Observation of a pressure limit in a gun-driven spheromak

Density in a spheromak is lowered by action of a m∕n=2∕4 island, after which the electrons heat rapidly to give profiles of nTbefore=nTafter, indicating that the spheromak heats up to a pressure limit. The spheromak core remains at Te∼150eV for ∼1∕2ms during a period exhibiting low amplitude incoherent fluctuations consistent with a pressure-driven mode, giving the volume-averaged electron beta ⟨βe⟩∼3% and peak βe of ∼6%. Calculations show that measured pressure profiles are close to the Mercier limit.