Conceptual design of the superconducting magnet for the 250 MeV proton cyclotron
© The Author(s). 2016
Received: 15 February 2016
Accepted: 12 May 2016
Published: 23 May 2016
The superconducting cyclotron is of great importance to treat cancer parts of the body. To reduce the operation costs, a superconducting magnet system for the 250 MeV proton cyclotron was designed to confirm the feasibility of the superconducting cyclotron.
The superconducting magnet system consists of a pair of split coils, the cryostat and a pair of binary high temperature superconductor current leads. The superconducting magnet can reach a central magnetic field of about 1.155 T at 160 A. The three GM cryocooler with cooling capacities of 1.5 W at 4.5 K and 35 W at 50 K and one GM cryocooler of 100 W at 50 K were adopted to cool the superconducting magnet system through the thermosiphon technology.
The four GM cryocoolers were used to cool the superconducting magnet to realize zero evaporation of the liquid helium.
Cancer is one of the main causes of death in the world today (Siegel et al. 2014). In the field of cancer treatment, proton beams often offer an improved dose distribution compared with the commonly used photon and electron beams, and thus enable dose escalation while sparing normal tissues (Schulz-Ertner and Tsujii 2007; Ma 2009). The proton beam therapy has the unique merits mentioned above that makes it particularly attractive for the treatment of pediatric cancers, cancers in the eye, cancer of skull base, and spine cancer (Efstathiou et al. 2013; Levin et al. 2005). The 250 MeV superconducting cyclotron for proton therapy is being designed due to the advantages with high magnetic field, low operation costs and more compactness compared with the conventional magnet technology (Kang et al. 2010; Newhauser and Zhang 2015; Choi et al. 2010). The design goals of the superconducting cyclotron include high reliability, low activation, easy maintenance and easy to use. To confirm the feasibility of the superconducting cyclotron, a superconducting magnet for the 250 MeV cyclotron is being designed to evaluate the electromagnetic and cryogenic properties. The superconducting magnet system consists of a pair of split coils, cryostat, a pair of HTS current leads, and four GM cryocoolers. The cryogenic system needs to be designed to realize zero evaporation of the liquid helium.
In this paper, the design of the superconducting magnet system is described. Also, the electromagnetic and thermal performance of the superconducting magnet and the thermal characteristics of the cryogenic system are analyzed.
Description of the superconducting coils
Design parameters of the superconducting magnet
Strand dimension (mm)
1.80 × 1.20
1.80 × 1.20
Inner radii (mm)
Outer radii (mm)
Operating current (A)
Stored energy (MJ)
Central field (T)
When energized, a large electromagnetic force and stress are generated in the superconducting magnet. To improve the mechanical stability of the superconducting magnet, a pretension of 80 MPa was exerted on the coils during winding (Chen et al. 2010).
Cryostat design of the 250 MeV superconducting cyclotron
The cryogenic system of the superconducting magnet system is composed of the cryostat, GM cryocooler, coldbox, and magnet feeder. The GM cryocooler was used to cool the superconducting magnet system through thermosiphon technology. The three GM cryocoolers with cooling capacity of 1.5 W at 4.2 K and 35 W at 50 K are located at the top of the cooling cryostat to recondense the helium gas from the superconducting coil cryostat. The fourth GM cryocooler with 100 W at 50 K was used to cool the thermal shield and the other structure. The flexible copper braids were used to connect the cold head of the GM cryocoolers to the conduction structure in order to avoid any damage due to thermal contraction.
To reduce the heat load from the residual gases, the vacuum pressure should be below 2 * 10−4 Pa.
In order to reduce the radiation heat load, an aluminum foil with a thickness of 20 μm has been applied to the surface of the coil case.
40 Layers of superinsulation material is required to reduce the radiation heat.
The Al 1100 material of 6 mm in thickness with high thermal conductivity was selected as the thermal radiation shield for the cryostat. The maximum temperature of the thermal shield should not exceed 60 K. The six copper tubes as cooling channels were welded on the surface of the thermal shield.
The 12 tie rods with Carbon fireglass material were adopted to support the cold mass of the superconducting coils and to reduce the thermal conduction. The support structure for the cold mass is composed of eight longitudinal and four radial tie rods.
