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PWI and PPPL

As long as the plasma does not...

Today I will introduce you to the wild Universe of Plasma Wall Interaction, or PWI (you know that physicists like to call their pets with nicknames). As long as the plasma does not go in contact with any material everything is terrific but when it gets close to any type of matter, things get complicated.

The idea of 'confining' a plasma in a vacuum chamber is quite clever in order to be able to rise its temperature and density and hence push D and T to hug and fuse, but it is way to be the case in a fusion device. Although a high vacuum is continously observed in a tokamak, the plasma is permanently in contact of residual gases and in case of a loss of the position of the plasma, a high heat load can suddently be lost to the wall -causing irreversible damage.

When the plasma goes in contact with these residual gases, the neutral molecules get ionised causing the plasma 'to delute' - decreasing its density. Radiation is the immediate phenomenon after ionisation of these residual gases then the temperature that we are trying to rise just does the opposite. The same happens when the plasma goes close to the first wall of the chamber, the material of which the wall is made gets eroded then ionized causing the same nasty effects.

Although radiation at the edge can be of some benefit, it is of paramount importance to understand and control either these interactions with the first wall or the residual gases and impurities in the plasma edge. My research activities include the understanding of the plasma wall interactions with the aim of controling the plasma density.

In all fusion devices, the wall of the machine is routinely conditioned to reduce the impurity influxes during plasma operation. One of the techniques that is used is the Lithization of the wall. For the past 10 years research on Lithium conditioning in fusion devices has been undertaken at the Princeton Plasma Physics Laboratory. Lithium has the miraculous effect to improve most of the plasma parameters when injected in the edge area of the plasma at a given controlled amount. One of the main studies that are getting me occupied during my PhD are the studies of the wall conditioning using Lithium in Fusion devices.

Since the month of November, I am working in a collaboration with PPPL to study the chemical properties of 'Lithiated' plasma first wall. It consists in looking at how Lithium is bonded with the Graphite wall (bulk material used in most fusion machines as first wall) in order to understand the underlying processes that improve the plasma performances.

X-rays can be used to probe the near surface of a sample exposed to a Lithium conditioning and obtain the chemical bondings of a fresh Lithium deposited on the Carbon matrix. XPS (X-ray Photoelectron Spectrocopy) is now commonly used to obtain such chemical information on solid samples such as the ones I analysed at PPPL. 

The tritium shot

During my visit, on December 9th 2013, the NSTX team celebrated the 20th anniversary of the tritium shot, a high-powered mixture of fuel that has been tested for the first time 20 years ago, a world-record burst of more than 3 million watts of fusion energy — enough to momentarily light some 3,000 homes — fueled by the new high-powered mixture. The beginning of the year at PPPL started with terrific news with a Congress's new spending bill, which would increase funding for fusion-energy research by more than $100 million over last year. Cuts in funding for Fusion in the US during the past year caused tens of researchers at PPPL to work on voluntary separation programs.