An apparatus (100) for obtaining ion energy distribution measurements in a plasma processing system (105) comprising a substrate (101) for placement in the plasma processing system and that is exposed to the plasma, an ion energy analyser (201) disposed in the substrate for measuring the ion energy distribution at the substrate surface during plasma processing, the analyser comprising a first conductive grid, G 0 , a second conductive grid, G 1 , a third conductive grid, G 2 , a fourth conductive grid G 3 , and a collection electrode, C, each grid separated by an insulating layer (205-207) and an insulation layer (209) also provided beneath the collector, wherein G 1 is biased with a negative potential relative to G 0 , G 2 is biased with a positive voltage sweep relative to G 0 and G 3 is biased negatively with respect to the collector G 4 and G 4 is biased negatively with respect to G 0 , a rechargeable battery power supply including control circuitry, integrated in the substrate, for supplying voltage to each of the grids and the collector of the ion energy analyser; and a Faraday shield encasing the ion energy analyser, the power supply and the control circuitry.