The physics assumptions on which the design and performance of ITER are based have been defined and developed by the Joint Central Team (now International Team) primarily through its Physics Unit together with fusion physicists throughout the Parties. A detailed summary of the current knowledge and understanding of ITER physics in particular and of tokamak physics in general was pub lished in 1999, and its application to the current ITER design is summarised in the ITER Final Design Report Technical Basis. ITER physics is based on data collected on the present experimental tokamaks, physics models for different plasma processes, and numerical codes created for extrapolation of the plasma performance to the ITER scale and parameter range.

To keep this information up to date, the whole fusion physics community both inside and outside the ITER Participants pools its knowhow and coordinates its development through the International Tokamak Physics Activity (ITPA). Topical Groups provide inputs in each of the main physics areas and report annually to the ITPA Coordinating Committee. This "Physics Committee" sets the R&D Research Priorities for Fusion Physics.

Based on this joint work, the following assumptions have been made to derive the plasma parameters:

1. the plasma will operate in the "ELMy H-mode", i.e, with edge-localised low amplitude mhd modes and in the "high confinement" regimes observed in present experiments: the extrapolation to be used for the parameters of an ITER-size plasma will be determined by the Confinement Topical Group; the ratio of plasma to magnetic pressure, normalised to the factor I/aB, b_N, shall be < 2.5, where I is plasma current, a minor radius, and B toroidal field on axis; the density shall be below the Greenwald density (defined as I/pa²); the degree of helical twist of the field lines, measured by the "safety factor", q, on the toroidal flux surface 5% from the plasma edge, shall be ~3; the impurity level in the plasma shall be such as to retain its effective atomic number below 2;
2. the plasma will be operated in a well-controlled divertor configuration.

The basic physics elements of the ITER plasma performance have been successfully tested in present day experiments. However, an integrated simulation of a fully ITER-like plasma is not possible in present tokamaks. The choice of plasma parameters to satisfy the above goals and conditions therefore depends on a consideration of the operating space available given the uncertainties of extrapolating from today's knowledge.

In summary, these predictions show that a device whose fusion power output is ~500 MW is the minimum size device that can achieve the energy multiplication requirements with reasonable margins. Such a device can satisfy the technical objectives and has the necessary flexibility to accommodate contingencies.

To maintain the programmatic objective, the ITER design includes provisions, wherever it is possible at reasonable cost, to accommodate the likelihood that some of today's predictions will turn out to be different in reality.