3.4.1 Superconductiong Magnet
3.4.1.1 General Consideration
The BESIII solenoid magnet is designed to provide an axial magnetic fie1d of about 1.2T overthe tracking volume. Particle detectors within this volume will measure the trajectoriesof charged tracks emerging from the co1lisions. Particle momentum is determined fromthe measured curvature of these tracks in the fie1d. To support our physics goals, themagnet tracking systems must meet the following performance requirements: the expectedmomentum resolution for charged particles in a 1.0 T magnetic fie1d is σp/p(Gev/c)=0.65%, superconducting magnet is selected. The iron absorber plates of the muon detector provide the magnetic flux return. Since all wide angle particles mustpass through the superconducting coil and cryostat before impacting the Mouncounter,the Mouncounter performance requires the material in the coil to be minimized in termsof radiation length (X0). In practice, this requirement mustbe ba1anced against the overall requirement that the superconducting coil must be robust,and have excellent longterm operational reliability.
Generalparameters of the BESIII detector solenoid are summarized in Table 3.4.6-1.
Table 3.4.6-1 Parameters of the BESIII solenoid Coil
CryostatInner radius / 1.0m
Outer radius / 1.3m
Length / 3.4m
Coil
Effective radius / 1.15m
Length / 3.0m
Conductor dimension / 3mm*33mm
Electrical parameters
Centerfield / 1.2T
Nominal current / 3000A
Stored energy / 9MJ
Cool Down Time / ≤10 days
3.4.1.2 Magnetic filed design
Using poisson program, when the diameter of the aperture of the end yoke is 50cm, and center filed is 1T, we get out the magnetic filed distribution below.
Figure3.4.6-1 BESIII flux display and center B along Z
3.4.1.3 Superconducting Coil
(1) The Conductor
The BTCF Superconduchng so1enoid consists of the coil in a cryostat. Its design will benefit from experience gained in producing similar systems for other detectors over the past l5 years. High purity(>99.99%) aluminum-stabi1ized NbTi/Cu superconductor may be employed to assure conductor stability with minimum radiation thickness.
Figure 3.4.6-2Cross section view of a typical conductor
(2) Coil winding
An outer high-strength aluminum cylinder restrains the conductor, and the fiberglass-tape insulated conductor is often wound directly into the bore of this cylinderand then vacuum impregnated for stable, effective electrical insulation.
(3)Cryostat
The BESIII Superconduchng solenoid consists of the coil in a cryostat. The outer high-strength aluminum hoop restrains the conductor, and it is the primary structural member of the cold mass. Coolant is supplied through tubes attached to the outer surface of the restraining hoop cylinder. This eliminates the large cryogen inventory and thick cryostat that is necessary for cooled coils.The conductor is cooled by thermal conduction through the thickness of the hoop restraint.A simple radiation shield will replace the inner bobbin after winding. We may benefit from the newly developed cooling techniques and materials in the coil and cryostat design.
Figure 3.4.6-3A reference figure shows a coil inside the cryostat
(4) Current leads and Chimney
A special chimney is needed to connect the coil and cryostat to the outside. Nearly 3000 ampere current is supplied through a pair of leads that are installed in the chimney, in order to minimize the heat leak and Joule heat generation, silver added copper or high TC material will be used for the leads. Inside the chimney, there should be a structure to let evaporating cold helium gas come out through the gap to cool the leads efficiently.
(5)Monitoring
Various types of monitoring devices are needed in the magnettomeasure a variety of temperatures, gas pressures, strains, the liquid helium level, the coil tap voltage and the vacuum pressure.
3.4.1.4 The Cryogenics
The BESIII detector cryogenic system consists of the helium refrigerator/liquefier, liquid and gas transfer lines, liquid and gas storage, and a nitrogen system. Its detailed heat loads will be calculated in R&D. The approximately estimation is 50l/l50W. The major components include: the compressors, oil removal systems, cool box and control system. All the helium supplied by this system, except for normal leakage and necessary venting, is circulated and reliquefied. The nitrogen system supplies liquid nitrogen. Possible liquid nitrogen usage includes the cooling of the transfer line thermal shield, the cooling of the thermal shield and thermal intercepts in the superconducting solenoid magnet, and the cooling of the high pressure helium feed stream in the refrigerator/liquefier, and ancillary uses. Allof the nitrogen supplied by this system vents to the atmosphere after use,major components include the storage dewar and subcooler.
