JT-60, JT-60U, and JT-60SA: tracing fusion through generations of machines

JT-60, JT-60U, JT-60SA. Their names look similar, but they are not three separate power plants operating simultaneously. They represent a history of carrying research questions forward by modifying and replacing equipment at the Naka research site. From first plasma in 1985 to a superconducting machine, what changed and what remained? Following past achievements also changes how we view today's research and development.

Design of JT-60 began in 1975. The machine was completed on April 3, 1985, and first produced plasma on April 8. The Atomic Energy Commission's monthly report at the time records the testing sequence: individual components, connections between components, and then the entire facility. 1985 record of completion and first plasma

A tokamak combines magnetic fields in a toroidal space to confine high-temperature plasma. Plasma is a state in which electrons have separated from atoms and electrically charged particles move around. Completing the machine does not immediately enable high-performance experiments. Vacuum, power supplies, controls, and other systems are checked in sequence, with first plasma serving as the starting point for expanding operating conditions.

Reading this first success as “the start of electricity generation” misses the subsequent story. What began here was experimental investigation of the high-temperature plasma needed for a fusion reactor.

In 1989, JT-60 stopped operating for a higher-current upgrade, moving into the JT-60U research stage in 1991. This was not merely a name change: machine modifications enabled exploration of higher performance and new operating conditions. QST's research history

An ion temperature of 520 million degrees in 1996 and an energy gain factor of 1.25 in 1998 are representative achievements in that history. However, the temperature refers to ions in the plasma, not the entire vessel. Nor should records obtained under different conditions be combined into a single experiment that achieved everything simultaneously. JAEA's FY2008 performance report

In particular, 1.25 is a “D-T-equivalent” value. It was calculated using temperatures and densities obtained in JT-60 deuterium experiments to assess what would happen if some fuel were replaced with tritium. It does not mean fusion output actually produced with D-T fuel, or electricity generated after subtracting the entire facility's power consumption. QST's explanation of Q

JT-60 completed its experiments on August 29, 2008. Moving to JT-60SA required removing the main machine to make room for installing its successor. Research knowledge was carried forward, while the hardware underwent extensive changes.

According to JAEA's 2014 report, dismantling began in 2009 and finished in October 2012. It involved equipment activated by neutrons from deuterium experiments and structures totaling approximately 5,400 tonnes. A diamond wire saw that required no cooling water proved useful for cutting the vacuum vessel and other components into sections. Record of JT-60 dismantling

This process is industrial work that is difficult to see in photographs of a completed machine. Experience combining cutting, lifting, storage, and safety management also provides a foundation for building the next generation of research facilities.

The Japan-Europe JT-60SA collaboration replaced the machine with one using superconducting magnets. F4E's 2020 assembly-completion report explains that existing on-site infrastructure was reused while many major components were newly designed and manufactured. F4E assembly-completion report

Superconductivity is the disappearance of electrical resistance when conditions such as sufficiently cooling a material are met. It is an important technology for operating magnets for long periods. It does not mean that the plasma itself becomes cold.

Assembly finished in 2020, and first plasma was produced on October 23, 2023. F4E first-plasma announcement

QST groups the objectives into three areas: supporting ITER, complementary research toward a demonstration reactor, and training people. Developing operating methods that maintain high-pressure plasma for around 100 seconds is a research goal; its inclusion in a description does not establish that it has been achieved. JT-60SA objectives and specifications

Simply placing JT-60U's historical peak temperature beside JT-60SA's first-plasma values does not determine which generation is superior. Their configurations, heating conditions, and target durations differ. For the current plan, also see our JT-60SA explainer.

The editorial team's suggestion from this history is to ask “What stage is it at now?” as well as noting the machine's name. Construction emphasizes manufacture and installation; operation emphasizes diagnostics, maintenance, and improvements; replacement emphasizes dismantling and connections to existing facilities. Even at the same research site, the technologies required change.

Companies considering entry should clarify whether their products serve new equipment, upgrades to existing equipment, or replacement works. Then check the purchaser's documentation for specifications, test methods, the presence of radiation environments, and scope of work. Past procurement records do not guarantee current orders or contracts.

What has continued since JT-60 is not the unchanged use of a single machine. It is the transfer of experimental knowledge and technologies for building, operating, and renewing machines to the next generation. Understanding that accumulation allows us to read fusion progress without judging it from one record alone.