Energy doubling of 42GeV electrons in a particle-driven, plasma-based accelerator

Summary form only given. Plasma-based accelerators have made remarkable progress in the last few years. In particular, the energy doubling of 42GeV incoming electrons has been demonstrated over a plasma length of only 85cm in a particle beam-driven plasma-based accelerator or plasma wakefield accelerator (PWFA). This distance is about 2000 times shorter than the conventional linear accelerator that produced these particles. In the plasma the accelerating gradient is ap 50GeV/m, suggesting that energies of the order of ITeV could be reached in only ap 20 m of plasma. In the experiment the energy gain scales linearly with the plasma length. The maximum energy gain is obtained at a plasma density such that the plasma wavelength is approximately equal to the electron bunch. This scaling is verified for long (ap 700 mum) and short (ap 20 mum) bunches with corresponding optimum plasma densities of ap1.8 x 10¹⁴cm^-3 and ap2.7 x 10¹⁷cm^-3. In the short bunch experiments, the plasma is produced through field ionization of a lithium vapor confined to the hot region heat- pipe oven by a helium buffer gas at room temperature. Above a wake field amplitude of ap 30 GV/m, trapping of plasma electrons is observed. The trapping was identified as resulting from the field-ionization ionization of neutral helium in the lithium to helium transition regions of the heat-pipe oven. These free electrons born at rest inside the wake and have a much lower trapping threshold that the lithium electrons born ahead of the wake. Spectral interference measurements show that the trapped electrons form one or more short bunches (a few fs). Simulations suggest that they have a peak current larger and an emittance lower that those of the drive bunch, possibly making them interesting for radiation source applications. Experimental results will be presented and compared to scaling laws. The prospects for the application of plasma-based accelerators to a future li- - near particle collider will be discussed.