33 research outputs found
Analytic 1D approximation of the divertor broadening S in the divertor region for conductive heat transport
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 SâÏâ„/Ïâ„ 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 âŒ30âeV.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
I-mode pedestal relaxation events at ASDEX Upgrade
The I-mode confinement regime can feature small edge temperature drops that
can lead to an increase in the energy deposited onto the divertor targets. In
this work, we show that these events are associated with a relaxation of both
electron temperature and density edge profiles, with the largest drop found at
the pedestal top position. Stability analysis of edge profiles reveals that the
operational points are far from the ideal peeling-ballooning boundary. Also, we
show that these events appear close to the H-mode transition in the typical
I-mode operational space in ASDEX Upgrade, and that no further enhancement of
energy confinement is found when they occur. Moreover, scrape-off layer
transport during these events is found to be very similar to type-I ELMs, with
regard to timescales ( 800 s), filament propagation, toroidally
asymmetric energy effluxes at the midplane and asymmetry between inner and
outer divertor deposited energy. In particular, the latter reveals that more
energy reaches the outer divertor target. Lastly, first measurements of the
divertor peak energy fluence are reported, and projections to ARC - a reactor
designed to operate in I-mode - are drawn