The largest plasma based acceleration experiment in the world has been approved at CERN. This experiment will use 0.5 TeV proton bunches to drive intense plasma waves capable to accelerate particles to high energies. One of the physical questions to be clarified is the role of the background ion motion. This work carried out by EPP team members Jorge Vieira, Ricardo Fonseca and Luís Silva, in collaboration with Warren Mori from University of California, Los Angeles, shows that the the background plasma ion motion can strongly reduce accelerating fields. The work also identifies gases that could be used to avoid the deleterious effects associated with the motion of the background plasma ions. More information can be found here.