Plasma potential formation and measurement in TMX‐U and MFTF‐B (invited)

Tandem mirrors control the axial variation of the plasma potential to create electrostatic ‘‘plugs’’ that improve the axial confinement of central cell ions and, in a thermal barrier tandem mirror, control the electron axial heat flow. Measurements of the spatial and temporal variations of the plasma potential are, therefore, important to the understanding of confinement in a tandem mirror. In this paper we discuss potential formation in a thermal barrier tandem mirror and examine the diagnostics and data obtained on the TMX‐U device, including measurements of the thermal barrier potential profile using a diagnostic neutral beam and charged‐particle energy spectroscopy. We then describe the heavy ion‐beam probe and other new plasma potential diagnostics that are under development for TMX‐U and MFTF‐B.