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Anastasia Dvornova - Master Thesis at IPP Prague

Master Thesis at IPP Prague, Czech Republic
Reported by Anastasia Dvornova. Powered by FuseNet.

In the framework of the Erasmus Mundus Fusion-EP programme, I spent 4 months at the IPP Prague in the Czech Republic, working on my Master’s thesis. My final project consisted of two parts, since I wanted to do both, experimental work and modelling.

Experimental determination of the ion temperature in the SOL of COMPASS

During the experimental part of my project, I worked on a method to measurement the ion temperature in the Scrape-Off Layer (SOL), which is the small layer of plasma just outside the last closed flux-surface. For that purpose, an retarding field analyzer (RFA) was used, which typically consists of a protective plate, which reduces the incoming plasma flux, followed by a slit, one or two grids, and a collector. Such a RFA has been installed on the horizontal manipulator of the COMPASS-tokamak in Prague, in order to measure the ion temperature, the electron temperatures, and radial electron density distributions.

When operating in a standard ion temperature regime, the slit of the RSA is negatively biased to repel most of the incoming electrons. A positive voltage is then applied to the first grid to reject the low-energetic ions that don’t have enough energy to overcome this bias. During a typical measurement, the bias voltage is swept between 0 and a certain maximal value, while measuring the ion current through the RSA. This results in ion I-V characteristics, which allow the determination of the ion temperature. In principle, the RFA can be operated without the use of a second grid, but it is convenient to use them since they suppress secondary electron emission from the collector, improving the accuracy of the measurement. The electron temperature can also be obtained with the RFA, by using the biased protective slit plate as a Langmuir probe, allowing measurements of the electron temperature, density, and floating potential.

Plasma transport simulations in the SOL

For the second part of my thesis, I performed simulations using the SolEdge2D code, which is a 2D plasma fluid transport code designed to simulate transport in the SOL and edge regions of tokamaks. The code assumes a toroidal axisymmetry, while currently an approximation of a first-principle 3D turbulent model is being developed. These kind of transport codes usually require diffusion coefficients as inputs, which govern the transport processes in the plasma. However, since these coefficients vary for different tokamaks and different plasma operation regimes, it is very difficult to determine them in such a way that the results of the simulation match the experimental data.

One of the main advantages of SolEdge2D is that it has a fitting algorithm included, which allows the user to input radial density profiles, as well as ion and electron temperature profiles. In that case, it's not necessary to manually specify the diffusion coefficients, since they are automatically generated based on the plasma parameters in such a way that the calculated profiles match the experimental data. During my thesis, I used the results of the RFA and Thomson Scattering data as an input for SolEdge2D to study plasma transport.

Prague: a city to fall in love with

One of the reasons that I was so eager to do my project at COMPASS was its location. Prague is a magnificent city, which I’ve visit a couple of times before and with which I  have fallen in love. I adore its small streets, cozy cafes, loud bars, and unique sights. It welcomes international students with a romantic atmosphere, a fun mix of languages, unusual Czech cuisine, culture, and, of course, beer. Situated in the very heart of Europe, Prague also makes it easy to spend a weekend in nearby cities like Vienna, Dresden, Budapest, and Munich.

Overall, my stay at IPP Prague has been an incredible experience and I have had lots of fun during my internship. I’d like to thank my awesome supervisors, Michael Komm and Claudia Norscini, for their support, patience, advice, kindness, and sense of humor. It was great to spend time at COMPASS. Finally, I'd like to thank FuseNet for their funding support!