Energy gain scaling with plasma length and density in the plasma wakefield accelerator

We present plasma wakefield acceleration experimental results showing that the energy gain by 28.5 GeV electrons scales with plasma length and reaches 14 GeV over a plasma with a density of 2.6 x 10(17) cm(-3) and a length of 31 cm. At this plasma density the average accelerating gradient is 36 Ge V m(-1). These results are in good agreement with the numbers obtained from particle in cell simulations describing the experiment. The linear scaling is also observed both at lower and higher plasma densities, at which smaller energy gains and accelerating gradients are measured. The systematic measurements of energy gain show the reproducibility and control of the acceleration process.