The contribution of the initial plasma to density profiles reconstructed from the d phi/df vs f curve obtained with reflectometry is studied. It is shown that the initial phase information determines to a great extent the accuracy of the inverted profiles at the edge. As it is difficult to measure the edge contribution models are required to initialize the profile evaluation. A novel method is presented that uses the phase information from the lowest frequency waves to obtain d phi/df below the first probing frequency F-1, by imposing the continuity with the measured d phi/df characteristic and its derivative at F-1. An approximate shape of the edge profile is obtained because low-frequency waves are sensitive to the initial plasma where they propagate without reflection. The accuracy of the inverted profiles is thereby improved, as shown by simulation studies performed for profiles with an exponential-like decay and with an edge density plateau (typical of H-mode regimes during ELMs). It is found that the contribution from the initial plasma decreases with density (or frequency); for densities of the order of 10X the first probed density n(el) it is reduced to values less than 10% in the case of a profile with a flat edge and 2% for a peaked one. For n(e)>10n(el) the profiles can be absolutely calibrated from reflectometry data alone with an accuracy of +/-2 mm independent of the initialization model. The numerical study also shows that profile deviations resulting from insufficient phase derivative data, e.g., due to discrete probing, can be more significant than those originated by the initialization process. (C) 1995 American Institute of Physics.
