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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 over the tracking volume. Particle detectors within
this volume will measure the trajectories of charged tracks emerging
from the co1lisions. Particle momentum is determined from the measured
curvature of these tracks in the fie1d. To support our physics goals,
the magnet tracking systems must meet the following performance
requirements: the expected momentum 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 must pass through
the superconducting coil and cryostat before impacting the Moun counter,
the Moun counter performance requires the material in the coil to be
minimized in terms of radiation length (X0). In practice, this
requirement must be ba1anced against the overall requirement that the
superconducting coil must be robust, and have excellent longterm
operational reliability.
General parameters of the BESIII detector solenoid are summarized
in Table 3.4.6-1.
Table 3.4.6-1 Parameters of the BESIII solenoid Coil
Cryostat
Inner radius
Outer radius
Length
1.0m
1.3m
3.4m
Coil
Effective radius
Length
Conductor dimension
Electrical parameters
Center field
Nominal current
Stored energy
Cool Down Time
1.15m
3.0m
3mm*33mm
1.2T
3000A
9MJ
≤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 3.4.6-2 Cross 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 cylinder and 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 3.4.6-3 A 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.
All of 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：2,100 K
3）Cryogenics, including control：1,100 K
4）Magnetic measurements：180 K
5）Power supply, protection：240 K
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%)：480 K
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