Technologies and Capabilities

Three different ways to cool a superconducting magnet:：Cryogen free, Small boil-off, zero boil-off.

Cryogen Free  There are 80% magnets we make are cryogen free magnet, utilizing GM or PT cryocooler. Using GM or PT cryocooler avoids the use of expensive liquid cryogens and can be convenience for customers to use.

Zero boil-off (helium bath)  Magnet is cooling by liquid helium, the boiling helium is recondensed using GM or PT cryocooler. The liquid helium will not loss during magnet operation. The cost of the magnet will be higher, because using liquid helium and cryocooler at the same time. And if the magnet is quench during operation, almost all the helium will be loss. This technique is widely using in MRI magnet and accelerator superconducting magnets. 

Small boil-off (helium bath)  Magnet is cooling by liquid helium. There is no Cryocooler in the system to recondense helium gas. So, there is a much lower level of vibration. Disadvantages is we have to fill up the system with liquid helium at regular intervals. And almost all the helium will be loss if the magnet is quench. This technique is widely using in NMR magnet and STM magnet. And the boil off rate is 50cc/h to 100cc/h.

Two traditional superconductors (NbTi and Nb3Sn) can be used to make high field magnet economically. At lower fields of up to 9.4 Tesla (T), niobium-titanium (NbTi) is the preferred superconducting material. At higher fields above 9.4T, Scientific Magnetics uses niobium-tin (Nb3Sn) wire which have much higher critical magnetic field. We can offer magnets up to 14T. Nb3Sn magnet is wound in a non-reacted state and must then undergo a heat treatment cycle in a vacuum furnace for many days.

After switch off the persistent current switch (superconducting switch), the magnet start to work in persistent mode. The stability of the magnet field is mainly depend on the resistance of the joint and the inductance of the magnet. So high quality Superconducting joints are essential for persistent-mode operation in a superconducting magnet to produce an ultra-stable magnetic field.

We are using the same superconducting joint technique as the MRI magnet. The stability we estimate is around 0.1ppm/h~10ppm/h;

*Test date: 1.5T MRI magnet contain 10 superconducting joints and the inductance is 26H, the field stability is 0.05~0.1ppm/h;

Homogeneity the magnetic field is a requirement for most systems to ensure that the field delivered by the magnet is the same across the entire sample. Different levels of uniformity are required and can be achieved. Generally, a simple split pair will have homogeneity levels of 1% over a region of a few millimeters. To improve the homogeneity, it is preferable to use thin wall solenoid magnets or Helmholtz coils, which will tend to have a better homogeneity of 0.1%. Further improvements can be made by adding several compensation coils to the solenoid. The MRI, NMR and FT-ECR magnet, the homogeneity is in ppm range.

Normally, the original field of MRI magnet is around 400ppm @45cm DSV due to manufacturing tolerances. we need using passive shim or active shim to correct the inhomogeneity less than 10ppm.

The vibration specification in some applications should be very strict. Various method or configuration to reduce the vibration. For example, the bellows，copper braids, pulse tube cryocooler(PT cryocooler), Gas-gap Unisock, etc.

The best way to reduce the vibration is not using the cryocooler, which will bring the vibration because it contains moving parts. So the vibration of small boil off magnet is lowest. But the helium will lost during operation, so the operation cost are significantly higher.

The vibration level of cryogen free magnet and zero-boil off magnet is much higher than small boil-off magnet because of using Cryocooler to cooling the magnet. However, the vibration level of PT cryocooler is much smaller than GM cryocooler. So, the PT cryocooler is widely used when we need lower vibration. Another way to reduce the vibration is using Gas-gap Unisock that the cryocooler in the Unisock is decoupled from the system. But this structure needs complex second support to hold the cryocooler.