Design Of Cryogenic System And Current Leads For Mice Coupling Superconducting Magnets

The cooling capacity of current commercial cryocoolers can reach 1.5W at 4.2K. A superconducting magnet system with heat load of several watts at 4.2K may be cooled by using cryocoolers. There are some advantages compared with a conventional superconducting magnet system cooled using refrigerator systems. That is, the cryocooler cooled SC magnet system is compact, single, easy handling and maintenance, and especially low cost. The superconducting coupling solenoid magnet for the Muon Ionization Cooling Experiment (MICE) is to be cooled by cryocoolers.The MICE to be operated at Rutherford Appleton Lab in UK will provide the first demonstration of the muon ionization cooling technique, which is critical to the success of a future muon-based accelerator system. The superconducting coupling magnet is an essential part of the MICE muon cooling section, in which reduction of emittance by energy loss in hydrogen absorbers is followed by replenishment of energy in RF cavities.A pair of binary current leads composed of conventional copper leads and high temperature superconductors (HTS) connect the superconducting coupling coil at liquid helium temperature region with the power supply at room temperature. The application of HTS leads can largely reduce the heat leakage from room temperature to the liquid helium temperature, which makes the use of cryocoolers practicable for the MICE project.The optimization of conventional conduction-cooled metal leads was deduced in detail in this thesis. The thermal analysis using theoretical method and numerical simulation were carried out. The stray field around the HTS leads of the coupling coil was calculated, which determines the position of HTS leads. The study shows that the HTS leas can operated without magnetic shield when the magnetic field around the warm end of the HTS leads is lower than 0.4T. Hence, the warm end of the HTS leads for the coupling magnet should be located farther than 1.40m along the radial direction with respect to the magnet central axis. The effects of the warm end temperature of the HTS leads and directions of the magnetic field on the performance of the HTS leads were also analyzed and the orientations of the HTS leads in the stray field were suggested. The cooling connection methods for the HTS leads were studied. Presently, the aluminum nitride ceramic thin plates were proposed to be applied for the thermal connection and electrical insulation between the cooler cold heads and the HTS leads. This paper also presents a method that one can use the cold vapor from the He-condenser to protect current leads from burning out in the event of power failure.Based on the detailed analysis of the magnet system, the heat leak into the 60K cold mass and the 4.2K cold mass were calculated. The results show one PT-415 can supply the cooling capacity needed by the magnet system. In order to increase the safety margin of superconducting, it is necessary to minimize the temperature difference from the magnet to the cryocooler cold head at 4.2K. Since the thermal siphon system can transfer more heat at small temperature drop, it was studied and the optimization of the helium re-condenser was presented.Heat generated within the DC magnet is the AC loss at unsteady state. This paper discusses the magnet loss during the normal charging and the fast discharging of the coupling magnet. Sine the charge and the discharge times for the coupling magnet are designed long, the AC losses in the coil is dominantly hysteretic losses. The temperature distributions within the coupling magnet at various event scenarios were simulated by ANSYS. The maximum hot spot temperature within the magnet is about 5.063K, at which the coupling magnet can operate without quench. However, there must be extra liquid helium to cool the magnet.The above studies and results can provide theoretic guidelines for the engineering design of the MICE superconducting coupling solenoid magnet system.