Plasma behavior and the performance of plasma technologies are studied during the startup and fractional power operation of tandem mirrors. Five phases of machine operation are identified, some of which require plasma. The plasma phases include plasma initiation and heating, a standby phase with plasma at the density and temperature characteristics of full design performance in reactors, a deuterium-tritium fractional power operating phase in which the fusion plasma undergoes staged power increases to full power, and rated power operating phase. Plasma initiation and heating uses electron cyclotron resonance heating preionization of background gas in the plug and ion cyclotron resonance heating in the central cell. Operation of the radio-frequency systems, the neutral beams, and the direct converter are studied to determine constraints affecting plasma operation. Studies of fractional power operation, carried out using a quasi-steady-state analysis, show that the plasma Q value can be made remarkably insensitive to the level of fusion power by controlling the plasma radius. Copper insert coils used to increase the maximum choke field require considerable power and cause the recirculating power fraction to increase sharply as the fusion power is reduced. Moreover, when an efficient drift pumping scheme is used, achieved improvements in plasma Q by using high-field choke coils must be weighed against their power consumption and other technological difficulties.