The tokamak is so far the most promising magnetic configuration for achieving a net production of fusion energy. The D-T fusion reactions result in 3.5 MeV alpha-particles, which may destabilize Alfvén eigenmodes through wave-particle interaction. These instabilities redistribute the alpha-particles from the central region of the plasma towards the edge, where they are thermalized, and hence result in a reduced heating efficiency. The high-energy alpha-particles may even be thrown out of the plasma and may damage the wall. To investigate the destabilization of Alfvén eigenmodes by high-energy ions, ion cyclotron resonance heating (ICRH) and neutral beam injection (NBI) are often used to create a high-energy tail on the distribution function. The ICRH does not only produce high-energy anisotropic tails, it also decorrelates the wave-particle interaction with the Alfvén eigenmodes. Without decorrelation of the wave-particle interaction an ion will undergo a superadiabatic oscillation in phase space and there will be no net transfer of energy to the mode. For the thermal ions the decorrelation from collisions dominates while for the high-energy ions the decorrelation from ICRH dominates. As the unstable modes grow up, the gradients in phase space, which drive the mode, are reduced, resulting in a weaker drive. The dynamics of the system becomes non-linear due to a continuous restoration of the gradients by D-T reactions and ICRH. In this thesis the non-linear dynamics of toroidal Alfvén eigenmodes (TAEs) during ICRH has been investigated using the SELFO code. The SELFO code, which calculates the distribution function during ICRH self-consistently using a Monte-Carlo metod, has been upgraded to include interactions with TAEs. The fast decay of the mode amplitude as the ICRH is switched off, which is seen in experiments, as well as the oscillation of the mode amplitude as the distribution function is repetetively built up by the ICRH and flattened by the TAE has been reproduced using numerical simulations. In the presence of several unstable modes the dynamics become more complicated. The redistribution of an alpha-particle slowing down distribution function as well as the reduced heating efficiency in the presence of several modes has also been investigated. / QC 20100628

The goal of nuclear fusion research is to create a clean and virtually limitless energy source. In order to that, a plasma must be heated to hundreds of millions degrees Celsius. A commonly used heating mechanism is ion cyclotron resonance heating, where antennas emit radio waves into the plasma. The wave can resonate with the ions at their cyclotron frequency, which leads to wave absorption. In order to investigate and improve the heating, one can perform computer simulations. FEMIC is a finite element model for ICRH that calculates the wave field created by the antennas. However, this code does not take into account how the wave modifies the velocity distribution of the plasma. Therefore, a time-independent Fokker-Planck solver is implemented that computes the fast ion distribution due to the incident wave field calculated with FEMIC. The novelty of this code is to include Doppler shift, which influences where ions resonate and how they are heated. / Målet med fusionsforskningen är att skapa en ren energikälla som kan producera obegränsade mängder energi. För detta krävs att ett plasma värms till hundratals miljoner grader Celsius. En vanlig teknik för att värma plasmat är joncyklotronuppvärmning, där en antenn emitterar radiovågor som propagerar in i plasmat. Om vågen är i resonans med jonernas cyklotronrörelse leder detta till att vågen absorberas av jonerna. För att studera och utveckla denna uppvärmningsteknik kan man använda datorsimuleringar. FEMIC är en kod baserad på den finita elementmetoden som beräknar vågfälten som skapas av antennen. Med denna kod kan vi dock inte beräkna hur vågen påverkar jonernas fördelningsfunktioner. Därför har en Fokker-Planck-lösare implementerats som kan beräkna fördelningen av snabba joner som accelererats av vågfältet från FEMIC. Det nya i denna modell är att koden tar hänsyn till Dopplerskiftet, vilket påverkar var jonerna är i resonans med vågen och hur de värms upp.