# Presentation: 802778

## Muon Colliders: Progress and Plans

- Steve Geer

- Introduction
- Muon Collider Ingredients
- Comaparison with Neutrino Factories
- Cooling Channel Design Progress
- Acceleration, Storage Ring and Detector Issues
- Next Steps
- Summary

## Motivation

-  We want high-luminosity multi-TeV lepton-lepton collisions!
-  we need accelerator R&D!

-  Circular (compact) multi-TeV Lepton Collider that would fit on an existing laboratory site   hope that Muon Colliders will be affordable.
-  Very small beam energy spread enabling precise scans and width measurements  Muon Colliders may have a special role for precision measurements.

-  The Muon Collider concept is attractive because muons do not radiate as readily as electrons (m / me ~ 207):

## The Challenge

-  To produce sufficient luminosity for an interesting physics program (L = 1034-1035 cm-2s-1 at s = 1-few TeV) will require very bright muon beams. This is challenging because:

-  If we can meet this challenge, we will also have the technology for neutrino factories and for low energy muon experiments using up to ~1021 muons/year !

-   Muons produced as a tertiary beam that occupies a large longitudinal & transverse phase space. The beam must be cooled by a large factor: a longitudinal emittance reduction of about 14 & a transverse emittance reduction of about 400  6D reduction of ~14400400 = 2 106 ; cf NF which requires transverse emittance reduced by factor of a few.
-  Muons decay (t0 = 2s). Beam manipulation & acceleration must be rapid.

##      Muon Collider Baseline Parameters

- Requires 1  1021 + / year
- Similar to (a little more than) number of muons expected from Neutrino Factory front-end
- Collider energy limited by
- “neutrino radiation” (high energymuon decays in straight sections produce collimated beam of neutrinos that interact in Earth
- to produce a radiation field at exit point). s=3 TeV consideredsafe, s=4 TeV marginal.
- If beam-beam tune shift can be corrected, lower emittance beams would enable use of fewer muons higher energy colliders plausible (but dose ~ E4 ... win slowly).

| Energy | 0.1 TeV | 3 TeV |
| --- | --- | --- |
| Proton Srce Power | 4 MW | 3.5 MW |
| Rate | 15 Hz | 30 Hz |
| Muons / bunch | 4  1012  | 2  1012  |
| Bunches | 1  1 | 2  2 |
| Circumference | 0.35 km | 6 km |
| Effective turns | 450 | 900 |
|  | 9.4 mm | 3 mm |
| (mm radians) | 0.195 | 0.05 |
|  | 5 mm | 72 mm |
|  | 0.022 | 0.044 |
| p/p | 0.01 % | 0.16 % |
| Luminosity (cm-2 s-1) | 2.2  1034 | 7  1034 |

## Muon Collider Ingredients

- Proton Driver
- primary beam on production target
- Target, Capture, and Decay
- create ; decay into 
- Bunching & Phase Rotation
- reduce E of bunch
- Cooling
- reduce 6D emittance
- Acceleration
- 130 MeV  up to 1.5 TeV
- Storage Ring
- store for ~1000 turs
- One IP

- ~ 4 km

- 3 TeV

## Neutrino Factory Ingredients

- Proton Driver
- primary beam on production target
- Target, Capture, and Decay
- create ; decay into 
- Bunching & Phase Rotation
- reduce E of bunch
- Cooling
- reduce transverse emittance
- Acceleration
- 130 MeV  20 GeV
- Storage Ring
- store for 500 turns; long straight section

- US Design schematic

##      News from the Recent Past

- US front-end MC & NF designs used to be very different. NF design:  The bunching/phase rotation/cooling  many muon bunches. MC design:  Want to end up with 1 or 2 muon bunches / cycle to maximize luminosity. We used to think the best way to achieve this was to make one bunch at the beginning, & keep hold of it through the entire front-end.
- This meant using low frequency rf systems. We did not succeed in producinga practical, self-consistent cooling channel that reduced the emittance by the O(106) factor needed for a MC.
- In the last 2 years it has been realized that it is easier to start with many bunches, & combine in the middle of the cooling scheme  first completeself-consistent MC cooling channel designs and ...the MC & NF front-ends (up to the beginning of the cooling) are the same !

- (See talks from the Low Emittance Muon Collider Workshop, Fermilab 6-10, 2006:  _[http://www.muonsinc.com/](http://www.muonsinc.com/)_ )

##      Neutrino Factory vs Muon Collider

|  | NF | MC |
| --- | --- | --- |
| Proton Beam | Yes | Same |
| Target | Yes | Same |
| Capture & Decay | Yes | Same |
| Buncher | Yes | Same |
| Phase Rotation | Yes | Same |
| Early Cooling | Yes | Same ? |
| More Cooling | No | Yes |
| Early Acceleration | Yes | Different |
| More Acceleration | No | Yes |
| Storage Ring | Yes | Different |
| Detector | Yes | Different |

- We will need to choose which neutrino source will best serve our long-term needs.
- The choice may not be independent of the bigger “we want a multi-TeV lepton collider” picture.
- Neutrino Factories & Muon Colliders are linked by their common R&D and possible staging path.

##      News: A Complete Cooling Scheme

- (Palmer et al)

##      Bunch Merging is Critical

- (Palmer et al)

##      Cooling Technologies

- There are competing ideas (using different technologies) for the various steps in the cooling chain:

- Palmer et al:
- RFOFO Ring
- Guggenheim
- 50-60T Solenoid Channel

- Muons Inc.High pressure gas-filled cavitiesHelical Cooling ChannelReverse Emittance ExchangeParametric Resonance Induced Cooling

- }

- }

- And new ideasare still emerging

- (I think) our task for the next couple of years will be to sort through the ideas with enough engineering input to identify the best bet(s) and define the required component R&D.

