Observation and analysis of a large banana orbit in the diocotron mode of a coaxial Malmberg–Penning trap

Abstract

The diocotron mode of an off-axis electron column is studied in a coaxial version of the Malmberg–Penning trap. Measurements of the diocotron frequency as a function of the bias on the central conductor agree well with a derived theory including finite-length corrections and confinement potential contributions. When the experimental parameters are adjusted to give a very low diocotron frequency, the column motion abruptly changes from an axis-encircling orbit to a large banana-shaped orbit in the [Formula: see text] plane with extent [Formula: see text] and [Formula: see text], where R is the wall radius. This banana motion is apparently in response to a previously unknown background construction asymmetry. The size of the asymmetric potential is deduced from orbit data and found to be 45–100 mV. Theoretical modeling shows this to be consistent with a small radial offset δ in the center wire position of [Formula: see text]. Implications and applications of these findings are discussed and a note on obtaining the line density of an electron column in a coaxial trap is given.