Frequency-Domain Interferometry of Electron Bunch Driven Wakefields

Beam-driven plasma wakefield accelerators (PWFA), such as the ``plasma afterburner'' can potentially greatly increase the particle energies of conventional accelerators . Various schemes using single and multiple bunches of electrons, positrons and protons have been investigated. Appropriately delayed witness bunches have been the usual method to probe the fields of such wakes, and indirectly, the corresponding plasma wake structures. However, the wake structure has not been observed directly in the PWFA. We will report our progress in the development of direct, optical interferometric methods of measuring the plasma density modulation in electron beam driven wakefields. Frequency Domain Holography (FDH), employing two chirped laser pulses (probe and reference) co-propagating with the particle drive-beam and its plasma wake, permits single shot observation of an extended section of the wakefield behind a drive bunch. The chirped, temporally stretched, probe samples several periods of the wake, while the undisturbed reference pulse propagates ahead of the electron drive bunch. The technique is being developed in the Accelerator Test Facility at the Brookhaven National Laboratory as a probe for two and multibunch driven plasmawakefield experiments