MANX: A 6D Ionization-Cooling Experiment

MANX: A 6D Ionization-Cooling Experiment

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Daniel M. Kaplan (for the MANX Collaboration)

arXiv:0711.1523v1 [physics.acc-ph] 9 Nov 2007

Illinois Institute of Technology, Chicago, IL 60616, USA Abstract. Six-dimensional ionization cooling of muons is essential for muon colliders and possibly beneficial for neutrino factories. An experiment to demonstrate six-dimensional ionization cooling using practical apparatus is presented. It exploits recent innovative ideas that may lead to six-dimensional muon-cooling channels with emittance reduction approaching that needed for high-luminosity muon colliders. Keywords: Muon cooling, muon collider, neutrino factory, helical cooling channel. PACS: 29.27.-a, 29.20.-c, 14.60.Ef, 41.85.Lc

INTRODUCTION Ionization cooling [1], in which a beam is cooled by energy loss in an absorber medium, is a key technique for future muon accelerator facilities, e.g., a neutrino factory [2] or muon collider [3]. It is unique in its ability to cool an intense beam of muons before a substantial fraction of them have decayed. Ionization cooling is essentially a transverse effect but can be made to cool the longitudinal degrees of freedom as well via emittance exchange [4]. An experiment to demonstrate transverse ionization cooling (the Muon Ionization Cooling Experiment, MICE) [5] is in progress. We describe a possible six-dimensional (6D) cooling experiment: the Muoncollider And Neutrino-factory eXperiment, MANX [6].

SIX-DIMENSIONAL MUON COOLING Several approaches to six-dimensional muon cooling have been devised. The first design shown to work in simulation was the Balbekov ring cooler [7]. Since then, several ring cooler designs have been studied, based on solenoid-focused “RFOFO” cells [8] and quadrupole[9] or dipole-edge-field-focused [10] cells. All can produce useful levels of 6D cooling, but injection and extraction are problematic. This problem is eliminated (at the expense of greater hardware cost) by extending an RFOFO ring into the third dimension, giving a helical, “Guggenheim” cooling channel [11].2 This can also alleviate problematic RF loading and absorber heating, and it allows the focusing strength at each step along the device to be tailored to the emittance at that point, enhancing the cooling efficacy. In all of these designs, bending mag-

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To appear in Proc. NuFact07 Workshop, Okayama, Japan (2007). The allusion is to the Guggenheim Museum in New York, rather than, say, that in Bilbao. 2

nets introduce the dispersion needed for longitudinal– transverse emittance exchange.

Helical Cooling Channel A more recent development is the Helical Cooling Channel (HCC) [12], employing a helical dipole field superimposed on a solenoid field. The helical dipole, known from “Siberian Snake” magnets used to control spin resonances in synchrotrons, provides the dispersion needed for emittance exchange. The solenoid field provides focusing, and helical quadrupole magnets are added for beam stability and larger acceptance. Figure 1 illustrates the beam motion, as well as two possible magnet configurations: a conventional one with three separate windings generating the required field components, and the recent “Helical Solenoid” invention [13], which achieves the same field components and acceptance using simple circular coils of half the radius, about onequarter the stored energy, and smaller fields at the conductors. The equilibrium beam orbit follows the centers of the coils. (The theory of the HCC, based on a Hamiltonian formalism that starts with the opposing radial forces shown in Fig. 1, is derived in [12].)

Continuous Absorber Six-dimensional muon coolers were first formulated with emittance exchange via wedge absorbers located at dispersive points in the lattice. The same effect may be achieved more simply by use of a continuous absorber [12, 14] (Fig. 2). This approach may be synergistic with the idea of maximizing the operating gradient of copper RF cavities in high magnetic fields by filling them with pressurized hydrogen [15]; the absorber needed for ionization cooling can thus be combined with the muon

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A +six-dimensional (6D) ionization cooling channel $( ) %! * "& 5$$#"&$%3(!4*#&( based on helical magnets surrounding RF cavities filled Emittance Exchange with Continuous Absorber with dense hydrogen gas is the basis!"#$!#%&'(!$)*%!"++"(,#%!(,-.(/ for the latest plans The simple idea that emittance exchange can occur in a !"#$"%&'()*'+)), -./00'+)),*'1023 FIGURE (left) Helical-channel principle; (top-right) “Siberian homogeneous snake”( solutionabsorber with individual windings without shapedprovidedges for 1.muon colliders. This helical cooling channelconventional (HCC) practical ing the has required solenoidal, helical dipole, and helical quadrupole fields; (bottom-right) Solenoid implementation the the observation that RF with cavities solenoidal, helical dipole, and helical quadrupole followed from Helical same acceptance and the three required fields produced using simple offset coils only half the diameter of the conventional magnet. magnetic fields, where emittance exchange is achieved by pressurized with a low Z gas are possible [6]. Figure 1 is using a continuous homogeneous absorber. Momentum- a schematic description of the new approach. dependent path length differences in the dense hydrogen Incident Muon Beam Incident Muon Beam re-acceleration, givingprovide a shorter and more adiabatic energy absorber the required correlation between channel. Anotherand possibility a “separated-function” momentum ionizationisloss to accomplish longitudinal Evacuated Absorber-Filled D. M. Kaplan@NuFact07 MANX: A 6D Ionization-Cooling Experiment 5 coolingcooling. channel in whichstudies pressurized-gasliquid-filled Recent of an 800or MHz RF cavity Dipole Magnet Dipole Magnet HCC segments are separated by linear-accelerator secpressurized with hydrogen, as would be used in this !p/p tions; application, in such an arrangement, fields of gradient each HCC show that thethemaximum is not limited by graded a large external field, unlike vacuum segment can be [14], somagnetic as to maintain constant HCC cavities. Two newbeam cooling ideas, Parametric-resonance focusing strength as the momentum is reduced by Effective 6D cooling (simulations: cooling factor Ionization Cooling and Reverse Emittance Exchange, will energy loss in the absorber medium. Such an arrange50,000 in 150 m) be employed to further reduce transverse emittances to a Wedge ment may be advantageous in that the acceleration could few mm-mr, which allows high luminosity with fewer Absorber1: LEFT: Older Wedge Absorber Technique Figure then be done using superconducting RF cavities, reducmuons than previously imagined. We describe these new RIGHT: Proposed Homogeneous Absorber Technique ing instantaneous-power requirements. ideas as well as a new precooling idea based on a HCC FIGURE where dispersion causes higher energyviaparticles to have 2. (left) Emittance exchange dispersion and with z dependent fields that is being developed for an wedge absorber; (right) continuous longer path length andemittance thus moreexchange ionizationvia energy loss. exceptional 6D cooling demonstration experiment. The absorber. status of the designs, simulations, and tests of the cooling HCC Example components for a high luminosity, low emittance muon collider will be Figure 3 shows thereviewed. results of a G4beamline [16] simInc. [17], Fermilab, and university groups [6]. The ap-

