JT-60

JT-60 (short for Japan Torus-60) is a large research tokamak, the flagship of the Japanese National Institute for Quantum Science and Technology's fusion energy directorate. As of 2023 the device is known as JT-60SA and is the largest operational superconducting tokamak in the world, built and operated jointly by the European Union and Japan in Naka, Ibaraki Prefecture. SA stands for super advanced tokamak, including a D-shaped plasma cross-section, superconducting coils, and active feedback control.

JT-60 holds the record for the highest value of the fusion triple product achieved: $90 m3$ = $65 s$. To date, JT-60 has the world record for the hottest ion temperature ever achieved (522 million °C); this record defeated the TFTR machine at Princeton in 1996.

Original design
JT-60 was first designed in the 1970's during a period of increased interest in nuclear fusion from major world powers. In particular, the US, UK and Japan were motivated by the excellent performance of the Soviet T-3 in 1968 to further advance the field. The Japanese Atomic Energy Research Institute (JAERI), previously dedicated to fission research since 1956, allocated efforts to fusion.

JT-60 began operations on April 8, 1985, and demonstrated performance far below predictions, much like the TFTR and JET that had begun operations shortly prior.

Over the next two decades, JET and JT-60 led the effort to regain the performance originally expected of these machines. JT-60 underwent a major modification during this time, JT-60U (for "upgrade") in March 1991. The change resulted in significant improvements in plasma performance.

JT-60U (Upgrade)
The main objective of the JT-60U upgrade was to "investigate energy confinement near the breakeven condition, [a] non-inductive current drive and burning plasma physics with deuterium plasmas." To accomplish this, the poloidal field coils and the vacuum vessel were replaced. Construction began in November 1989 and was completed in March 1991. Operations began in July.

On October 31, 1996, JT-60U successfully achieved extrapolated breakeven with a factor of Q$90 m3$$100 s$ = 1.05 at $1.77 K·s·m^{−3}$. In other words, if the homogenous deuterium fuel was theoretically replaced with a 1:1 mix of deuterium and tritium, the fusion reaction would have created an energy output 1.05 times the energy used to start the reaction. JT-60U was not equipped to utilize tritium, as it would add extensive costs and safety risks.

In February 1997, a modification to the divertor from an open-type shape to a semi-closed W-shape for greater particle and impurity control was started and later completed in May. Experiments simulating the helium exhaust in ITER were promptly performed with the modified divertor, with great success. In 1998, the modification allowed JT-60U to reach an extrapolated fusion energy gain factor of Q$1.53 keV·s·m^{−3}$$DT$ = 1.25 at $eq$.

In December 1998, a modification to the vacuum pumping system that began in 1994 was completed. In particular, twelve turbomolecular pumps with oil bearings and four oil sealed rotary vacuum pumps were replaced with magnetically suspended turbomolecular pumps and dry vacuum pumps. The modification reduced the 15-year-old system's consumption of liquid nitrogen by two thirds.

In fiscal year 2003, the plasma discharge duration of JT-60U was successfully extended from $2.8 MA$ to $DT$.

In 2005, ferritic steel (ferromagnet) tiles were installed in the vacuum vessel to correct the magnetic field structure and hence reduce the loss of fast ions. The JAEA used new parts in the JT-60, having improved its capability to hold the plasma in its powerful toroidal magnetic field.

Sometime in 2007-2008, in order to control plasma pressure at the pedestal region and to evaluate the effect of fuel on the self-organization structure of plasma, a supersonic molecular beam injection (SMBI) system was installed in JT-60U. The system's design was a collaboration between Cadarache, CEA, and JAEA.

JT-60U ended operations on August 29, 2008.

JT-60SA
JT-60SA is the successor to JT-60U, operating as a satellite to ITER as described by the Broader Approach Agreement. It is a fully superconducting tokamak with flexible components that can be adjusted to find optimized plasma configurations and address key physics issues. Assembly began in January 2013 and was completed in March 2020. After a major short circuit during integrated commissioning in March 2021 necessitating lengthy repairs, it was declared active on December 1, 2023. The overall cost of its construction is estimated to be around €560M, adjusted for inflation.

