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Mmon Cooling
RINGBERG
21-25 July, 2003
Studies at Columbia University/Nevis Labs & MPI
Raphael Galea, Allen Caldwell
• Why Muons, why Muon Collider?
• Muon Cooling, Ionization or Frictional
Cooling
• Frictional Cooling with protons
• Conclusions and future work
How to reduce beam
Emmittance by 106?
6D,N = 1.7 10-10 (m)3
Why a Muon Collider ?
No synchrotron radiation
problem (cf electron)
P a (E/m)4
Can build a high energy
circular accelerator.
P
R. Galea
P
Ringberg Workshop
21-25 July, 2003
Collide point
particles rather
than complex
objects
Dimensions of Some Colliders discussed
R. Galea
Ringberg Workshop
21-25 July, 2003
Why Cooling?
• All Colliding beams have some cooling to reduce the
large phase space of initial beam
STOCHASTIC
• sample particles
ELECTRON
• principle of
RADIATIVE
• lose the hot
passing cooling
system
• correct to mean
position
heat exchanger
particles
R. Galea
Ringberg Workshop
21-25 July, 2003
Physics at a Muon Collider
Muon Collider Complex:
• Proton Driver 2-16GeV; 1-4MW leading to
10 22 p /year
•  production target & Strong Field Capture
• COOLING resultant m beam
• m acceleration
•Storage & collisions
• Stopped m physics
From target, stored m
•  physics
mm= 40000 ee
• Higgs Factory
• Higher Energy Frontier
R. Galea
Ringberg Workshop
21-25 July, 2003
HIGH ENERGY MUON COLLIDER
PARAMETERS
Baseline parameters for high energy muon colli ders. From “Status of Muon Colli der
Research and Development and Future Plans,” Muon Colli der Collaboration, C. M.
Ankenbrandt et al., Phys. Rev. ST Accel. Beams 2, 081001 (1999).
COM energy (TeV)
p energy ( GeV)
p’s/bunch
Bunches/fill
Rep. rate (Hz)
p power (MW)
m/ bunch
m power (MW)
Wall power (MW)
Colli der circum. (m)
Ave bending field (T)
rms p/p (%)
6D  (m)3
rms n ( mm mrad)
* (cm)
z (cm)
r spot (mm)
 IP (mrad)
Tune shift
nturns (effective)
Luminosity (cm2 s1)
R. Galea
0.4
16
2.5  1013
4
15
4
2  1012
4
120
1000
4.7
0.14
1.7  1010
50
2.6
2.6
2.6
1.0
0.044
700
1033
Ringberg Workshop
21-25 July, 2003
3.0
16
2.5  1013
4
15
4
2  1012
28
204
6000
5.2
0.16
1.7  1010
50
0.3
0.3
3.2
1.1
0.044
785
7  1034
’s in red
m’s in green
Drift region for  decay  30 m
P beam (few MW)
Solenoidal Magnets: few T … 20 T
Target
Simplified emittance estimate:
At end of drift, rms x,y,z approx 0.05,0.05,10 m
Px,Py,Pz approx 50,50,100 MeV/c
Normalized 6D emittance is product divided by (mmc)3
drift6D,N 1.7 10-4 (m)3
Emittance needed for Muon Collider
collider6D,N  1.7 10-10(m)3
This reduction of 6 orders of magnitude must be done with reasonable efficiency !
R. Galea
Ringberg Workshop
21-25 July, 2003
Some Difficulties
• Muons decay, so are not readily available – need multi MW
source. Large starting cost.
• Muons decay, so time available for cooling, bunching,
acceleration is very limited. Need to develop new techniques,
technologies.
• Large experimental backgrounds from muon decays (for a
collider). Not the usual clean electron collider environment.
• High energy colliders with high muon flux will face critical
limitation from neutrino induced radiation.
R. Galea
Ringberg Workshop
21-25 July, 2003
Muon Cooling
Muon Cooling is the signature challenge of a Muon
Collider
Cooler beams would allow fewer muons for a given
luminosity, thereby
• Reducing the experimental background
• Reducing the radiation from muon decays
• Allowing for smaller apertures in machine elements, and so
driving the cost down
R. Galea
Ringberg Workshop
21-25 July, 2003
Cooling Ideas : Ionization Cooling
RF
m beam
B
Gas cell
Ionization cooling (Skrinsky, Neuffer,
Palmer, …): muons are maintained
at ca. 200 MeV. Transverse cooling
of order x20 seems feasible (see factory feasibility studies 1-2).
Longitudinal cooling not solved.
R. Galea
Ringberg Workshop
21-25 July, 2003
Longitudinal Cooling via Emittance
Exchange
Transform longitudinal phase space into
transverse (know how to cool transverse)
Wedge shaped absorber
Bent solenoid produces dispersion
R. Galea
Ringberg Workshop
21-25 July, 2003
‘Balbekov Ring’
There are
significant
developments in
achieving 6D phase
space via ionization
cooling (R. Palmer,
MUTAC03), but still
far from 106 cooling
factor.
R. Galea
Ringberg Workshop
21-25 July, 2003
Frictional
Cooling
Nuclear scattering, excitation,
charge exchange, ionization
• Bring muons to a kinetic
energy (T) where dE/dx
increases with T
• Constant E-field applied
to muons resulting in
equilibrium energy
• Big issue – how to
maintain efficiency
• First studied by Kottmann
et al., PSI
R. Galea
Ionization
stops, muon
too slow
1/2 from
ionization
Ringberg Workshop
21-25 July, 2003
Problems/comments:
•
•
large dE/dx @ low kinetic energy
 low average density (gas)
Apply E  B to get below the dE/dx
peak
F  q(E + v  B) 
•

