Density dependence of SOL power width in ASDEX upgrade L-Mode

AbstractUnderstanding the heat transport in the scrape-off layer (SOL) and the divertor region is essential for the design of large fusion devices such as ITER and DEMO. Current scalings for the power fall-off length λq in H-Mode [1] are available only for the outer divertor target at low densities with low recycling divertor conditions. For the divertor power spreading S only an empirical scaling for ASDEX Upgrade L-Mode is available based on global plasma parameters [2]. Modelling using SOLPS shows a dependence of S on the divertor electron temperature [3]. A more detailed analysis of the heat transport forming λq and S is presented for ASDEX Upgrade L-Mode discharges in hydrogen (H), deuterium (D) and helium (He). For low densities the power fall-off length λq,o on the outer divertor target in H and D is described by the same parametric dependencies as the H-Mode scaling [1] but with a larger absolute size of the power fall-off length in L-Mode.The divertor power spreading S is studied using the local divertor measurements of the target electron temperature Te,tar and density ne,tar. It is found that the competition of the diffusive transport parallel and perpendicular to the magnetic field forming S∝χ⊥/χ∥ is dominated by the temperature dependence of parallel electron conduction. For high divertor temperatures the ion gyro radius has a significant contribution to S, resulting in a minimum of S at ∼30 eV.A recent study [4] with an open divertor configuration found an asymmetry of the power fall-off length between inner and outer target with a smaller power fall-off length λq,i on the inner divertor target. Measurements with a closed divertor configuration find a similar asymmetry for low recycling divertor conditions. It is found, in the experiment, that the in/out asymmetry λq,i/λq,o is strongly increasing with increasing density. Most notably the heat flux density at the inner divertor target is reducing with increasing λq,i whilst the total power onto each divertor target stays constant. It is found that λq,o exhibits no significant density dependence for hydrogen and deuterium but increases with about the square root of the electron density for helium. The difference between H,D and He could be due to the different recycling behaviour in the divertor. These findings may help current modelling attempts to parametrize the density dependence of the widening of the power channel and thus allow for detailed comparison to both divertor effects like recycling or increased upstream SOL cross field transport

ELM divertor peak energy fluence scaling to ITER with data from JET, MAST and ASDEX upgrade

A newly established scaling of the ELM energy fluence using dedicated data sets from JET operation with CFC & ILW plasma facing components (PFCs), ASDEX Upgrade (AUG) operation with both CFC and full-W PFCs and MAST with CFC walls has been generated. The scaling reveals an approximately linear depen- dence of the peak ELM energy with the pedestal top electron pressure and with the minor radius; a square root dependence is seen on the relative ELM loss energy. The result of this scaling gives a range in parallel peak ELM energy fluence of 10–30 MJm −2 for ITER Q = 10 operation and 2.5–7.5 MJm −2 for in- termediate ITER operation at 7.5 MA and 2.65 T. These latter numbers are calculated using a numerical regression ( ε II = 0 . 28 MJ m 2 n 0 . 75 e T 1 e E 0 . 5 ELM R 1 geo ). A simple model for ELM induced thermal load is introduced, resulting in an expression for the ELM energy fluence of ε II ∼= 6 πp e R geo q edge . The relative ELM loss energy in the data is between 2–10% and the ELM energy fluence varies within a range of 10 0.5 ∼3 con- sistently for each individual device. The so far analysed power load database for ELM mitigation experi- ments from JET-EFCC and Kicks, MAST-RMP and AUG-RMP operation are found to be consistent with both the scaling and the introduced model, ie not showing a further reduction with respect to their pedestal pressure. The extrapolated ELM energy fluencies are compared to material limits in ITER and found to be of concern.RCUK Energy Programme P012450/1EURATOM 63305

Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade

Sustained ELM mitigation has been achieved on MAST and AUG using RMPs with a range of toroidal mode numbers over a wide region of low to medium collisionality discharges. The ELM energy loss and peak heat loads at the divertor targets have been reduced. The ELM mitigation phase is typically associated with a drop in plasma density and overall stored energy. In one particular scenario on MAST, by carefully adjusting the fuelling it has been possible to counteract the drop in density and to produce plasmas with mitigated ELMs, reduced peak divertor heat flux and with minimal degradation in pedestal height and confined energy. While the applied resonant magnetic perturbation field can be a good indicator for the onset of ELM mitigation on MAST and AUG there are some cases where this is not the case and which clearly emphasise the need to take into account the plasma response to the applied perturbations. The plasma response calculations show that the increase in ELM frequency is correlated with the size of the edge peeling-tearing like response of the plasma and the distortions of the plasma boundary in the X-point region.Comment: 31 pages, 28 figures. This is an author-created, un-copyedited version of an article submitted for publication in Nuclear Fusion. IoP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i