MuPAC Review CloseoutJanuary 9th, 2014

Attending: Sergey Belomestnykh (BNL), Tor Raubenheimer (SLAC, chair), Mike Syphers (MSU), Tom Taylor (CERN), Frank

Zimmermann (CERN)

Remote: Tom McManamy (ORNL), Ritchie Patterson (Cornell)

Introduction• Lots of progress since last MuPAC Review (refocusing of program)

– Projectized MAP and MICE and established firmer basis for ongoing program– Established Staging Strategy (MASS) and Baseline Selection process (IBS)– NuStorm concept developed more fully as path to physics and cooling demo– Begun global optimization of systems using HPC resources and the Technical

R&D program has continued to make good progress

• Charge:1. Whether the structures implemented within the MAP effort are suitable and capable of completing the

MAP Feasibility Assessment by the end of the decade, taking the proposed budget profile into account; 2. The quality and significance of the MASS staging concepts as guidance for design efforts during the

MAP Feasibility Assessment; 3. The technical validity of the Initial Baseline Selection process for carrying forward the critical muon

accelerator designs and in guiding the ongoing R&D program; 4. The status and effectiveness of MAP efforts to support MICE; 5. The effectiveness and direction of the R&D program being carried out to demonstrate the required

technologies; 6. Progress that has been achieved in each area since the last review and the program’s response to

recommendations from the last review.

MASS (Patterson)• Finding: MASS establishes a sequence of accelerator facilities together with

physics experiments that gradually develops the muon accelerator complex at Fermilab. It has a number of attractive features, such as:– It includes experiments at both the intensity frontier, with NuSTORM, NuMAX, and NuMAX+, and at

the energy frontier, with the Higgs Factory and a multi-TeV Muon Collider. – Each stage builds on the previous one, often leveraging infrastructure and facilities, and taking

advantage of PIP-II and SURF. – Each steps allows preparing and validating the new technology required for the following one,

including major system tests and decision points based on technical achievement.

• Comments– MASS provides a welcome roadmap. The committee applauds the MAP team for this achievement.– nuSTORM would provide the first “real” muon beam with a pulse current of order 1 mA (to be

compared with fA currents at MICE) allowing important progress in muon beam operation, beam diagnostics, injection, separation, radiation benchmarking, etc.

• Recommendations– Continue exploration of the dual linac possibility and coordinate planning for MASS and PIP-II and

its upgrades.– Further develop the physics user community for the MASS facilities, and the physics case for

nuSTORM as results from other experiments emerge.– Further explore possibilities for an adiabatic transition from nuSTORM to NuMAX.

Initial Baseline Selection (Syphers)• Finding:

– The initial baseline selection (IBS) is a process for arriving at a site-specific set of staged facilities at Fermilab leading ultimately to a muon collider. The initial design, to be based upon knowledge at the time, is to be completed by the end of FY16.

– IBS will include a schedule and resource budget, with sign-offs on lattice designs and technology choices and will culminate in an overall review of the selected choices. Further optimizations and new choices will thus be tracked incrementally from the “basline” selection.

– MASS has helped to enhance and evolve the design inputs for IBS.– Clear example of the IBS process is seen in the 6D cooling efforts, where the number of approaches

has been reduced to two, and criteria and a method for final selection are being established.– Schedules have been established for identifying lattices, interfaces, technologies and appropriate

specifications for the seven major areas (driver, front end, cooling, acceleration, collider ring, MDI, decay rings)

• Comments– The change from exploring “all ideas” to the development of specific concepts for a coherent plan is

appropriate and welcome.

