Search

The triggering of rapid small edge localized modes (ELMs) by high frequency pellet injection has been proposed as a method to prevent large naturally occurring ELMs that can erode the ITER plasma facing components (PFCs). Deuterium pellet injection has been used to successfully demonstrate the on-demand triggering of edge localized modes (ELMs) at much higher rates and with much smaller intensity than natural ELMs. The proposed hypothesis for the triggering mechanism of ELMs by pellets is the local pressure perturbation resulting from reheating of the pellet cloud that can exceed the local high-n ballooning mode threshold where the pellet is injected. Nonlinear MHD simulations of the pellet ELM triggering show destabilization of high-n ballooning modes by such a local pressure perturbation. A review of the recent pellet ELM triggering results from ASDEX Upgrade (AUG), DIII-D, and JET reveals that a number of uncertainties about this ELM mitigation technique still remain. These include the heat flux impact pattern on the divertor and wall from pellet triggered and natural ELMs, the necessary pellet size and injection location to reliably trigger ELMs, and the level of fueling to be expected from ELM triggering pellets and synergy with larger fueling pellets. The implications of these issues for pellet ELM mitigation in ITER and its impact on the PFCs are presented along with the design features of the pellet injection system for ITER. [ABSTRACT FROM AUTHOR]

Strong support for the critical pressure gradient model for the heat flux width has been obtained, in that the measured separatrix pressure gradient lies below and scales similarly to the pressure gradient limit obtained from the ideal, infinite-n stability codes, BALOO and 2DX, in all cases that have been examined. Predictions of a heuristic drift model for the heat flux width are also in qualitative agreement with the measurements. These results have been obtained using an improved high rep-rate and higher edge spatial resolution Thomson scattering system on DIII-D to measure the upstream electron temperature and density profiles. In order to compare theory and experiment, profiles of density, temperature, and pressure for both electrons and ions are needed as well values of these quantities at the separatrix. A simple method to identify a proxy for the separatrix has been developed to do so. [ABSTRACT FROM AUTHOR]

The compatibility of detached radiative divertor operation with a robust H-mode pedestal is examined in DIII-D. A density scan produced low temperature plasmas at the divertor target, T e ⩽ 2 eV, with high radiation leading to a factor of ⩾4 drop in peak divertor heat flux. The cold radiative plasma was confined to the divertor and did not extend across the separatrix in X-point region. A robust H-mode pedestal was maintained with a small degradation in pedestal pressure at the highest densities. The response of the pedestal pressure to increasing density is reproduced by the EPED pedestal model. However, agreement of the EPED model with experiment at high density requires an assumption of reduced diamagnetic stabilization of edge Peeling–Ballooning modes. [ABSTRACT FROM AUTHOR]

Recent DIII-D experiments assessed the snowflake divertor (SF) configuration in a radiative regime in H-mode discharges with D 2 seeding. The SF configuration was maintained for many energy confinement times (2–3 s) in H-mode discharges ( I p = 1.2 MA, P NBI = 4 – 5 MW, and B × ∇ B down (favorable direction toward the divertor)), and found to be compatible with high performance operation (H98y2 ⩾ 1). The two studied SF configurations, the SF-plus and the SF-minus, have a small finite distance between the primary X-point and the secondary B p null located in the private flux region or the common flux region, respectively. In H-mode discharges with the SF configurations (cf. H-mode discharges with the standard divertor with similar conditions) the stored energy lost per the edge localized mode (ELM) was reduced, and significant divertor heat flux reduction between and during ELMs was observed over a range of collisionalities, from lower density conditions toward a higher density H-modes with the radiative SF divertor. [ABSTRACT FROM AUTHOR]

Abstract: A previous experiment in the National Spherical Torus Experiment (NSTX) showed that pre-discharge lithium deposition gradually suppressed edge-localized modes (ELMs) and had nearly continuous relationships with reduced recycling and transport. In this paper, additional data filled gaps in the earlier experiment, and demonstrates that recycling, confinement, and pedestal structure continued to improve with additional lithium, even after ELMs were completely suppressed. New analysis shows that toroidal rotation and ion temperature also increased continuously with additional lithium. Besides its evolution with additional lithium, we also characterize the time evolution of the ELM-free H-mode pedestal as average density rose and impurities accumulated. We find that the pedestal structure, divertor heat flux and D α profiles, and inferred recycling coefficient did not change significantly, at least until radiative losses become dominant. This suggests that the low-recycling properties of lithium were not significantly degraded over the duration of the discharge. [Copyright &y& Elsevier]

Abstract: A cross machine analysis of heat flux profile measurements has produced a particularly simple scaling law for the scrape-off-layer (SOL) width, going as with e ∼−1.0 where Bp , mp is the poloidal magnetic field at the outer midplane separatrix. This result is in very good agreement with a model based on drift-induced transport. We are also developing a model that extends the kinetic ballooning mode pedestal paradigm to the separatrix and into the SOL. We have calculated the critical pressure gradient at the separatrix using the BALOO code and found that it scales proportionately with the measured pressure gradient. Using an improved high rep-rate and higher edge resolution Thomson scattering system on DIII-D, it is now possible to make detailed comparisons of SOL profile characteristics with the heat flux profile. We find that a two-point flux-limited model is in better agreement with measurements than a conduction-limited model. [Copyright &y& Elsevier]