A bar magnet sets plasma aglow

Plasmas often host dipole magnetic fields. Earth’s own dipolar field protects us from the plasma-filled solar wind. Plasma vapor deposition systems commonly boost efficiency by using fields from permanent magnets to confine electrons and ions at desired locations. Many investigations have explored magnetized plasmas, but few have focused on the nonlinear dynamics of a plasma glow discharge (think neon lights) in the presence of dipolar fields. To help fill in the research gap, Sekar Iyengar and his colleagues at the Saha Institute of Nuclear Physics in Kolkata, India, took an ingeniously simple approach: They placed a bar magnet next to a plasma discharge chamber. The magnetic field induced fluctuations in the floating potential—that is, the electric potential of a probe immersed in the plasma. When Iyengar and his colleagues increased the field strength by moving the magnet incrementally closer to the cathode, the fluctuations went from being periodic, through a cascade of period-doubling bifurcations, to being chaotic. The transit to chaos, heralded by an irregular, broadband power spectrum, was accompanied by a localized glow near the cathode surface that grew brighter with increasing field. The photographs show the brightening localized glow at various fields. In the presence of curved dipolar field lines, electrons emitted by the cathode follow the looping lines and ping-pong above the cathode surface. That motion increases the number of collisions with neutral atoms and enhances ionization. In turn, the usually monotonic plasma potential develops a local minimum near the cathode. From numerical models of the ion dynamics, the Saha group surmises that the chaotic floating-potential fluctuations reflect the ion oscillations and nonlinear interactions between the trapped ions and the bulk plasma. (P. K. Shaw et al., Phys. Plasmas, in press.)