The path of the laser is affected by user input and enemies present on the screen.
Here is a video, at 5:00 minutes the laser in question is shown : [Raiden II (PS) - 1 Loop Clear - Part 2][1]

**UPDATE** 

Here is a test using Inkscape, ship is at bottom, the first 4 enemies are targeted by the plasma.
There seems to be a sort of pattern. I moved the ship first, then the handle from it to form a 45° angle, then while trying to fit the curve I found there are series of 2 parallel handles.

![v1][2]

Another version with better aligned angles (now enemy 5 is aligned)

![v2][3]


  [1]: http://www.youtube.com/watch?v=qd1EuxE-LIs&feature=player_detailpage#t=280s
  [2]: https://i.sstatic.net/pppas.png
  [3]: https://i.sstatic.net/OEoec.png