Search for: All records

An interesting aspect of complex plasma is its ability to self-organize into a variety of structural configurations and undergo transitions between these states. A striking phenomenon is the isotropic-to-string transition observed in electrorheological complex plasma under the influence of a symmetric ion wake field. Such transitions have been investigated using the Plasma Kristall-4 (PK-4) microgravity laboratory on the International Space Station. Recent experiments and numerical simulations have shown that, under PK-4-relevant discharge conditions, the seemingly homogeneous direct current discharge column is highly inhomogeneous, with large axial electric field oscillations associated with ionization waves occurring on microsecond time scales. A multi-scale numerical model of the dust–plasma interactions is employed to investigate the role of the electric field in the charge of individual dust grains, the ion wake field and the order of string-like structures. Results are compared with those for dust strings formed in similar conditions in the PK-4 experiment. 

The excitation of low frequency dust acoustic (or dust density) waves in a dusty plasma can be driven by the flow of ions relative to dust. We consider the nonlinear development of the ion–dust streaming instability in a highly collisional plasma, where the ion and dust collision frequencies are a significant fraction of their corresponding plasma frequencies. This collisional parameter regime may be relevant to dusty plasma experiments under microgravity or ground-based conditions with high gas pressure. One-dimensional particle-in-cell simulations are presented, which take into account collisions of ions and dust with neutrals, and a background electric field that drives the ion flow. Ion flow speeds of the order of a few times thermal are considered. Waveforms of the dust density are found to have broad troughs and sharp crests in the nonlinear phase. The results are compared with the nonlinear development of the ion–dust streaming instability in a plasma with low collisionality. 

Abstract Many gamma-ray bursts (GRBs) have been observed from radio wavelengths, and a few at very high energies (VHEs, >100 GeV). The High Altitude Water Cherenkov (HAWC) gamma-ray observatory is well suited to study transient phenomena at VHEs owing to its large field of view and duty cycle. These features allow for searches of VHE emission and can probe different model assumptions of duration and spectra. In this paper, we use data collected by HAWC between 2014 December and 2020 May to search for emission in the energy range from 80 to 800 GeV coming from a sample of 47 short GRBs that triggered the Fermi, Swift, and Konus satellites during this period. This analysis is optimized to search for delayed and extended VHE emission within the first 20 s of each burst. We find no evidence of VHE emission, either simultaneous or delayed, with respect to the prompt emission. Upper limits (90% confidence level) derived on the GRB fluence are used to constrain the synchrotron self-Compton forward-shock model. Constraints for the interstellar density as low as 10 −2 cm −3 are obtained when assuming z = 0.3 for bursts with the highest keV fluences such as GRB 170206A and GRB 181222841. Such a low density makes observing VHE emission mainly from the fast-cooling regime challenging.