Large acceptance angle retarding-potential analyzers

Abstract

Electrostatic retarding-potential gridded analyzers have been used to measure the current and the axial energy distributions of ions escaping along magnetic field lines in the 2XIIB magnetic fusion experiment at Lawrence Livermore National Laboratory (LLNL). Two analyzers are discussed: a large scanning analyzer with a movable entrance aperture that can measure ion losses from a different segment of the plasma diameter on each shot, and a smaller analyzer that mounts in 5-cm-diam. ports. Careful electromagnetic shielding and grounding were necessary in order to reduce spurious signals. Accurate measurements of ion current require grids that attenuate the plasma flow and isolate it from the analyzer grid potentials, several techniques to suppress secondary and primary electrons, and consideration of the space-charge limits as well as techniques to increase these limits. Accurate measurements of ion energy require, in addition, confining large-angle ion orbits within the analyzer by an axial magnetic field, limiting the sweep rate of the ion-repeller grid, and aligning the analyzer so that it is normal to the magnetic field lines. (The analyzers are also used on the Tandem Mirror Experiment, TMX, where the angular acceptance requirements are reduced). The analyzers measure the axial energy, which in 2XIIB and TMX is much greater than the transverse energy. Several pitfalls and their cures are identified in the design, construction, and operation of these analyzers.