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==Introduction==
==Introduction==


The first in the chain of the Mu2e magnets is the Production Solenoid (PS), shown in Figure 1. The role of PS is to collect and focus pions and muons generated in interactions of the 8-GeV proton beam with a tilted high-Z target (tungsten) by supplying a peak axial field between 4.6 T and 5.0 T and an axial gradient of ~1 T/m within the 1.5 m diameter warm bore.  
Mu2e requires a high intensity, pulsed proton beam to produce large numbers muons. The experiment relies on Fermilab’s Booster, which is also used by other Fermilab experiments, such as g-2, to provide the protons. The beam for Mu2e can be delivered while other experiments are running simultaneously. While the beam of protons must be intense during these pulses in order to deliver enough muons to the stopping target for Mu2e to reach its design sensitivity, the beam of protons must also disappear between pulses. This allows for the muons captured by the experiment to decay over time, and these decay products to be measured. Protons delivered between pulses will contribute to the experiment’s backgrounds, and their rate will be measured with the extinction monitor.  




[[File:Production solenoid.png |thumb|center|600px|Figure 1. Cross-section through the PS cryostat with a part of the Transport Solenoid (TS) cryostat shown. HRS is not shown.]]
[[File:ProtonBeamlineArial.jpg|thumb|center|500px|Figure 1. Layout of the Mu2e facility (lower right) relative to the accelerator complex that provides proton beam to the detector. Protons are transported from the Booster through the MI-8 beamline to the Recycler Ring where they will circulate while they are re-bunched by a 2.5 MHz RF system. The reformatted bunches are kicked into the P1 line and transported to the Delivery Ring where they are slow extracted to the Mu2e detector through a new external beamline. ]]


==General Requirements==


The PS is a challenging magnet because of the relatively high magnetic field and a harsh radiation environment that requires state-of-the-art conductor both in terms of the current-carrying capacity and structural strength. The PS coils are protected from radiation by a massive Heat and Radiation Shield (HRS) made of bronze, placed within the warm magnet bore. The HRS volume and cost were minimized while keeping the absorbed dose, peak power density, total power dissipation and number of displacements per atom (DPA) within the acceptable limits.


The magnet consists of three superconducting coils made of Al-stabilized NbTi cable surrounded by support shells made of structural Al and bolted together to form a single cold-mass assembly. The cold mass is supported inside the cryostat using a system of axial and radial support rods. During normal operation of the Mu2e magnet system, the axial force on the cold mass is about 1.3 MN acting toward the TS. The axial force reverses the direction in the case of a quench during stand-alone PS operation, which requires a 2-way axial support system.
Protons designated for Mu2e are acquired from the Booster during the available portions of the Main Injector timeline when slip-stacking operations are underway for NOνA. Two Booster proton batches, each containing 4.0 × 10<sup>12</sup> protons with a kinetic energy of 8 GeV, are extracted into the MI-8 beamline and injected into the Recycler Ring. After each injection, the beam circulates for 90 msec while a 2.5 MHz bunch formation RF sequence is performed. This RF manipulation coalesces each proton batch into four 2.5 MHz bunches occupying one seventh of the circumference of the Recycler Ring. Each of these bunches will be synchronously transferred, one at-a-time, through existing transfer lines to the Delivery Ring, where the beam is held in a 2.4 MHz RF bucket during resonant extraction to the experiment through a new external beamline. To help control the spill rate uniformity during resonant extraction a technique known as RF knockout will be used. RF knockout will allow for fast transverse heating of the beam. It will also serve as a feedback tool for fine control of the spill rate. The resonant extraction process will not completely remove the entire beam, so what remains must be disposed of in a controlled way. Therefore, a beam abort system will be required for the Delivery Ring to “clean up” beam that remains after resonant extraction is complete. The resonant extraction system will inject ~ 3 × 10<sup>7</sup> protons into the external beamline every 1.7 µs (the revolution period of the Delivery Ring). An extinction system, in the form of a high frequency AC dipole , is required to suppress unwanted beam between successive pulses that can generate experimental backgrounds. After transiting the extinction system the proton pulses are delivered to the production target located in the evacuated warm bore of a high-field superconducting solenoid. The proton beam will have a transverse radius of about 1 mm (rms) and will be about 250 ns in duration. The proton beam deflects in the magnetic field of the solenoid before striking the production target, complicating the final focus beamline optics and steering. The production target is a radiatively cooled tungsten rod about the size and shape of a pencil. Not all of the proton beam interacts in the production target. The unspent beam is absorbed in an air-cooled beam absorber downstream of the production target. A monitor, located above the beam absorber, will measure scattered protons as a function of time to provide a statistical measure of the residual beam between pulses that traverses the extinction system. The proton delivery scheme is shown in Figure 1. The Mu2e proton beam requirements are described in [1].


