Spectroscopic measurements of Be erosion at JET ILW and interpretation with ERO modelling
Introduction
The erosion of beryllium (Be) determines the life time of first wall components in some fusion devices including ITER, the Be influx into the plasma (dilution) and affects tritium retention due to co-deposition. The 3D Monte-Carlo (MC) impurity transport and plasma–surface interaction code ERO was applied for simulation of the ITER blanket modules (BMs) life time and compared with earlier LIM calculations [3] as well as for the ITER divertor duty cycle, which is dominantly limited by retention [4]. For instance, it was shown that the uncertainties (Section 2, Table 1) in the physical sputtering data lead to BM life time estimations varying within a factor of 4. Therefore, an assessment of Be sputtering in existing experiments is of great importance. In particular it is important to validate the models and underlying data at the JET ITER-like wall [1], whose plasma facing parts have similar power-load optimised geometry as in ITER in combination with similar 3D configuration of plasma and electromagnetic field. The same ERO code is also continuously applied to erosion experiments at the linear plasma device PISCES-B [5].
The focus of this paper are Be erosion measurements using passive spectroscopy at JET ILW during a plasma density scan. The limiter configuration plasmas used in this experiment are suitable for assessing Be erosion, as it is significantly larger than in the divertor configuration due to smaller wall clearance. In addition, limiter plasmas are relevant for the ITER start-up phase. The spectroscopic system measures integrated light in the vicinity of the solid shaped Be limiter. It provides time-resolved simultaneous recording of several BeI, BeII and D (deuterium) lines. The line emission can be converted into the respective particle fluxes using reverse photon efficiencies S/XB [6] provided by ADAS [7]. The ratio between Be and D fluxes gives an estimate for an effective physical sputtering yield, which is assumed to be the main erosion mechanism. The precision of spectroscopic measurements as well as S/XB values is usually about 20%.
However, the effective yield includes an additional erosion contribution due to self-sputtering by Be impurity contained in the plasma (this effect is changing, most important at large electron temperatures and low densities). In addition, local geometry and other factors pertain to the actual experiment. Moreover, sputtering yields are known to depend on the incident energy and angle [8]. Distributions of these particle impact parameters are unique for every surface point due to complex limiter shape and varying plasma parameters. Thus, the interpretation of the measured effective yield and its extrapolation for ITER is elaborate.
Therefore, to obtain the physical sputtering yield, which should be universal for all experimental situations, detailed 3D modelling is necessary, for example by the ERO code mentioned above. MC simulations allow reproducing the observed line emission assuming an appropriate sputtering yield, however it is challenging to apply such approach for solution of the inverse problem – to determine the yield from the measured light. Therefore, we use high and low estimates (Section 2) for the yield deduced on the basis of various simulated data in the form of the recent fit [8] and aim to prove that existing experiments do not violate these limits. This is a first ERO application to the situation at hand. The main aim at the current stage is thus to test the model reasonability by reproducing the observed Be line intensities and also their ratios varying during plasma density scan as a benchmark for ERO simulations.
Section snippets
Effective yield measurement
The general geometry is illustrated in Fig. 1. The observation system is directed on the solid Be tile (#7 of the octant 7X) of the JET inner wall as shown in Fig. 1b. Fig. 1a shows the tile position inside the poloidal cross-section of JET. The tile is positioned in the scrape-off-layer (SOL) several cm away from the separatrix. The line-of-sight (“spot”) is nearly cylindrical with a radius of about 60 mm and directed at an angle of 58° to the torus radius taken at the limiter centre tip z-axis
Physical sputtering model and data
The available in literature empirical and various simulated sputtering yields (Fig. 2) can differ by orders of magnitude [11], [12]. The simulated data show less scattering and, more importantly, follow similar trends allowing for a function fit, including both incident ion energy and angle as variables. In our view, the scatter in the empirical data are often not due to measurement techniques, but rather due to the challenging interpretation of the observed effective yields as universal
ERO simulations of the density scan experiment
Fig. 1 illustrates the ERO simulation box and coordinate system. The toroidal x-axis is chosen to be perpendicular to the torus radius at the shaped limiter central tip. The z-axis is chosen perpendicular to x in the toroidal plane. This makes it nearly perpendicular to the limiter surface. The orthogonal to x and z coordinate y is parallel to the torus axis. The spectroscopic system line of sight is nearly cylindrical with radius of about 60 mm and directed at an angle of 58° to the z axis. The
Conclusions
Passive spectroscopy has been used for erosion assessment of a shaped solid Be limiter of the recently installed JET ILW [1]. Physical sputtering is assumed to be the main erosion mechanism. Effective yields of about 10% and smaller were observed for a wide range of plasma parameters in the series of limiter discharges especially aimed for erosion study.
The interpretation of the experiment aimed in determining of universal physical sputtering yields demands detailed modelling including the
Acknowledgements
This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.
The authors are thankful to D. Harting and S. Jachmich for helpful discussions.
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Improved ERO modelling of beryllium erosion at ITER upper first wall panel using JET-ILW and PISCES-B experience
2019, Nuclear Materials and EnergyCitation Excerpt :The normal incidence part of the yield (Fig. 1, left) is often at the focus of the data validation efforts at various devices. However, due to large scattering of the experimental data, which are partially explainable by the difficulty of measurement interpretation (angular part is often neglected despite its significance), ERO utilizes the fits [13] based on the combination of the BCA and molecular dynamics (MD) simulated points which are of particular significance at the lower impact energies close to the sputtering threshold. The “ERO-max” fit is based on simulations for pure Be surface (it is close to earlier BCA simulations [8]) and for the “ERO-min” 50% D content is assumed [13].
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2017, Nuclear Materials and EnergyCitation Excerpt :Therefore it is of importance to validate both the code and the underlying data (e.g. for physical erosion) at existing devices like PISCES-B [1,10] capable to operate with Be. Recently the ERO code was also extensively applied to various erosion experiments at JET ITER-like wall (ILW) [6,7,8,24] which have led to extensions and refinements of the model and data in ERO. Multiple improvements were implemented including an analytical treatment of the distributions of angles and energies on impact including additional surface biasing [11].
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Presenting author.
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See App. of F. Romanelli et al., Proc. of the 23rd IAEA Fusion Energy Conf. 2010, Daejon, Korea.