Stephen pjs already asked this question, wakefield acceleration has been around for long time
Interesting paper pjs
They are using more focusing, shorter pulse 50 fs vs 9 ns and therefore almost 10000 times energy/cm2/ time and can therfore create a highly ionised H2 plasma
They are not looking for the ions in the measurements they could replicate Leif Work in a new way if they included more diagnostic measurements for the ions and other particles
Greetings
Sveinn