Detector Solenoid and Iron Structure

Transcription

1 Chapter 11 Detector Solenoid and Iron Structure 11.1 Iron Yoke TheironstructureoftheBelledetector servesasthereturnpathofmagneticfluxandanabsorber material for KLM. 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. 11.2, 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 2) tons for the barrel yoke and end-cap yokes, respectively Solenoid Magnet A superconducting solenoid provides a magnetic field of 1.5 T in a cylindrical volume of 3.4 m in diameter and 4.4 m in length [1]. The coil is surrounded by a multi layer structure consisting of iron plates and calorimeters, which is integrated into a magnetic return circuit. The main coil parameters are summarized in Table The overall structure of the cryostat and the schematic drawing of the coil cross section are shown in Fig Cryogenic System A fundamental flow diagram of the Belle II cryogenic system is shown in Fig. 11.6, where most of the hardware like the compressor, the refrigerator or the sub-cooler, will be reused since the heat load onto the solenoid is same in the new system. In a viewpoint of efficiency, the operation condition shouldbeimprovedas folows. Inthenormalmode, aheaterpower of120 Wis supplied steadily into the liquid helium buffer tank in the sub-cooler to offset redundant cooling power. This corresponds to almost half of the refrigeration capacity of 240 W, which is produced by consuming an electric power of 250 kw. Power saving operation has been developed for J-PARC neutrino cryogenic system[2]. The improved cryogenic operation will be realized for Belle II to save running cost. A field map of 100,000 points was made for a period of a month[3]. Fig shows a contour plot of fields measured inside the tracking volume for the nominal magnet settings. The field strength is shown in Fig as a function of z for various radii. The tracking performance was evaluated with the reconstructed mass peak of J/ψ particles from B meson decays. The fitted 337

2 Table 11.1: Main parameters of the iron structure. Items Parameters Belle Iron Yoke Height 9.57 m Beam level 5.72 m Total weight kn Barrel yoke Shapes octagonal Material S10C iron Height 7.7 m Width 7.7 m Length 4.4 m Total Weight 6240 kn Number of iron plates 15 Thinckness of iron plate 47 mm Thicknesss of gap 44 mm End Yoke Material S10C iron Height 7.7 m Width 7.7 m Length in beam direction 1321 mm Total Weight 5254 kn Number of iron plates 15 Thinckness of iron plate 47 mm Thicknesss of gap 44 mm 338

3 Table 11.2: 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 t Effective cold mass 6 t Length 4.41 m Coil Effective radius 1.8 m Length 3.92 m Conductor dimensions 3 33 mm 2 Superconductor NbTi/Cu Stabilizer % 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 339

4 Service port Chimney Cooling Tube Support Cylinder Superconductor with Al Stabilizer Cryostat Pure Al strip 3 1 (a) Outlook of the magnet (b) Cross sectional view of the coil Figure 11.1: An outlook of the solenoid and the cross sectional view of the coil. value of the peak mass was GeV/c 2 compared with the established value of GeV/c 2. Thus, the uncertainty in the absolute calibration of the present measurement is estimated to be approximately 0.25% Possible rotation of the Belle structure At the time of writing, there is a possiblity for superkekb to have a different IR configuration from the current one with respect to the Belle solenoid axis. In the current IR, the axis is aligned to the LER direction which is tilted by 22 mrad from the KEKB tunnel axis as shown in Fig.??. For the new configuration, the whole Belle structure with its weight of 1500 ton might be aligned to the median line of LER and HER direction by rotating the whole Belle structure of 1500 ton total weight by several tens mili-radians. In the technical view point the rotation could be made on the floor level with some specail hydraulic jacks and rollers to be inserted under the platform on whcih the current strucutre are mounted as depicted in Fig.??. The decision will be made during the optimaization of the accelerator design. 340

5 Figure 11.2: Barrel part of the iron yoke. 341

6 Figure 11.3: The iron endyoke. 342

7 0.02 Tesla/ line r(mm) Tesla QCS-L QCS-R z(mm) Figure 11.4: Contour plot of magnetic fields measured in the Belle coordinate system with the origin at the interaction point. 343

8 Bz (KGauss) r = 0 cm r = 50 cm r = 80 cm Br (KGauss) Figure 11.5: Field strength as a function of z for r = 0, 50, and 80 cm. 344

9 Figure 11.6: A flow diagram of cryogenic system. 345

10 Figure 11.7: Plan view of the IR and Tsukuba experimental hall. 346

11 Figure 11.8: Belle supporting structures 347

12 References [1] Y. Makida et al., Development of superconducting solenoid magnet system for the B-factory detector (BELLE), in ADVANCES IN CRYOGENIC ENGINEERING, VOL 43 PTS A AND B, edited by Kittel, P,, ADVANCES IN CRYOGENIC ENGINEERING Vol. 43, pp , 233 SPRING ST, NEW YORK, NY USA, 1998, PLENUM PRESS DIV PLENUM PUBLISHING CORP, Joint Cryogenic Engineering / International Cryogenic Materials Conference, PORTLAND, OR, [2] T. Ogitsu et al., IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 15, 1175 (2005), 2004 Applied Superconductivity Conference, Jacksonville, FL, OCT 03-08, [3] N. Tan et al., IEEE TRANSACTIONS ON NUCLEAR SCIENCE 48, 900 (2001). 348