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- Fehér, Tamás, 1984
(författare)
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Simulation of runaway electron generation during plasma shutdown by impurity injection
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Disruptions are dangerous instabilities in tokamaks that should be avoided or mitigated. One possible disruption mitigation method is to inject impurities into the plasma to shut it down in a controlled way. Runaway Electrons (REs) can be generated after the plasma is cooled down bythe impurities and these electrons can damage the tokamak. In this work a simulation code is developed to investigate different disruption mitigation scenarios. The response of the bulk plasma, more precisely the temperature evolution of electrons, deuterium and impurity ions are described by energy balance equations in a 1D cylindrical plasma model. The induction and resistive diffusion of electric field is calculated. RE generation rates are used to calculate the runaway current. The Dreicer, hot-tail and avalanche effect is taken into account and a simple model for RE losses is also included. RE generation is studied in JET-like plasmas during pellet injection. Carbon pellets cause effective cooling but these scenarios are prone to runaway generation. A mixture of argon and deuterium gas could be used for safe shutdown without RE generation. In ITER the hot-tail RE generation process becomes important, and the simulation is thereforeextended to take this into account. Shutdown scenarios with different concentration of neon and argon impurities were tested in ITER-like plasmas. To simplify the problem the impurity injection into the plasma is not modeled in these cases, only the response of the bulk plasma. The avalanche process cannot be suppressed in a simple way and wouldproduce high runaway current. It can be avoided if some runaway loss phenomenon is included in the simulations, like diffusion due to magnetic perturbations.
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- Fehér, Tamás, 1984, et al.
(författare)
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Simulation of runaway electron generation during plasma shutdown by impurity injection in ITER
- 2011
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Ingår i: Plasma Physics and Controlled Fusion. - 1361-6587 .- 0741-3335. ; 53:3, s. 035014-
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Tidskriftsartikel (refereegranskat)abstract
- Disruptions in a large tokamak can cause serious damage to the device and should be avoided or mitigated. Massive gas or killer pellet injection are possible ways to obtain a controlled fast plasma shutdown before a natural disruption occurs. In this work, plasma shutdown scenarios with different types of impurities are studied for an ITER-like plasma. Plasma cooling, runaway generation and the associated electric field diffusion are calculated with a 1D-code taking the Dreicer, hot-tail and avalanche runaway generation processes into account. Thin, radially localized sheets with high temperature can be created after the thermal quench, and the Dreicer and avalanche processes produce a high runaway current inside these sheets. At high impurity concentration the Dreicer process is suppressed but hot-tail runaways are created. Favorable thermal and current quench times can be achieved with a mixture of deuterium and neon or argon. However, to prevent the avalanche process from creating a significant runaway current fraction, it is found to be necessary to include runaway losses in the model.
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- Papp, Gergely, 1985, et al.
(författare)
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The effect of magnetic perturbations on runaway dynamics
- 2013
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Ingår i: Proceeding of the13th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems. ; , s. IAEA-F1-TM-44766 / I12-
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Konferensbidrag (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway (RE) electron beam that may cause serious damage to the first wall. To suppress the RE beamthe application of resonant magnetic perturbations (RMP) has been suggested. We investigate the effect of resonant magnetic perturbations by simulating the RE drift orbits inmagnetostatic perturbed fields and calculating the transport and orbit losses for various particle energies and different magnetic perturbation configurations. In the simulations we use model configurations with the planned ITER RMP system and solve the relativistic, gyro-averaged drift equations for the runaway electrons including radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where thenormalised perturbation amplitude δB/B is larger than ∼0.1% in a properly chosen perturbation geometry. This corresponds to the outer half of the confinement volume in ITER. We show that despite the chaotic magnetic topology the ensemble behaviour can only be approximated by a diffusion process.
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| 10. |
- Papp, Gergely, 1985, et al.
(författare)
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The effect of the ITER-like wall on runaway electron generation in JET
- 2013
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 0029-5515 .- 1741-4326. ; 53:12
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the effect of the ITER-like wall (ILW) on runaway electron (RE) generation through a comparative study of similar slow argon injection JET disruptions, performed with different wall materials. In the carbon wall case, a RE plateau is observed, while in the ITER-like wall case, the current quench is slower and the runaway current is negligibly small. The aim of the paper is to shed light on the reason for these differences by detailed numerical modelling to study which factors affected the RE formation. The post-disruption current profile is calculated by a one-dimensional model of electric field, temperature and runaway current taking into account the impurity injection. Scans of various impurity contents are performed and agreement with the experimental scenarios is obtained for reasonable argon and wall impurity contents. Our modelling shows that the reason for the changed RE dynamics is a complex, combined effect of the differences in plasma parameter profiles, the radiation characteristics of beryllium and carbon, and the difference of the injected argon amount. These together lead to a significantly higher Dreicer generation rate in the carbon wall case, which is less prone to being suppressed by RE loss mechanisms. The results indicate that the differences are greatly reduced above ~50% argon content, suggesting that significant RE current is expected in future massive gas injection experiments on both JET and ITER.
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