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• Rekaa, Vegard Lundby; Chapman, Sandra & Dendy, Richard O. (2014). Ion pre-acceleration in fully self-consistent particle-in-cell simulations of supercritical perpendicular reforming shocks in multiple ion species plasmas. The Astrophysical Journal (ApJ). ISSN 0004-637X. 791(1). doi: 10.1088/0004-637X/791/1/26. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2013). Self-similar space-time evolution of an initial density discontinuity. Physics of Plasmas. ISSN 1070-664X. 20(7). doi: 10.1063/1.4816953. Fulltekst i vitenarkiv Vis sammendrag

  The space-time evolution of an initial step-like plasma density variation is studied. We give particular attention to formulate the problem in a way that opens for the possibility of realizing the conditions experimentally. After a short transient time interval of the order of the electron plasma period, the solution is self-similar as illustrated by a video where the space-time evolution is reduced to be a function of the ratio x/t. Solutions of this form are usually found for problems without characteristic length and time scales, in our case the quasi-neutral limit. By introducing ion collisions with neutrals into the numerical analysis, we introduce a length scale, the collisional mean free path. We study the breakdown of the self-similarity of the solution as the mean free path is made shorter than the system length. Analytical results are presented for charge exchange collisions, demonstrating a short time collisionless evolution with an ensuing long time diffusive relaxation of the initial perturbation. For large times, we find a diffusion equation as the limiting analytical form for a charge-exchange collisional plasma, with a diffusion coefficient defined as the square of the ion sound speed divided by the (constant) ion collision frequency. The ion-neutral collision frequency acts as a parameter that allows a collisionless result to be obtained in one limit, while the solution of a diffusion equation is recovered in the opposite limit of large collision frequencies. © 2013 AIP Publishing LLC.

• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2012). Numerical studies of a plasma diode with external forcing. Physics of Plasmas. ISSN 1070-664X. 19(8). doi: 10.1063/1.4747620. Fulltekst i vitenarkiv Vis sammendrag

  With reference to laboratory Q-machine studies we analyze the dynamics of a plasma diode under external forcing. Assuming a strong axial magnetic field, the problem is analyzed in one spatial dimension by a particle-in-cell code. The cathode is assumed to be operated in electron rich conditions, supplying an abundance of electrons. We compare different forcing schemes with the results obtained by solving the van der Pol equation. In one method of forcing we apply an oscillation in addition to the DC end plate bias and consider both amplitude and frequency variations. An alternative method of perturbation consists of modelling an absorbing grid at some internal position. Also in this case we can have a constant frequency with varying amplitude or alternatively an oscillation with chirped frequency but constant amplitude. We find that the overall features of the forced van der Pol equation are recovered, but the details in the plasma response need more attention to the harmonic responses, requiring extensions of the model equation. The analysis is extended by introducing collisional effects, where we emphasize charge exchange collisions of ions, since these processes usually have the largest cross sections and give significant modifications of the diode performance. In particular we find a reduction in oscillator frequency, although a linear scaling of the oscillation time with the system length remains also in this case. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.

• Miloch, Wojciech Jacek; Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2010). Ion acoustic double layers forming behind irradiated solid objects in streaming plasmas. Journal of Plasma Physics. ISSN 0022-3778. 76, s. 429–439. doi: 10.1017/S002237780999064X. Fulltekst i vitenarkiv

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• Bjorå, Charlotte Sletten; Rekaa, Vegard Lundby & Dietrichs, Erik Sveberg (2025). Abels tårn. [Radio]. NRK P2. doi: https:/radio.nrk.no/podkast/abels_taarn/sesong/2025/l_a2081198-2116-4c1c-8811-9821162c1cbb. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby (2014). Self-consisten kinetic PIC simulations of collisionless supercritical shocks in astrophysical plasmas with multiple ion species. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Chapman, Sandra c. & Dendy, Richard O. (2013). 40th European Physical Society Conference on Plasma Physics in astrophysical plasmas. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Chapman, Sandra c. & Dendy, Richard O. (2013). Perpendicular shock reformation and multi-ion specie acceleration. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Chapman, Sandra c. & Dendy, Richard O. (2013). Perpendicular shock reformation and multi-ion species acceleration. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2012). Numerical studies of a plasma diode with external forcings. Fulltekst i vitenarkiv Vis sammendrag

  With reference to laboratory studies we analyze the dynamics of a plasma diode under external forcing. Assuming a strong axial magnetic field, the problem is analyzed in one spatial dimension by a Particle in Cell code. The cathode is assumed to be operated in electron rich conditions, supplying an abundance of electrons. We compare different forcing schemes with the results obtained by solving the van der Pol equation. In one method of forcing we apply an oscillation in addition to the DC end plate bias, and consider both amplitude and frequency variations. An alternative method of perturbation consists of modelling an absorbing grid at some internal position. Also in this case we can have a constant frequency with varying amplitude, or alternatively an oscillation with chirped frequency but constant amplitude. We find that the overall features of the forced van der Pol equation are recovered, but the details in the plasma response require more attention to the harmonic responses, requiring extensions of the model equation. The analysis is extended by introducing collisional effects, where we emphasize charge exchange collisions of ions, since these processes usually have the largest cross sections. Charge exchange collisions give significant modifications of the diode performance when the effects are compared with the influence of elastic electron-neutral and ion-neutral collisions with the same collisional mean free paths.

• Rekaa, Vegard Lundby; Pecseli, Hans & Trulsen, Jan Karsten (2011). Numerical simulations of anomalous resistivity in the ion acoustic and Buneman regime. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby (2010). Numeriske simuleringer av Dobbeltlag i Plasma. Fulltekst i vitenarkiv
• Rekaa, Vegard Lundby (2014). Numerical Simulations of Kinetic Plasmas. Universitetet i Oslo. Fulltekst i vitenarkiv

Se alle arbeider i NVA