Pre-cooling the superconducting magnet can be accomplished through two different methods. In the first option, approximately 1000 L of liquid nitrogen is used to cool the magnet to 77 K. Subsequently a combination of nitrogen gas and helium gas is used to exhaust the liquid nitrogen. Finally, liquid helium is admitted into the cryostat. In the second option, only helium gas, cooled by the cryocooler is circulated through the superconducting magnet. The second option avoids the challenge of removing all the nitrogen, but it requires a longer time.
The calculated heat load from the thermal shield and support structure is about 30 W at 60 K. The heat load from the HTS current leads is below 0.3 W at 4.2 K and 25 W at 60 K, which will be described in the next section. The maximum heat load from proton beam losses is below 2.0 W at 4.2 K. The heat loads from the proton losses are not a real source in the present study, which can be simulated with the heater during the magnet operation. Therefore, there are sufficient cooling capacity with four GM cryocoolers of 4.5 W at 4.2 K and 205 W at 50 K to cool the superconducting magnet system to realize zero evaporation of the liquid helium.
Design of a pair of binary HTS current leads
To reduce the heat leak from the room temperature to the liquid helium temperature, a pair of binary HTS current leads were adopted. The binary current leads consists of two parts, i.e., the warm end section and the cold end section. The OFHC copper material with RRR of about 50 was adopted for the warm end to connect the power supplies. To gain enough safety margin, the maximum allowable current density of the OFHC copper is limited to 10 A/mm2. The four Bi-2223/Ag–Au HTS tapes were adopted to reduce the heat load for the cold end. The stacks of Bi-2223/Ag–Au tapes need to be mechanically supported by stainless steel tube for the cold end section. The stainless steel can be used as a current shunt during a quench due to its larger heat capacity.
Design parameters of a pair of HTS current leads
Maximum operating current (A)
Ground insulation (V)
4 × Bi-2223/Ag–Au
Joint resistance between HTS and NbTi/Cu (μΩ)
Operating temperature (K)
Heat load (W)
<0.3 @4.2 K
<25 @60 K
The temperature sensor of Pt was adopted to monitor the operating temperature of the HTS. The HTS should be operated at a temperature below 70 K. The quench protection system for the HTS element is activated if the operating temperature exceeds 90 K, or the threshold voltage of the HTS element exceeds 2 mV.
Quench protection design of the 250 MeV superconducting cyclotron
The superconducting magnet is powered with a power supply. The stored energy of the superconducting magnet is about 4.406 MJ. To protect the superconducting magnet against damage during a quench, the appropriate quench protection is required. The quench protection circuit needs to be designed to limit the quench hot spot temperature and the quench voltage. The two split coils are subdivided into 4 sections to limit the quench voltage. Each section is in parallel with a back-to-back diode and a dump resistor. To accelerate the quench propagation, the quench heater was adopted. As a further study, we will describe the quench protection design and the relevant quench analysis in detail in the following paper.
In this paper, the conceptual design of a superconducting magnet for the 250 MeV proton cyclotron has been described. The superconducting magnet consists of two split coils made of NbTi superconductor with a large stored energy of 3.84 MJ. The relevant performance analysis of the superconducting magnet system were described. The thermal analysis shows that the superconducting magnet can realize zero evaporation of the liquid helium. Design of the superconducting magnet system is in progress and will be fabricated in future.
YR, XL and XG carried out the design and analysis of the superconducting cyclotron. YR, XL and XG helped to draft the manuscript. All authors read and approved the final manuscript.
This work was supported in part by the National Natural Science Foundation of China under (Grant No. 51406215), the Anhui Provincial Natural Science Foundation under Grant No 1408085QE90, by the National Magnetic Confinement Fusion Program of China (Grant Nos. 2014GB106000, 2014GB106003, and 2014GB105002) and by the Science Foundation of Institute of Plasma Physics, Chinese Academy of Sciences (Grant No. Y45ETY2305). The views and opinions expressed herein do not necessarily reflect those of the CFETR Organization and EAST Team.
The authors declare that they have no competing interests.
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