Implementation of laser-plasma-based acceleration stages in user-oriented facilities requires the definition and deployment of appropriate diagnostic methodologies to monitor and control the acceleration process. An overview is given here of optical diagnostics for density measurement in laser-plasma acceleration stages, with emphasis on well-established and easily implemented approaches. Diagnostics for both neutral gas and free-electron number density are considered, highlighting real-time measurement capabilities. Optical interferometry, in its various configurations, from standard two-arm to more advanced common-path designs, is discussed, along with spectroscopic techniques such as Stark broadening and Raman scattering. A critical analysis of the diagnostics presented is given concerning their implementation in laser-plasma acceleration stages for the production of high-quality GeV electron bunches.

Quasi-monoenergetic electron beams of energies 12 MeV to over 200 MeV are generated from both nitrogen and helium gas targets with 7TW laser pulses. Typically nitrogen gas interactions lead to electron bunches in the range of 12 to 50 MeV varying from shot to shot. Helium gas leads to higher energy electron bunches from 25 to 100 MeV. Occasionally exceptionally high energy bunches of electrons up to 200 MeV are observed from nitrogen and helium. Initial full two-dimensional simulations indicate the production of 20–30 MeV electron energy bunches for the typical interaction conditions as the electrons are injected from wave breaking in the plasma wake behind the laser pulse and injected into the strong electric field gradient propagating with the optical pulse. This is consistent with the experimental observations from the majority of shots. Pulse compression during propagation in the high density plasma does not allow the threshold conditions for the full bubble regime to be reached. However, the electric acceleration field in the wakefield cavity is still sufficient to lead to the formation of a bunch of electrons with an energy peak in the range of 20 to 30 MeV. In order to explain the occasional high energy shots most likely a lower density channel created by the laser prepulse may occasionally form a natural low density electron guide channel giving ideal conditions for acceleration over much longer lengths leading to the high energies observed.