Heat and particle transport onto plasma-facing components is a key issue for next generation tokamaks, as it will determine the erosion levels and the heat loads at the main chamber first wall. In the scrape-off layer (SOL), this transport is thought to be dominated by the perpendicular convection of filaments. In this work, we present recent experiments which have led to an improved picture of filamentary transport, and its role on the onset of a density profile flattening, known in the literature as the density "shoulder" r1s. First, L-mode experiments carried out in the three tokamaks of the ITER stepladder (COMPASS, AUG and JET) showed how normalized divertor collisionality r2s can be used to scale both filament size and the density e-folding length in the far SOL. Furthermore, a transition in the filament regime is found to be the reason for the formation of the density shoulder, as it coincided with a change in the scaling of filament size with propagation velocity from Sheath Limited regime to Inertial regime r3s. This result was later confirmed in AUG by independent experiments which showed how the polarization term in the charge conservation equation became dominant after the onset of the shoulder and how the transition was reversed as filaments propagate radially across regions of decreasing collisionality. Besides, measurements carried out in AUG with a Retarding Field Analyzer in equivalent discharges have led to the discovery of a strong reduction of T i in the far SOL after the onset of the shoulder, both in filaments and background plasmas, which can not be explained by the minor reduction of T i at the separatrix. Finally, equivalent experiments in H-mode carried out in AUG have shown how inter-ELM filaments follow the same general behaviour as L-mode filaments, and how a density profile flattening reminiscent of the density shoulder is observed when collisionality is increased over a similar threshold. Besides, Thomson Scattering data indicate the same sharp increase on the e-folding length of density and electron temperature in the near SOL above a critical collisionality. Abstract. A summary of recent experiments on filamentary transport is presented: L-mode density shoulder formation is explained as the result of a transition between sheath limited and inertial filamentary regime. Divertor collisionality is found to be the parameter triggering the transition. A clear reduction of the ion temperature takes place in the far SOL after the transition. This mechanism seems to be generally applicable to inter-ELM H-mode plasmas, although some refinement is still required.

The optical emission spectra of high pressure CO2 microwave plasmas are usually dominated by the C2 Swan bands. In this paper, the use of the C2 Swan bands for estimating the gas temperature in CO2 microwave plasmas is assessed. State by state fitting is employed to check the correctness of assuming a Boltzmann distribution for the rotational and vibrational distribution functions and, within statistical and systematic uncertainties, the C2 Swan band can be fitted accurately with a single temperature for rotational and vibrational levels. The processes leading to the production of the C2 molecule and particularly its d 3 Π g state are briefly reviewed as well as collisional relaxation times of the latter. It is concluded that its rotational temperature can be associated to the gas temperature of the CO2 microwave plasma and the results are moreover cross-checked by adding a small amount of N2 in the discharge and measuring the CN violet band system. The 2.45 GHz plasma source is analyzed in the pressure range 180–925 mbar, for input microwave powers ranging from 0.9 to 3 kW and with gas flow rates of 5–100 l min−1. An intense C2 Swan bands emission spectrum can be measured only when the plasma is operated in contracted regime. A unique temperature of about 6000 ± 500 K is obtained for all investigated conditions. A spectroscopic database is constructed using the recent compilation and calculations by Brooke et al (2013 J. Quant. Spectrosc. Radiat. Transfer 124 11–20) of the line strengths and molecular constants for the C2 (d 3 Π g −a 3 Π u ) Swan bands system and made available as supplementary material in a format compatible with the open source MassiveOES software.

Microwave plasmas are a promising technology for energy-efficient CO2 valorization via conversion of CO2 into CO and O2 using renewable energies. A 2.45 GHz microwave plasma torch with swirling CO2 gas flow is studied in a large pressure (20–1000 mbar) and flow (1–100 L min−1) range. Two different modes of the plasma torch, depending on the operating pressure and microwave input power, are described: at pressures below 120 mbar the plasma fills most of the plasma torch volume whereas at pressures of about 120 mbar an abrupt contraction of the plasma in the center of the resonator is observed along with an increase of the gas temperature from 3000 K to 6000 K. The CO outflow is generally found to be proportional to the plasma effective power and exhibits no significant dependence on the actual CO2 flow injected into the reactor but only on the input power at certain pressure. Thermal dissociation calculations show that, even at the lowest pressures of this study, the observed conversion and energy efficiency are compatible with a thermal dissociation mechanism.

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A mass spectrometer with a custom sampling system comprising one fixed and one variable orifice is presented. The custom sampling system allows the determination of the gas composition in the pressure range from 5 mbar to 1000 mbar, with low gas-demixing (<1.5%). A case study of mass spectrometer optimization and calibration for the measurement of relative concentration of CO2, CO, O2, and N2 gases is presented, together with an example of the CO2 conversion at a microwave plasma torch. The absolute error of the measured conversion of CO2 in CO is found to be less than 1.6% in the complete pressure range. The conversion determination routine presented here allows us to determine relative molar flows of CO2, CO, O2, and N2 and to distinguish between CO and N2 gases, which is important for the determination of the CO2 conversion in the case of air impurities or in the case of CO2/N2 mixtures.