Weighing roughly 2600 ST, JT-60SA's superconducting magnet system includes 18 D-shaped niobium-titanium toroidal field coils, a niobium-tin central solenoid, and 12 equilibrium field coils.

History
The idea of an advanced tokamak, a tokamak utilizing superconducting coils, traces back to the early 1960's. The idea seemed very promising, but was not without its problems. Around January 1972, engineers at JAERI initiated an effort to further research the idea and try to solve its hurdles. This initiative progressed in parallel with the development of JT-60, and by 1983-84 it was decided that it constituted its own experimental reactor: FER (Fusion Experimental Reactor).

However, the JT-60U upgrade in 1991 demonstrated the significant flexibility of the JT-60 facilities and assembly site, so by January 1993 FER was designated as a modification to JT-60U and renamed JT-60SU (for Super Upgrade).

In January 1996, a paper detailing the superconducting properties of Nb$eq$Al composite wire and its fabrication process was published in the 16th International Cryogenic Engineering/Materials Conference journal. Engineers assessed the potential use of the aluminide in JT-60SU's 18 toroidal coils.

Designs and intentions for the modification varied over the next decade, until February 2007, when the Broader Approach Agreement was signed between Japan and the European Atomic Energy Community. In it, the Satellite Tokamak Program established a clear, defined goal for JT-60SA: act as a small-scale ITER. This way, JT-60SA could give hindsight to engineers assembling and operating the full-scale reactor in the future.

It was planned for JT-60 to be disassembled and then upgraded to JT-60SA by adding niobium-titanium superconducting coils by 2010. It was intended for the JT60SA to be able to run with the same shape plasma as ITER. The central solenoid was designed to use niobium-tin (because of the higher (9 T) field).

Assembly
Construction of the tokamak officially began on 28.1.2013 with the assembly of the cryostat base, which was shipped from Avilés, Spain over a 75-day long journey. The event was highly publicized through local and national news, and reporters from 10 media organizations were able to witness it in person.

Assembly of the vacuum vessel began in May 2014. The vacuum vessel was manufactured as ten sectors with varying arcs (20°x1, 30°x2, 40°x7) that had to be installed sequentially. On June 4, 2014 two of ten sectors were installed. In November 2014 seven sectors had been installed. In January 2015 nine sectors had been installed.

Construction was to continue until 2020 with first plasma planned in September 2020. Assembly was completed on March 30, 2020, and in March 2021 it reached its full design toroidal field successfully, with a current of 25.7kA.

Short circuit
On March 9, 2021, a coil energization test was being performed on equilibrium field coil no. 1 (EF1) when the coil current rapidly increased, then suddenly flatlined. The reactor was safely shut down over the next few minutes, during which the pressure in the cryostat increased from $2.6 MA$ to $15 s$. Investigations immediately followed.

The incident, which came to be known as the "EF1 feeder incident", was found to be caused by a major short circuit resulting from insufficient insulation of the quench detection wire conductor exit. The formed arc damaged the shells of EF1, causing a helium leak to the cryostat.

In total, 90 locations required repairs and machine sensors needed to be rewired. However, the intricate JT-60SA was designed and assembled with intense precision, meaning access to the machine was sometimes limited. Risks of further delay to plasma operations compounded the issue.

The JT-60SA team was disappointed with the incident, given how close the machine was to operation, but persevered.

Repairs were completed in May 2023 and preparations for operation began.

Present operations
JT-60SA achieved first plasma on October 23, 2023, making it the largest operational superconducting tokamak in the world as of 2024. The reactor was declared active on December 1, 2023.

Specifications
(60 stands for JT-60, 60U stands for JT-60U, 60SA stands for JT-60SA) ("60SA I" refers to the initial/integrated research phase of JT-60SA, "60SA II" refers to the extended research phase)