december 10, 2008 tjrparticle refrigerator1 the particle refrigerator tom roberts muons, inc. a...
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December 10, 2008 TJR Particle Refrigerator 1
The Particle Refrigerator
Tom Roberts
Muons, Inc.
A promising approach to using frictional coolingfor reducing the emittance of muon beams.
Introduction
• Frictional cooling has long been known to be capable of producing very low emittance beams
• The problem is that frictional cooling only works for very low energy particles, and its input acceptance is quite small in energy:– Antiprotons: KE < 50 keV– Muons: KE < 10 keV
Key Idea: Make the particles climb a few Mega-Volt potential, stop,
and turn around into the frictional cooling channel. This increases the acceptance from a few keV to a few MeV.
• So the particles enter the device backwards; they come back out with the equilibrium kinetic energy of the frictional cooling channel regardless of their initial energy.
• Particles with different initial energies turn around at different places.• The total potential determines the momentum (energy) acceptance.December 10, 2008 TJR Particle Refrigerator 2
Frictional Cooling
• Operates at β ~ 0.01 in a region where the energy loss increases with β, so the channel has an equilibrium β.
• In this regime, gas will break down – use many very thin carbon foils.• Hopefully the solid foils will trap enough of the ionization electrons in
the material to prevent a shower and subsequent breakdown.
Experiments on frictional cooling of muons have beenperformed with 10 foils (25 nm each).
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FrictionalCooling
IonizationCooling
Simulation of a Thin Carbon Foil, Muons
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Useful Range
< 2.2 keVStopsin Foil
OperatingPoint
2.4 kV/foil
G4beamline / historoot
Compared to antiprotons, the useful range is smaller, and theoperating point is closer to the upper edge of the useful range.
Variance is large
Muon Refrigerator – Diagram
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Solenoid
μ− In(3-7 MeV)
μ− Out(6 keV)
…Resistor DividerGnd
HV Insulation First foil is at -2 MV, so outgoing μ− exit with 2 MeV kinetic energy.
Solenoid maintains transverse focusing.
μ− climb the potential, turn around, and come back out via the frictional channel.
10 m
20cm
1,400 thin carbon foils (25 nm), separated by 0.5 cm and 2.4 kV.
-5.5 MV
Device is cylindrically symmetric (except divider); diagram is not to scale.
Remember that 1/e transverse cooling occurs by losing andre-gaining the particle energy. That occurs every 2 or 3 foilsin the frictional channel.
Refrigerator Output – KERight after first foil
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Refrigerator Output – tRight after first foil
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Refrigerator Tout vs Kein
Right after first foil
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Output in the Frictional Channel
“Lost” muonsat higher energy
Background: Muon ColliderFernow-Neuffer Plot
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R.B.Palmer, 3/6/2008.
Why a Muon Refrigeratoris so Interesting!
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RefrigeratorTransmission=12%
RefrigeratorTransmission=6%
G4beamline simulations,ecalc9 emittances.(Same scale)
Difference is just input beam emittance
Muon Losses
Input Transverse Emittance
Loss Mechanism 0.75 π mm-rad 1.6 π mm-rad
Decay while moving 23% 20%
Escape out the end 0% 0%
Scraping (radial) 0% 0%
Stop in a foil 23% 9%
Lose too little energy 42% 65%
Survive in frictional channel 12% 6%
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Higher transverse emittance input beam was due to larger σx’, σy’. Larger-angle particles have larger β at turn-around, and can already be out of the frictional regime at the first foil.
Challenge: can we use all those higher-energy muons?
Dominant Loss Mechanism
• The dominant loss mechanism is particles losing too little energy in a foil and leaving the frictional-cooling channel.
• This happens much more frequently for muons than for antiprotons.• Many are lost right at turn-around.
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Incoming(going right)
Turn Around
In the FrictionalChannel
(going left)
Lost
Outgoing(going left)
One μ+
Track
Those “Lost” muons Have Also Been Cooled
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“Lost” muonsTransmission=65%
This can surely be optimized to
do better.(Same scale)
Comments onSpace charge
• Be wary in applying the usual rules of thumb
• Low normalized emittance is achieved by low momentum, not small bunch size:
σx 25 mmσy 25 mmσz 673 mm<pz> 1.1 MeV/c (β=0.01)
• Clearly a careful computation including space charge is needed.
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An Inexpensive ExperimentUsing Alphas
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Resistor Divider
-50 kVSupply
+50 kVSupply
• Shows feasibility andmeasures transmission,not emittance or cooling
• Uses two 50 kV suppliesto keep costs down.
• The source must bedegraded to ~100 keV.
• Hopefully the sourcecollimation will avoid theneed for a solenoid (asshown).
This is just a concept −lots of details need tobe worked out.
This is a simple, tabletop experiment that should fit within an SBIR budget.
100 nm Carbon
Foils
Collimated Alpha
Source(degrader?)
Detector
Vacuum Chamber
Typical Alpha Track
LOTS more work to do!
• Investigate space charge effects• Investigate electron cloud effects
– Will electrons multiply in the foils and spark?
• Investigate foil properties, handling, etc.• Engineer the high voltage• Will foils degrade or be destroyed over time?• Design the input/output of the refrigerator (kicker, bend?)• Design the following acceleration stages
There are many unanswered questions, but the sameis true of most current cooling-channel designs.
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Conclusions
• This is an interesting device that holds promise to significantly improve the design of a muon collider.
• Much work still needs to be done to validate that.
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