| 1. |
- Guszejnov, David, et al.
(författare)
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Three-dimensional modeling of beam emission spectroscopy measurements in fusion plasmas
- 2012
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Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 83:11, s. 113501-
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Tidskriftsartikel (refereegranskat)abstract
- One of the main diagnostic tools for measuring electron density profiles and the characteristics of long wavelength turbulent wave structures in fusion plasmas is beam emission spectroscopy (BES). The increasing number of BES systems necessitated an accurate and comprehensive simulation of BES diagnostics, which in turn motivated the development of the Rate Equations for Neutral Alkali-beam TEchnique (RENATE) simulation code that is the topic of this paper. RENATE is a modular, fully three-dimensional code incorporating all key features of BES systems from the atomic physics to the observation, including an advanced modeling of the optics. Thus RENATE can be used both in the interpretation of measured signals and the development of new BES systems. The most important components of the code have been successfully benchmarked against other simulation codes. The primary results have been validated against experimental data from the KSTAR tokamak.
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| 2. |
- Kómár, A, et al.
(författare)
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Electromagnetic waves destabilized by runaway electrons in near-critical electric fields
- 2013
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Ingår i: Physics of Plasmas. - : AIP Publishing. - 1089-7674 .- 1070-664X. ; 20:1, s. 012117-
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Tidskriftsartikel (refereegranskat)abstract
- Runaway electron distributions are strongly anisotropic in velocity space. This anisotropy is a source of free energy that may destabilize electromagnetic waves through a resonant interaction between the waves and the energetic electrons. In this work, we investigate the high-frequency electromagnetic waves that are destabilized by runaway electron beams when the electric field is close to the critical field for runaway acceleration. Using a runaway electron distribution appropriate for the near-critical case, we calculate the linear instability growth rate of these waves and conclude that the obliquely propagating whistler waves are most unstable. We show that the frequencies, wave numbers, and propagation angles of the most unstable waves depend strongly on the magnetic field. Taking into account collisional and convective damping of the waves, we determine the number density of runaways that is required to destabilize the waves and show its parametric dependences.
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| 3. |
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| 4. |
- Kómár, A, et al.
(författare)
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Interaction of electromagnetic waves and suprathermal electrons in the near-critical electric field limit
- 2012
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Ingår i: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 401:1
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Konferensbidrag (refereegranskat)abstract
- The velocity-space anisotropy of suprathermal electron distributions is a source of free energy that may destabilize plasma waves through a resonant interaction between the waves and the energetic electrons. In this work we use a suprathermal electron distribution appropriate for the case when the accelerating electric field is near-critical and we investigate the frequencies, wave numbers and propagation angles of the most unstable waves using a general dispersion relation. It is shown that if the electric field is sub-critical, the anisotropy is not enough to drive electromagnetic waves unstable, as the Landau damping of the waves overwhelms the drive through the anomalous Doppler resonance. In the case when the electric field is supercritical, two types of electromagnetic waves will be destabilized, the electron-whistler and the extraordinary electron wave. The number of electrons for destabilization of the latter is several orders of magnitude lower than for the electron-whistler wave. Consequently, the threshold for destabilization of the extraordinary electron wave is much lower.
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| 5. |
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| 6. |
- Papp, Gergely, 1985, et al.
(författare)
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Low frequency sawtooth precursor activity in ASDEX Upgrade
- 2011
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53:6, s. 065007-
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes the precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes in various neutral beam heated plasmas, utilizing the soft x-ray diagnostic. In addition to the well-known (m, n) = (1,1) internal kink mode and its harmonics, a lower frequency mode is studied in detail. Power modulation of this mode is found to correlate with the power modulation of the (1, 1) kink mode in the quasistationary intervals indicating possible nonlinear interaction. Throughout the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash. In addition to its temporal behaviour, its spatial structure was estimated and the most likely value was found to be (1, 1). A possible role of this mode in the mechanism of the sawtooth crash is discussed.
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| 7. |
- Papp, Gergely, 1985, et al.
