  Laser-driven plasma wakefields can provide hundreds of MeV electron beam in
mm-range distances potentially shrinking the dimension of the actual particle
accelerators. The plasma density plays a fundamental role in the control and
stability of the acceleration process, which is a key development for the
future electron injector proposed by EuPRAXIA. A gas cell was designed by LPGP
and LIDYL teams, with variable length and backing pressure, to confine the gas
and tailor the gas density profile before the arrival of the laser. This cell
was used during an experimental campaign with the multi TW-class laser at the
Lund Laser Centre. Ionization assisted injection in a tailored density profile
is used to tune the electron beam properties. During the experiment, we filled
the gas cell with hydrogen mixed with different concentration of nitrogen. We
also varied the backing pressure of the gas and the geometrical length of the
gas cell. We used a transverse probe to acquire shadowgraphic images of the
plasma and to measure the plasma electron density. Methods and results of the
analysis with comparisons between shadowgraphic and interferometric images will
be discussed.
