Plasma-wall interactions impurities

In concluding this section we should like to give a brief overview of the processes involved in the release of impurities due to plasma-wall interactions. These processes will be discussed in some detail in Sect. 5. 

The control of impurity release and transport requires a better understanding of the complex phenomena of plasma-wall interactions including the processes occuring in the scrape-off layer in the limiter shadow. In order to establish the feasibility of suggested solutions such as divertors or surface modifications, experiments have to be performed not only in the laboratory but also in-situ in fusion devices. The latter... 

Simulating erosion and re-deposition processes in fusion devices lead to a better understanding of the processes involved. The 3-dimensional Monte-Carlo code ERO-TEXTOR [35,36] has been developed to model the plasma-wall interaction and the transport of eroded particles in the vicinity of test limiters exposed to the edge plasma of TEXTOR. Important problems concerning the lifetime of various wall materials (high Z vs. low Z) under different plasma conditions and the transport of eroded impurities into the main plasma can be treated with the ERO-TEXTOR code. Recently, the divertor geometries have been implemented to carry out simulations for JET, ASDEX and ITER [37], In addition, first attempts have been made to simulate erosion and re-deposition processes in the linear plasma device PISCES to analyze the effect of beryllium. 

Futhermore, boron also plays a key role in thermonuclear fusion research, belonging to the most abundant impurities in the plasmas of several presently operating tokamaks. Thus, boron is a constituent in carbon-based wall materials which face the plasma, and it can enter the plasma by various plasma-wall interaction mechanisms, sputtering, erosion, etc. [12], As in the case of astrophysics, fusion plasma diagnostics, by means of optical spiectroscopy, utilizes basic experimental and theoretical data on the structure and interaction of boron in various stages of ionization. 

The requirements for long pulse operation in the next step fusion device ITER and beyond, like acceptable power exhaust, peak load for steady state, transient loads, sufficient target lifetime, limited long term tritium retention in wall surfaces, acceptable impurity contamination in central plasma and efficient helium exhaust, depend on complex processes. The input to the numerical codes, which are used for the optimization of divertor and wall components, relies to a large extend on our understanding of the major processes related to erosion and deposition, tritium retention, impurity sources and erosion processes. The reliability of predictions made with these codes depends crucially on the accuracy of the atomic and plasma-material interaction data available. 

The interaction of the plasma with the first wall is, in the current Tokamaks, significantly reduced by the so called limiter which is typically a metallic ring mounted inside the torus in order to keep the plasma away from the first wall. In turn, the interaction of the plasma with the limiter is a significant source of plasma impurities. In many of today s Tokamaks the plasma impurity level is dominated by limiter erosion. 

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