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I believe those are Mach/Shock diamonds, which are common in rocket engine exhaust. It wouldn't surprise me if the propellant and the time of the test made them more visible.

https://en.wikipedia.org/wiki/Shock_diamond

As also seen on page 2 of "Ignition" with page 3 showing some other interesting rocket effects. http://library.sciencemadness.org/library/books/ignition.pdf

They are called shock diamonds and are caused by standing shock waves in the exhaust plume: https://en.wikipedia.org/wiki/Shock_diamond

Its supersonic shock waves. https://en.m.wikipedia.org/wiki/Shock_diamond