7 - JT-60U

Abstract

JT-60 (JAERI Tokamak-60) started operating at the Naka Fusion Institute in 1985 as one of the world’s largest tokamaks with an outboard diverter configuration using hydrogen plasmas in Japan. Its modification to achieve higher plasma current with a single null open divertor was completed in 1991 (JT-60U, U=Upgrade), allowing it to use deuterium plasmas. Since then, JT-60U has addressed the physics issues of reactor plasmas, including ITER (International Thermonuclear Experimental Reactor) physics R&D (Research and Development), and has explored advanced tokamak regimes with high-βp and reversed shear modes for a commercially attractive fusion reactor—work which led to the conception of a Steady-State Tokamak Reactor (SSTR). A variety of additional heating and current drive systems to inject lower hybrid, ion cyclotron RF (Redio Frequency) waves, and positive-ion-based neutral beams have been equipped from the beginning to address the issues. A negative-ion-based neutral beam injector with 500-keV deuterons and electron cyclotron RF system were added to JT-60U in 1996 and 1999, respectively, and in-vessel modifications necessary to the accommodation of a W-shaped pumped divertor were completed in 1997. Ferritic steel tiles were installed inside the vacuum vessel to reduce the toroidal field ripple in 2005. The results produced in JT-60U made a significant impact on the design of ITER, and paved the promising way for the development of DEMO (DEMOnstration fusion reactor) toward early realization of fusion energy with not only world-record achievements in performance such as the highest fusion triple product and DT (Deuterium-Tritium fusion reaction)-equivalent fusion gain, but also important physics findings relevant to the advanced tokamak such as high bootstrap current fraction, internal transport barrier, and current hole with making the JT-60U equipment enhanced. Through the perspective of the reactor relevance, JT-60U has innovated the plasma confinement with finding the physics nature, enhanced the performance at a maximum, and sustained the performance integrated so as to resolve the issues in particular for steady-state operation. JT-60U operations ended in August 2008 to start the modifications for a fully superconducting tokamak, the JT-60SA (SA=Super Advanced), which is being constructed at the Naka site. Thus, JT-60U has paved the way for a prosperous future for the coming eras of fusion developments emphasized on long and stable sustainment of high performance plasmas in JT-60SA, ITER, and DEMO.