In order to map the complex plasma footprint observed on the lower divertor of the WEST experiment after its first phase of operation, we analysed deposits collected on ITER-like Plasma Facing Units ...(PFUs) exposed in the C3 (deuterium plasma) and C4 (deuterium and helium plasma) campaigns. Our results show that these deposits have multilayer structures mainly composed of tungsten, oxygen, boron and carbon, whose texture and composition vary along the radial direction. These traits allowed us to identify three types of deposits: deposits rich in boron (conditioning) in the low plasma flux area further away from the strike point, deposits rich in tungsten with traces of metallic compounds (Cu, Fe, Cr, Ni, Ag) in the high plasma flux area and deposits rich in boron and tungsten in the private flux area. In addition, we found more nanoparticles, voids and tungsten oxide layers in the deposits formed during C4 in comparison to that of C3.
In a reactor grade device, the role of core fueling is to replace the D and T consumed in the fusion reactions (almost negligible) and to compensate the plasma losses through the separatrix - ...including the material expelled out by the ELMs. For this purpose, deep material deposition is an advantage and pellet injection the best candidate for fueling the future machines. Fueling by pellet injection consists in two phases: First, the pellet ablation itself, then the ablated material homogenization and drift in the discharge. The former is a self-regulated process, which depends only of the local plasma characteristics. The second is a global phenomenon, which depends on the whole magnetic configuration. In this paper, we discuss first the basics of the ablation physics, emphasizing the role of the fast particles - ions and electrons - resulting from NBI or wave heating; then we describe the homogenization process and associated ∇B-induced drift. The drift acceleration and damping processes are described as well as the influence of the magnetic configuration (tokamak, stellarator and reversed field pinch) on the predominance of a given damping process and its consequence on the resulting deposition profile. We finally review the last results relative to pellet fueling in these different kind of devices and present the ongoing projects for future large-scale machines.
Impact of the helium plasma exposure on the surface modification in tungsten and reduced activation ferritic/martensitic (RAFM) steel have been investigated on the linear plasma device PSI-2 assuming ...the condition of DEMO first wall. In tungsten, a nanoscale undulating surface structure, which has a periodic arrangement, is formed under low temperature conditions below fuzz nanostructure formation threshold ∼1000 K. Interval and direction of the undulation shows dependence on the crystal orientation. A large variation in surface level up to 200 nm has been observed among grains at a fluence of 3 × 10 26 He m−2 showing dependence of the surface erosion rate on the crystal orientation. The {100} plane in which the undulating surface structure is not formed shows the highest erosion rate. This significant erosion is due to the multistage sputtering through impurity. In RAFM steel, sponge-like nanostructure is developed and it grows with increasing helium fluence beyond 1 m. In the sponge-like nanostructure, a composition change from the base material is observed in which the tungsten ratio increases while the iron ratio decreases showing differences in sputtering ratio depending on the atomic mass.
Optimizing the EU-DEMO pellet fuelling scheme Lang, Peter Thomas; Cismondi, Fabio; Day, Christian ...
Fusion engineering and design,
July 2020, 2020-07-00, 20200701, Letnik:
156
Journal Article
Recenzirano
Odprti dostop
•Elaborated suitable solution for the core particle fuelling of EU-DEMO fusion reactor.•Inboard pellet injection identified as suitable technique.•Conservative conventional approach, relying only on ...proven technical potential.•Next step: additional integration of plasma feedback control requirements.
Efficient fuelling will be an essential task in the EU-DEMO. The basic requirement here is to establish the target plasma core density with a minimum particle flux by injecting mm-size solid fuel pellets. Modelling showed this requires a pellet launch from the vessel inboard. Optimization can be achieved by the pellet parameters and the injection geometry; the latter however taking into account boundary conditions resulting from system integration needs. Design activities integrating the pellet transfer system into the vessel and the breeding blanket unveiled several possible variants requiring different levels of technical efforts. Basically, all extra efforts bear the benefit of a deeper and hence more favourable particle deposition. To quantify the potential gain, a full closed loop modelling was performed calculating the required pellet particle flux for any solution considered. Results allow now to balance potential advantages against related efforts required. Furthermore, the analysis tools developed can be employed for even more refined optimization of the pellet actuator tool by e.g. taking into account the interplay of pellet fuelling with burn control requirements.