• Recommendations– Continue to develop the IBS, taking advantage of opportunities presented by MASS– Clearly establish selection criteria (perhaps using a TRL scheme or equivalent)– Ensure that the Technology R&D program supports the down selection process– Continue evaluating pragmatic limits as well as technical (e.g. safety, availability, maintenance, …)

MICE (Raubenheimer)• Finding:

– The MICE experiment will demonstrate the concept of ioniation muon cooling in the parameter regime needed for a Neutrino Factory or Muon Collider

– MICE is an international collaboration sited in the UK with a strong US component– The experimental program has slipped further and the expected conclusion of Step IV is now in

2015 and Step VI is after 2020– The US contribution has delivered the detectors and one of the spectrometer magnets. A new

deliverable of magnetic shielding was also added– The US MICE project was put on a more solid footing with contingency and risk analysis– The US spending on MICE will be capped at 5M$ per year

• Comments– MICE is a technically challenging project with high visibility; it is important to conclude the

demonstration successfully– The US deliverables of the SSM and the CCM have been further delayed– The establishment of a projectized format for MICE with an international project team is very

welcome – The inclusion of contingency and a risk evaluation for the US program is very important; the

establishment of a funding cap on the US program will protect the other components of MAP and 5M$ per year for the US construction project seems appropriate

• Recommendations– Maintain the focus on the MICE construction project

Target/Capture (McManamy)• Findings:

– A 4 MW target design option using a mercury jet stabilized by a high magnetic field was presented

– Design features included• Large 15T Solenoid Nb3Sn superconducting magnet ( 3 GJ)• Option of 5T radiation resistant copper magnet insert• Tungsten bead shielding to reduce SC magnet heat load to < 0.1mW/g• Mercury pool beam stop• 15 m exit taper with analysis to show improvement with 5 m taper• Downstream chicane to reduce scattered proton power

– Proof of Principal for Jet stabilization demonstrated in MERIT experiments– MARS15 pion production curves developed for a gallium, mercury and graphite at 3 and

7 GeV – MARS15(2014) just released, indicates reduced production by high-Z targets, but Hg still

favored over C at 7 GeV.– Initial mercury loop layout and target module handling concepts were shown– MAP Management has preselected a carbon-target module for initial use.

• This module is to have the same envelope as a possible later mercury-target module.• A carbon-target module could include a 5-T copper-coil insert which would compensate somewhat for the

lower muon yield from a carbon target at 7 GeV.

Target/Capture (2)• Findings (cont):

– No carbon target designs were presented but a 2001 Neutrino Factory study at 1.5 MW was referenced

– Initial estimate of a carbon target lifetime due to radiation damage was 4 to 6 weeks at 1 MW.

• Comments– Initial operation with carbon target appears to be a good choice– Carbon target design options with associated systems and remote handling should be

developed for initial operation and a power/target upgrade plan developed– It may be worth evaluating other shielding options that could give higher packing

fractions than the spheres in low heat generation zones. Also, a mix of different materials sometimes gives a more efficient shielding solution.

Target/Capture (3)• Comments (cont.)

– The mercury pool beam dump containment structure may be exposed to cavitation damage. Early SNS testing with static mercury targets at Los Alamos showed cavitation damage on the wall even with a free surface and also when distributed structures were included in the pool. In SNS the damage is complicated and there is some indication of cracking starting at the cavitation damage sites but probably enhanced by liquid metal embrittlement.

– The front double wall at SNS helps to greatly reduce damage to the outer wall and gas injection at J-PARC seems to help.

– The magnetic field may also help to suppress the jets from bubble collapse when normal to the field.

– It is likely that a separate shroud around the target would be desired to contain mercury leaks such as done with SNS and J-PARC targets

– The external flange mercury supply and return connections will likely be a source of dips and airborne contamination and the balance of the loop components will require full remote handling maintenance.

– An anticipated event is likely to be one or more beam pulses which do not intercept the mercury stream either due to misalignment or mercury system delivery problems. This should not cause permanent damage requiring component replacement.

Target/Capture (4)• Recommendations

– Develop the carbon target and associated facility designs for initial operation and for the planned power upgrade.

– Evaluate the thermal response to loss of helium cooling to the shielding, assuming the beam is tripped.

– Calculate peak pressures in the mercury pool after a pulse and evaluate the potential for cavitation damage to the structure and review SNS and J-PARC experience

– Identify an R&D program and schedule to test splash and cavitation damage mitigation methods

– Consider adding a separate shroud around the primary mercury structure to contain potential leaks. This may require a separate cooling circuit.

