Electron rings around magnetized electrodes

When a permanent dipole magnet is biased strongly negative (-800~V) in the presence of sufficient neutrals ($>5$~mTorr) a ring of light, i.e. hot electrons, is formed in the equatorial plane of the dipole. The phenomenon has been explained by the physics of a magnetron discharge: Energetic ions impact on the magnet, release secondary electrons, which are confined by the magnetic field, $\mathbf{E\times B}$ drift, efficiently ionize and gradually diffuse toward the anode to close the discharge current. Time-resolved measurements have shown that a weak electron ring forms on a fast time scale compared to ion transit or electron diffusion times. It oscillates coherently near the ion plasma frequency ($f\simeq1~$MHz) and exhibits the characteristics of an ion-rich sheath instability. In contrast, under steady-state conditions the instability becomes broadband and turbulent wave packets propagate in the $\mathbf{E\times B}$ direction. While the focus is on the basic physics sputtering applications will also be addressed. The cold-cathode discharge can be operated in reactive gases.