The Mirror Fusion Test Facility, or MFTF, was an experimental magnetic confinement fusion device built using the magnetic mirror, or so-called "yin-yang" design. It was designed and built at the Lawrence Livermore National Laboratory (LLNL), one of the primary research centers for mirror fusion devices. It cost 372 million dollars to construct, making it at the time, the most expensive project in the lab's history. It opened on February 21, 1986 and was promptly shut down. The reason given was to balance the United States federal budget.