AbstractHigh perveance ion beams are required for heavy-ion inertial fusion drivers. The current limit of a low energy beam transport line is enhanced by space-charge compensation. It rise time is several hundreds of microseconds under the supposition that electrons are produced by residual gas ionization only. Therefore the rise time depends on the residual gas pressure and ionization cross-section. This is of special interest for pulsed ion beams, with a pulse length close to or below that range. To study this compensation process, we developed a time-resolving residual gas ion energy spectrometer with single-particle detection; the available time resolution is about 2 μs. Experimental results are presented and compared with a theoretical prediction.