To inject a plasma, the first gate (called the "inject gate") is grounded to allow electrons produced at the cathode to stream along the magnetic field lines, bounce off the last gate, and then exit the trap (back to the left).
To trap and hold a plasma, the voltage on the inject gate is lowered. Electrons are now trapped in the middle gate (called the "hold gate"). The plasma then evolves according to the E x B motion. (The E is the self-electric field of the electrons)
To dump the plasma, the last gate (called the "dump gate") is grounded allowing the plasma to stream along the magnetic field lines and out to the acceleration region. The electrons are then accelerated into the phosphor screen, producing light much like in a tv or oscilloscope. A CCD camera then takes a picture of the image, brightness is proportional to electron density. Note that the 3D plasma becomes a 2D image.