JET methane screening experiments

Methane screening experiments were performed in JET L-Mode plasmas. Potentially, methane puffing quantifies the SOL processes governing the intrinsic carbon, its penetration to the plasma core, and its removal to the divertor. We measured the perturbations to the core and the SOL plasmas as a function of CD4 and D2 injection rates. The deduced screening was independent of the methane injection rate indicating that the methane itself did not significantly perturb the deduced screening. Identification of appropriate reference plasmas is important. For JET L-Modes, the reference plasma should have enough deuterium puffing to achieve the same density as that which occurred with the methane. The screening results are susceptible to systematic errors in the visible and charge exchange determination of the core carbon density. For JET, these two measurements yield screening values that differ by a `factor-of-threeʹ.