Download Particle Refrigerator

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
1
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
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).
December 10, 2008 TJR
Particle Refrigerator
3
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.
December 10, 2008 TJR
Particle Refrigerator
4
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.
December 10, 2008 TJR
Particle Refrigerator
5
Refrigerator Output – KE
Right after first foil
December 10, 2008 TJR
Particle Refrigerator
6
Refrigerator Output – t
Right after first foil
December 10, 2008 TJR
Particle Refrigerator
7
Refrigerator Tout vs Kein
Right after first foil
Output in the
Frictional
Channel
“Lost” muons
at higher energy
December 10, 2008 TJR
Particle Refrigerator
8
Background: Muon Collider
Fernow-Neuffer Plot
R.B.Palmer, 3/6/2008.
December 10, 2008 TJR
Particle Refrigerator
9
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)
December 10, 2008 TJR
Particle Refrigerator
10
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?
December 10, 2008 TJR
Particle Refrigerator
11
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)
December 10, 2008 TJR
Particle Refrigerator
12
Those “Lost” muons Have
Also Been Cooled
“Lost” muons
Transmission=65%
This can surely
be optimized to
do better.
(Same scale)
December 10, 2008 TJR
Particle Refrigerator
13
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.
December 10, 2008 TJR
Particle Refrigerator
14
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.
December 10, 2008 TJR
Particle Refrigerator
15
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.
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