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We are introducing an effort to complete a conceptual design of the large solenoid magnet that can contain all trackers and calorimeters required by JLC experiments. In the present design the magnet has a overall length of 10 m, which is divided into three parts in axial direction, and its bore diameter is 9 m. The inductance of the coil is 5.5 H and, consequently, the magnet stores an energy of 1.1 GJ when it is operated at 20 kA to produce a central field of 2 Tesla.

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What's New?

Introduction

One of the most characteristic features of a JLC experiment is that all physics processes can be recognized in terms of known fundamental particles such as leptons, quarks and gauge bosons. To make a good use of this advantage for new physics searches, we need a detector which is capable of exclusively reconstructing all final state particles except neutrinos.
Reconstruction of W and Z via jet invariant masses is very important in order to use their large branching fractions into quarks. To improve the jet invariant mass, we rely primarily on the tracking information for charged particles, though neutral particles are only detectable by the calorimeter. In this scheme, a good track-cluster matching is essential to determine the event topology correctly.
To meet these requirements above, as a possible detector, we are considering a general purpose detector that allows precision tracking in a solenoid field, hermetic calorimetry, and high resolution vertex detection. The field strength of the solenoid is required to be 2 Tesla in the central tracking region.
The best way to realize harmetic calorimetry is to construct a large magnet that can be placed outside the calorimeter. This type also produces a uniform field with a well matched iron yoke, although the construction cost is consequently higher than that of the small magnet which can be placed inside the calorimeter. We are now studieng a technological feasibility for constructing such a large superconducting solenoid for the JLC experiment.

General Characteristics

The bore diameter is 9 m and the overall length is 10 m. We divide the magnet into three parts in order to simplify the assembly procedure and also provide a rigid support for the electromagnetic and hadron calorimeters. The supports are taken through the gaps between each coil.

The coil length of each part is 2.5 m and is equipped with its own separate cryostat.

Parameters of the superconductiong solenoid

The central part and both of the end parts consist of 257 and 278 turns of coaxial coil windings with double layers. This configuration is to compensate the field drop along the beam axis. The superconducting magnet requires a relatively thick iron yoke to ensure the 2 Tesla uniform field in the central tracking region. In order to keep the maximum field in the iron yoke below 2 Tesla, we need a total amount of 11,500 tons of iron. The overall inductance of the magnet is 5.5 H. Consequently, the total stored energy is 1.1 GJ when the magnet is operated at 20 kA to produce the 2 Tesla central field.
One of the most important issues in the design of the superconducting coil is the method of cooling. For the JLC detector magnet, a direct cooling system is applied with the flow of two-phase helium through a cooling path in the conductor. In this case, it is very important to reduce the pressure drop since the cooling path is very long. Three modules are cooled in parallel. Also, in our design, two shields cover the coil at 20 K and 80 K. The 20 K shield is essential to reduce the helium amount further by controlling the pressure drop to be less than 0.2 kg/cm2.

The cryostat is a vacuum chamber with superinsulation. Two shields at 20 K and 80 K are cooled by the helium gas flow from the refrigerator. The heat flow to each coil is 4 W/coil, resulting in 12 W in total. This corresponds to a liquid helium flow of 6 g/sec. It is expected to take a long time for initial cooldown since the path is very long. To reduce the cooldown time, we are considering extra cooling channels on the outer cylinder of the coil. The total heat load for this magnet at 4.2 K is estimated to be about 330 W. Therefore, a medium class refrigerator with a cooling power of 500 W (corresponding to electric power consumption of 500 kW) is enough to operate the magnet.

Conductor

Parameters of the superconductiong solenoid

The operation current and maximum temperature are 20 kA and 5.0 K, assuming that the helium pressure at the inlet is 0.3 kg/cm2 and the local temperature rise is 0.5 K. On the other hand, the critical current of the conductor is 40 kA at 5.0 K in a 3 Tesla field (50 kA at 4.2 K in the same field).
The coil is made of NbTi/Cu multifilamentary superconducting cables. The cable consists of 10200 filaments with 50Ám in diameter. The NbTi/Cu ratio is chosen to be 1/2 for easy mechanical handling. The cross section of the NbTi is conservatively designed so that the current density is 2500 A/mm2 at 4.2 K in 3 Tesla. The superconductor is soldered to Cu stabilizer in which the cooling path is placed. The diameter of the path is 14 mm and the length in one module is about 8 km. The overall NbTi/Cu ratio is 1/36 and the current density of this Cu part is about 25 A/mm2 when the magnet gets quenched. This current density is expected to be low enough, from a simple quench simulation, to keep the local maximum temperature rise less than 80 K, so that the magnet is safe from local mechanical stress.

Energy Extraction

The stored energy has to be safely extracted on encounter with a quench. In our magnet design, all of the three modules are connected to a single dump resistor of 0.1 W, which absorbs all of their stored energy even when only one of them gets quenched. This is to avoid unbalanced force between the coils and the iron yoke. The maximum voltage at the energy extraction is 2 kV between the current leads.

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matsui@jlcux1.kek.jp Apr 1, 1995