nonlinear phase
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Author(s):  
Matteo Faganello ◽  
Manuela Sisti ◽  
Francesco Califano ◽  
Benoit Lavraud

Abstract A 3D two-fluid simulation, using plasma parameters as measured by MMS on September 8th 2015, shows the nonlinear development of the Kelvin-Helmholtz instability at the Earth’s magnetopause. It shows an extremely rich dynamics, including the development of a complex magnetic topology, vortex merging and secondary instabilities. Vortex induced and mid-latitude magnetic reconnection coexist and produce an asymmetric distribution of magnetic reconnection events. Off-equator reconnection exhibits a predominance of events in the southern hemisphere during the early nonlinear phase, as observed by satellites at the dayside magnetopause. The late nonlinear phase shows the development of vortex pairing for all latitudes while secondary Kelvin-Helmholtz instability develops only in the northern hemisphere leading to an enhancement of the occurrence of off-equator reconnection there. Since vortices move tailward while evolving, this suggests that reconnection events in the northern hemisphere should dominate at the nightside magnetopause.


2021 ◽  
Author(s):  
Sizhe Duan ◽  
Guoyong Fu ◽  
Huishan Cai

Abstract Based on the experimental parameters in HL-2A tokamak, hybrid simulations have been carried out to investigate the linear stability and nonlinear dynamics of BAE. It is found that the (m/n=3/2) beta-incuced Alfvén eigenmode (BAE) is excited by co-passing energetic ions with qmin=1.5 in linear simulation, and the mode frequency is consistent with experimental meuasurement. The simulation results show that the energetic ions βh, the injection velocity v0 and orbit width parameter ρh of energetic ions are important parameters determining the drive of BAE. Furthermore, the effect of qmin (with fixed shape of q profile) is studied, and it is found that: when qmin ≤ 1.50, the excited modes are BAEs, which are located near q=1.50 rational surfaces; when qmin > 1.50, the excited modes are simillar to the reversed-shear Alfvén eigenmodes (RSAEs), which are mainly localized around q=qmin surfaces. Nonlinear simulation results show that the nonlinear dynamics of BAE is sensitive to the EP drive. For strongly driven case, firstly, redistribution and transport of engetic ions are trigged by (m/n=3/2) BAE, which raised the radial gradient of energetic ions distribution function near q=2 rational surface, and then an EPM (m/n=4/2) is driven in nonlinear phase. Finally, these two instabilities triggered significant redistribution of energetic ions, which results in the twice-repeated and mostly-downward frequency chirping of (m/n=3/2) BAE. For weakly driven case, there are no (m/n=4/2) EPM being driven and twice-repeated chirping in nonlinear phase, since the radial gradient near q=2 rational surface is small and almost unchanged.


2021 ◽  
Author(s):  
Hanzheng Li ◽  
Y Todo ◽  
Hao Wang ◽  
Malik Idouakass ◽  
Jialei Wang

Abstract Kinetic-magnetohydrodynamic hybrid simulations were performed to investigate the linear growth and the nonlinear evolution of off-axis fishbone mode (OFM) destabilized by trapped energetic ions in tokamak plasmas. The spatial profile of OFM is mainly composed of m/n = 2/1 mode inside the q = 2 magnetic flux surface while the m/n = 3/1 mode is predominant outside the q = 2 surface, where m and n are the poloidal and toroidal mode numbers, respectively, and q is the safety factor. The spatial profile of the OFM is a strongly shearing shape on the poloidal plane, suggesting the nonperturbative effect of the interaction with energetic ions. The frequency of the OFM in the linear growth phase is in good agreement with the precession drift frequency of trapped energetic ions, and the frequency chirps down in the nonlinear phase. Two types of resonance conditions between trapped energetic ions and OFM are found. For the first type of resonance, the precession drift frequency matches the OFM frequency, while for the second type, the sum of the precession drift frequency and the bounce frequency matches the OFM frequency. The first type of resonance is the primary resonance for the destabilization of OFM. The resonance frequency which is defined based on precession drift frequency and bounce frequency of the nonlinear orbit for each resonant particle is analyzed to understand the frequency chirping. The resonance frequency of the particles that transfer energy to the OFM chirps down, which may result in the chirping down of the OFM frequency. A detailed analysis of the energetic ion distribution function in phase space shows that the gradient of the distribution function along the E′ = const. line drives or stabilizes the instability, where E′ is a combination of energy and toroidal canonical momentum and conserved during the wave-particle interaction. The distribution function is flattened along the E′ = const. line in the nonlinear phase leading to the saturation of the instability.


Photonics ◽  
2021 ◽  
Vol 8 (11) ◽  
pp. 520
Author(s):  
Yasser Nada ◽  
Efim Khazanov

We showed that the nonlinear Mach–Zehnder interferometer may be used not only for enhancing temporal contrast, as proposed earlier, but also for increasing pulse power due to efficient pulse compression. The interferometer introduces into the output pulse a nonlinear phase equal to π/2. This allows increasing laser power by a factor of 1.5 only by means of a chirped mirror. Use of an additional nonlinear plate leads to a multi-fold power increase retaining the contrast enhancement.


2021 ◽  
Vol 119 (19) ◽  
pp. 192405
Author(s):  
Alexey B. Ustinov ◽  
Nikolai A. Kuznetsov ◽  
Roman V. Haponchyk ◽  
Erkki Lähderanta ◽  
Taichi Goto ◽  
...  

2021 ◽  
Author(s):  
Jonathan Rodrigues Da Silva ◽  
Joao Batista Rosa Silva ◽  
Rubens Viana Ramos

Photonics ◽  
2021 ◽  
Vol 8 (9) ◽  
pp. 387
Author(s):  
Luka Černe ◽  
Jaka Petelin ◽  
Rok Petkovšek

In this manuscript, an implementation of a tunable nonlinear phase compensation method is demonstrated on a typical femtosecond hybrid laser consisting of a fiber pre-amplifier and an additional solid-state amplifier. This enables one to achieve constant laser pulse parameters over a wide range of pulse repetition rates in such a laser. As the gain in the solid-state amplifier is inversely proportional to the input power, the shortfall in the solid-state gain at higher repetition rates must be compensated for with fiber pre-amplifier to ensure constant pulse energy. This increases the accumulated nonlinear phase and consequently alters the laser pulse parameters such as pulse duration and Strehl ratio. To overcome this issue, the nonlinear phase must be compensated for, and what is more it should be compensated for to a different extent at different pulse repetition rates. This is achieved with a tunable CFBG, used also as a pulse stretcher. Using this concept, we demonstrate that constant laser pulse parameters such as pulse energy, pulse duration and Strehl ratio can be achieved in a hybrid laser regardless of the pulse repetition rate.


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