INITIALIZATION OF PLASMA-DENSITY PROFILES FROM REFLECTOMETRY

The contribution of the initial plasma to density profiles reconstruct ed 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 di fficult to measure the edge contribution models are required to initia lize 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 ap proximate 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 expone ntial-like decay and with an edge density plateau (typical of H-mode r egimes during ELMs). It is found that the contribution from the initia l plasma decreases with density (or frequency); for densities of the o rder of 10X the first probed density n(el) it is reduced to values les s than 10% in the case of a profile with a flat edge and 2% for a peak ed one. For n(e)>10n(el) the profiles can be absolutely calibrated fro m 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., du e to discrete probing, can be more significant than those originated b y the initialization process. (C) 1995 American Institute of Physics.