In the case of a quench, the quench protection system extracts the stored energy to a dump resistor located outside of the magnet cryostat. Because of the relatively low resistivity of the Al support shells, a considerable fraction of the stored energy dissipates there due to eddy currents, accelerating the quench propagation in the coil volume.


The cold mass as shown in Figure2 is instrumented with voltage taps, temperature gauges, strain gauges, and displacement sensors that monitor the magnet parameters during operation. Witness samples made of Al and Cu are placed at strategic locations on the inner cold mass surface to monitor the degradation of the cable stabilizer under irradiation. Once the critical level of degradation is detected, the magnet will be thermo-cycled to room temperature to restore the stabilizer resistivity.


The production solenoid does not have an iron return yoke.


Most of the infrastructure required to deliver proton beam to the Mu2e production target already exists or will exist before Mu2e needs it. The g-2 experiment, scheduled to take data before Mu2e, requires much of the same infrastructure. To satisfy the common needs of both projects a program to develop a Muon Campus through a series of Accelerator Improvement Plans (AIP) and General Plant Projects (GPP) has been initiated. The accelerator infrastructure required exclusively by Mu2e is part of the Mu2e Project and includes:


[[File:Cold mass.jpg|thumb|center|600px|Figure2. General cold mass view. ]]
*Resonant Extraction System
*MHz Delivery Ring RF system
*Mu2e external beamline • Extinction System
*Extinction Monitor System • Production Target
*Radiation Safety and Shielding
*Beamline instrumentation and controls
*Diagnostic Beam Absorber
*Proton Target Beam Absorber.


==Requirements==
These elements are described in detail in this Technical Design Report[2].


The Mu2e Production Solenoid (PS) magnet must meet the specific design and operational requirements as defined in [1]. Because of the relatively high cost, complexity and difficulty to replace once installed into the system, the PS should have adequate operating margins and be as fail-resistant as practically achievable.
[1] R. Bernstein et al., “Mu2e Proton Beam Requirements,” [http://mu2e-docdb.fnal.gov/cgi-bin/ShowDocument?docid=1105 Mu2e-doc-1105].<br>
[2] Mu2e Mu2e Technical Design Report [http://mu2e-docdb.fnal.gov/cgi-bin/ShowDocument?docid=4299 Mu2e-doc-4299].


In particular, the magnet must be able to operate with the other adjacent magnets or standalone, survive the worst-case quench, tolerate reversal of the axial force direction, and return to the nominal operating parameters without retraining after each thermal cycle. The main magnet requirements to be used as the design key point are discussed below.


