Wikipedia
DIII-D is a tokamak developed in the 1980s by General Atomics in San Diego, USA, as part of the ongoing effort to achieve magnetically confined fusion. The mission of the DIII-D Research Program is to establish the scientific basis for the optimization of the tokamak approach to fusion energy production.
DIII-D pioneered new technology including the use of beams of neutral particles to penetrate the confinement field of the device and heat the plasma within. It achieved several milestones including the highest plasma β parameter ever achieved at the time (early 1980s) and the highest neutron flux (fusion rate) ever achieved at the time (early 1990s). DIII-D continues to be operated by General Atomics, focusing primarily on exploration of the advanced tokamak regime.
Advanced tokamaks are characterized by operation at high plasma β through strong plasma shaping, active control of various plasma instabilities, and achievement of current and pressure profiles that produce high performance. DIII-D is the third-largest operating shaped tokamak in the world (after JET in the UK and JT-60U in Japan).