The ISX-A graphite limiter experiment

Abstract

ATJS graphite rail limiters were installed in the ISX tokamak in order to compare the resulting discharges to those obtained with the existing stainless steel limiters and to investigate operation of the graphite limiters at elevated temperatures using heating elements inside the graphite. No large systematic differences were observed in the electron temperature profiles, electron density profiles, nor Zeff between successive runs with the stainless steel (SS) limiters and graphite (G) limiters. There was, however, a monotonic decrease in Zeff for both cases from ∼5.6 in the first run following the installation of the graphite limiters to –2.8 after a two week period when the experiment was terminated due to a scheduled shutdown. Normal incidence UV spectroscopic measurements of C, N, O, and H Radiation showed a factor of 1.5–4 greater light for the SS case over the G case. Arc tracks were observed on the graphite limiters upon removal and SEM analysis was performed so that the amount of material removed could be estimat, ed. As the graphite limiter was heated during a series of discharges, no increase was observed in either the plasma carbon contamination nor RGA hydrocarbon signals until the temperature rose above ~400°C.