– For the mercury or carbon target evaluate the potential and consequences for beam pulses which miss the target and identify diagnostic systems.

– Evaluate the impact of limiting the maximum power on target to 1 ~ 2 MW

Cooling (Zimmermann)• Finding: – Effort focuses on completing MICE for a first experimental demonstration of basic ionization

cooling principles, together with UK collaborators, and on simulation studies of two more elaborate cooling systems under consideration for the muon collider, namely helical cooling channel

– A MICE construction subproject has been performed and several reviews were held. A budget plan, including contingency and UK+US funding for operation has been established. It should ensure completion of MICE construction by 2020/21.

– Cooling technology includes either vacuum or gas-filled RF cavities at 325 and 650 MHz, LiH and LH2 disks or wedges, high field large aperture solenoid coupling coils.

– Criteria for down selection between the two cooling channels have been defined.

• Comments– The MuPAC concurs with the plan to complete MICE, and commends the establishment of a

coherent schedule and solid budget plan till the end of this project– The solenoid magnets for the cooling channels appear feasible ; the RF cavities at 325 and 650 MHz

are bracketed by prototype tests at 201 and 805 MHz– The required 6D cooling could be demonstrated using a collimated muon beam from nuSTORM– The simulation effort for both cooling channels is impressive and the HPC addition is critical. The

exact dynamics of the extremely rapid space charge neutralization over 40 ps was not clear.

• Recommendations– Ensure that the Technical R&D program supports the IBS downselect– Further explore possible two-stream effects , as well single- and coupled-bunch wake fields in the

cooling channels , through refined simulations and also further explore the possibility of some benchmarking experiment(s), e.g. using proton beams to validate different aspects of the dynamics

Acceleration and MTA (Belomestnykh)• Findings:

– Compared to MuPAC12 meeting, the FFAG option was eliminated. – The acceleration scenario is now a bit more streamlined with three options remaining

only for an acceleration stage from 63 GeV to ~375 GeV.– There following design challenges have been identified: RLA switchyard layout;

Injection/extraction; Time of flight correction; Collective effects.– No showstoppers so far, but cost escalation an issue.– The key technologies are: RF (superconducting and normal conducting, both requiring

high gradients); SC magnets (high fields and protection from decays); kickers for injection/extraction.

– The number of different RF frequencies at the front end was significantly reduced as compared to MAC12.

– There was progress reported with cavity testing at MTA.– All Season cavity (805 MHz) was successfully tested under vacuum in high magnetic

field. It reached 30 MV/m w/o magnetic field and >20 MV/m at 5 T. The gradient seems to reach saturation. These gradients are very close to what will be required, but at a wrong frequency.

Acceleration and MTA (2)• Findings (cont):

– A PRL paper was published summarizing results of experiments obtained with hydrogen-filled cavities.

– Fabrication of a modular cavity is nearly complete and experiments are expected to start this FY. The cavity will be heavily instrumented.

– A prototype MICE cavity (201 MHz) is on site. The cavity assembly is in final stages. Testing will commence soon. There will be an opportunity to test this cavity in a high magnetic field.

• Comments– Reduction of the number of different frequencies required at the front end is a welcome

development.– While there was good progress at MTA, there still appears to be a lack of understanding

of factors limiting the performance of cavities (both vacuum and high-pressure). The heavily-instrumented modular cavity should be a good tool to address this.

– Testing of the prototype MICE cavity will be a very important milestone for both MICE and MAP in general.

Acceleration and MTA (3)• Recommendations

– Further explore possible two-stream effects , as well single- and coupled-bunch wake fields in the cooling channels , through refined simulations and also further explore the possibility of some benchmarking experiment(s), e.g. using proton beams to validate different aspects of the dynamics

– Continue the cavity testing program at MTA: modular cavity, MICE prototype cavity.– Develop a clear set of goals to be achieved during cavity testing, e.g. What is the “safe”

accelerating gradient?– Develop a better understanding of the effects limiting cavity’s performance in high

magnetic fields and how to scale the test results to other rf frequencies.– Make sure that the MTA test plans and goals are aligned and closely coupled with the

overall MAP requirements and schedule.

