Magnetics Business & Technology - July/August 2020 - 18

RESEARCH & DEVELOPMENT

Figure 4. The GA NI HTS magnet out of the housing, showing the HTS leads (left). These connect to the Room Temperature busbar that is intercepted thermally at 40K by the Sumitomo RDK400B cryocoolers on the top of the housing (right).

Applications
At the start of GA's HTS magnet program, the goal was to
develop engineering for HTS magnets, demonstrate the critical elements of fabrication, and test magnet performance using
immersive cooling to 4K, without a specific application in mind.
However, a very appropriate application arose even before fabrication was started.
Developing materials capable of surviving the intense neutron
flux produced by fusion is one of the key challenges of associated with energy production from magnetic fusion reactors. There
are no existing materials that can withstand this neutron fluence
over the life of a commercial fusion power plant.
Researchers at the University of Wisconsin (UW) Fusion
Technology Institute have been developing a device known as
a gas dynamic trap (GDT) that is capable of simulating the fusion environment and can be used to qualify sufficiently robust
plasma-facing materials. The GDT concept was first developed
at the Budker Institute for Nuclear Physics in Russia. It comprises a long, axially symmetric, high-β, magnetic solenoid pinched
by high-field mirror magnets at both ends, inside which a fusion
plasma is confined. The UW machine, called the Wisconsin HTS
Axisymmetric Mirror (WHAM), will also have applications for
medical isotope production.
During the design phase of GA's HTS magnet program, researchers from UW approached GA regarding the development
of a small warm-bore magnet capable of producing a 6 Tesla
magnetic field on axis for the WHAM experiment. Though GA's
HTS magnet was engineered to deliver the requisite magnetic
field, the original immersive cooling method would not have
been compatible with the UW experiment. In part because of
the close alignment between the UW research and GA's overall
fusion program, GA decided to explore UW's experiment as an
application for the HTS magnet.

Magnetics Business & Technology *

July/August 2020

As an alternative to immersive cooling, GA engineers at the MTC
examined the possibility of a conductively cooled magnet that
would employ cryocoolers. With additional engineering design
and analysis, it was determined that conductive cooling was
feasible and would maintain the coil magnetic performance.
However, this required re-designing the magnet to handle the
high heat generation in the non-superconducting section of the
bus bars, given the high (700 amp) current. In the previous design, the heat generated was extracted with immersive cooling,
and switching to conductive cooling with cryocoolers presented a
challenge because of their limited cooling capacity.
To have sufficient cooling power, two cryocoolers RDK415D and
RDK400B were selected to extract the conductive and electrical heat losses. Both cryocoolers and their compressors were
provided by Sumitomo Heavy Industry (Figure 4). Other design
changes were made to improve the thermal coupling of the leads
to provide the requisite cooling.
The completed GA HTS magnet with conductive cooling was
tested at 4K in the fall of 2019. Figure 5 shows the results of one
operational cycle. The magnet current was increased stepwise at
rates of 0.3 A/s up to 500 A and 0.1 A/s from 500A-700 A.

Figure 5. Test results for the GA HTS Magnet.

www.MagneticsMag.com

http://www.MagneticsMag.com

Magnetics Business & Technology - July/August 2020

Table of Contents for the Digital Edition of Magnetics Business & Technology - July/August 2020