This is called a Lichtenberg Figure, and it was created by a charge of 2.5 MV by first passing it through a 5 million volt electron beam. Right after the charge is applied you can see pockets of residual charge flashing as a form of discharge. Lichtenberg Figures can be created within solid insulating materials, such as acrylic or glass by injecting them with a beam of high speed electrons from a linear electron beam accelerator. Inside the accelerator, electrons are focused and accelerated to form a beam of high speed particles.
Captured Lightning Sculptures
Video by nutekcorp.com and capturedlightning.com