We have recently incorporated the occupation probability formalism (OPF) in the simulation model C. Stehlé and S. Jacquemot, Astron. Astrophys. 271, 348 (1993) to have a smooth transition from ...discrete lines to continuum spectrum in the wavelength range near the Balmer series limit. We have analyzed spectra measured for the hydrogen pellet ablation cloud in the Large Helical Device with the revised model, and have found that the electron density in the ablation cloud has a close correlation with the electron temperature of the background plasma. This type of correlation is first confirmed in the present analysis and should give a new insight in the simulation studies of pellet ablation for the magnetically confined fusion plasma.
Abstract
WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source ...and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix,
n
sep
, similarly to ASDEX Upgrade (AUG) tokamak and Joint European Torus (JET) tokamak in H-mode (Verdoolaege
et al
2021
Nucl. Fusion
61
076006). Reflectometry in the midplane provides
n
sep
, while the temperature at the separatrix,
T
sep
is inferred by the ‘two-point model’ using Langmuir probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations (Eich
et al
2020
Nucl. Fusion
60
056016). As expected in the presence of an efficient neutral source due to recycling fluxes,
n
sep
correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density
n
ˉ
, lower
n
sep
correlates with a larger density gradient around the separatrix as well as a larger global density peaking,
n
ˉ
/
⟨
n
⟩
, measured by interferometry. The latter correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes (Angioni
et al
2007
Nucl. Fusion
47
1326). The correlations reported allow phrasing the subsequent causality question: what is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature and better core energy confinement quality? Understanding the causality chain is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumped flux with respect to the plasma volume will be lower than presently operating tokamaks.
The SYCOMORE code is a modular system code which aims at modelling future fusion power plants with all subsystems and to provide a global view of the whole plant. The code consists in different ...modules handling the different subsystems of the plant, from the core plasma to the conversion of heat to electricity. Among them, the divertor is one of the most important components and must withstand high heat load. While the complex magnetic configuration in tokamaks and the peculiar transport in the scrape-off layer (SOL) give rise to an asymmetry in the high field and low field energy fluxes, this issue should be properly addressed in SYCOMORE for quick and reliable predictions. In this work, the SOLDIV code which is a scrape-off-layer and divertor module in SYCOMORE has been used to investigate this asymmetry problem based on an extended two-point model. When the outgoing fluxes of particles and heat from the plasma core enter the SOL at the stagnation point, they split into two parts: one transporting to the inner divertor, and the other transporting to the outer divertor. By introducing the imbalance factor of the energy flux between the two divertor plates, the transport equations become a set of nonlinear equations that can be numerically solved for the densities and temperatures at both divertor plates and the stagnation point. Strong temperature and density differences at the targets can be found. The analysis results are validated with the transport code SolEdge2D-EIRENE for WEST test discharges. The simulation results for ITER are also investigated.
Quantitative characterization of the porosity structure of the carbon reinforced carbon fibre (CFC) materials is carried out by high resolution cone-beam X-ray micro-tomography (CBμCT). It is shown ...that CBμCT provides useful information pertaining to the in-depth fuel migration into carbon tiles function of the CFC material structure. The investigated materials comprised two types of non-irradiated CFC samples (former ITER reference CFC NB31 and JET CFC DMS780) and a series of CFC N11 samples in the frame of the Deuterium Inventory in Tore Supra (DITS)
post-mortem analysis. A procedure for the quantitative evaluation of the CFC porosity factor has been introduced and tested. Useful information about the pores connectivity as well as the metal impregnation inside the CFC macroscopic pores, in case of heat sink region of the TS CFC, can be retrieved. The method can be used for the quality control monitoring of the new CFC ITER reference materials.
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