Three images, from left to right: 

typical sand beach in San Diego County, sample inserted in the DiMES system, SEM imaging of pre-exposure C-microspheres.

Nom	Laboratoire	Mobilité	Pays	Laboratoire
Luca Cappelli	M2P2	2 mois	San Diego, USA	General Atomics

A collaboration between the IRFM, CEA research institute in France and the DIII-D National Fusion Facility, operated by General Atomics (San Diego) was made possible thanks to the ISFIN mobility project (mobilité international pour les étudiants doctorants). The objective was to verify and validate the predictive capabilities of codes currently used for estimating W sources at divertors and impurity transport in SOL plasma through an experiment. Five biased samples were inserted into DIII-D lower divertor using the Divertor Material Evaluation System (DiMES) manipulator and exposed to constant L-mode attached plasma conditions. The samples were manually coated with Carbon microsphere in the weeks prior to the experiment. Plasma conditions at the lower divertor were optimized to obtain the best compromise between emissivity of eroded material and high screening above the target. W sources were monitored with in situ visible spectroscopy, imaging, VUV, LP, and Thomson scattering. The in situ measurements were then complemented by post-mortem analysis for measuring the net erosion of W. The experimental erosion data were finally compared with simulations done with the ERO2.0 code. The experience was enriching not only from a technical point of view but also from a personal point of view, offering the opportunity to visit and learn about a similar but at the same time distant culture as that of Western America.

PhD Topic and scientific context

The PhD topic is mainly concerned with the study of erosion and redeposition of W using quasi-analytical models. The main objective is to understand what are the main drivers underlying erosion in order to implement them in reduced models. In particular, the focus is on the effect of sheath potential drop, ionization coefficients, and incident distributions in the final estimation of erosion and redeposition. The experiment performed in the project is a key step in the thesis. In fact, this fits perfectly into the study plan, enriching it with the possibility of planning and co-conducting an experiment. The DIII-D tokamak was particularly suitable for the purpose. In fact, the uniqueness of DIII-D is that it is equipped with the DiMES system through which it is possible to insert and remove W surfaces at each shot at the lower divertor. Moreover, DIII-D is a reactor that does not normally employ W hence it is easy to isolate the contribution of local sources. To give an idea, in the WEST tokamak currently in use at CEA by IRFM this would not have been possible. In fact, since the machine is completely covered in W and without systems such as DiMES, figuring out the cause of erosion at the divertor is a very complex challenge linked to integrated effects.

TIMELINE SUMMARY

The project lasted about six weeks. The timeline observed during the tenure is proposed in this section. In the first week an erosion study was devised based on sample design, the purpose was to predict where the W might be most eroded/redeposited. In the second and third weeks, a so-called mini-proposal was drafted in which the reasons for requesting the experiment were justified and the required plasma conditions were engineered (such as Power in input, which heating system to use, which probes, etc.). At the same time, once the samples were received, they were analyzed under an optical microscope and covered with microspheres with an average diameter of 6 μm, by hand. The morphology of the samples was analyzed by electron microscope (SEM) before erosion. In the fourth week, some preliminary meetings were made and the experiment was finally carried out. In the last two weeks, the post-exposure sample morphology was compared with the pre-exposure morphology, at the same time a comparison study was made between temperature and electron density data measured with Langmuir probes and Thomson scattering. Finally, a set of simulations with ERO2 was produced by taking as input the experimental electronic density and temperature measures.

CULTURAL ASPECTS

America has amazed me on a personal level. The work culture is full of optimism, positivity, and team building. It really pushed me to give my best and enjoy the work there. At the same time, San Diego special microclimate makes it an oasis in the desert. Temperatures are almost always pleasant and thanks to the lack of rainfall it seems always summer. However, there are not only positive sides, in particular I noticed an extreme use of private vehicles (a problem also present in Europe), one must always go by car, both for safety and for lack of public transportation. Finally, there are obvious problems of homelessness especially around downtown. Still though, to have a research experience I think it is an excellent place.