*Magnetic Field<br>
[[Category:Experiment]]
The magnetic field requirements for the Mu2e experiment are described in [2]. The PS field varies with the axial position. The maximum axial field on the axis is required to be at least 4.5 T. The axial field shall monotonically decrease with no more than ±5% non- linearity from the peak value to 2.5 T over the length of 2.8 m. In order to guarantee meeting the peak field requirement, the magnet shall be designed and fully operational at the maximum axial field of 5.0 T while meeting all other requirements under the static thermal load, excluding the particle radiation load. Note that energizing the magnet to 5.0 T requires using a trim power supply in TS to maintain the 2.5 T field at the interface with the TS magnet.
[[Category:Tutorial]]
 
*Particle Radiation Load<br>
The magnet will experience particle irradiation due to interaction of the proton beam with the fixed target in the magnet aperture. The HRS is designed to limit the main radiation quantities to the following values: peak coil power density - 30 μW/g, total heat load on the cold mass - 100 W, peak lifetime absorbed dose - 7 MGy, and displacements per atom - (DPA) – (4-6)•10-5 year-1 [3]. The magnet shall be designed to operate under this maximum radiation load while meeting all other requirements.
 
*Degradation of RRR<br>
The residual resistivity ratio (RRR) of the cable stabilizer degrades under irradiation at cryogenic temperatures. The minimum allowable values for the RRR of Al and Cu stabilizers are 100 and 50, which will be reached in about a year of continuous operation for the given DPA values with a sufficient safety margin [3]. Once the critical degradation of RRR is detected, the magnet will be thermo-cycled to recover the RRR. The magnet shall be designed to operate at the minimum allowable values of RRR while meeting all other requirements.
 
*Cooling<br>
The magnet shall be cooled by heat conduction to the thermo-siphon system connected to the cryogenic plant, which provides the nominal liquid helium temperature of 4.70 K. In order to maintain the required 1.5 K temperature margin, the maximum allowable temperature of the superconducting coils is 5.10 K when operating at 4.6 T peak axial field and 4.85 K when operating at 5.0 T peak axial field.
 
*Quench Protection<br>
The magnet will be protected from quenches by an active quench protection system, which continuously monitors the coil voltages and extracts the energy to an external dump resistor once a quench is detected. The maximum coil temperatures and voltages during a protected quench must be limited to 130 K and 600 V.
 
*Structural Criteria<br>
The superconducting coils shall be reinforced against Lorentz forces by external support shells made of structural Al, and the cryostat shells will be made of austenitic stainless steel. The thicknesses of the coil support shells and cryostat shells must be chosen according to the 2013 ASME Boiler and Pressure Vessel Code for operation at 5.0 T peak axial field. Plastic deformation of the cable stabilizer shall be considered during calculation of the shell stresses.
 
The maximum allowable stress in all other structural elements, including the cold mass assembly bolts and the cryostat suspension rods, is the lesser of 2/3 of the minimum specified Yield Strength (Sy) or 1/3 of the minimum specified Ultimate Strength (Su). An additional safety factor of 2 will be included in calculation of the gravity loads on the cold mass suspension components.
 
A separate shipping restraint system shall be devised for supporting the cold mass during transportation. The cold mass suspension components shall not be used to support the shipping and handling loads.
 
[1] M. Lamm, "Production Solenoid Requirements",<br>
[http://mu2e-docdb.fnal.gov:8080/cgi-bin/ShowDocument?docid=945 Mu2e-doc-945.]<br>
[2] R. Coleman et al., "Mu2e Magnetic Field Specifications"<br>
[http://mu2e-docdb.fnal.gov:8080/cgi-bin/ShowDocument?docid=1266 Mu2e-doc-1266.]
[3] M. Lamm, "Detector Solenoid Requirements",<br>
[http://mu2e-docdb.fnal.gov:8080/cgi-bin/ShowDocument?docid=946 Mu2e-doc-946.]

Latest revision as of 01:58, 23 September 2022

Introduction

Mu2e requires a high intensity, pulsed proton beam to produce large numbers muons. The experiment relies on Fermilab’s Booster, which is also used by other Fermilab experiments, such as g-2, to provide the protons. The beam for Mu2e can be delivered while other experiments are running simultaneously. While the beam of protons must be intense during these pulses in order to deliver enough muons to the stopping target for Mu2e to reach its design sensitivity, the beam of protons must also disappear between pulses. This allows for the muons captured by the experiment to decay over time, and these decay products to be measured. Protons delivered between pulses will contribute to the experiment’s backgrounds, and their rate will be measured with the extinction monitor.


