Neutral pathways and heat flux widths in vertical- and horizontal-target EDGE2D-EIRENE simulations of JET

This paper further analyses the EDGE2D-EIRENE simulations presented by Chankin et al (2017 Nucl. Mater. Energy 12 273), of L-mode JET plasmas in vertical-vertical (VV) and Vertical-horizontal (VH) divertor configurations. As expected, the simulated outer divertor ionisation source peaks near the separatrix in VV and radially further out in VH. We identify the reflections of recycled neutrals from lower divertor tiles as the primary mechanism by which ionisation is concentrated on the outer divertor separatrix in the VV configuration. These lower tile reflection pathways (of neutrals from the outer divertor, and to an even greater extent from the inner divertor) dominate the outer divertor separatrix ionisation. In contrast, the lower-tile-reflection pathways are much weaker in the VH simulation and its outer divertor ionisation is dominated by neutrals which do not reflect from any surfaces. Interestingly, these differences in neutral pathways give rise to strong differences in the heat flux density width λq at the outer divertor entrance: λq = 3.2 mm in VH compared to λq = 11.8 mm in VV. In VH, a narrow channel exists in the near scrape-off-layer (SOL) where the convected heat flux, driven by strong Er × B flow and thermoelectric current, dominates over the conducted heat flux. The width of this channel sets λq and is determined by the radial distance between the separatrix and the ionisation peak in the outer divertor

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

The low temperature boundary layer plasma (Scrape-Off-Layer or SOL) between the hot core and the surrounding vessel determines the level of power-loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces – and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νei Ωi ]/cs Ωe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in upstream SOL shoulder amplitude, As only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv with As was also found for H-mode discharges. Thus, while Λdiv above a threshold of ~1 may be necessary for shoulder formation and/or growth, another shoulder mechanism is required. More significantly we find that in contrast to parallel resistivity, outer divertor recycling as quantified by the total outer divertor Balmer Dα emission, I-Dα, does scale with shoulder amplitude where Λdiv does and even where Λdiv fails. Divertor recycling could lead to SOL density shoulder formation through: a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and b) changes in radial electric fields which lead to ExB poloidal flows as well as potentially affecting the SOL turbulence birth characteristics. Thus changes in divertor recycling may be the sole process in bringing about SOL density shoulders or in tandem with parallel resistivity

Observations and modelling of ion cyclotron emission observed in JET plasmas using a sub-harmonic arc detection system during ion cyclotron resonance heating

Peer reviewe

The activation spectrometry for the purpose of the Second Deuterum-Tritium Experimental Campaign on JET tokamak

Metoda aktywacyjna stosowana do charakteryzacji różnych źródeł neutronów, należy do jednej z najstarszych metod stosowanych w neutronice. Pomimo tego jest wciąż ważna, gdyż prowadzi do otrzymywania wartości bezwzględnych wydajności neutronów, a czasami pozwala na dekonwolucje ich rozkładu energetycznego. Druga Deuterowo- Trytowa Kampania Eksperymentalna na Wspólnym Europejskim Tokamaku JET jest wielkim wyzwaniem. Po jej zakończeniu nastąpi zamknięcie i odstawienie JETa, a tym samym zakończenie jakże ważnego rozdziału w historii fizyki plazmy. Przed kampanią deuterowo-trytową nastąpi kalibracja diagnostyk neutronowych tokamaka i to z niepewnością niższą niż 5%. Technika aktywacyjna posłuży do charakteryzacji generatora neutronów, który będzie zastosowany, jako ustandaryzowane źródło neutronów 14 MeV, a następnie będzie zastosowana w czasie ostatniej kampanii eksperymentalnej do monitorowania wydajności neutronów.The activation technique used for characterization of many different neutron sources belongs to the oldest one applied in neutronics area. It is still important because it leads to obtainment of obsolete value of neutron yield and from time to time allows neutron spectrum deconvolution. The Second Deuterium-Tritium Experimental Campaign that is held on Joint European Torus is a great challenge. After completing the above campaign the shut down and decommissioning of the JET is expected. Thus, the important chapter in history of plasma physics will be definitely closed. But before the deuterium-tritium campaign starts, the calibration of neutron diagnostic should be completed with uncertainty better than 5%. The activation technique is used for neutron diagnostics characterization that is chosen as the source of 14 MeV neutrons. Afterwards, the activation technique will be applied as neutron yield monitor during last experimental campaign on JET tokamak

Preliminary determination of angular distribution of neutrons emitted from PF-1000 facility by indium activation

This paper presents a new method applied to measure the angular neutron emission from Plasma Focus (PF) - type deuterium discharges performed within the PF-1000 facility. Neutrons were recorded by activation of especially optimized (mass and shape) indium samples with subsequent gamma spectrometry to measure the induced activity of the samples. The neutron fluence rate on every indium sample used was determined using neutron transport calculation and measured activity of the samples