Modeling the power flow in normal conductor-insulator-superconductor junctions

Normal conductor-insulator-superconductor (NIS) junctions promise to be interesting for x-ray and phonon sensing applications, in particular due to the expected self-cooling of the N electrode by the tunneling current. Such cooling would enable the operation of the active element of the sensor below the cryostat temperature and at a correspondingly higher sensitivity. It would also allow the use of MS junctions as microcoolers. At present, this cooling has not been realized in large area junctions (suitable for a number of detector applications). In this article, we discuss a detailed modeling of the heat flow in such junctions; we show how the heat flow into the normal electrode by quasiparticle back-tunneling and phonon absorption from quasiparticle pair recombination can overcompensate the cooling power. This provides a microscopic explanation of the self-heating effects we observe in our large area NIS junctions. The model suggests a number of possible solutions

Spectroscopic investigations of divertor detachment in TCV

The aim of this work is to provide an understanding of detachment at TCV with emphasis on analysis of the Balmer line emission. A new Divertor Spectroscopy System has been developed for this purpose. Further development of Balmer line analysis techniques has allowed detailed information to be extracted from the three-body recombination contribution to the n = 7 Balmer line intensity.During density ramps, the plasma at the target detaches as inferred from a drop in ion current to the target. At the same time the Balmer 6 → 2 and 7 → 2 line emission near the target is dominated by recombination. As the core density increases further, the density and recombination rate are rising all along the outer leg to the x-point while remaining highest at the target. Even at the highest core densities accessed (Greenwald fraction 0.7) the peaks in recombination and density may have moved not more than a few cm poloidally away from the target which is different to other, higher density tokamaks, where both the peak in recombination and density continue to move towards the x-point as the core density is increased.The inferred magnitude of recombination is small compared to the target ion current at the time detachment (particle flux drop) starts at the target. However, recombination may be having more localized effects (to a flux tube) which we cannot discern at this time. Later, at the highest densities achieved, the total recombination does reach levels similar to the particle flux

Impact of Divertor Shape on Divertor Performance in strongly Baffled Divertors on MAST Upgrade

Harnessing fusion energy efficiently requires optimising heat and particle exhaust in the edge from the fusion core plasma, which can be achieved through magnetic shaping of the divertor into Alternative Divertor Configurations (ADCs). In this study, we leverage MAST-U's unique shaping capabilities, which allow for a $\sim \times 2$ variation in the ratio of the magnetic field at the X-point and target ($B_{xpt}/B_t$), to investigate the power exhaust and core-edge compatibility of ADCs. Experiments show ADCs with large $B_{xpt}/B_t$ ratios drastically enhance divertor performance, with heat and particle loads reduced by factors up to $\sim 20$ and a 120 \% reduction in detachment onset. Notably, these benefits are achieved without compromising core plasma conditions. Our analysis attributes these improvements to the extra volume available below the ionisation front in longer leg-length divertors. This facilitates power dissipation and reduced particle loads through ion sinks from atomic (Electron-Ion Recombination) and molecular (Molecular-Activated Recombination) processes. The onset of divertor detachment and the evolution of the detachment front agrees with analytic models and divertor exhaust simulations. These insights emphasise the potential minor divertor geometry adjustments can have on power exhaust. This study illuminates pathways for devising optimised exhaust strategies in future fusion devices

Results from recent detachment experiments in alternative divertor configurations on TCV

Divertor detachment is explored on the TCV tokamak in alternative magnetic geometries. Starting from typical TCV single-null shapes, the poloidal flux expansion at the outer strikepoint is varied by a factor of 10 to investigate the X-divertor characteristics, and the total flux expansion is varied by 70% to study the properties of the super-X divertor. The effect of an additional X-point near the target is investigated in X-point target divertors. Detachment of the outer target is studied in these plasmas during Ohmic density ramps and with the ion ∇B drift away from the primary X-point. The detachment threshold, depth of detachment, and the stability of the radiation location are investigated using target measurements from the wall-embedded Langmuir probes and two-dimensional CIII line emissivity profiles across the divertor region, obtained from inverted, toroidally-integrated camera data. It is found that increasing poloidal flux expansion results in a deeper detachment for a given line-averaged density and a reduction in the radiation location sensitivity to core density, while no large effect on the detachment threshold is observed. The total flux expansion, contrary to expectations, does not show a significant influence on any detachment characteristics in these experiments. In X-point target geometries, no evidence is found for a reduced detachment threshold despite a 2-3 fold increase in connection length. A reduced radiation location sensitivity to core plasma density in the vicinity of the target X-point is suggested by the measurements

Nitrogen-seeded divertor detachment in TCV L-mode plasmas

Most of the detachment experiments done to date on the tokamak à configuration variable (TCV), both in standard and alternative divertor geometries, focused on L-mode integrated core density ramps. In view of extending these studies to high-power, high-confinement regimes, where impurity seeding will be necessary for detachment, the properties of nitrogen seeded L-mode detachment in TCV are assessed here with the extensive set of edge and divertor diagnostics and similarities and differences with integrated core density ramp detachment experiments are elucidated. It is found that in high current, reversed field plasmas, detachment at the outer target is achieved with N2-seeding and density ramps, with target heat flux reductions of up to 90%, while the inner target only detaches with seeding. The Scrape-Off Layer radiation fraction reaches values of 60%-80% and in all situations, a stable radiator can form around the X-point. The most striking difference between seeding and density ramp is the behavior of the upstream quantities. During the ramp, a broadening of the upstream density profile (density 'shoulder') occurs, concurrent with the outer target ion flux roll-over, while no such behavior occurs during nitrogen seeded detachment. Separatrix density, electron temperature and pressure also evolve strongly with increasing density, and are largely unaffected by the injection of nitrogen. Comparison of upstream and target pressures reveals that, in all cases, the outer target ion flux reduction coincides with the development of a parallel gradient of the total pressure. Common to all cases is also a reduction of energy confinement time with detachment, although this effect is weak for seeding at relatively high density. Studying the impact of the ∇B-drift direction in both nitrogen seeding and core density ramps reveals that drifts mainly affect the behavior at the inner strike point, highlighting the need to include them in edge transport simulations

Operation of Alcator C-Mod with high-Z plasma facing components and implications

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TCV divertor upgrade for alternative magnetic configurations

The Swiss Plasma Center (SPC) is planning a divertor upgrade for the TCV tokamak. The upgrade aims at extending the research of conventional and alternative divertor configurations to operational scenarios and divertor regimes of greater relevance for a fusion reactor. The main elements of the upgrade are the installation of an in-vessel structure to form a divertor chamber of variable closure and enhanced diagnostic capabilities, an increase of the pumping capability of the divertor chamber and the addition of new divertor poloidal field coils. The project follows a staged approach and is carried out in parallel with an upgrade of the TCV heating system. First calculations using the EMC3-Eirene code indicate that realistic baffles together with the planned heating upgrade will allow for a significantly higher compression of neutral particles in the divertor, which is a prerequisite to test the power dissipation potential of various divertor configurations

The Lantern Vol. 67, No. 1, Fall 1999

• I Write Poetry • So Many Poems Start This Way... • Mantra 99 for A. Joe • Indecision • Last Night • My Body Cut Off at the Back • Brittany Sestina • The Hearth • To Jane • Attention • Chemistry • Recombination • Buried • Going Home • Bound for Happiness • Stumble • Punk Rock • Ghost Spray and a Slingshot • Distant Voices • Beyond the Gear Shift • Damn It • Poppy Hands • Waist Deep • The Lie • Paying Your Dueshttps://digitalcommons.ursinus.edu/lantern/1155/thumbnail.jp