An in-situ spectroscopic erosion yield measurement of material erosion and redeposition in plasma

Use of the line radiation of material eroded by plasma bombardment is investigated as an in-situ erosion yield measurement in the PISCES plasma facility. PISCES produces plasma with parameters which are similar to those of the edge-plasma of fusion devices. The experiments are done with material sample of 2.5 cm in diameter held in the center of the 10 cm diameter circular plasma. The typical magnetic field is 250 gauss. A Langmuir probe is inserted in front of the material sample to measure electron temperature and density from time to time during the experiments. The typical electron temperature ranges from 4 eV to 20 eV. The plasma density can be varied from 10/sup 11/ to 10/sup 13/ cm/sup -3/. The incident plasma flux on the material sample ranges from 10/sup 17/ to 10/sup 18/ ion cm/sup -2/s/sup -1/. The bombarding ion energy is set by the electrical bias of the sample with respect to the plasma. A 1.33 m Czerny-Turner monochromator is used to view the plasma in front of the sample perpendicular to the direction of the magnetic field. The line radiation from the eroded material sample in the plasma is found to follow the materials erosion behavior. An example is shown in Fig. 1. In this case, the material is copper while the background plasma is argon. The measured electron temperature and density are 8.5 eV and 1.4x10/sup 12/ cm/sup -3/ respectively. The normalized intensity of the CuI (3247.5 A) line in the plasma column is plotted against the measured erosion yield at different ion bombarding energies. The normalized line intensity shows a linear relation with the erosion yield as independently evaluated by the weight loss method. It follows that this method allows us to quantitatively monitor in-situ the erosion yield with proper calibration