A series of experimental emission images for low-density SMB. Ddistance between nozzle and skimmer is 15 mm, stagnation pressure 50 bar, central density of SMB about 2.73× 10¹³ cm^–3, RF power 2500 W, initial central plasma density about 7× 10¹² cm^–3, at time (a) 0.3125 ms, (b) 1.2500 ms, (c) 2.1875 ms, and (d) 3.1250 ms after SMB has hit plasma column, and exposure time for each image is 0.3125 ms.