Plasma Injection and Trapping in a Caulked Stuffed Cusp Magnetic Field

Abstract

A minimum-average-B-stable toroidal magnetic well with high shear is obtained from the combined fields of a solenoid, axial conductors, and an internal ring conductor. Results of measurements of the injection and trapping of high-energy plasma from a coaxial gun are described. The confinement times are dependent on both particle energy and magnetic field. The density e-folding time is independent of the ion velocity under conditions of low shear and well depth. As they are increased the e-folding time varies roughly as 1/vi2. Trapped 0.9-keV plasma decays with an e-folding time that increases by a factor of 6 to 110 μsec as shear and magnetic well depth increase. The initial density of the plasma (0.5 to 3 keV) is ∼ 5 × 1011 cm−3 for field magnitudes resulting in a 40-μsec lifetime of 0.9-keV deuterons. This lifetime is independent of density over the range from ∼ 5 × 1011 cm−3 to 109 cm−3. The range of magnetic field magnitude is limited by the decreasing amount of plasma injected from the gun with increasing magnetic fields. The maximum lifetime of 110 μsec is commensurate with the combined effects of the ∼ 370 μsec support lifetime, a calculated ∼ 1000-μsec charge exchange time, and an anomalous lifetime of ∼ 190 μsec. A search for fluctuating electric fields that might be associated with such an anomalous loss shows the presence of low-amplitude 2-MHz oscillations but their possible role as a loss mechanism is not identified.