##      One Example from Palmer et al.

- Steps 3 & 4 inPalmer et alscheme

- RFOFO RING &
- The GUGGENHEIM

##      One Example from Muons Inc.

- MUCOOL R&D  achievable in a normal rf cavity may be limited if the cavity operates within a significant magnetic field.  If this turns out to be the case we will have to redesign our baseline cooling channel lattices.Muons Inc have proposed using cavities filled with hydrogen (or helium) at high pressure (suppresses breakdown and provides energy-loss medium).  Test cell results at 805 MHz are encouraging.

- Latest News:
- Performance
- seems
- unaffected
- by magnetic
- Field.

##      The End of The Cooling Channel

- The last few meters of cooling will (probably) require the most challenging cooling channel technology.Presently favored idea is to use liquid hydrogen absorber in ~50T solenoids.
- These could be like the 45T solenoid at the High Field Magnet Lab in Florida ... but we need a handful of these & their power consumption is phenomenal  need a new technology.
- One approach to explore is to use high-Tc Superconductor, run cold to get to very high fields.  This technology is developing fast, but we need some basic engineering studies to understand if 50T high-Tc cooling channel solenoids are plausible.

##      Acceleration

- After the cooling, cost-effective (affordable) acceleration is next on the list of TeV-scale Muon Collider challenges.
- Over the last couple of years there have been several dreams of usingILC accelerating structures to accelerate muons to TeV energies, reconfiguring some of the ILC into an on-site RLA.Latest dream:  use ILC as-is:

- More studies needed before we know if this is plausible.  If not, there are alternative schemes using linacs and rapid cycling synchrotrons.

##      Storage Ring

- Needs to store beam for ~1000 turns
- 3TeV & Higgs Factory lattices have been studied (PRST-AB 2,  081001 (1999))
- To achieve required luminosity at 3 TeV will need * ~ 3 mm at Interaction Point  bunch lengths no larger than this  almost isochronous ring.
- IR studied in detail (including shielding & detector backgrounds)
- Dynamic Aperture ~OK (but needs some more study)

##      Detector Issues

- Detailed GEANT & MARS studies of detector backgrounds were made in 1996-98 for a 4 TeV muon collider: 2  1012 muons/bunch  2  105 decays/m.Mean electron energy = 700 GeV.
- With careful design of final focus, most electrons can be swept into shielding,
- but the forward physics (20 deg) is sacrificed.

| R (cm) |  | n | p |  | e |  |
| --- | --- | --- | --- | --- | --- | --- |
| 5 | 2700 | 120 | 0.05 | 0.9 | 2.3 | 1.7 |
| 10 | 750 | 110 | 0.20 | 0.4 | - | 0.7 |
| 15 | 350 | 100 | 0.13 | 0.4 | - | 0.4 |
| 20 | 210 | 100 | 0.13 | 0.3 | - | 0.1 |
| 50 | 70 | 120 | 0.08 | 0.05 | - | 0.02 |
| 100 | 31 | 50 | 0.04 | 0.003 | - | 0.008 |

- GEANT Results:
- Radial fluxes / cm2 / crossing
- Thresholds:E = 25 keV;  En = 40 keVEp = 10 MeV; E = 10 MeV

- Corresponds to 0.4% occupancy in 300 x 300 m2 pixels at r = 10 cm & 1.3% occupancy at r = 5 cm; with doses comparable to LHC at 1034 cm-2 s-1

- Calorimeter backgrounds also look OK provided spikes from Bethe-Heitler muons can be removed by pattern recognition

##      Next Steps

- Muon Collider & Neutrino Factory R&D have tremendous overlap. It is important that we succeed with the NF R&D program (MICE,
- MERIT, ISS follow-on ... ).
- Given the recent progress on Muon Collider cooling channel ideas, and steady progress with the relevant NF R&D, now is a good time to revisit the overall Muon Collider concept with an emphasis on the cooling channel design, acceleration scheme, and storage ring.

- Which of the cooling channel ideas are feasible and what  component R&D is required ?Is a high-Tc 50T final-cooling section plausible ?
- Is using ILC accelerating structures OK ?  (Great idea if it is!)

- To make significant progress on these questions in the next couple of years will require more people and more accelerator R&D resources than we presently have.

##      News from Fermilab

- “ ...  a Task Force to develop a plan for an advanced R&D
- program aimed at the technologies required to support the
- long term prospects of a Muon Collider. “

- As a step towards establishing an enhanced Advanced Accelerator R&D Program at Fermilab, the Fermilab Director has requested:

- It is hoped this will bring in new people and advanced accelerator R&Dresources so that significant progress can be made on the critical issues.
- The first step, to be completed by end of September, is to develop an R&D proposal.

##      Summary

- There is tremendous overlap between Neutrino Factory R&D and Muon Collider R&D.
- Muon Colliders require much more beam cooling than Neutrino Factories,
- and (right now) this presents the biggest Muon Collider challenge.
- New Muon Collider cooling channel ideas have emerged. The front-ends
- (proton source, targetry & collection, decay channel, phase rotation, andpossibly early cooling) are now the same for Muon Colliders & NeutrinoFactories.
- Its time to look deeper at the competing candidate cooling  technologies.
- Which are practical ?  What component R&D is needed ? This needs a critical study with more people & resources.
- Its also time to revisit the overall Muon Collider design, and explore  further the idea of using ILC accelerating structures.
- Lots of challenges, but also lots of progress. Keep tuned !

## Muon Collider Parameter Table

- C. Ankenbrandt et al., PRST-AB 2,  081001 (1999)