Figure 1. Layout of the Mu2e facility (lower right) relative to the accelerator complex that provides proton beam to the detector. Protons are transported from the Booster through the MI-8 beamline to the Recycler Ring where they will circulate while they are re-bunched by a 2.5 MHz RF system. The reformatted bunches are kicked into the P1 line and transported to the Delivery Ring where they are slow extracted to the Mu2e detector through a new external beamline.

General Requirements

Protons designated for Mu2e are acquired from the Booster during the available portions of the Main Injector timeline when slip-stacking operations are underway for NOνA. Two Booster proton batches, each containing 4.0 × 1012 protons with a kinetic energy of 8 GeV, are extracted into the MI-8 beamline and injected into the Recycler Ring. After each injection, the beam circulates for 90 msec while a 2.5 MHz bunch formation RF sequence is performed. This RF manipulation coalesces each proton batch into four 2.5 MHz bunches occupying one seventh of the circumference of the Recycler Ring. Each of these bunches will be synchronously transferred, one at-a-time, through existing transfer lines to the Delivery Ring, where the beam is held in a 2.4 MHz RF bucket during resonant extraction to the experiment through a new external beamline. To help control the spill rate uniformity during resonant extraction a technique known as RF knockout will be used. RF knockout will allow for fast transverse heating of the beam. It will also serve as a feedback tool for fine control of the spill rate. The resonant extraction process will not completely remove the entire beam, so what remains must be disposed of in a controlled way. Therefore, a beam abort system will be required for the Delivery Ring to “clean up” beam that remains after resonant extraction is complete. The resonant extraction system will inject ~ 3 × 107 protons into the external beamline every 1.7 µs (the revolution period of the Delivery Ring). An extinction system, in the form of a high frequency AC dipole , is required to suppress unwanted beam between successive pulses that can generate experimental backgrounds. After transiting the extinction system the proton pulses are delivered to the production target located in the evacuated warm bore of a high-field superconducting solenoid. The proton beam will have a transverse radius of about 1 mm (rms) and will be about 250 ns in duration. The proton beam deflects in the magnetic field of the solenoid before striking the production target, complicating the final focus beamline optics and steering. The production target is a radiatively cooled tungsten rod about the size and shape of a pencil. Not all of the proton beam interacts in the production target. The unspent beam is absorbed in an air-cooled beam absorber downstream of the production target. A monitor, located above the beam absorber, will measure scattered protons as a function of time to provide a statistical measure of the residual beam between pulses that traverses the extinction system. The proton delivery scheme is shown in Figure 1. The Mu2e proton beam requirements are described in [1].



Most of the infrastructure required to deliver proton beam to the Mu2e production target already exists or will exist before Mu2e needs it. The g-2 experiment, scheduled to take data before Mu2e, requires much of the same infrastructure. To satisfy the common needs of both projects a program to develop a Muon Campus through a series of Accelerator Improvement Plans (AIP) and General Plant Projects (GPP) has been initiated. The accelerator infrastructure required exclusively by Mu2e is part of the Mu2e Project and includes:

  • Resonant Extraction System
  • MHz Delivery Ring RF system
  • Mu2e external beamline • Extinction System
  • Extinction Monitor System • Production Target
  • Radiation Safety and Shielding
  • Beamline instrumentation and controls
  • Diagnostic Beam Absorber
  • Proton Target Beam Absorber.

These elements are described in detail in this Technical Design Report[2].

[1] R. Bernstein et al., “Mu2e Proton Beam Requirements,” Mu2e-doc-1105.
[2] Mu2e Mu2e Technical Design Report Mu2e-doc-4299.