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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. 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