•
•
dT
rˆ
dx
m+ has the problem of Muonium
formation
(Mm) dominates over e-stripping in
all gases except He
m has the problem of Atomic capture
 small below electron binding
energy, but not known
Slow muons don’t go far before
decaying
d = 10 cm sqrt(T) T in eV
so extract sideways (E  B )
R. Galea
Ringberg Workshop
21-25 July, 2003
Trajectories in detailed simulation
Transverse motion
Motion
controlled
by B field
Lorentz angle drift, with nuclear scattering
Fluctuations in energy
results in emittance
Final stages of muon trajectory in gas cell
R. Galea
Ringberg Workshop
21-25 July, 2003
Phase rotation sections
Results of
simulations to
this point
Cooling cells
 He gas is used for m+, H2 for m-.
There is a nearly uniform 5T Bz
field everywhere, and Ex =5 MeV/m
in gas cell region
 Electronic energy loss treated as
continuous, individual nuclear
scattering taken into account since
these yield large angles.
Full MARS target simulation,
optimized for low energy
muon yield: 2 GeV protons on
Cu with proton beam
transverse to solenoids
(capture low energy pion
cloud).
Not to scale !!
R. Galea
Ringberg Workshop
21-25 July, 2003
Yields & Emittance
Results as of NUFACT02
Look at muons coming out of 11m cooling cell region after
initial reacceleration.
Yield: approx 0.002 m per 2GeV proton after cooling cell.
Need to improve yield by factor 3 or more.
Emittance: rms
R. Galea
x = 0.015 m
y = 0.036 m
z = 30 m ( actually ct)
Px = 0.18 MeV
Py = 0.18 MeV
Pz = 4.0 MeV
6D,N = 5.7 10-11 (m)3
Ringberg Workshop
21-25 July, 2003
6D,N = 1.7 10-10 (m )3
RAdiological
Research
Accelerator
Facility
Perform TOF measurements with
protons
2 detectors START/STOP
Thin entrance/exit windows
for a gas cell
Some density of He gas
Electric field to establish
equilibrium energy
NO B field so low acceptance
R. Galea
Look for a bunching in time
Can we cool protons?
Ringberg Workshop
21-25 July, 2003
 4 MeV p
R. Galea
Ringberg Workshop
21-25 July, 2003
Accelerating grid
Contains O(10-100nm) window
Si detector
To MCP
R. Galea
Proton beam
Gas
Ringbergcell
Workshop
21-25 July, 2003
Vacuum chamber
Assumed initial conditions
•20nm C windows
•700KeV protons
•0.04atm He
TOF=T0-(Tsi-TMCP)
speed
Kinetic energy
Summary of Simulations
•Incorporate scattering cross sections into the cooling program
•Born Approx. for T>2KeV
•Classical Scattering T<2KeV
•Include m- capture cross section using calculations of Cohen (Phys. Rev. A. Vol 62
022512-1)
•Difference in m+ & m- energy loss rates at dE/dx peak
•Due to extra processes charge exchange
•Barkas Effect parameterized data from Agnello et. al. (Phys. Rev. Lett. 74 (1995) 371)
•Only used for the electronic part of dE/dx
•Energy loss in thin windows
•For RARAF setup proton transmitted energy spectrum is input from
SRIM, simulating protons through Si detector
(J.F. Ziegler http://www.srim.org)
R. Galea
Ringberg Workshop
21-25 July, 2003
Cool protons???
MC exp
Flat constant Background
750ns
# Events
N
i
 58  82(55)
i
 5  45(49)
i
 55  194(77)
i
 42  124(77)
i 300ns
400ns
# Events
N
i 300ns

750ns
# Events
N
i 200ns
400ns
# Events
N
i 200ns
Background exponential with m>0
R. Galea

Ringberg Workshop
21-25 July, 2003
Conclusions
No clear sign of cooling but this is expected from
lack of Magnetic field & geometric MCP
acceptance alone
The Monte Carlo simulation can provide a
consistent picture under various experimental
conditions
Can use the detailed simulations to evaluate
Muon Collider based on frictional cooling
performance with more confidence….still want to
demonstrate the cooling
Work at MPI on further cooling demonstration
experiment using an existing 5T Solenoid and
develop the m- capture measurement
A lot of interesting work and results to come.
R. Galea
Ringberg Workshop
21-25 July, 2003
Lab situated at MPI-WHI in
Munich
R. Galea
Ringberg Workshop
21-25 July, 2003
Problems/Things to investigate…
• Extraction of ms through window in gas cell
•Must be very thin to pass low energy ms
•Must be reasonably gas tight
• Can we apply high electric fields in gas cell without
breakdown (large number of free electrons, ions) ?
Plasma generation  screening of field.
• Reacceleration & bunch compression for injection into
storage ring
• The m- capture cross section depends very sensitively on
kinetic energy & falls off sharply for kinetic energies greater
than e- binding energy. NO DATA – simulations use
theoretical calculation
• +…
R. Galea
Ringberg Workshop
21-25 July, 2003
Future Plans
• Frictional cooling tests at MPI with 5T Solenoid, alpha source
• Study gas breakdown in high E,B fields
• R&D on thin windows
• Beam tests with muons to measure m capture cross section
m-+H  Hm+ e+’s
• muon initially captured in n=15 orbit, then cascades down to n=1.
Transition n=2n=1 releases 2.2 KeV x-ray.
Si drift detector
Developed by MPI
HLL
R. Galea
Ringberg Workshop
21-25 July, 2003
Summary of Frictional Cooling
•Works below
the Ionization
Peak
•Detailed
simulations
•Cooling factors
O(106) or more?
•Still
unanswered
questions being
worked on but
early results are
encouraging.
Kinetic Energy(KeV)
Proton cooling test
Time(ns)
X(cm)