What type of wave is produced when an electron beam is removed quickly from its target surface?

When an electron beam is removed quickly from its target surface, a pulse wave is generated. This occurs because the sudden removal of the beam creates a rapid change in current, resulting in a quick surge followed by a return to zero current. The pulse wave reflects this sharp transition, characterized by distinct peak and trough segments, which indicates the brief duration of the electron beam's presence on the target.

In contrast, a sine wave represents a continuous and smooth oscillation, which is not applicable in this scenario as there is no regular periodicity when the beam is abruptly turned off. A square wave is defined by its consistent high and low states, suggesting a steady on-off pattern, which again does not match the characteristics of a single quick removal. Similarly, a pulse train consists of a series of pulses occurring in succession rather than a solitary event, which describes a different scenario involving repetitive pulses rather than a single swift removal of an electron beam.

Thus, a pulse wave is the most fitting answer as it corresponds directly to the rapid change in current caused by the quick removal of the electron beam from the target surface.