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Performance of JET ICRH system since installation of the metal ITER-Like Wall (ILW) has been assessed statistically. The data demonstrate steady increase of the RF power coupled to plasmas over recent years with the maximum pulse-average and peak values exceeding respectively 6MW and 8MW in 2016. Analysis and extrapolation of power capabilities of conventional JET ICRH antennas is provided and key performance-limiting factors are discussed. The RF plant operational frequency options are presented highlighting the issues of efficient ICRH application within a foreseeable range of DT plasma scenarios.

A dedicated study on JET-ILW, deploying two types of ICRH antennas and spectroscopic observation spots at two outboard, beryllium limiters, has provided insight on long-range (up to 6m) RFenhanced plasma-surface interactions (RF-PSI) due to near-antenna electric fields. To aid in the interpretation of optical emission measurements of these effects, the antenna near-fields are computed using the TOPICA code, specifically run for the ITER-like antenna (ILA); similar modelling already existed for the standard JET antennas (A2). In the experiment, both antennas were operated in current drive mode, as RF-PSI tends to be higher in this phasing and at similar power (∼0.5 MW). When sweeping the edge magnetic field pitch angle, peaked RF-PSI effects, in the form of 2-4 fold increase in the local Be source,are consistently measured with the observation spots magnetically connect to regions of TOPICAL-calculated high near-fields, particularly at the near-antenna limiters. It is also found that similar RF-PSI effects are produced by the two types of antenna on similarly distant limiters. Although this mapping of calculated near-fields to enhanced RF-PSI gives only qualitative interpretion of the data, the present dataset is expected to provide a sound experimental basis for emerging RF sheath simulation model validation.

The works undertaken to reinstate the JET ICRF ILA are reviewed. The vacuum matching capacitors were replaced, an extensive calibration of all the measurements in the RF circuit was carried out, new simulation tools were created and new control algorithms were implemented for the – toroidal and poloidal – phase control of the array as well as for the matching of the second stage. A review of the contribution of the reinstated ILA to the JET programme during the last campaigns is given showing namely that the new controls allowed extending the range of the operation to lower (29MHz) and higher (51MHz) frequencies than previously achieved and allowed more flexible and reliable operation. Operation with coupled power levels up to 2.8MW and voltages up to 40kV was achieved. ILA results on plasma are discussed and emphasis is given to the features of interest for ITER.

Inverted 3He and D ion cyclotron minority heating scenarios were recently tested in JET-ILW. They confirm the good heating efficiency at low concentrations of ∼3%. The 3He minority heating scheme is only modestly affected by the change from a carbon (JET-C) to a Beryllium (JET-ILW) wall but unlike what was the case in JET-C, the intrinsic Be ions D-like particles in terms of charge-over-mass ratio do not prevent the D (or 4He) minority regime from being exploited. Direct and indirect evidence of the existence of fast particle subpopulations was found in both cases.

Multi-megawatt Ion Cyclotron Range of Frequencies (ICRF) heating is routinely used in the JET tokamak. To increase the ICRF heating power available from the A2 antennas, the ICRF ITER-Like Antenna (ILA) was reinstalled for the 2015 JET ITER-like wall experimental campaign. The application of high levels of ICRF power typically results in increased plasma wall interaction which leads to the observation of enhanced influx of metallic impurities in the plasma edge. It is assumed that the impurity production is mainly driven by the parallel component of the Radio-Frequency (RF) antenna electric near-field, E// (parallel to the confinement magnetic field of the tokamak), that is rectified in a thin boundary layer (RF sheath). Torino Polytechnic Ion Cyclotron Antenna (TOPICA) code was used to compute E// field maps in front of the ILA and between its poloidal limiters in the presence of plasma using measured density profiles and various antenna feedings. E// field maps calculated between the poloidal limiters were used to estimate the poloidal distribution of RF-sheath Direct Current (DC) potential in this private region of the ILA and make relative comparison of various antenna electrical settings. For this purpose we used the asymptotic version of the Self-consistent Sheaths and Waves for Ion Cyclotron Heating Slow-Wave (SSWICH-SW) code. These estimations can help to study the formation of RF sheaths around the antenna and even at distant locations (∼3m away).