Figure 3.4.6-4 Typical configuration of the Cryogenics System
3.4.1.5 The Power Supply and Protection Systems
The function of the power supply and protection systems are to supply a controlled current to the coil, to detect magnet quenches, and to dissipate the energy stored in the magnet during normal discharge and emergency dump situations. As the operating current is about 3000A, the stored energy of the magnet is about 10MJ. The power supply system will provide an l0V dc, 3000A current. Besides the power supply, this system also includes a voltage filter, a high-current buswork, a magnet discharge resistor, a charge/discharge switch, a quench resistor, redundant dc current interruptors and local controls. The protection system consists of magnet sensors, instrumentation modules and a local control station. The high current buswork runs from the power supply to the magnet, it needs a cooling system, coo1ed by de-ionized water or by air.
3.4.1.6 Field Mapping
A field-mapping device must be constructed to measure the magnet precisely, this can be readily reached by the current techniques, using precisional machining, air bearing, high precision sensors, and etc. The characteristics of the mapping device are shown below.
Design field 1.2T
Size Length 3.2m, diameter 2m
Measuring accuracy 0.2%
Step Axial 50㎜, radial 50㎜, angle 5
Positioning accuracy Axial<±0.5㎜,radial<±0.25㎜,angle 3’
Speed 3000 points per day
3.4.1.7 Iron yoke
The BESIII magnet consists mainly of three parts: a superconducting coil, a vacuum tank and the magnet yoke. The solenoid produces an axial field whereas the yoke is responsible for the return of the magnetic flux. Due to the general request of the Muon detector, the yoke is split into the barrel and two endcaps. The barrel yoke includes several layers of steel plates with octagon configuration.
The material for the yoke will be low-carbon steel. This is the material of choice for its magnetic properties, however it has relatively low strength, a balance between good magnetic properties and moderate strength has to be achieved.
The fabrication process is very dependent on the design, and we should design special assembly tools and supporting system.
3.4.1.8 Magnet R&D
During R&D, we will finish the magnetic filed design, ensure the iron yoke structure, the solenoid design, and caculate the distribution of the magnetic field in 3-D.
Several techniques need detail research, include coil material selection, coil structure, coil winding, coil cooling, and quench protection. We will also investigate the mapping device, power supply, install method.
It's desirable to build a superconducting solenoid magnet for BESIII based on wide international collaborations.
3.4.1.9 Milestone
Contents / Time（year）R&D / 1
Iron Yoke(in Parallel) / 2
Coil、Cryostat(in Parallel)
Field Mapping Device(in Parallel)
Power system(in Parallel)
Integration & Test / 0.5
Installation & Mapping / 0.5
Total / 4
3.4.1.10 Cost Estimate（Chinese RMB Yuan）
1）R&D：240 K
2）Coil, cryostat and chimney：2,100K
3）Cryogenics, including control：1,100 K
4）Magnetic measurements：180 K
5）Power supply, protection：240K
6）Liquid and dower: 180 K
7）Iron flux return yoke：550 K
8）Miscellaneous(jacks etc.)：60 K
9）Monitor：25 K
10）Lab. restoration：25 K
11）Intercommunion & Others：35 K
12）Contingency(10%)：480K
Total：5,215 K。
References:
1 北京谱仪研制报告,中国科学院高能物理所，1989年3月
2 Y.Doi et al., A 3T Superconducting Magnet for the AMY Detector, NIM,1989
3 A.Yamamoto et al., Performance of the TOPAZ Thin Superconducting Solenoid Wound with Internal Winding Methods, Japanese Journal of Applied Physics, 1986
4 低温物理实验的原理和方法，阎守胜等，科学出版社，1985
5 超导磁体系统的稳定化，焦正宽等译，国防工业出版社，1992年8月
6 The European Physical Journal C,Review of Particle Physics,Volume 3,1998