What is it about?
In summary, we, in this work, have considered an e-i plasma in the presence of an external, Gaussian-shaped, charge perturbation (debris) moving through the plasma in an ion-acoustic timescale. We find that the response of the plasma differs significantly depending on the nature and the magnitude of debris charge density and its velocity. The simulation is being carried out with the well-tested h-PIC-MCC code which can take into account dust and dust-charge fluctuation self-consistently. We have shown that while a positively charged external perturbation produces DSWs in the precursor region, a negatively charged perturbation causes an IICSI, which quickly becomes turbulent, giving rise to the pinned solitons. In the ion density plot as well as in the scatterplot of the ion phase-space for various debris velocities, we see that when debris velocity increases; it causes the widening of the phase space vortices causing well-separated pinned solitons, which merge to form one single soliton when debris velocity reduces to zero. In the opposite extreme, when debris velocity becomes highly supersonic, the vortices are widened up to a limit causing the pinned solitons to disappear altogether. We further show that as demonstrated through linear analysis, the counterstreaming ions must exceed a critical velocity in order for the IICSI to be excited, which in this case is directly related to the strength of negatively charged debris charge density, which causes the ions to counter stream. So, in order to have pinned soliton formation, the debris charge density must be above a certain value. This value as determined from the simulation closely agrees with the analytical estimated value. Beyond this value of the debris charge density, the higher is the value of debris charge density, the quicker the IICSI develops. Through this work, we have shown that the pinned solitons are actually manifestations of the ion phase-space vortices formed in the turbulent regime of the ion-ion counterstreaming instability (IICSI), where ions are effectively trapped in the potential structure. Our simulation results are supported by the linear kinetic theory, through which we have shown the existence of critical debris charge density for the instability to turn turbulent. We have shown that a Kolmogorov-type energy cascading scaling exists in the turbulent regime which supports the formation of pinned solitons. In this context, we have the validity of MMT model in the case of 1D turbulence which also supports the scaling law for energy cascading. Toward the end, we have shown the effect of negatively charged dust particles on the pinned solitons, which causes the amplitude of the solitons to decrease and, thus, requires a relatively large debris velocity to make the pinned solitons appear as compared to the case when there is no dust particle. These results largely agree with the fluid simulation results.
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Why is it important?
In this work, we have primarily shown that the pinned solitons produced due to external charge perturbation are a manifestation of turbulence-induced ion-ion counter-streaming instability and can only occur when the type of external charge distribution is negative. We have also drawn a parallel between our findings and the 1D turbulence resulting from the famous MMT model.
Perspectives
In this work, we have tried to look into the basic physics involving the fundamental problem of plasma interactions with embedded charged debris. Naturally, it is quite tempting to see whether these findings can be extrapolated toward certain applicability namely, the detection of space debris. However, at this point, it is worth noting that the science of detection of space debris through possible plasma activities is still an open question and research in this direction is in a nascent stage. Though there are other theoretical works involving plasma interaction with externally charged debris, the scientific community as a whole, has not been able to detect such plasma activities so far, resulting out of space debris and we need more insights into the problem including the effect of stochastic and periodic variations of cumulative charge of such debris on plasma waves, which are some possible interesting extensions to this work. Toward this, we believe that this work provides a renewed look into the fundamental nature of such problems.
Mridusmita Das
Gauhati University
Read the Original
This page is a summary of: External charge perturbation in a flowing plasma and electrostatic turbulence, Physics of Plasmas, March 2025, American Institute of Physics,
DOI: 10.1063/5.0249425.
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