[en] Drift surface geometry and the trapped particle fraction in the H-1NF heliac are investigated, using cold ion orbits to trace out the surfaces. In the absence of electric fields, the drift surfaces of the helically trapped orbits are unclosed, but when electric fields of thermal magnitude are present the helically trapped drift surfaces are effectively closed. The drift surface geometries are used to suggest an ECH heating configuration, which may have improved heating efficiency. A simple, quantitative method of classifying the geometrically different subclasses of trapped orbit is presented. It is used to show the effect of radial electric field on the trapped particle populations. (author)