Iron Yoke

The iron structure of the Belle detector serves as the return path of magnetic flux and an absorber material for KLM [4,6]. It also provides the overall support for all of the detector components. It consists of a fixed barrel part and movable end-cap parts, both on the base stand. The barrel part, shown in Fig. , consists of eight KLM blocks and 200-mm thick flux-return plates surrounding the outermost layers of of the KLM blocks. Neighboring KLM blocks are joined using fitting blocks. Each end-cap part can be retracted for an access to the inner detectors.
The weight of the iron yoke is 608 and 524 (= 262 $\times$ 2) tons for the barrel yoke and end-cap yokes, respectively.

Table: Main parameters of the solenoid coil.

Items	Parameters
Cryostat
Radius: outer/inner	2.00 m/1.70 m
Central field	1.5 T
Total weight	23
Effective cold mass	$\sim$ 6
Length	4.41 m
Coil
Effective radius	1.8 m
Length	3.92 m
Conductor dimensions	3 $\times$ 33 mm$^2$
Superconductor	NbTi/Cu
Stabilizer	99.99 % aluminum
Nominal current	4400 A
Inductance	3.6 H
Stored energy	35 MJ
Typical charging time	0.5 h
Liquid helium cryogenics	Forced flow two-phase
Cool down time	6 days
Quench recovery time	1 day

Figure: An outlook of the solenoid and the cross sectional view of the coil.

Figure: Barrel part of the iron yoke.