The time behaviour of radial electric fields at the L-H transition from the observation of ripple-trapped ions

The density of ripple-trapped ions is strongly influenced by negative radial electric fields that can compensate the grad B drift. This density can be monitored with a time resolution of about 100 μs by the measurement of charge-exchange neutral particle fluxes. According to Monte Carlo and Fokker-Planck calculations these fluxes should respond with a time delay of less than 200 μs, if the halfwidth of the electric field is reasonably large (a few centimetres). In the experiment, rise times of the fluxes after the L-H transition are longer or about 1 ms; jumps of the electric field in much shorter times can be excluded under such circumstances. The same is found for the H-L transition, where the flux measurement does not show any indication for a fast drop of the fields. (author)