For the past few months I have had the opportunity to be an intern at the DIII-D National Fusion Facility in San Diego, California. Here General Atomics, partnering with the US department of energy, operates the DIII-D tokamak. It is the largest in operation of its kind in the US. The goal of this large international program is to establish the basis for the optimization of the tokamak approach to fusion energy production. With the help of EUROfusion and FuseNet I was able to spend 10 weeks working in this greatenvironment. This has helped me to obtain experience in the fusion field by coming in contact with the tokamak and the numerous diagnostic and heating devices in action.

The neutral beam injection (NBI) systems at DIII-D are crucial for the research done there. By injecting high energy neutral particles into the tokamak, the plasma is heated and other potentially beneficial effects such as additional plasma current and rotation can be achieved.The ion sources at very start of NBI chain have not been revised since their construction and some operational issues have revealed themselves as a result. To study the observed failure modes and make further improvements the Miniature Arc Chamber Experiment (MACE) was constructed last year. This table-top, easily adjustable ion source is meant to resemble the actual sources.

My goal during my stay at DIII-D was to revise the Langmuir probes which are used in the sources to ensure that the right plasma parameters are reached. The ones currently in use are water-cooled and unfortunately show leaks from time to time. The approach here was to construct a finite element model to simulate the probe heating and verify these results by experiments in the MACE. The experiments were conducted in good cooperation with my lab partner and supervisor who worked on getting the MACE ready for the experimental phase of the project. After verification of the model it has been used to make recommendations on a new probe design for the source.

DIII-D is a wonderful environment to work in with many dedicated scientists and engineers who are always willing to help you out whenever possible. It has been a great to work on my own project here, seeall the research progress from nearby and beable to talk to many experts about their experience in the field. Apart from the research at DIII-D California is of course also a wonderful place to be and the hiking trip and others activities with the staff members also contributedto the great time I have had there.Iwould like to thank EUROfusion, FuseNet and the DIII-D crew for the amazing opportunity and experience.

- Tijs Wijkamp