The new Ariane 6's upper stage engine "Vinci" is expected to challenge the RL-10C, which is currently the most efficient chemical rocket engine on the market. The European aerospace conglomerate aims at achieving a much better thrust-to-weight ratio while realizing approximately the same specific impulse.
While that sounds great, the published test run video does not look good at all, at least from my armchair engineer perspective:
The exhaust looks very unstable, especially during the close-up side views. This is in contrast to the test run of the comparable ISRO engine CE20 (same fuel, approximately same thrust) which looks much more steady and smooth:
Is a deep-throttled state of operation the only possible explanation for the appearance of the exhaust in the Vinci video? Or is it the lack of a propper nozzle? What's the most likely explanation?