Q2. How can the particle flux be inferred from the atomic line intensity?
Assuming that no atoms escape from the plasma before being ionized, the particle flux can be inferred from an absolutely measured atomic line intensity using the so-called S/XB method [1].
Q3. What is the effect of nitrogen seeding on the tungsten particle flux?
Starting from the highest temperatures, the tungsten sputtering increases steeply upon nitrogen seeding, evidently due to sputtering by nitrogen.
Q4. What is the effect of impurity seeding on divertor spectra?
Impurity seeding to reduce power and energy loads on divertor targets (e.g. [14, 15] and references therein) will add extrinsic impurities to the species mix.
Q5. What is the effect of nitrogen seeding on the tungsten particles?
The nitrogen seeding starts to have a net beneficial effect, i.e. a reduction in the tungsten erosion, when the temperature drops below 15 eV.7
Q6. What is the spectroscopic system used to view the horizontal target plate?
The KS3 and PMT systems view 10 chords of 20 mm diameter using fiber optics, therewith also covering the horizontal target plate.
Q7. What is the effect of redeposited tungsten on the surface?
A calculation of the promptly redeposited tungsten fraction predicts an order of magnitude lower net erosion,i.e. number of particle that really leave the target surface, compared to the gross erosion that has been characterized in the present work.
Q8. Why have the authors not applied the calculated S/XB values?
For the present analysis, the authors have not applied the calculated S/XB values because the uncertainty in the tungsten ground state temperature introduces an uncertainty in the S/XB value of an order of magnitude.
Q9. What is the effect of the ELM on the sputtering?
The top panel shows that this leads initially to a steep increase in the tungsten sputtering due to higher concentrations impurities (i.e. nitrogen), whereafter plasma cooling takes over to decrease the sputtering.
Q10. What was the spectroscopic system used to analyze the divertor particle flux?
The authors performed a consistency check on the integrated outer divertor particle flux (i.e. integrated over the strike point on the horizontal solid tungsten target plate) inferred from probe measurements and from Balmer epsilon intensity profiles.
Q11. What is the error bar for the effective erosion yields in JET?
The error bar drawn in the figure for the effective erosion yields in JET reflects the two main uncertainties in the analysis: the value of the photon efficiency S/XB (including the underlying uncertainty in the plasma temperature) and the determination of the plasma flux.
Q12. What was the ELM frequency of the sputtering?
For the specific example, the inter-ELM sputtering amounted to 6.3 × 1018 atoms/s (following from integration over the entire outer strike point).
Q13. What is the tungsten particle flux densities at the peak of the profile?
The tungsten particle flux densities at the peak of the profile were normalized to the saturation current measured by Langmuir probes at the same radial position.
Q14. What is the dependence plotted in Fig. 6?
For a magnetic field of 2.0 T, which was typical for the JET experiments discussed in the present paper, this resulted in the dependence plotted in Fig.
Q15. What was the perspective of sputtering by carbon impurities?
These were determined before any boronization had been performed and were put in the perspective of sputtering by carbon impurities as a placeholder for all low-Z impurity ions present in the divertor target particle flux.