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From the Netherlands to PPPL: Student reflects on his study of light

Dutch graduate student Jasper van Rens recently completed a three-month assignment at PPPL to study a diagnostic technique that will be crucial to the success of ITER, the huge international fusion facility under construction in France. Working with Fred Levinton and Howard Yuh of PPPL subcontractor Nova Photonics, Van Rens investigated the impact of reflected light on the ITER Motional Stark Effect (MSE) instrument, which  measures the internal magnetic configuration of fusion plasmas. Van Rens, who has completed the first year of a two-year master’s degree program at the Eindhoven University of Technology, discussed his findings and experiences at the Laboratory with PPPL science writer John Greenwald.

What brought you to PPPL?

I’m an applied physics major and everybody has to do an external internship abroad or with a company, and I just wanted to go to America. I asked my professor [Maarten de Bock], “Do you have any connections in America?” He’s one of the people responsible for the MSE system in ITER and he came up with a project that people have been struggling with for a couple of years.

What was that project?

It’s about reflections in a tokamak. The MSE system measures the direction of the polarized light from neutral beams in a tokamak. [The waves in polarized light all point in the same direction.] But there’s a lot of background light too. Because ITER is such a big machine there will be a lot of this background noise and that’s bad for the signal-noise ratio. And, on top of that, these reflections could polarize the background light and that would be disastrous for the MSE measurement because it also looks at the polarization of light.

How did you do your research?

I used a software program called LightTools to retrace light rays through a model of the ITER system. I looked at the light from both thermal radiation and Bremsstrahlung [another type of radiation].

What did you discover?

I found out that this thermal light is actually not that bad compared to the Bremsstrahlung noise. We didn’t know how big a problem the thermal light would be and it turns out not to be a showstopper. That’s one of my results. I also found out that there are no specular [mirror-like] paths by which a big part of the thermal radiation could polarize the light. Specular reflections would be very dangerous because they would polarize the background light and that would be very bad.

How would you describe your experience at PPPL?

I could recommend this place. All the people are very nice and friendly and helpful. I got a great office with a big computer screen and it was a very pleasant office to work in. I got some good results and made a group of friends. Every lunch I come to PPPL [from Nova Photonics] and have lunch with grad students. I also joined their softball team—the Tokabats. We are first in the league and are in the playoffs. [The Tokabats went on to win the University summer league softball championship.]

What’s next for you this summer?

Four of my friends from the Netherlands will come over and we’ll do a giant journey through America. We’ll spend five days in New York and fly to Los Angeles and rent a car and go. We’ll be traveling for three-and-a-half weeks.

And then?

Next year I will start research about cold electron beams or cold ion beams, I’m not very clear about which one yet. And after that I will have my master’s and could see myself doing a Ph.D.


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