Plasma Production by Molecular Ion Injection into a Magnetic Mirror Supplemented with a Radio Frequency Electric Field

Observations with a 27‐keV molecular ion injection experiment are described. In the experiment, an rf electric field is applied transversely to a containing magnetic mirror field in the mid‐region. The rf‐field intensity is proportional to $r^4$ , and the frequency relation is ${\text{ω\hspace{0.17em}=\hspace{0.17em}2.5ω}}_{\mathrm{ci}}{\text{\hspace{0.17em}=\hspace{0.17em}5ω}}_{\mathrm{cm}}$ , where $r$ is the radius, and ${\omega }_{\mathrm{ci}}$ and ${\omega }_{\mathrm{cm}}$ are the cyclotron frequencies of protons and hydrogen molecular ions, respectively. Accumulation of plasma greatly increases with an rf potential of about 1.4 kV, and a density of ${\text{7×10}}^7\mathrm{\hspace{0.17em}particles/}{\mathrm{cm}}^3$ is obtained with an injection current of 2 mA. Perpendicular proton velocity distribution is well spread with an rf potential of about 1.4 kV. The spreading is caused during the process of deceleration of the injected molecular ions. Emissions at the second harmonic and the fourth harmonic of the proton cyclotron frequency are observed. Intensity of the fourth harmonic is reduced with a rise in strength of the rf field. As the rf field is switched off, there are plasma loss processes other than charge exchange. The loss may be due to a low‐frequency drift instability. In the presence of the rf field, however, there is no evidence of plasma loss by processes other than charge exchange. The rf field may set up a sufficiently deep well of equivalent quasipotential for ions of the present low‐density plasma.