MICE Tapered B1 Study

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Transcript MICE Tapered B1 Study

The Particle Refrigerator
A promising approach to using frictional cooling
for reducing the emittance of muon beams.
Tom Roberts
Muons, Inc.
December 10, 2008 TJR
Particle Refrigerator
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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.
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Frictional Cooling
Frictional
Cooling
Ionization
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 been
performed with 10 foils (25 nm each).
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Simulation of a Thin
Carbon Foil, Muons
Variance is
large
< 2.2 keV
Stops
in Foil
Useful Range
Operating
Point
2.4 kV/foil
G4beamline / historoot
Compared to antiprotons, the useful range is smaller, and the
operating point is closer to the upper edge of the useful range.
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Muon Refrigerator – Diagram
10 m
Solenoid
1,400 thin carbon foils
(25 nm), separated by
0.5 cm and 2.4 kV.
μ− climb the potential, turn
around, and come back out via
the frictional channel.
…
μ− In
(3-7 MeV)
μ− Out
(6 keV)
Gnd
First foil is at -2 MV, so
outgoing μ− exit with
2 MeV kinetic energy.
Resistor Divider
20
cm
-5.5 MV
HV Insulation
Solenoid maintains
transverse focusing.
Device is cylindrically symmetric (except divider); diagram is not to scale.
Remember that 1/e transverse cooling occurs by losing and
re-gaining the particle energy. That occurs every 2 or 3 foils
in the frictional channel.
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Refrigerator Output – KE
Right after first foil
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Refrigerator Output – t
Right after first foil
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Refrigerator Tout vs Kein
Right after first foil
Output in the
Frictional
Channel
“Lost” muons
at higher energy
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Background: Muon Collider
Fernow-Neuffer Plot
R.B.Palmer, 3/6/2008.
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Why a Muon Refrigerator
is so Interesting!
Difference is just
input beam
emittance
Refrigerator
Transmission=12%
Refrigerator
Transmission=6%
G4beamline simulations,
ecalc9 emittances.
(Same scale)
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Muon Losses
Input Transverse Emittance
Loss Mechanism
Decay while moving
Escape out the end
Scraping (radial)
Stop in a foil
Lose too little energy
Survive in frictional channel
0.75 π mm-rad
1.6 π mm-rad
23%
0%
0%
23%
42%
12%
20%
0%
0%
9%
65%
6%
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?
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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.
Incoming
(going right)
One μ+
Track
Outgoing
(going left)
Lost
Turn Around
In the Frictional
Channel
(going left)
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Those “Lost” muons Have
Also Been Cooled
“Lost” muons
Transmission=65%
This can surely
be optimized to
do better.
(Same scale)
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Comments on
Space charge
• Be wary in applying the usual rules of thumb
• Low normalized emittance is achieved by low
momentum, not small bunch size:
σx
25 mm
σy
25 mm
σz
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 Experiment
Using Alphas
Vacuum
Chamber
100 nm
Carbon
Foils
Typical
Alpha
Track
Detector
Collimated
Alpha
Source
(degrader?)
Resistor Divider
-50 kV
Supply
+50 kV
Supply
• Shows feasibility and
measures transmission,
not emittance or cooling
• Uses two 50 kV supplies
to keep costs down.
• The source must be
degraded to ~100 keV.
• Hopefully the source
collimation will avoid the
need for a solenoid (as
shown).
This is just a concept −
lots of details need to
be worked out.
This is a simple, tabletop experiment that should fit within an SBIR budget.
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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 same
is 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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