2024 Seminar Series

11:00 a.m. – 12:00 p.m. Pacific Time (unless otherwise noted)
Join Zoom Here | Meeting ID: 982 2718 1770


May 8, 2024 | 11 a.m. PT

Simon Bolaños
Postdoctoral Scholar, UC San Diego

Talk Title: Laboratory investigations of the initial stages of quasi-parallel collisionless shocks at high Alfvén Mach number

Abstract: Collisionless shocks are ubiquitous in astrophysics and a possible source of the highest-energy cosmic rays in our universe. Recent experimental and numerical efforts [1,2] have shown that ion-Weibel instability (IWI) is a leading candidate mechanism for collisionless shock formation in unmagnetized astrophysical objects. In a magnetized environment, ion beam instabilities, such as the right-hand instability or the non-resonant instability (NRI), can dominate the dynamics and mediate the development of collisionless shocks. Of particular interest, is the quasi-parallel configuration wherein the plasma flow is parallel to the ambient magnetic field.

We will present experimental results from OMEGA-EP during the formation stage of a quasi-parallel collisionless shock at a high-Alfvén Mach number, MA ~ 200. In those experiments, a laser-driven super-Alfvénic plasma flow interpenetrates a premagnetized background plasma at a high velocity, vflow ~1800 km/s. The background magnetic field (10 T) is aligned with the bulk ion velocity. As these kinetic ions interact with the background plasma and magnetic field, streaming instabilities develop and produce magnetic fields, which can eventually mediate a collisionless shock. Proton deflectometry was used to visualize the electromagnetic fluctuations at different times during the evolution, allowing the determination of the streaming instabilities involved in collisionless dissipative processes. A thorough analysis of the proton images indicates the development of the ion-Weibel instability followed by the non-resonant parallel streaming instability, also known as the Bell instability [3], after an ion gyroperiod.  Three-dimensional hybrid PIC simulations were performed and support the growth of the non-resonant instability in the experimental conditions.  Simulations suggest that NRI provides an efficient source of dissipation for the formation of a shock, especially in the non-linear regime reached after eight ion gyroperiods.

Finally, we will introduce the redesigned platforms adapted for the Janus laser at JLF and OMEGA laser facility in order to investigate the effect of weak collisions on the NRI and during the initial stages of shock formation. 

[1] C. M. Huntington et al., Physics of Plasmas 24, 041410 (2017).

[2] Tsunehiko N. Kato and Hideaki Takabe, the Astrophysical Journal, 681 L93 (2008).

[3] A. R. Bell, Monthly Notices of the Royal Astronomical Society 353, 550 (2004).

This material is based upon work supported by the Department of Energy, National Nuclear Security Administration under Award Number(s) DE-NA0003842. This work was supported by the DOE Office of Science, Fusion Energy Sciences under Contract No. DE-SC0021061: the LaserNetUS initiative at the Omega Laser Facility.

Bio: Simon Bolaños is an expert in magnetized HEDP and laboratory astrophysics. He received his Ph.D. degree from École Polytechnique (France) in 2019. His Ph.D. work focuses on the laboratory I investigation of magnetic reconnection driven by high-power lasers. He joined the group in 2020 as a post-doctoral fellow. Today, his research is related to the laser-plasma instabilities in a magnetized environment and magnetized collisionless shock.

June 12, 2024 | 11 a.m. PT

Roman Lee
Graduate Student, UC LA

Talk Title: Effect of small normalized magnetic fields on stimulated Raman scattering in the kinetic, strongly-driven regime

Abstract: We have previously shown [1] that small normalized magnetic fields (ωc/ωp ≪ 1) can suppress backward stimulated Raman scattering (SRS) in the kinetic, inflationary regime due to the enhanced dissipation of nonlinear electron plasma waves propagating perpendicular to magnetic fields. This work considered parameters of direct relevance for SRS in inertial confinement fusion devices. Specifically, moderate laser intensities were considered: on the order of those found in the average speckle of an ICF beam. However, speckles of up to an order of magnitude higher intensity also exist in this context and are important. In this work we use particle-in-cell simulations to show that, while small magnetic fields suppress SRS for weak or moderate drivers, the effects of the B field are more complex at higher intensities (stronger drives). Reflectivity can be similar, or even enhanced. due to competing effects from the B field. On the one hand, it can sometimes, as described in [1], enhance dissipation of nonlinear electron plasma waves and suppress kinetic, inflationary SRS. On the other hand, it reduces non-linear frequency shift of non-linear electron plasma waves. This can lead to less detuning of SRS, which sometimes acts as a saturation mechanism, and can also lead to reduced dissipation of plasma waves and therefore provide a more coherent source from which the instability can grow. At moderate or low laser intensities these effects are not on equal footing, so a magnetic field generally suppresses SRS. But as intensity increases the balance shifts, and the effect of a magnetic field becomes more complex. We show both driven electron plasma wave simulations and self-consistent SRS simulations to illustrate these effects.

[1] B. Winjum, F. Tsung, W. Mori, Mitigation of stimulated raman scattering in the kinetic regime by external magnetic fields, Physical Review E 98 (2018) 043208.

Bio: Roman Lee is a PhD student at UCLA under advisor Warren Mori. His research has focused on using particle-in-cell simulations to study laser plasma interactions in the context of inertial confinement fusion, as well as developing algorithms and software for particle-in-cell codes which run on the largest high performance computing systems in the world. He received a BSc in Physics and a BSc in Mathematics from The University of Michigan, Ann Arbor, in 2017.