Magnets (Taylor)• Findings:

– The program requires various types of large aperture high field magnets . Direct studies are presently focused on developing solenoids using HTS (REBCO) at BNL, and leveraging development of Bi-2212 material at FSU and FNAL. The magnets being proposed are based on material properties that are extrapolated from present state-of-the-art values.

• Comments: .– The REBCO material is well adapted for winding high-field solenoids, and the present

work can lead to useful demonstration. A problem is that only small coils can be made due to the present cost of the material. However, in the long term this cost could potentially come down significantly as the raw materials are not expensive.

– The Bi-2212 material , which is conveniently adapted for use in flat Rutherford cable has recently benefited from a boost in performance with heat treatment under pressure.

– This program requires very high performance magnets. The cost (and time) involved in the full development of such magnets puts it outside the possibilities of present MAP.

• Recommendations:– Continue leveraged work on HTS– Identify a contact within MAP to evaluate and monitor the potential for the muon

program of work going on elsewhere on high field magnets– Evaluate the cost ($ and risk) benefit of choice of field level

MDI (Zimmermann)• Findings:

– Detector background is an important potential showstopper; for the muon-collider Higgs factory (HF) half of the machine can be the source of background; in the HF there are 5e8 decays in the ring per bunch crossing and 3e11 decays/m/s for 2 beams, to be compared with 1.3e10 /m/s/ at a 1.5 TeV muon collider.

– To reduce background and heat load at 1.9 K, the magnets are equipped with a liner and individually tailored small-aperture W masks (down to 5 sigma) are installed in the magnet interconnects; as a result 1 kW/m is deposited on the “warm” components (cooled by liquid nitrogen), and 10 W/m on the cold mass at 1.9 K

– Particle fluxes due to muon decay have been simulated for the various detector components. The neutron flux in the vertex detector is estimated to be only about 10% of the one for CMS at LHC design luminosity

– Most MDI calculations were performed on the FNAL GRID• Comments:

– The successful suppression of beam related backgrounds in the Higgs Factory is an important accomplishment.

– Many mask-related transitions in the beam aperture introduce a geometric broad band impedance which could be significant for the high bunch charges and short bunch length considered, despite the large absolute aperture and the small number of turns

• Recommendations: – Pursue ANSYS calculations to model temperatures and stresses in absorbers and

magnets including cooling

Detector (Patterson)• Finding: Recent progress includes

– Studies showing that with 0.5ns resolution in the tracker, timing cuts can suppress the background by a factor of 300-500 with high signal efficiency. Timing will also be important for the calorimeter.

– Parametric studies of the Higgs mass and width precision at a Higgs Factory, and of the Higgs trilinear self-coupling at a multi-TeV muon collider. These studies do not include backgrounds but do account for limited solid angle due to shielding cone.

– A full ILCroot simulation of a SiD-like detector with MARS backgrounds, showing that timing gates and energy corrections have little impact on the dijet mass resolution, but the addition of background degrades the resolution by about 50%.

• Comments:– Progress in understanding the reach of a Higgs Factory and the impact of machine-

related backgrounds has been impressive, given the very limited manpower.– We congratulate the full detector simulation including backgrounds, which is an

important step toward demonstrating the viability of physics measurements at a muon collider.

– It is encouraging tracker timing demands are challenging but plausible• Recommendations:

– Ensure that manpower and computer resources are adequate to study Higgs Factory detector performance with full MARS backgrounds.

– Continue attempts to form a joint lepton collider detector effort.

Summary• The staging study has helped focus the MAP activities and provides more

realistic expectations• The MAP program has made good technical progress over the past 1.5 years

and the program has been well focused to develop a baseline configuration• Management structures have been put in place to maintain the focus and

align the R&D program to address the critical questions in the baseline design• The US MICE construction project structure appears convincing• The MAP program is a broad effort. The potential increase in the funding to

16M$/yr would provide badly needed support to continue the feasibility study. A future design effort would require a large funding increase.