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In free range grazed pastures, NEE estimations are supposed to also include cattle respiration. However, cattle respiration measurement by an EC system is challenging as animals act as moving points emitting CO2 that are more or less captured by the EC tower depending on their presence in the footprint. Often it is supposed that, over the long term, cattle distribution in the pasture is homogeneous so that fluctuations due to moving sources are averaged and NEE estimates are reasonably representative of cattle respiration. In this study, we test this hypothesis by comparing daily cow respiration rate per livestock unit (LU) estimated by postulating a homogeneous cow repartition over the whole pasture with three other estimates based on animal localization data, animal scale carbon budget and confinement experiments. We applied these methods to an intensively managed free range grassland and showed that the NEE estimate based on a homogeneous cow repartition was systematically lower than the three other estimates. Consequently, in order to allow estimating the validity of this hypothesis but also to improve inter site comparisons, we advocate to compute separately pasture NEE and grazer's respiration. In the presentation, we will propose a method based on cattle presence detection using CH4 fluxes, elimination of data with cattle and gap filling on the basis of data without cattle. For the second we propose three independent methods (animal localization with GPS, animal scale carbon budget, confinement experiments) to estimate the cattle respiration rate and discuss their use depending on site specificities. [ABSTRACT FROM AUTHOR]

Eddy covariance is increasingly used to monitor cattle emissions. However, these sources are only intermittently present in the footprint area, due to the movement of the source and to wind characteristics variations within one averaging period. The assumption of flux stationarity is thus breached and it is unclear how well the covariance of the scalar concentration and the vertical wind component at the measurement point is representative of the true flux. Moreover, the calculation of the source emission from the measured flux relies on the use of a footprint model and those models are insufficiently validated. In this study (Dumortier et al., 2019), we used a single known artificial point source placed at cow's muzzle height in order to assess the impact of the flux calculation method (averaging method, averaging period, quality filters) and of the footprint model on the emission estimates. The selected optimal combination (running mean, 15 minute averaging periods, no application of the Foken & Wichura (1996) stationarity filter, and the use of the KM (Kormann and Meixner, 2001) footprint model) led to estimated emissions between 90 and 113% of the true emission, leading to the conclusion that the use of eddy-covariance for point-source emission estimation is feasible provided an adequate calculation method is selected. However, the two tested footprint models, KM and FFP (Flux Footprint Prediction tool (Kljun et al., 2015)), chosen for their popularity and simplicity of use both force a source at ground level and not at the actual release height of 80 cm. This element is not trivial and might largely influence the choice of the best footprint model. For instance, the better performance of the KM footprint model over FFP might come from the cancellation of two systematic errors associated with the KM footprint model, one intrinsic to the model and the other due to the influence of the source height. As a follow up of the previous study, the impact of the source height will therefore be addressed through the use of a footprint model allowing source height modification, namely the FIDES footprint model (Loubet et al., 2001).REFERENCES:Dumortier, P., Aubinet, M., lebeau, F., Naiken, A., Heinesch, B., 2019. Point source emission estimation using eddy covariance: Validation using an artificial source experiment. Agricultural and Forest Meteorology 266–267, 148–156. https://doi.org/10.1016/j.agrformet.2018.12.012Foken, T., Wichura, B., 1996. Tools for quality assessment of surface-based flux measurements. Agricultural and Forest Meteorology 78, 83–105. https://doi.org/10.1016/0168-1923(95)02248-1Kljun, N., Calanca, P., Rotach, M.W., Schmid, H.P., 2015. A simple two-dimensional parameterisation for Flux Footprint Prediction (FFP). Geosci. Model Dev. 8, 3695–3713. https://doi.org/10.5194/gmd-8-3695-2015Kormann, R., Meixner, F., 2001. An Analytical Footprint Model For Non-Neutral Stratification. Boundary-Layer Meteorology 99, 207–224. https://doi.org/10.1023/A:1018991015119Loubet, B., Milford, C., Sutton, M.A., Cellier, P., 2001. Investigation of the interaction between sources and sinks of atmospheric ammonia in an upland landscape using a simplified dispersion-exchange model. Journal of Geophysical Research: Atmospheres 106, 24183–24195. https://doi.org/10.1029/2001JD900238 [ABSTRACT FROM AUTHOR]