Improved break-in-slope analysis of the plasma energy response in tokamaks

The break-in-slope method is a simple-although powerful-data analysis technique that is commonly used to determine the power absorption profiles of the plasma species during auxiliary heating experiments in tokamaks. It is based on the study of the energy response of the particles to sudden changes in the external power applied to the plasma. Even though some experimental conditions are favorable for the straightforward application of the break-in-slope analysis in its most simple form ( linear fit of the experimental temperature signals), most situations require the retention of additional terms in the linearized energy conservation equation for a successful use of this technique. In this paper, important corrections necessary to extend the applicability of the traditional break-in-slope technique will be presented: ( i) the numerical determination of the break-in-slope instants in the plasma energy response allowing the study of indirect ( collisional) heating scenarios; ( ii) the inclusion of the density variations due to the external power step based on fast density measurements; ( iii) the exponential representation of the plasma energy evolution after the power break, describing the saturation of the experimental signals in slow modulation or single power step experiments; ( iv) a first assessment of the influence of the change in the radiated power on the break-in-slope results. As will be shown, these corrections are particularly important in low absorption scenarios and in single power step studies, where the change in the external power cannot be considered 'non-perturbative' and the standard break-in-slope analysis usually leads to integrated power levels well below the actual power injected into the plasma.