Unprecedented confinement time of electron plasmas with a purely toroidal magnetic field in SMARTEX-C

Abstract

Confinement time of electron plasmas trapped using a purely toroidal magnetic field has been found to exceed 100s in a small aspect ratio (Ro/ a∼ 1.59 , Ro and a are device major and minor radius, respectively), partial torus. It improves upon the previously reported confinement time by nearly two orders of magnitude. Lifetime is estimated from the frequency scaling of the linear diocotron mode launched from sections of the wall, that are also used for mode diagnostics. Confinement improves as neutral pressures are reduced to < 5 × 10 - 10mbar in the presence of a steady state magnetic field of 200 Gauss (∼60s with droop < 0.1 %) at 100V electron injection energies. With reduced pressures the role of (ion driven) instability diminishes and loss mechanisms resulting from elastic electron–neutral (e–n) and the ubiquitous electron–electron (e–e) scattering seem to play an important role which suggests low electron temperatures. The contribution to electron population resulting from the ionization of background neutral gas at these temperatures and pressures are expected to be insignificant and is corroborated in our experiments.