The double ionization of helium is arguably the most fundamental process in strong-field physics. We plan to probe for the first time the ionization yields at intensities where the classical description should fail. This will highlight in an unambiguous way the importance of a quantum approach. Since this occurs at relatively low intensities, the ion yields are extremely low. A spectrometer capable of operation at high repetition rates, and therefore a high data stream, must be constructed. Peter de Castro has designed and constructed such a device (see below), which will rely on a static magnetic field to spatially separate the singly and doubly charged ions. A computer simulation (below) shows the splitting of He⁺ from He²⁺ when the ions pass through a region of static magnetic and electric fields.

Adam Holman (above) fine tunes the Ti:S laser oscillator. The results from his Java computer code (inset) show the effects of a longitudinal electric field on the trajectories of electrons in an electromagnetic field. The black curve is the result of no longitudinal field, while the blue and red curves show the effects of an in-phase and out-of-phase longitudinal term, respectively. Clearly, the out-of-phase component results in the most dramatic effect on the electron path. In this example, the electron represented by the red curve is much more likely to reencounter the parent ion, in principle leading to an increased rate of double ionization.