Verbesserung des Energie- und Teilcheneinschlusses nach Pelletinjektion

In this work, the influence of pellet injection on the energy confinement time in different types of discharges has been investigated. During the ablation of a pellet, the electron density is increased. At the same time the electron temperature is decreased because the energy of the plasma is used to ionize and heat the ablated particles of the pellet. After the pellet injection the plasma is reheated. In ohmic plasmas with high density particles are transported into the core during this phase. In this way the density and the temperature in the core are increased leading to a pressure increase and an increase in the energy content of the plasma and to an increase in the energy confinement time. The inward transport of particles can be stopped by reappearing of sawtooth activity which was stopped by the pellet injection; the plasma relaxes to the initial state. In strongly heated plasmas with low confinement (L-mode) and ohmic discharges with low density, the increased transport causes a rapid loss of the additional density coming from the pellet. Because of this the energy increase is much smaller or vanishes. In radiation cooled discharges with improved confinement (RI-mode), the pellet injection leads to a quasi stationary improved energy confinement provided that no mode activity or MARFE's are exited destroying the confinement improvement. Also here the initial increase in plasma density is reduced similar to L-mode discharges however in contrast to all other discharge types the reduction stops. Later, particles are transported from outside into the plasma core establishing and enhanced core density for the rest of the discharge. Because the temperature has been recovered quickly after the the pellet injection, the energy in the plasma is increased together with the density. Furthermore, no additional impurities are transported into the plasma core so that the Z'ff is decreased and the neutron production rate is increased. The change in plasma parameters takes place only in the core plasma and the plasma edge is not influenced significantly. Model calculations show that the ITG (Ion-Temperature-Gradient instability) can be stabilized by impurities and a steep electron density profile. Because the pellet injection increases the central density and the density profile steepens more - meaning the positive characteristics of the RI-mode are improved - the first explanations for the observed improvement after the pellet injection are based on stabilizing the ITG mode further by changing the electron density profile due to pellet injectio