ST 40 : Plans and status of construction

In magnetic confinement fusion devices, fusion power is proportional to t Bt0 , where βt is the ratio of the plasma pressure to magnetic pressure ppl/pmag=nkBT/(Bt0 /2μ0) [1]. Spherical tokamaks (STs) have operated at high βt but, thus far, only at relatively low toroidal field (TF) of Bt0=1T or below. ST40, a new high field spherical tokamak, is currently under construction at Tokamak Energy Ltd (TE). Its main design parameters are: R0~0.4m, R/a~1.8, Ip=2MA, Bt0=3T, and κ=2.5. ST40 will demonstrate (i) the feasibility of constructing and operating a high field ST, (ii) the benefits of a high field in a ST, (iii) conditions close to burning plasma requirements (n~1–5·10m, T~5–10keV, τE~100–500ms). ST40 may also be suitable for DT operations in future. ST40 will only have a small central solenoid, which maximises the size of the TF magnet while maintaining the small aspect ratio. For plasma start-up, ST40 will use a process called merging compression (MC) [2], utilized in START [3] and MAST [4]. In MC start-up, plasma rings are formed around two in-vessel MC coils by rapidly ramping down the current in them. When the rings merge, their magnetic energy is converted into plasma thermal energy. The resulting hot plasma is compressed along the major radius using the PF coils. The central solenoid and up to two 1MW neutral beam injectors may be used to heat the plasma and to extend the pulse. The first experiments on ST40 will focus on optimising the MC start-up. The PSUs for the TF and MC coils have been installed and the stainless steel vacuum vessel tested to ~10 mbar. The liquid nitrogen cooled copper magnets, powered by capacitor-based power supply units (PSUs), will allow ST40 to have a current flat top of ~1.5s at full TF current (250 kA). Complete PSU installation will have a total stored energy exceeding 300MJ, roughly half of which will be stored in the TF PS. The first MC coil tests will be reported.