  Event shape distributions in $e^+e^-$ annihilation are determined from the
DELPHI data taken between 183 and 207\gev. From these the strong coupling
$\alpha_s$ is extracted with several techniques. Together with the results from
other LEP2 energies and at about $m_Z$ this allows both, a combined measurement
of $\alpha_s$ and a test of the scale dependence of the strong interaction.
Alternatively the renormalisation group invariant (RGI) perturbation theory is
applied to measure the $\beta$ function of strong interaction. The results are
good agreement with the QCD expectation and allow to exclude the existence of
light gluinos with a mass below 30\gev in a model independent way.
