Many experts believe that the z \sim 3 HeII Ly \alpha forest will suffer from the same saturation issues as the z \sim 6 HI Ly \alpha forest and , therefore , will not be a sensitive probe of HeII reionization .
However , there are several factors that make HeII Ly \alpha absorption more sensitive than HI Ly \alpha .
We show that observations of HeII Ly \alpha and Ly \beta Gunn-Peterson troughs can provide a relatively model-independent constraint on the volume-averaged HeII fraction of { x } _ { HeII,V } \gtrsim 0.1 .
This bound derives from first using the most underdense regions in the HeII forest to constrain the local HeII fraction and , then , assuming photoionization equilibrium with the maximum allowed photoionization rate to calculate the ionization state of nearby gas .
It is possible to evade this constraint by a factor of \sim 2 , but only if the HeII were reionized recently .
We argue that HeII Ly \alpha Gunn-Peterson troughs observed in the spectra of Q0302-003 and HE2347-4342 signify the presence of \gtrsim 10 comoving Mpc patches in which { x } _ { HeII,V } > 0.03 .
This is a factor of 20 improvement over previous constraints from these spectra and 100 times stronger than the tightest constraint on the HI volume-filling fraction from the z > 6 HI Lyman forest .