Energy confinement and magnetic field generation in the SSPX spheromaka)

The Sustained Spheromak Physics Experiment (SSPX) [Hooper et al., Nuclear Fusion 39, 863 (1999)] explores the physics of efficient magnetic field buildup and energy confinement, both essential parts of advancing the spheromak concept. Extending the spheromak formation phase increases the efficiency of magnetic field generation with the maximum edge magnetic field for a given injector current (B /I) from 0.65T_{/MA previously to 0.9T/MA. We have achieved the highest electron temperatures (Te) recorded for a spheromak with Te>500eV, toroidal magnetic field ∼1T, and toroidal current (∼1MA) [Wood et al., "Improved magnetic field generation efficiency and higher temperature spheromak plasmas," Phys. Rev. Lett. (submitted)]. Extending the sustainment phase to >8ms extends the period of low magnetic fluctuations (<1%) by 50%. The NIMROD three-dimensional resistive magnetohydrodynamics code [Sovinec et al., Phys. Plasmas 10, 1727 (2003)] reproduces the observed flux amplification ψpol/ψgun}. Successive gun pulses are demonstrated to maintain the magnetic field in a quasisteady state against resistive decay. Initial measurements of neutral particle flux in multipulse operation show charge-exchange power loss <1% of gun input power and dominantly collisional majority ion heating. The evolution of electron temperature shows a distinct and robust feature of spheromak formation: A hollow-to-peaked T_e(r) associated with q ∼1/2.