Such machines include particle accelerators and tokamak reactors. In 2017 UK's fusion reactor ST40 operated by Tokamak Energy generated "first plasma". It also studies the materials and engineering technology of tokamak fusion reactors. The Plasma Physics Laboratory operates a tokamak on campus. The current leading designs are the tokamak and inertial confinement (ICF) by laser. The spherical tokamak is an offshoot of the conventional tokamak design. A spherical tokamak is a type of fusion power device based on the tokamak principle. The advantage of this design was demonstrated in the Lithium Tokamak Experiment. Maximising the bootstrap current is a major goal of advanced tokamak designs. It was followed by the NSTX spherical tokamak. These experiments, especially PLT, put the US far in the lead in tokamak research. The ITER tokamak will use three interconnected cooling systems. They named this layout the "spherical tokamak", or ST. In 1991, START was built at Culham, UK, as the first purpose-built spherical tokamak. In December it was shut down and reopened in May as the Symmetric Tokamak (ST).