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The Particle Refrigerator. A promising approach to using frictional cooling for reducing the emittance of muon beams. Tom Roberts Muons, Inc. Introduction. Frictional cooling has long been known to be capable of producing very low emittance beams - PowerPoint PPT Presentation
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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).
December 10, 2008 TJR Particle Refrigerator 3
FrictionalCooling
IonizationCooling
Simulation of a Thin Carbon Foil, Muons
December 10, 2008 TJR Particle Refrigerator 4
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
December 10, 2008 TJR Particle Refrigerator 5
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
December 10, 2008 TJR Particle Refrigerator 6
Refrigerator Output – tRight after first foil
December 10, 2008 TJR Particle Refrigerator 7
Refrigerator Tout vs Kein
Right after first foil
December 10, 2008 TJR Particle Refrigerator 8
Output in the Frictional Channel
“Lost” muonsat higher energy
Background: Muon ColliderFernow-Neuffer Plot
December 10, 2008 TJR Particle Refrigerator 9
R.B.Palmer, 3/6/2008.
Why a Muon Refrigeratoris so Interesting!
December 10, 2008 TJR Particle Refrigerator 10
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%
December 10, 2008 TJR Particle Refrigerator 11
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.
December 10, 2008 TJR Particle Refrigerator 12
Incoming(going right)
Turn Around
In the FrictionalChannel
(going left)
Lost
Outgoing(going left)
One μ+
Track
Those “Lost” muons Have Also Been Cooled
December 10, 2008 TJR Particle Refrigerator 13
“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.
December 10, 2008 TJR Particle Refrigerator 14
An Inexpensive ExperimentUsing Alphas
December 10, 2008 TJR Particle Refrigerator 15
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.
December 10, 2008 TJR Particle Refrigerator 16
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.
December 10, 2008 TJR Particle Refrigerator 17