A. Lasa

Loop punching and bubble rupture causing surface roughening —A model for W fuzz growth

We develop a multi-scale computational model for studying tungsten fuzz formation under low-energy He irradiation. The molecular dynamics and kinetic Monte Carlo results show that the W fuzz growth mechanism is the following: the He atoms are trapped in W, forming bubbles and causing growth by loop punching. The bubbles close to the surface rupture. The balance between these processes leads to a stochastic surface growth, causing the surface roughness and fuzz thickness growth to scale as . The growth rates agree with the experimental results.

Multiscale modelling of plasma-wall interactions in fusion reactor conditions

The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten.

Developing and validating advanced divertor solutions on DIII-D for next-step fusion devices

A major challenge facing the design and operation of next-step high-power steady-state fusion devices is to develop a viable divertor solution with order-of-magnitude increases in power handling capability relative to present experience, while having acceptable divertor target plate erosion and being compatible with maintaining good core plasma confinement. A new initiative has been launched on DIII-D to develop the scientific basis for design, installation, and operation of an advanced divertor to evaluate boundary plasma solutions applicable to next step fusion experiments beyond ITER. Developing the scientific basis for fusion reactor divertor solutions must necessarily follow three lines of research, which we plan to pursue in DIII-D: (1) Advance scientific understanding and predictive capability through development and comparison between state-of-the art computational models and enhanced measurements using targeted parametric scans; (2) Develop and validate key divertor design concepts and codes through innovative variations in physical structure and magnetic geometry; (3) Assess candidate materials, determining the implications for core plasma operation and control, and develop mitigation techniques for any deleterious effects, incorporating development of plasma-material interaction models. These efforts will lead to design, installation, and evaluation of an advanced divertor for DIII-D to enable highly dissipative divertor operation at core density (nmorexa0» e/n GW), neutral fueling and impurity influx most compatible with high performance plasma scenarios and reactor relevant plasma facing components (PFCs). In conclusion, this paper highlights the current progress and near-term strategies of boundary/PMI research on DIII-D.«xa0less

Atomistic simulations of Be irradiation on W: mixed layer formation and erosion

In the recent ITER-Like Wall experiment at JET, tungsten (W) and beryllium (Be) are used as the first wall plasma-facing materials. Due to the plasma–wall interactions, these materials will erode, be transported, re-deposit and mix. We present the first computational, atomistic, systematic study on the W–Be material mixing under fusion-relevant conditions. To this end, W surfaces were irradiated by Be, varying the impacting energy and angle, followed by annealing the mixed W–Be layers. At low energies, a Be layer is deposited on W, suppressing the W erosion. The materials mix as the W atoms migrate towards the Be layer due to the heat of mixing. Be2 and BeW molecules eroded, both physically (dimer sputtering) and chemically (sputter etching). All the mixed layers show an underlying hcp-like Be structure and the Be : W ratios are close to those in the intermetallic phases (Be2W—Be12W). However, no crystalline alloy structure formed, even after annealing. Further, we present a geometrical model for the angular dependence of the Be reflection, which strongly affects the W sputtering.

Estimates of RF-Induced Erosion at Antenna-Connected Beryllium Plasma-Facing Components in JET

