The capability of a power plant to operate at a wide range of output power is essential for initial commissioning and normal maintenance. In this work, we explore critical physics issues related to operating a tokamak fusion reactor at fractions of its rated power and identify methods for power control.Analysis is carried out with a steady-state, profile-dependent, zero-dimensional power balance model of the plasma in which several empirical transport scalings appropriate to tokamaks are used. It is found that reactor operation depends strongly on the confinement model, plasma 6 limit, and the effect of alpha power on transport. Parametric calculations indicate that density, auxiliary heating power and an effective external confinement control mechanism are the key control elements, and burn control is required in most cases. Transition between power plateaus is facilitated by operating in the hybrid transformer mode. In general, the impact of fractional power operation on full power reactor designs appears to be small. Conceptual designs of fusion reactors have been based primarily on achieving rated power output by maintaining the plasma parameters at their optimum operating levels. However, as with fossil and fission power plants, the commissioning of fusion power reactors will entail a series of plasma and system component performance checks at low power «5% of rated power) and subsequent testing at progressive power stages. In addition, we can envision normal start-up/shutdowns of the plant for scheduled maintenance and when system components malfunction. In these procedures, the fusion power can vary from below 5% to 100% of the plant rated power, and the rate of change must be controlled to satisfy physics and engineering constraints on the plasma and its support systems. What this itnnlles is that, in addition to operating at full load, the plant should have the capability of operating at fractions of its full power and of making controlled transitions between power levels.In this paper, we explore methods to operate a tokamak fusion reactor at fractions of its rated power, identify the more effective control "knobs" and assess the impact of the requirements of fractional power operation on full power reactor design. As a prerequisite to this task, we carefully examine The paper is organized in the following manner. Section II gives a brief description of the physics basis, both theory and experiment, on which the
