Non-Ideal Hall MHD Rayleigh–Taylor Instability in Plasma Induced by Nanosecond and Intense Femtosecond Laser Pulses

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In their article entield "Non-ideal Hall MHD Rayleigh-Taylor instability in plasmas induced by nanosecond and intense femtosecond laser pulses, Zemskov et al. present a comprehensive comparative analysis on the behavior of plasma flows generated by high-intensity femtosecond and nanosecond laser pulses with comparable fluences. The study explores how these plasma flows interact with a transverse, uniform magnetic field exceeding 14 T.

Main results:

The authors found that in vacuum, the femtosecond plasma exhibits initial collimation, maintaining a narrow propagation path, while the nanosecond plasma diverges significantly.

Under an external magnetic field, the femtosecond plasma displays distinct behavior when compared with the nanosecond case. While the nanosecond plasma develops quasi-spherical cavities and side flutes, these features are absent in the femtosecond plasma. Instead, the latter rapidly transitions into a narrow plasma sheet or "tongue" that propagates across the magnetic field at a steady velocity, mimicking at first glance the behavior seen in nanosecond plasma.

However, beyond t > 40 ns, the femtosecond plasma tongue exhibits significantly different dynamics. Its tip twists in the direction of ion motion induced by the magnetic field, in contrast to nanosecond plasma tongues, which remain randomly oriented. This twisting suggests a more structured interaction between the femtosecond plasma and the magnetic field.

The study also examines the instability of this elongated plasma slab geometry. Linear theory confirms that both femtosecond and nanosecond plasmas undergo fragmentation into tongues as a result of Rayleigh-Taylor instability in a transverse magnetic field. Nevertheless, a key difference lies in the behavior at the tips of these tongues: the femtosecond plasma tongues twist consistently in the ion motion direction, while those in the nanosecond plasma show no preferred orientation.

The authors validate this contrast using magnetohydrodynamic simulations carried out with the FLASH code. 

They demonstrate that the random orientation of nanosecond tongues is typical of classical MHD Rayleigh-Taylor instability, and largely unaffected by the external magnetic field. In contrast, the femtosecond tongues appear to be influenced by Hall effects, particularly at their tips, due to the plasma's lower density and reduced flow velocity.

Simulation-based estimates further suggest that these Hall effects are significantly amplified in femtosecond plasmas, aligning well with earlier experimental measurements of Hall fields in larger-scale plasmas. 

Comments :

- The authors claim that the underlying mechanism behind the femtosecond plasma's collimation is beyond the scope of this work and will be addressed in future studies. Could they nevertheless say a few words about it in the present article ?

- A more detailed description of the FLASH code would improve the readability of the paper and the interpretation of the results.

- An important, and rather unexpected point, is that the influence of these ion kinetic effects in large-scale femtosecond plasmas depends predominantly on macroscopic parameters such as density, temperature, and velocity, rather than on the hot electron population generated during intense femtosecond laser interaction. Is it really unexpected for large-scale plasmas ? Has such a property already been observed by other simulations in the past, in particular by other groups in the world? Could this be justified by a simple back-of-the-envelope calculation? Comparing characteristic lengths/times?

- In the discussions, the authors state that they consider only Rayleigh-Taylor Instability and Hall-Modified Rayleigh-Taylor Instability. I understand that the large Larmor-radius instability can be neglected since the Larmor radius is larger than the ion mean free paths, but could the helicoidal trajectories of the charged particles have an influence here?

- Are there some velocity anisotropies here, which could lead to Weibel instability?

- Some figures should be enlarged, such as the subfigures of Fig. 3, or Fig. 6.

- Table 1 was much appreciated.

General assessment:

In conclusion, the article presents very interesting results, which are the outcome of well-executed experimental campaigns and the work of a substantial group of researchers. The paper is clearly written and rather well organized. The measurements are compared to simulations, guided by order-of-magnitude estimates, and the analyses demonstrate a sharp physical intuition. Moreover, astrophysical applications are mentioned (flares in active galactic nuclei). I therefore recommend the article for publication in the journal "Plasma" without hesiation, provided that the aforementioned minor comments are taken into account.".

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Plasma flows generated by high-power nanosecond and high-intensity femtosecond laser pulses are investigated, based on the PEARL facility. Dynamics of fs and ns plasma flows are found to be different. The observed twisting of femtosecond tongues is related to the Hall effects. MHD simulations are performed to support the measurement. My comments are as follows.

1. Can authors further explain why two shapes of the laser spot on the target (circular and rectangular) are compared in the present work.
2. Can authors briefly explain the density reconstruction algorithm for obtaining fig 1.
3. “which redirect the plasma flow into a narrow «sheet» from the very early stages” would it be possible to mark the sheet in the figure to improve the readability.
4. “In the case of the nanosecond flow, the «tongues» move at a constant velocity, while the base of the flow where the tongues begin to form remains almost stationary.” Can authors further discuss how to judge that the tongues move at a constant velocity.
5. Can authors make a statement on how to make sure the simulation input parameters are consistent with the experiment setup, e.g. selection of pulse energy, duration, target temperature, etc.
6. The MHD simulation does not include fs flow (only ns simulations in fig.6), or there was a misunderstanding.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf