A quench protection circuit is designed to protect the magnet from quench, in which ten coils are connected together in series with a 0.185 Ω by-pass dump resistor. A quench
simulation is preformed by connecting the three-dimensional thermal model presented in chapter 3 with an electrical circuit model, in which the pure aluminum strips are also taken into account. Despite the electrical and thermal conductivity of the aluminum stabilizer is degraded by the irradiation, the hot spot temperature of manget is suppressed below an allowable temperature of 200 K. Thereby, all coils in PCS is capable of being protected from magnet quench with the contributions of aluminum strips and a by-pass dump resistor.
105
Chapter 6
Prospect for Future Muon Beam Line
In previous chapters, we have discussed the irradiation effect on the thermal character-istics of conduction-cooled superconducting magnets for the COMET muon beam line, in which all magnets are wound with the aluminum-stabilized NbTi superconducting cable.
Owing to the degradation on thermal conductivity of thermal path, the temperature margin is vanished due to the nuclear heating for a long term operation, thus the super-conducting magnet has to be annealed before the magnet quench is occurred. However, for a future muon beam line, more muons, more compact or longer continuous beam operation will be required for the experiment such as PRISM [109] or muon collider [110].
To improve the thermal characteristics of a conduction-cooled superconducting magnet for future high intense muon beam line, a high temperature superconductor (HTS) based magnet could be a candidate.
At the first step, in order to examine the feasibility of a HTS based magnet, a design study is performed in this chapter for which the muon beam intensity of 109 µ−/sec can be obtained with a compact HTS solenoid coil. The conceptual design of a coil using a commercial ReBCO coated conductor introduced in Sec. 6.1. In Sec. 6.2, the thermal characteristic under the beam operation, including the estimation of radiation with PHITS simulation and thermal analysis, is described to validate the feasibility for which the HTS coil can be operated in a higher irradiation environment.
6.1 Conceptual Design
Aiming to achieve a muon beam intensity of 109 µ−/sec, a compact HTS test coil, which provides a magnetic field on target of 3 Tesla, is designed at the first step. As given in Fig. 6.1, this compact HTS coil, of which overall length is about 340 mm, are made from 34 double pancake coils with an inner diameter of 340 mm. Each pancake coil is wound with a commercial ReBCO coated conductor (SuperPower, SCS4050) of which the size is 0.1 mm thick and 4 mm wide with the insulation layer.
The ReBCO coated conductor consists of the total 40 µm thick copper stabilizer at top and bottom, about 4 µm thick silver overlayer, 1 µm thick ReBCO, about 0.2 µm
buffer stack and 50 µm thick substrate made of hastolly. The ReBCO coated conductor is insulated with a 25 µm polyimide. The number of turns in each coil is 166 turns along the transverse resulting in about 25 mm thick coil. In between each double pancake coil, the thermal path of a 1.9 mm thick pure aluminum sheet (RRR = 2000), which is thermally linked to an aluminum shell of 10 mm, is installed to remove the nuclear heating during the beam operation. The polyimide film with a thickness of 25 µm is employed as the ground insulation between aluminum sheet and double pancake coil.
ReBCO coated HTS tape (copyright holder: SuperPower)
c a
b
Figure 6.1: Schematic view of the compact pion capture solenoid made of the HTS based double pancake coils. ab-axis: direction as same as the current flow,c-axis: perpendicular to the surface of HTS tape. (copyright: IOP science [111])
The maximum field parallel to the conductor surface is about 3.5 Tesla and that vertical field to the conductor surface is about 2.5 Tesla as calculated in Fig. 6.2. The critical current of ReBCO coated conductor relies on the direction of magnetic field on the surface of conductor, in which the critical current is 700 A at 3.5 Tesla forB//ab-axis, and 330 A at 2.5 Tesla for B//c-axis as given in Ref. [112], therefore, the critical current is limited by the magnetic field parallel to the c-axis. Using a measured critical surface of the HTS tape made by SuperPower (SCS4050) for B//c-axis as shown in Fig. 6.3, an operation current of the HTS coil is 105 A resulting to a central field on target of about 3 Tesla corresponding to about 20% of the critical current. To thermally stabilize the HTS coil during the operation, the outer shell of the coil is conduction-cooled to 20 K because the thermal conductivity of pure aluminum as well as copper has a peak at this temperature regime, for which the HTS coil is expected to be very thermally stable. A pulse tube cryocooler (Sumitomo, RP-082B2S) may be a candidate because the cold head and valve unit can be separated so that the valve unit can be settled in a low radiation environment to avoid the radiation damage on cryocooler.
The production target is made of graphite with a length of 300 mm and a diameter of 40 mm. Between the production target and the HTS coil, a tungsten alloy radiation shielding with an outer diameter of 300 mm and maximum thickness of 50 mm is installed to shield the secondary particles. Proton beam should be tilted by 12 degree to be injected from a gap of the magnet system. The tilted injection has a benefit to result in more
Chapter 6. Prospect for Future Muon Beam Line
B [Tesla] Br [Tesla] Bz [Tesla]
340 mm 25 mm
Figure 6.2: Magnetic field calculated with an asymmetrical model. The coil has a length l of 340 mm, inner diameter of 340 mm and a thicknesstof 25 mm. The current density is calculated to 139 A/mm2 from the equation ofJ =nturn×nlayer×Iop/(t×l). The total magnetic field, that ofr component and that of z component are shown on left, middle and right.
pion yield by around 10%, while the secondary particles and effect of the irradiation have asymmetric distributions in the pion capture solenoid.