Role of core radiation during slow oscillations in LHD

During experiments in LHD using stainless steel divertor plates, a slow (∼1 s) cyclic oscillation in the plasma parameters known as 'breathing' plasma was observed during NBI heated long pulse discharges. Using an average ion, corona equilibrium model for the iron impurity cooling rate, the iron impurity density profile is calculated for 0.0 < ρ < 0.8 from the measured electron temperature and density profiles and radiation profile data for these discharges in LHD. This calculated iron density oscillates out of phase with the electron density and peaks near ρ = 0.4 at a fraction of the electron density that varies by a factor of 4. This is in qualitative agreement with spectroscopic measurements of iron that show a similar oscillation. The correlation of the iron impurity concentration with the change in electron temperature and with the local power balance between radiation and beam deposition indicates that when radiation from the iron impurity dominates the local power balance the core plasma is cooled. The increase in the calculated iron density during the phase of the oscillation when the divertor electron temperature exceeds the sputtering threshold suggests that sputtering of the stainless steel divertor plate may be the source of the iron impurity. Evidence of changing transport points to the need for a closer examination of the role of impurity transport in this oscillation.