A co-axial velocity map imaging spectrometer for electrons

We present the design of an electron velocity map imaging (VMI) spectrometer where the ionizing laser source propagates along the symmetry axis of the spectrometer. The co-axial geometry is useful in a variety of experiments, because it provides a unique 2-dimensional projection of the 3-dimensional electron momentum distribution. Initial simulations show that this co-axial VMI can work with both high energy (more than 100 eV) and low energy (tens of eV) electrons. We demonstrate the performance of this co-axial VMI spectrometer at the Linac Coherent Light Source X-ray Free Electron Laser facility.