(författare)
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Low frequency sawtooth precursor in ASDEX Upgrade
- 2011
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Ingår i: Proceedings of 5th IAEA Technical Meeting on the Theory of Plasma Instabilities. ; , s. B4.1-
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Konferensbidrag (refereegranskat)abstract
- The present paper describes the precursor activity observed in the ASDEX Upgrade tokamakbefore pronounced sawtooth crashes in various neutral beam heated plasmas, utilizing the soft X-raydiagnostic. Besides the well-known (m, n) = (1, 1) internal kink mode and its harmonics, a Low FrequencySawtooth Precursor (LFSP) mode is studied in detail. Indications of a second, lower frequency sawtoothprecursor have been reported on JET and HT-7 as well. Throughout the studied sawtooth crashes, thepower of the lower frequency mode rose by several orders of magnitude just before the crash with a growthrate of 400 1/s, which is shown to be consistent with the growth rate of a resistive core mode. Besidesits temporal behaviour, its spatial structure was estimated with a wavelet based method used on SXRmeasurements, and the most likely value was found to be (m, n) = (1, 1). Power modulation of this modeis found to correlate with the power modulation of the (1,1) kink mode in the quasi-stationary intervals,and significant bicoherence was measured, both indicating non-linear interaction. The frequency ratio ofthe two modes was calculated with an instantaneous frequency following algorithm and was found to bein the 0.5-0.7 range. The LFSP is expected to play a role in the partial magnetic reconnection process,hence every sawtooth crash model involving such reconnection may be affected by the existence of theLFSP.
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| 8. |
- Papp, Gergely, 1985, et al.
(författare)
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Runaway electron drift orbits in magnetostatic perturbed fields
- 2010
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Ingår i: Proceedings of 23rd IAEA Fusion Energy Conference, Korea 2010.
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Konferensbidrag (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To mitigate the disruption and suppress the runaway beam the application of resonant magnetic perturbations has been suggested. In this work we investigate the effect of resonant magnetic perturbations on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the orbit losses for various initial energies and magnetic perturbation magnitudes. In the simulations we use a TEXTOR-like configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including synchrotron radiation and collisions. The results indicate that runaway electrons are well confined in the core of the device, but the onset time of runaway losses closer to the edge is dependent on the magnetic perturbation level and thereby can affect the maximum runaway current. However, the runaway current damping rate is not sensitive to the magnetic perturbation level, in agreement with experimental observations.
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| 9. |
- Papp, Gergely, 1985, et al.
(författare)
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Runaway electron dynamics and transport anisotropy due to resonant magnetic perturbations in ITER
- 2012
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Ingår i: Proceedings of the 24th IAEA Fusion Energy Conference. ; , s. TH/P4-05
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Konferensbidrag (refereegranskat)abstract
- In this paper the effect of resonant magnetic perturbations (RMP) on the net radial transport of runaway electrons (RE) is calculated by simulating the RE drift orbits in magnetostatic perturbed fields. Through the transport, RMP influences the time dynamics andpreferred loss directions of the REs. The distribution of the field mesh exit points of therunaway electrons become more localised compared to the unperturbed case, since the losspattern depends on the geometric properties of the RMP configuration such as periodicityor helicity. On the other hand, the loss patterns do not depend on the particle energiesand starting positions. The particle radial steps are correlated to the local radial magneticperturbation component, which makes the transport chaotic, but deterministic.
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| 10. |
- Papp, Gergely, 1985, et al.
(författare)
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Runaway electron losses caused by resonant magnetic perturbations in ITER
- 2011
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 1361-6587 .- 0741-3335. ; 53:9, s. 095004-
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Tidskriftsartikel (refereegranskat)abstract
- Disruptions in large tokamaks can lead to the generation of a relativistic runaway electron beam that may cause serious damage to the first wall. To suppress the runaway beam the application of resonant magnetic perturbations (RMPs) has been suggested. In this work we investigate the effect of RMPs on the confinement of runaway electrons by simulating their drift orbits in magnetostatic perturbed fields and calculating the transport and orbit losses for various initial energies and different magnetic perturbation configurations. In the simulations we model the ITER RMP configuration and solve the relativistic, gyro-averaged drift equations for the runaway electrons including a time-dependent electric field, radiation losses and collisions. The results indicate that runaway electrons are rapidly lost from regions where the normalized perturbation amplitude δB/B is larger than 0.1% in a properly chosen perturbation geometry. This applies to the region outside the radius corresponding to the normalized toroidal flux ψ = 0.5.
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