Load the game as usual and don't do anything until the high score
screen appears. Type 'COMMANDER' and then press the fire button. After
a   few  moments  press  F10.  If  your  ship  flickers  you  are  now
invulnerable, and have infinite expansions and smart bombs.

F1-F5 = Other Six Levels of 'expansion' (you can still capture the capsules)
F8 = Advance a level (There are three)
F10 = Turn cheat mode on/off

     You cannot get a high score with the cheat mode on.