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Nick Maassen & Mathijn Bernards - Master Projects at the Culham Centre for Fusion Energy

Master Thesis at JET - United Kingdom 
Report by Nick Maassen. Powered by FuseNet

My Name is Nick Maassen and I’ve spend half a year at the Joint European Torus (JET) experiment in Culham, England. Culham is located near Oxford and is by far the most interesting place to stay. I tried to find a room before I came to England via the website www.gumtree.com and, after about 40 replies, I found a room for only 520 pounds per month near the centre of the city (Buckingham street 20). In hindsight I have been rather lucky because it turned out to be rather uncommon to find a place without meeting the owner first. Even though 520 pounds per month is relatively cheap for Oxford I am glad that FuseNet was able to provide this money otherwise I wouldn’t have been able to afford it. I’ve travelled to England with my motorcycle so I would have my own transportation and could ride around the surroundings of Oxford during weekends.

Holidays
During holidays I took flights back to the Netherlands via Ryanair (costs only around 40 pounds for a return flight) from Stansted Airport. The first time I went there on my motorcycle but I don’t recommend that. During the return journey it was dark and there was fog and I could hardly see more than 20 meters so it was quite dangerous on the highway. The journey also took around 2 hours and was during the winter so I had to stop quite often to find a place to get a coffee and warm up. The other times I went to Stansted I took the Oxford Tube and the EasyJet bus, which were much slower than my motorcycle but also much warmer.

In Culham there is a student group, most of them being PhDs, with whom I spend some time playing football and softball outside working hours as well as going to some nice bars in Oxford.

Project
The project that I worked on is the following: ITER, the largest fusion reactor to date is under construction. It will employ an all-metal wall which has not been used in a high power reactor before. For this reason, a so called ITER-Like Wall (ILW) has been installed on JET, currently the largest operating fusion reactor, to perform an integrated test of such a wall. The ILW is less forgiving than the old carbon wall so the surface temperature requires constant monitoring. The primary monitoring system consists of CCD cameras equipped with near-infrared filters. This system may, however, not always be available in which case a secondary, model-based system called WALLS takes over. The Upper Dump Plate (UDP) is an area of the wall that has not yet been incorporated into the WALLS system. In this report a model is therefore developed to calculate the surface temperature of the UDP tiles in real-time. This model consists of two sub-models that are placed in series. The first is a plasma model that directly relates the power exhausted by the main plasma to a power density impinging on the UDP, and the second is a recursive heat diffusivity model that uses this power density to calculate the surface temperature of the UDP in real-time. The plasma model contains two unknown parameters, namely the scrape-off length and the power fractions. A method is developed to calibrate these parameters using the thermocouples located inside the UDP tiles. First, a transfer function is constructed that, when applied to the spectrum of the thermocouple temperature, recovers the low-frequency part of the power density post-experimentally. This low-frequency part of the power density is subsequently used to determine the scrape-off length and the power fractions. Finally, a collective validation of the used methods and models is made by comparing the measured and real-time calculated thermocouple temperature.

 

Master Internship at CCFE - United Kingdom 
Report by Mathijn Bernards. Powered by FuseNet

In a tokamak experiment it is essential to measure the plasma density, or more specific, the edge plasma density since it is often used as input for plasma reconstruction codes. The Lithium beam is an active beam emission diagnostics providing electron densities in the plasma  edge with good temporal and good spatial resolution. Calibration of the Lithium beam directly influences the accuracy of the calibrated intensities, hence an accurate calibration is necessary.

In my report it will be demonstrated that the fitting error of the Lithium Beam-Into-Gas calibration fitting routine can be reduced significantly, by using a double Gaussian instead of a single gauss fit.  The second Gaussian can be connected characterized with a fixed amplitude ratio with the main Gaussian, a fixed displacement from the main Gaussian, and a fixed width, so the number or free parameters stays the same in comparison to the single Gaussian fit. The presence of the second Gaussian peak can be explained due to a characterization observed in the parameters of the second Gaussian in the double fit, which means the spectrometer causes it, or more specific, the off-axis imaging of the spectrometer.

Big differences
I had a good time in England. It is so close to my home country, the Netherlands, but still vastly different. I really enjoyed the culture, the area, and the people. I used to sit together with my supervisor and his colleagues during lunch breaks, and learned a lot from talking with them. I also spent time with other students working on projects there. There is however one big downside to England in my opinion and that’s the public transport! Do not count on those ever being on time! All in all England still is a good country with good people.