The spoke instability in an E × B Penning discharge is shown to be strongly affected by the boundary that is perpendicular to B field lines. The instability is the strongest when bounded by dielectric walls. With a conducting wall, biased to collect electron current from the plasma, the spoke becomes faster, less coherent and localised closer to the axis. The corresponding anomalous cross-field transport is assessed via simultaneous time-resolved measurements of plasma potential and density. This shows a dominant large-scale E×B anomalous character of the electron cross-field current for dielectric walls reaching 40-100% of the discharge current, with an effective Hall parameter β eff ∼ 10. The anomalous current is greatly reduced with the conducting boundary (characterised by β eff ∼ 10 2 ). These experimental measurements are shown to be qualitatively consistent with the decrease of the E field that triggers the collisionless Simon-Hoh instability.

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