During high-power, ion cyclotron resonance heating (ICRH), RF sheath rectification and RF induced plasma-wall interactions (RF-PWI) can potentially limit long-pulse operation. With toroidally-spaced ICRH antennas, in an ITER-like wall (ILW) environment, JET provides an ideal environment for ITER-relevant, RF-PWI studies. JET pulses combining sequential toggling of the antennas with q95 (edge safety factor) sweeping were recently used to localize RF-enhanced Be I and Be II spectral line emission at outboard poloidal (beryllium) limiters. These measurements were carried out in the early stages of JET-ILW and in ICRF-only, L-mode discharges. The appearance of enhanced emission spots was explained by their magnetic connection to regions of ICRH antennas associated with higher RF-sheath rectification [1]. The measured emission lines were the same as those already qualified in ERO modelling of inboard limiter beryllium erosion in JET limiter plasmas [2]. In the present work, we revisit this spectroscopic study with the focus on obtaining estimates of the impact of these RF-PWI on sputtering and on net erosion of the affected limiter regions. To do this, the ERO erosion and re-deposition code [2] is deployed with the detailed geometry of a JET outboard limiter. The effect of RF-PWI on sputtering is represented by varying themorexa0» surface negative biasing, which affects the incidence energy and the resulting sputtering yield. The observed variations in line emission, from [1], for JET pulse 81173 of about factor 3 can be reproduced with ~ 100 200 V bias. ERO simulations show that the influence of the respective E-field on the local Be transport is localized near the surface and relatively small. Still, the distribution of the 3D plasma parameters, shadowing and other geometrical effects are quite important. The plasma parameter simulated by Edge2D-EIRENE [3] are extrapolated towards the surface and mapped in 3D. These initial modelling results are consistent with the range of potentials anticipated through RF sheath rectification (see, e.g., [4]). Shortcomings from both the modelling and experimental side will be discussed, as will be plans for improvements in both areas method for the upcoming 2015 - 2016 JET campaign. [1] C.C. Klepper et al., J. Nucl. Mater. 438 (2013) S594 S598 [2] D. Borodin et al., Phys. Scr. T159 (2014) 014057 [3] M. Groth et al., Nucl. Fusion 53 (2013) 093016 [4] Jonathan Jacquot et al., Phys. Plasmas 21 (2014) 061509 *Corresponding author: presently at CCFE (UK) tel.: +44 1235 46 4304, e-mail: kleppercc@ornl.gov **See the Appendix of F. Romanelli et al., Proc. of the 25th IAEA Fusion Energy Conference 2014, Saint Petersburg, Russia Work supported, in part, by US DOE under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.«xa0less

Modelling of W?Be mixed material sputtering under D irradiation

We present an atomistic study on the D irradiation on W?Be mixtures, including a comparison between molecular dynamics (MD) and binary collision approximation methods. We compared the D reflection and Be erosion yields after the non-cumulative D impacts, concluding that both methods agree qualitatively, but low-energy irradiation related chemical effects can be recognized in MD. We also followed the evolution of W?Be mixtures under cumulative D irradiation. At low energies, the surface deuterates, quickly saturating the D reflection and suppressing the Be erosion.

The effect of beryllium on deuterium implantation in tungsten by atomistic simulations

Tungsten (W) and beryllium (Be) have been chosen as plasma-facing materials for the ITER reactor and the main fuel component will be deuterium (D). Due to plasma–wall interactions, these materials will immediately mix via erosion, transport and re-deposition. We present the first atomistic study on the effect of D co-implanted with Be into W, by modelling D plus Be irradiation of W surfaces, at projectile energies and compositions relevant for the plasma–wall interactions. The D implantation and Be sticking yields increased with the Be fraction in the system, especially at the lowest energies, as a Be layer was deposited on the surface. Tungsten was sputtered by Be, although the yield was partially suppressed by the deposited Be–D layer, and the Be erosion was determined by the balance between the Be concentration at the surface and projectile energy. Molecules were also sputtered: a large fraction of the D is reflected as D2, and purely metallic molecules (Be2, BeW) as well as different Be–D compounds were sputtered. On the other hand, the D clustered when implanted in or beneath a pre-deposited Be–W layer.

SURFACE BIASING INFLUENCE ON THE PHYSICAL SPUTTERING IN FUSION DEVICES

A new simplified analytical expression for the electromagnetic field in the Debye sheath in the presence of an oblique magnetic field including surface biasing effect is suggested. It is in good agreement with the numerical solution of the integral equation for the potential distribution in the Debye sheath. The energy and angular impact distributions and corresponding surface sputtering yields were analyzed in the presence of an oblique magnetic field and surface biasing. The analytical expression was used to estimate a) the effective sputtering yield of the W target with a varying negative voltage against plasma in PSI-2 linear device and b) erosion of the JET outer wall Be limiter near the ICRH antenna enhanced during RF emission.