A particle simulation with plasma source is carried out on plasma structures generated by an electron emissive electrode floated in a collisionless plasma. When low‐temperature, high‐density thermal electrons are emitted, there appears a negative potential dip in front of the electrode, which is always accompanied by a low‐frequency oscillation. On the other hand, three regimes of plasma structures appear for an electron beam injection. When a high‐flux electron beam is injected, an electron sheath is generated in front of the electrode. The sheath reflects ions flowing to the electrode, providing an increase in the plasma density. When a low‐flux electron beam is injected, no electron sheath is generated. When an intermediate‐flux beam is injected, the electron sheath structure appears periodically in time. The lifetime of the sheath is proportional to the system length. These results of beam injection are almost consistent with those of a Q‐machine experiment.