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Jonathan van den Berg - Internship at JT-60SA

Internship at JT-60SA in Naka, Japan
Reported by Jonathan van den Berg. Powered by FuseNet

Last summer, I joined the department of advanced plasma research of the Naka Fusion Institute in Japan for an internship, during which I worked on a Thomson scattering (TS) diagnostic, under supervision of Hiroshi Tojo.

The Naka Fusion Institute and JT-60SA: Japan's first superconducting tokamak

The Naka Fusion Institute, which is part of the Japanese National Institutes for Quantum and Radiological Science and Technology (QST), is home to the JT-60SA tokamak (in which JT stands for Japan Torus). JT-60SA is the upgraded version of the JT-60 tokamak, which is currently under construction in Naka and which is expected to produce its first plasma in 2019. The machine is part of the ‘Broader Approach agreement’ between the EU and Japan, which aims to support the ITER project and to develop advanced operation scenarios for DEMO.

The site of the Naka Fusion Insitute is designed with a large tokamak in mind and, as such, the area and buildings are decently sized, surrounded by lots of greenery. The offices aresimilar to the ones you might expect to encounter in Europe, with one major difference: the tables and chairs are quite a bit lower than I was used to! At first this looked and felt funny, but I got used to it soon enough.

Developing a real-time Thomson Scattering diagnostic

During my internship I have investigated two main topics related to the Thomson Scattering setup:

Firstly, I investigated how the finite size of the collection optics of the core TS setup affected on the measured spectra, electron temperature and density. For this purpose, I made a model of the setup, taking its geometry into account. The effects of this geometry turned out to be very small in comparison to several other error sources, making it unnecessary to include these effects in the TS model.

Secondly, I evaluated the computation times for different methods to calculate the electron temperature and density from the TS measurements. In the integrated research phase of JT-60SA, real-time diagnostics will be crucial for the purpose of controlling the advanced operation regimes, which is why fast computations are essential for the development of a real-time TS diagnostic. With that in mind, I considered three calculation methods: a least-mean-square fit of the data with a model function, a similar fit of the data with the tabulated results of the model function and a direct estimation method. For each of these three methods, I determined the calculation time and its accuracy. The first method was accurate but not fast enough for real-time diagnostics, while the second method met the demanded maximum calculation time of 10ms but was slightly less accurate. The third method turned out to be very fast (0.1 ms) and accurate as well, but it was not implemented for the full range of possible temperatures.

QST's hospitality and exploring Japan during the well-earned weekends

QST offered me an apartment to stay in the Minouchi Housing of QST Naka, which was very kind of them. The room had a real “shoji” window, which is a traditional Japanese inner window made of wood and paper! QST also took care of transportation, allowing me to commuted using an awesome ”QST bike”; a typical Japanese granny bike that QST lends to its visitors.

After a long week of work, you deserve some well-earned leisure time, especially when you are 8000 km from home. As such, I spent most of my weekends exploring the beautiful country of Japan. Of course I had to visit Tokyo, which I did a couple of times, because Naka is not too far from the Japanese capital. In addition, I also went to the following places: Nikko, the beaches in Ibaraki, the Large Helical Device in Toki, and Takayama. To round things off, I spent 7 days on the road with an open and unlimited ride pass for the Japan Rail at the end of my stay in Japan! Inbetween my travels by train, I went for some nice hikes to Mt. Nantai in Nikko, Mt. Hakkoda in Aomori, Mt. Misen in Miyajima (Hiroshima) and, of course, to the beloved Mt. Fuji.

Altogether, my stay in Japan, especially at QST, has been a wonderful experience for me, and I would love to come back one day in the future!