JT-60
Japan Torus-60
Device typeTokamak
LocationNaka, Ibaraki Prefecture, Japan
AffiliationJapan Atomic Energy Agency
Technical specifications
Major radius3.4 m (11 ft)
Minor radius1.0 m (3 ft 3 in)
Plasma volume90 m3
Magnetic field4 T (40,000 G) (toroidal)
Discharge duration65 s
History
Year(s) of operation1985–2010
Preceded byJFT-2M
Succeeded byJT-60SA
Related devicesTFTR
Links
Websitewww.qst.go.jp/site/jt60-english/
JT-60SA
Japan Torus-60 Super Advanced
Device typeTokamak
LocationNaka, Ibaraki Prefecture, Japan
AffiliationQST + F4E
Technical specifications
Discharge duration100 s
History
Date(s) of construction2013 - 2020
Year(s) of operation2023–present
Preceded byJT-60U
Related devicesITER
Links
Websitewww.jt60sa.org/wp/

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,[1] built and operated jointly by the European Union and Japan in Naka, Ibaraki Prefecture.[2][3] SA stands for super advanced tokamak, including a D-shaped plasma cross-section, superconducting coils, and active feedback control.

As of 2018, JT-60 holds the record for the highest value of the fusion triple product achieved: 1.77×1028 K·s·m−3 = 1.53×1021 keV·s·m−3.[4][5] 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.[6]

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,[7] 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.[8] 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.[9] Operations began in July.[10]

On October 31, 1996, JT-60U successfully achieved extrapolated breakeven with a factor of QDTeq = 1.05 at 2.8 MA. 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.[11][12][13] 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 QDTeq = 1.25 at 2.6 MA.[14][15][16]

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.[17]

In fiscal year 2003, the plasma discharge duration of JT-60U was successfully extended from 15 s to 65 s.[18]

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.[19][20] 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.[21]

JT-60U ended operations on August 29, 2008.[22]

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.[23] 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 €560000000, adjusted for inflation.[24]

Weighing roughly 2,600 short tons (2,400 t),[25] 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.[26] This initiative progressed in parallel with the development of JT-60,[27] and by 1983-84 it was decided that it constituted its own experimental reactor: FER (Fusion Experimental Reactor).[28]

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).[29]

In January 1996, a paper detailing the superconducting properties of Nb3Al composite wire and its fabrication process was published in the 16th International Cryogenic Engineering/Materials Conference journal.[30] Engineers assessed the potential use of the aluminide in JT-60SU's 18 toroidal coils.[31]

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.[32] 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.[4][33] It was intended for the JT60SA to be able to run with the same shape plasma as ITER.[33]: 3.1.3  The central solenoid was designed to use niobium-tin (because of the higher (9 T) field).[33]: 3.3.1 

Assembly

Construction of the tokamak officially began on 28 January 2013 with the assembly of the cryostat base, which was shipped from Avilés, Spain over a 75-day long journey.[a] The event was highly publicized through local and national news, and reporters from 10 media organizations were able to witness it in person.[34]

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.[35] Assembly was completed on March 30, 2020,[36] and in March 2021 it reached its full design toroidal field successfully, with a current of 25.7kA.[37]

Short circuit

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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 10×10−3 Pa to 7000 Pa. 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.[38]

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.[39]

Present operations

JT-60SA achieved first plasma on October 23, 2023, making it the largest operational superconducting tokamak in the world as of 2024.[1] The reactor was declared active on December 1, 2023.[40]

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)

Plasma[41][42][43]
Volume Current Major radius Minor radius Aspect ratio Height Pulse length Elongation Triangularity
60 2.1 MA - 2.6 MA 3 m 0.85 m - 0.95 m 3.52 - 3.15 5 s
60U 90 m3 3 MA 3.4 m 1 m 3.4 1.5±0.3 m 65 s 1.5±0.3
60SA I 5.5 MA 2.97 m 1.17 m 2.54 2.14 m 100 s 1.83 0.50
60SA II 5.5 MA 2.97 m 1.18 m 2.52 2.28 m 100 s 1.93 0.57
Vacuum Vessel[44][42]
Material Baking temp. One-turn resistance
60 Inconel 625 500 °C > 1.3 mΩ
60U Inconel 625 300 °C 0.2 mΩ
60SA SS 316L 200 °C 16 µΩ
Toroidal Field Coils[42]
# Turns Material Coil current Inductance Resistance Time constant
60 18 1296 52.1 kA 2.1 H 84 mΩ 25 s
60U 18 1296 AgOFCu 52.1 kA 2.1 H 97 mΩ 21.65 s
60SA

Notes

  1. ^ The ship IYO (IMO number: 9300879) routed through the Panama Canal

References

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  31. ^ 青柳, 哲雄 (1997). 定常炉心試験装置の設計研究,第5編; 電源設備 [Design and research of constant furnace core test device, Part 5; Power supply equipment] (Report) (in Japanese). 日本原子力研究開発機構. doi:10.11484/jaeri-research-97-010.
  32. ^ European Commission. Directorate General for Energy. (2020). Broader approach :cutting edge fusion energy research activities. LU: Publications Office. doi:10.2833/62030. ISBN 978-92-76-16659-7. Archived from the original on 2024-02-25. Retrieved 2024-02-05.
  33. ^ a b c "JAEA 2006-2007 annual report". Archived from the original on 2013-01-06. Retrieved 2016-02-16. 3.1.3 Machine Parameters : A bird's eye view of JT-60SA is shown in Fig. I.3.1-1. Typical parameters of JT-60SA are shown in Table I.3.1-1. The maximum plasma current is 5.5 MA with a relatively low aspect ratio plasma (Rp=3.06 m, A=2.65, κ95=1.76, δ95=0.45) and 3.5 MA for an ITER-shaped plasma (Rp=3.15 m, A=3.1, κ95=1.69, δ95=0.36). Inductive operation with 100s flat top duration will be possible within the total available flux swing of 40 Wb. The heating and current drive system will provide 34 MW of neutral beam injection and 7 MW of ECRF. The divertor target is designed to be water-cooled in order to handle heat fluxes up to15 MW/m2 for long time durations. An annual neutron budget of 4x1021 neutrons is foreseen lots of detail on JT-60SA in section 3
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  43. ^ 核融合研究開発部門 (2011). "Annual report of Fusion Research and Development Directorate of JAEA for FY2008 and FY2009" (in Japanese). 日本原子力研究開発機構. doi:10.11484/jaea-review-2011-009. ((cite journal)): Cite journal requires |journal= (help)
  44. ^ Ishida, S.; Barabaschi, P.; Kamada, Y. (2011-09-01). "Overview of the JT-60SA project". Nuclear Fusion. 51 (9): 094018. Bibcode:2011NucFu..51i4018I. doi:10.1088/0029-5515/51/9/094018. ISSN 0029-5515. S2CID 122120186.

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