Recent progress in the PI3 spherical tokamak program

Achieving net energy gain with a Magnetized Target Fusion (MTF) system requires the plasma to satisfy a set of performance goals, such as particle inventory (~10^{21} ions), sufficient magnetic flux (~0.3 Wb) to confine the plasma without MHD instability, and initial energy confinement time several times longer than the compression time. To explore the physics of reactor-scale plasmas General Fusion (GF) has constructed Plasma Injector 3 (PI3). MTF relies on flux conservation by metal walls and so requires solenoid-free startup with no vertical field coils or toroidal field coils.  Therefore the toroidal magnetic field in PI3 is produced by driving current along a single central conductor using a pulsed power supply that also provides a long low-voltage pulse to compensate resistive losses on multi-millisecond timescale. Once the toroidal field is established PI3 uses a short (20us) pulse coaxial helicity injection, from a magnetized Marshall gun, to produce a self-organized spherical tokamak plasma with minor radius 0.65m (the flux conserver radius is 1m). Plasma diagnostics include Mirnov probes, visible imaging, interferometers, optical spectroscopy, Doppler thermometry, Thomson scattering, and FIR polarimetry.