Continuously Injected Plasma Columns

Electron plasma columns continuously injected into a Malmberg-Penning trap display a rich evolution. Electrons emitted from an illuminated photocathode are trapped in the electrostatic well formed by the negatively biased photocathode and the trap end cylinder. Initially, the injections form cylinders of uniform density. As the density increases, the columns hollow in an attempt to match the potential profile of the equipotential cathode. The hollow columns are subject to the diocotron instability, and as the evolution becomes increasingly turbulent, the columns slowly expand to the trap wall. We present preliminary results and analysis of the trapping mechanism and the 2D dynamics of a continuously injected system. INTRODUCTION Malmberg-Penning traps consist of a series of collimated conducting cylinders, or gates, aligned along a strong magnetic field. Electron plasmas are confined in these traps by appropriately biasing the trap cylinders to form an axial electrostatic well. Radial confinement is provided by a strong magnetic field. Electrons are injected into the trap by momentarily grounding an "inject" gate near the cathode, and allowing electrons from the negatively biased cathode to enter the trap (Fig. 1). The plasma can be imaged by briefly grounding the "dump" gate. The image formed on the phosphor screen is recorded by a CCD camera. Typically, electrons are injected into a trap for only a short time; here we study this injection process and examine the columns formed by long-term injection. For these studies we used the Berkeley Photocathode trap [1], in which electrons are created by photoemission. The trap is otherwise similar to most other Malmberg-Penning traps, and employs a magnetic field of 3T. PHOTOCATHODE PHOSPHOR