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 claimed that it held 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. The product quoted is not a valid fusion triple product since the plasmas did not satisfy the steady state of the Lawson criterion as discussed below. JT-60 also claimed without proof that it held the record for the hottest ion temperature ever achieved (522 megakelvins). In reality the TFTR machine at Princeton routinely measured higher ion temperatures during the 1993-1996 campaign, as discussed below.
Original design JT-60 was first designed in the 1970s 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, TFTR, 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-60/TFTR disputed records
By 1996, JT-60 had achieved its record ion temperature of 45 keV, which is claimed to have exceeded the highest temperatures measured at that time in the TFTR tokamak in Princeton. Detailed measurements of the ion temperatures analyzed during TFTR's experimental campaign with deuterium-tritium plasmas in 1993–1996, found numerous discharges with temperatures greater than 50 keV in both deuterium-only and deuterium-tritium plasmas. A 2025 publication of a reanalysis of TFTR transport and confinement results for a selected scan of discharges mentions that several "supershots", not in the scan, had ion temperatures of 70 keV with a measurement error bar of 28%. The TFTR team did not highlight these high temperatures for several reasons. The ion temperature measurements in JT-60, TFTR, and JET measured only singly ionized trace carbon impurity ions, not the temperatures of the hydrogenic ions. The carbon ions do not fuse, and displace the deuterium and tritium ions which can fuse. The hydrogenic ion temperatures can be calculated in the TRANSP analysis code. The methods used are published and widely used in analysis of experimental results. These temperatures are the relevant ones for calculating the deuterium and tritium fusion reactions. They generally are less than the carbon temperatures. Secondly, the end goal of this research, practical minimally polluting fusion energy, does not require ion temperatures greater than about 25 keV. An example of simulation of a burning plasma in ITER is
The fusion triple product metric applies only to plasmas in steady state, as stated explicitly in the Lawson criterion. The JT-60 plasmas with high values were far from steady state; in fact, their conditions rose rapidly in time to those values, and then suffered major disruptions, which extinguished the plasmas abruptly. Examples are in.
Also the derivation of the fusion triple product assumes that the fusion power results from thermonuclear fusion (from thermal deuterium and tritium). Instead the high fusion power in past tokamak experiments resulted dominatly from beam-thermal reactions. Thus the JT-60's claimed record for the triple product is not a 'fusion triple product'. Steady state discharges have been achieved in other devices such as Tore Supra and WEST have announced results for the fusion triple product.
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. JT-60U researchers claimed that on October 31, 1996, they achieved an estimated breakeven 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 is estimated to have created an energy output 1.05 times the energy injected into the tokamak. An estimate based on a discharge in 1968 gave QDTeq = 1.25. The record of the central ratio Qcore achieved in a tokamak discharge is 1.3 in JET in 1998.
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