Collisionless Larmor Coupling and Blob Formation in a Laser-Plasma Expanding into a Magnetized Ambient Plasma

TL;DR

This study experimentally demonstrates collisionless Larmor coupling during the expansion of a laser-produced plasma into a magnetized ambient plasma, forming a diamagnetic cavity and an energized He$^+$ blob at the leading edge. Using a high-repetition-rate LAPD configuration with time-resolved magnetic field mapping, filtered self-emission imaging, Doppler spectroscopy, and 2D PIC simulations, the authors show that background ions are transversely accelerated by the laminar electric field and subsequently gyrotate under the ambient magnetic field, transferring momentum to the debris plasma. The results—including redshifted then blueshifted He$^+$ emission and a phase-space-rotated ion population—confirm Larmor coupling and its role in blob formation, with simulations capturing the essential dynamics and revealing the underlying phase-space evolution. Overall, the work provides a detailed, kinetic-level understanding of blob formation in collisionless, magnetized plasmas and connects laboratory results to space plasma processes.

Abstract

Collisionless Larmor coupling is a fundamental process in space and astrophysical plasmas that enables momentum transfer between an expanding plasma and a magnetized ambient medium. In this paper, we report on the laboratory experimental study of Larmor coupling leading to the formation of a plasma blob associated with a laser-driven, super-Alfvénic plasma flow on the Large Plasma Device at the University of California, Los Angeles. The high-repetition rate enables systematic spatial and temporal scans of the plasma evolution using Doppler spectroscopy, as well as measurements of the magnetic field, electrostatic field, and self-emission of both debris and ambient ions using filtered imaging. We observe the self-focusing of the laser-produced plasma and the formation of a secondary diamagnetic cavity associated with a blob composed of background ions. Doppler spectroscopy reveals the transverse velocity distribution of the background ions, providing direct evidence of ion energization via Larmor coupling. The systematic spatial and temporal scans enabled by the high-repetition rate experiment allow for a detailed characterization of the ion dynamics. These experimental observations are supported by numerical simulations that provide more insight into the kinetic-scale physics associated with blob formation as well as the role of the ambient plasma density.

Figures (7)

• Figure 1: Schematic of the experimental setup on the LAPD.
• Figure 2: Variation of the z component of the magnetic field $\Delta B_z$ relative to the initial background field, for: a)-c) An expansion of the LPP in vacuum d)-f) An expansion of the LPP in the background plasma. The probe could not access the top left corner of the plane, as this region intersected the laser beam path.
• Figure 3: (a)-(d) Variation of the magnetic field measured with a B-dot probe, (e)-(h) Filtered imaging of the plasma using a bandpass filter at 468.6 nm with a 0.5 nm bandwidth, corresponding to an excited He$^{+}$ emission line, with an exposure time of 50 ns (i)-(l) Filtered imaging at 227 nm with a 10 nm bandwidth, corresponding to a C$^{4+}$ emission line, with an exposure time of 4 ns. The direction of the electrostatic field, derived from plasma potential measurements using the emissive probe, is depicted by arrows whose length and color indicate the field's magnitude.
• Figure 4: (a) Doppler-shifted spectra and corresponding transverse velocity $V_y$ at different times at x = 6 cm (left) and x = 9 cm (right). The $t<0$ case corresponds to the He$^+$ line measured in the resting background plasma. It provides the impulse response of the spectrometer, with a full-width half max of $\delta\lambda=$ 0.04 nm, corresponding to a velocity resolution $\delta V_y =$ 25 km/s. The spectra were acquired with an exposure time of 200 ns and averaged on 200 shots. (b) Principle of Larmor coupling and its Doppler shift signature.
• Figure 5: (a) He$^+$ ions positive transverse velocity at half-maximum $V_{+50\%}$ and (b) Negative transverse velocity at half-maximum $V_{-50\%}$ obtained from Doppler spectroscopy.