The impact of divertor detachment on carbon sources in JET L-mode discharges

Hydrocarbon injection experiments have been performed to investigate the chemical sputtering yield of carbon-fibre composites at elevated temperatures (Tsurface≃500K) and detached plasma conditions in the JET outer divertor. A plasma scenario in L-mode with the outer strike-point positioned on the load bearing septum replacement plate was developed to detach the outer divertor leg and to provide the strong recombining target plasma. The operational window was explored in a set of discharges with strong deuterium puffing into the private-flux region. Langmuir probes showed a clear roll-over in the ion flux up to complete plasma detachment at the target plate during the density ramp of the discharge. Paschen recombination line analysis shows strong volume recombination with electron temperatures below 2.0 eV prior to the appearance of a radiation instability – an X-point Marfe. A strong reduction of the intrinsic CD   photon flux was observed under these conditions, whereas local CD4CD4 injection still led to CD photon flux emission. This provides conclusive evidence for a reduction of the hydrocarbon flux and the chemical sputtering under detached plasma conditions.