measurable
  limitation for magnetic field!

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Understanding RF Breakdown
Scanning electron microscope images Be (top) and
Mo (bottom).
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Innovation Parametric Resonance Ionization
Cooling

• Clever method to greatly reduce ?? without
  increased magnetic fields.
• Excite ½ integer parametric resonance (in Linac
  or ring)
• Like vertical rigid pendulum or ½-integer
  extraction
• Elliptical phase space motion becomes hyperbolic
• Use xxconst to reduce x, increase x
• Use IC to reduce x
• Detuning issues are being addressed (chromatic
  and spherical aberrations, space-charge tune
  spread). Simulations are underway.
• Smaller beams from 6D HCC cooling are essential
  for this to work!

X
X
X
X
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Innovation Reverse Emittance Exchange

• p(cooling)200MeV/c, p(colliding)2.5 TeV/c ?
  room in ?p/p space
• After cooling and acceleration, the beam has much
  smaller longitudinal emittance than necessary.
• Reduce transverse emittance to increase
  luminosity, trading it for increased longitudinal
  emittance (limited by accelerator acceptance and
  interaction point ?).

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Innovation Bunch Coalescing

• Start with 100 MeV/c cooled bunch train.
• Accelerate to 20 GeV/c with high-frequency RF.
• Apply low-frequency RF to rotate the bunches
  longitudinally.
• Permit them to drift together in time.
• Avoids space charge problems at low energy.

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Innovation Dual-Use Linac

• Fermilab is considering Project X, a
  high-intensity 8 GeV superconducting linac
• Use it also to accelerate muons (after cooling)

NeutrinoFactoryaimed atSoudan, MN
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Innovation Pulsed Recirculating Linac

• Accelerating from 20 GeV to 2,500 GeV requires a
  lot of RF!
• Muon decay dictates high ratio of RF/length.
• A dogbone recirculating linac is a reasonable
  trade-off between cost, size, and muon decay.
• By pulsing the quadrupoles of the linac, more
  passes can be made without losing transverse
  focusing.
• This linac is several km long, so pulsing is
  feasible.
• With careful design this can handle both µ and
  µ (time offset in RF cavities, FODO vs DOFO
  lattice, travel opposite directions in arcs).

Injection
Extraction
Linac
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Innovation High-Field HTS Superconducting Magnets

• The high-temperature superconductors have a
  remarkable property at low temperature (2-4 K)
  they sustain a high current density at large
  magnetic fields.
• Measured up to 40 T, expected to hold to even
  higher fields.
• It is likely that solenoids in the range of 30 T
  to 50 T can be constructed.
• Higher field ? lower ??, so lower emittance can
  be achieved via ionization cooling.
• These materials are a challenge to work with

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Many New Arrows in the Quiver

• New Ionization Cooling Techniques
• Helical Cooling Channel
• Momentum-dependent Helical Cooling Channel
• Guggenheim cooling channel
• Ionization cooling using a parametric resonance
• Methods to manipulate phase space partitions
• Reverse emittance exchange using absorbers
• Bunch coalescing (neutrino factory and muon
  collider share injector)
• Technology for better cooling
• Pressurized RF cavities
• High Temperature Superconductor for up to 50 T
  magnets
• Acceleration Techniques
• Dual-use Linac
• Pulsed Recirculating Linac

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Conceptual Block Diagram of a Muon Collider
Proton Driver(8-40 GeV)
ProductionTarget
Pion Capture, Decay Channel,Phase Rotation, and
Pre-Cooling
Muon Ionization Cooling
Acceleration(0.2 to 20 GeV)
Reverse EmittanceExchange
Bunch Coalescing
Acceleration (20 to 2,500 GeV)
Storage Ring andInteraction Regions
Experiments
Must of course deal with both µ and µ-.
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Fernow-Neuffer Plot
Start Cooling After Capture, Decay, Phase
Rotation, Pre-Cooling
End CoolingStart Acceleration to2.5 TeV
HCC 400 MHz
REMEX Coalescing
HCC 800 MHz
PIC
HCC 1600 MHz
AccelerationTo 20 GeV
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Viewgraph-level Design
2.5 2.5 TeV muon storage ring with two IRs 1 km
radius ( Fermilab Main Ring,but its not deep
enough)
L 1035 cm-2 s-1
µ
2.5 km ILC-like linacs
10 recirculating arcs In one tunnel
µ
Final cooling, preacceleration
Helical cooling channel
Target, pion capture, Phase rotation
Proton driver
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Related Facility Neutrino Factory

• Muons in a storage ring with a long straight
  section aimed at the far neutrino detector
• Concept is more fleshed out that a muon collider
• Cheaper, of striking current interest, perhaps
  more feasible
• Thousands of times more neutrino intensity than
  alternatives
• Higher energy neutrinos, with narrower energy
  spectrum
• Essentially perfect purity (no p decays) great
  for wrong-sign appearance measurements of
  oscillation
• Near detector looks a lot like old fixed-target
  hadron experiments
• 30 cm liquid hydrogen target
• Event rate 1-100 Hz
• Must be careful about material (spontaneous
  muons!)

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Neutrino Factory
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Current RD Efforts

• Six different (but greatly overlapping)
  collaborations, more than 200 physicists
• Neutrino Factory and Muon Collider Collab.
• Umbrella U.S. collaboration
• MERIT Collab.
• Mercury jet target in 15 Tesla solenoid
• 24 GeV protons at CERN
• Analyzing data
• MuCool Collab.
• Engineering studies for individual components
• 4 years of studies so far, at Fermilab
• Test beam (400 MeV H-) SUMMER
• MICE Collab.
• Single-particle demonstration of emittance
  reduction
• First muon Beam (140-300 MeV/c µ) Real Soon
  Now
• MANX Collab.
• Just forming
• Fermilabs Muon Collider task Force
• Plus other Neutrino Factory organizations

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Merit Target Test

• High-power target test using a mercury jet in a
  15 T solenoid, at CERN
• Data taking completed last fall, data analysis in
  progress
• Preliminary conclusion concept validated up to 4
  MW at 50 Hz

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MuCool
Tests in progress at Fermilab MuCool Test Area
(MTA) near Linac, with full-scale (201 MHz) and
1/4-scale (805 MHz) closed-cell (pillbox)
cavities with novel Be windows for higher
on-axis field
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MICE(10 4d Cooling in 5.5 m)

• Installation in ISIS R5.2 is progressing
• Beamline commissioning Real soon now (2-3
  weeks)
• A month or two until beamline is complete
• Summer or fall until trackers are complete

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The MANX Experiment(500 6d Cooling in 4 m)

• Purpose is to demonstrate the Helical Cooling
  Channel.
• Could well become a Phase III of MICE
  (total is 2.5 m longer than MICE Stage VI
  fits in hall).

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Summary

• A number of clever innovations have made a Muon
  Collider much more feasible than previously
  thought.
• To make it possible to actually construct such a
  new facility, an ongoing program of research and
  development is essential.
• We are hosting a Low Emittance Muon Collider
  Workshop, at Fermilab in April.
• There is lots to do come join us!
• http//www.muonsinc.com