  A novel gas target for interactions between high power lasers and gaseous
medium, especially for laser wakefield accelerators, has been designed,
manufactured and characterized. The gas target has been designed to provide a
uniform density profile along the central gas cell axis by combining a gas cell
and slit nozzle. The gas density can be tuned from 10^16 atoms/cm^3 to 10^19
atoms/cm^3 and the gas target length can be varied from 0 to 10 cm, both
changes can be made simultaneously while keeping the flat-top gas profile. The
gas distributions inside the gas cell have been measured using interferometry
and validated using computational fluid dynamics.
