  The ESTHER shock tube is a new state-of-the-art facility at Instituto
Superior T\'ecnico designed to support future ESA planetary exploration
missions. Its driver is a high-pressure combustion chamber using a mixture of
He:H2:O2 ignited by a high-power Nd:YAG laser. Both hydrogen as an energy
vector and laser ignition are promising techniques with applications in
high-pressure combustion. The influence of gas mixture and laser parameters,
namely the air:fuel ratio, filling pressure, inert gas dilution and ignition
mode, on the combustion and thus shock tube performance were extensively
studied. A second, low-velocity driver mixture with nitrogen in place of helium
as a dilutant was also studied and experimental shots are done.
  Our results show that the filling pressure and helium dilution are the most
dominant parameters in both peak pressure, acoustic oscillation and combustion
velocity. The gas mixture peak pressure and acoustic wave amplitude increase
with the increased filling pressure. Yet, the increased filling pressure lowers
the combustion velocity. The helium in the mixture had a dilution effect, with
it lowering the overall effectiveness of combustion. Having higher dilution
factors lowers the combustion compression ratio, acoustic waves amplitude and
flame velocity. The air:fuel equivalence ratio influence was expected with
faster flame and peak pressures at the stoichiometric region. Nitrogen diluted
shots have drastically lower compression ratios and flame velocity when
compared to the helium ones, besides it, the acoustic perturbation was
stronger. ``tulip" flames and deflagration to detonation transitions phenomena
were identified in some of the experiments.
