The Particle Refrigerator

1. The Particle Refrigerator A promising approach to using frictional coolingfor reducing the emittance of muon beams. Tom Roberts Muons, Inc. Particle Refrigerator

2. 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. Particle Refrigerator

3. 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). FrictionalCooling IonizationCooling Particle Refrigerator

4. Simulation of a Thin Carbon Foil, Muons Variance is large Operating Point 2.4 kV/foil < 2.2 keVStopsin Foil Useful Range G4beamline / historoot Compared to antiprotons, the useful range is smaller, and theoperating point is closer to the upper edge of the useful range. Particle Refrigerator

5. 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) 20cm μ− Out(6 keV) -5.5 MV Gnd Resistor Divider HV Insulation First foil is at -2 MV, so outgoing μ− exit with 2 MeV kinetic energy. Solenoid maintains transverse focusing. 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. Particle Refrigerator

6. Refrigerator Output – KERight after first foil Particle Refrigerator

7. Refrigerator Output – tRight after first foil Particle Refrigerator

8. Refrigerator Tout vs KeinRight after first foil Output in the Frictional Channel “Lost” muonsat higher energy Particle Refrigerator

9. Background: Muon ColliderFernow-Neuffer Plot R.B.Palmer, 3/6/2008. Particle Refrigerator

10. Why a Muon Refrigeratoris so Interesting! Difference is just input beam emittance RefrigeratorTransmission=12% RefrigeratorTransmission=6% G4beamline simulations,ecalc9 emittances. (Same scale) Particle Refrigerator

11. Muon Losses 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? Particle Refrigerator

12. 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) Turn Around Lost In the FrictionalChannel (going left) Particle Refrigerator

13. Those “Lost” muons Have Also Been Cooled “Lost” muonsTransmission=65% This can surely be optimized to do better. (Same scale) Particle Refrigerator

14. 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 mm σy 25 mm σz 673 mm <pz> 1.1 MeV/c (β=0.01) • Clearly a careful computation including space charge is needed. Particle Refrigerator

15. An Inexpensive ExperimentUsing Alphas • 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. Vacuum Chamber 100 nm Carbon Foils Typical Alpha Track Detector Collimated Alpha Source (degrader?) Resistor Divider -50 kV Supply +50 kV Supply This is a simple, tabletop experiment that should fit within an SBIR budget. Particle Refrigerator

16. 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. Particle Refrigerator

17. 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. Particle Refrigerator