Impurity fuelling has been studied in TEXTOR by gas puffing under a wide range of experimental conditions. First, neon has been injected into discharges with a range of densities during both Ohmic and neutral injection heating. Then, helium, neon and argon have been injected into similar discharges and the increase in radiated power and Zeff studied as a function of the number of impurity atoms injected. Similar experiments using carbon monoxide and methane yielded markedly different results from those of the rare gases. The fraction of injected atoms entering the confined plasma has been estimated and found to vary widely, from less than 1% for carbon to about 100% for helium. Detailed Monte Carlo calculations performed using the LIM code to simulate the impurity behaviour have shown that good agreement with experiment can be obtained for all species except oxygen, where charge exchange processes are thought to be important
Oxygen gas injection during the current plateau of a TFR tokamak plasma