With a careful choice of the viewing parameters,neutral-particle analysers could be used to detect small changesin the radial electric field Er immediately next to the separatrixwith a submillisecond (50-100 µs) resolution. In particular,the poloidal viewing angle has to be optimized according to theripple- and plasma geometry. In addition, the use of a smalldiagnostic beam is proposed to improve the signal level andquality.