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RF to restore dE/dx losses!z-indep. Hamiltonian or use as precooler w/ z-dep. B fields & no RF or separated-function cooler

ulation of a 160 m, 4-section HCC carried out by K. proach taken is to design a separated-function, gradedINTRODUCTION Yonehara [14]. The 6D emittance reduction factor of HCC segment of modest length which nevertheless de6 required for a high5×104 isNew a bigdevelopments step towards have the ∼10 revived the hopes generated livers an impressive (≈ 3–5) 6D cooling factor. Such by themuon pioneering work of Skrinsky and Parkhomchuk luminosity collider. Cooling approaches capable of [1]. a device might be suitable for use as a precooler to a The the enthusiasm existed 10 years ago for providing additionalthat factor of 10–100 needed are aun-muon combined-function HCC incorporating pressurized RF collider was[14]. dampened by the failure to come up with a cavities, or as a first segment in a separated-function der development credible scheme to achieve fast longitudinal cooling. HCC. It may also be capable of increasing substantially Consequently, the idea that a neutrino factory based on a the rate of muons stopping in a thin target, e.g., in a muon storage ring would be an easier first step toward a muon-to-electron-conversion experiment [18]. MANX muon collider, has meant that efforts for the last 10 years have been focused on neutrino factory designs [2,3]. But By eliminating the RF cavities, the cost is substantially These the innovative muon-cooling require large number of muons approaches required for will a factory has led reduced and the attention is focused on the dynamics and experimental demonstration before a facility employto large emittance accumulation and storage schemes engineering issues of the HCC magnet itself. While this is not the only approach that might be taken in such ing them canthan be the approved construction. such rather small 6Dfor emittances neededSince for a collider. Recently, advantages of small(typically 6D emittance demonstrations aremany potentially expensive com-for a a demonstration experiment, it may be a sensible one haveto become apparent HEP [4], where, for example, in that it “factorizes” the engineering challenges: with parablecollider in cost medium-scale experiments), cost oftomuon acceleration canhow be reduced which the aspects demonstrate, and best to by dousing so, the hydrogen-absorber operation in close proximity to RF high frequency RF structures being developed for the cavities and high-field solenoids already being tackled must be considered with care. International Linear Collider (ILC). We believe that A proposal for a 6D HCC demonstration experiment the by MICE, arguably this need not be demonstrated again muon collider has now become an upgrade path for the before a full2:muon accelerator is engineered. is under development by a collaboration among Muons, Figure Simulation resultsfacility of a series of 4 pressurized ILC or its natural evolution if the LHC finds that the ILC energy is too low or its cost is too great. Effective 6D cooling and the recirculating of muons in the same RF structures that are used for the proton driver may enable a powerful new way to feed a storage ring for a neutrino factory [5]. This would put neutrino factory and muon collider development on a common path.

HCC segments which are matched to the beam by having smaller cavities and stronger fields as the beam cools.

Gas-filled HCC The HCC is an attractive example of a cooling channel based on this idea of energy loss dependence on path

milab are also under consideration. It is hoped to carry < 5 years. out the experiment within the next ∼

ACKNOWLEDGMENTS I thank my collaborators at Muons, Inc., who devised many of the innovations discussed here. This work is supported by the US Dept. of Energy. The G4beamline program is in ongoing development by IIT and Muons, Inc. under STTR grant DE-FG02-06ER86281.

REFERENCES 1. 2. 3. 4. 5. FIGURE 3. Simulation of emittance reduction in a 4segment, 160 m HCC filled with high-pressure hydrogen gas.

6. 7. 8. 9. 10.

11. 12. 13. FIGURE 4. Simulation of possible MANX HCC section between matching sections. The solenoid and rotating-dipole fields gradually turn on (off) in the upstream (downstream) matching section. The overall length in this example is 12 m.

The MANX apparatus will include muon-measurement sections and (Fig. 4) matching sections into and out of the cooling section; it may also be possible to operate thin tracking detectors within the HCC section as indicated in Fig. 4. Various venues for MANX are being explored. The MICE muon beamline and detectors might be re-usable for MANX; options involving a new muon beam at Fer-

14. 15.

16.

17. 18.

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