Explosion plasma is a transient intermediate state generated during explosive detonation and plays an important role in electromagnetic radiation and detonation physics. Electron density is one of the key parameters for characterizing explosion plasma, but its rapid evolution on the microsecond timescale makes accurate time-resolved measurement challenging. Moreover, while conventional optical diagnostics and charge-probe techniques can provide information about plasma characteristics, they have limited ability to resolve the temporal evolution of electron density.
To address this challenge, a team of researchers in China developed a microwave Rayleigh scattering diagnostic system adapted specifically for microscale explosive charges. HMX (high melting explosive, also known as octogen) was selected as the test explosive, while an exploding foil initiator (EFI) was used to provide highly synchronized initiation. The study establishes a fast, accurate, and non-intrusive approach for time-resolved diagnostics of electron density in microscale explosive plasma.
Microwave measurements were integrated with ultra-high-speed imaging, optical emission spectroscopy, thermodynamic calculations, and numerical simulations to obtain the parameters required for electron-density inversion.
“Combining microwave Rayleigh scattering with complementary experimental and computational diagnostics provides a rapid and non-intrusive means of quantitatively resolving the transient electron density of plasma generated by microscale explosive detonation,”explains co-corresponding author Zhe Zhai.
A High-Precision Diagnostic Platform for Microscale Explosive Plasma
Microwave Rayleigh scattering determines plasma properties through the interaction between microwave electromagnetic waves and free electrons. The scattered microwave signal is related to the electron number density, while the plasma volume and electron–neutral collision frequency provide additional parameters required for quantitative inversion. In the present study, the diagnostic system was calibrated using quartz dielectric scatterers. “The explosive plasma signal was measured using orthogonally arranged microwave antennas, while an ultra-high-speed camera recorded the evolution of the plasma cloud,” says Zhai. “The independently developed EFI substantially reduced the interference associated with conventional electric detonators and improved synchronization accuracy. Its firing-time jitter was less than 50 ns, while the discharge interference lasted approximately 1.35 μs.”
The researchers further combined several complementary measurements to determine the parameters required for electron-density reconstruction. “Plasma temperature was obtained from optical emission spectroscopy, the molecular radius of the gaseous detonation products was evaluated using thermodynamic calculations and detonation-pressure measurements, and the time-dependent pressure of the detonation products was reconstructed through finite-element simulations,” adds Zhai.
Resolving the Microsecond Evolution of Electron Density
The measurements reveal that the plasma generated by microscale HMX detonation has a very short lifetime of approximately 1.91 μs. “Its electron density reaches the order of 10²⁵ m⁻³ and exhibits nonlinear temporal evolution with multiple peaks,” shares Zhai. “The first electron-density peak reaches 5.50 × 10²⁵ m ⁻ ³ at approximately 1.7 μs , followed by a decrease and a second rise to 3.13 × 10²⁵ m ⁻ ³ at approximately 2.35 μs . The density subsequently decreases continuously.”
The experimentally observed plasma motion was also compared with numerical simulations. The maximum deviation between the simulated plasma trajectory and the ultra-high-speed imaging results was only 4.27%, demonstrating that the numerical model can effectively reproduce the dynamic motion of the detonation products and their pressure evolution.
“The combination of these results provides a quantitative picture of the evolution of explosive plasma over the microsecond timescale, overcoming the limitations associated with measurements based solely on temporally averaged or literature-derived plasma parameters,” says Zhai.
Detonation-Product Pulsation Drives the Multi-Peak Plasma Evolution
The study further reveals the physical origin of the multi-peak electron-density evolution. Plasma formation during condensed-explosive detonation involves several successive processes, including plasma generation at the detonation front, evolution of the detonation products, and interaction between the detonation products and the surrounding air shock wave.
“At the initial detonation stage, plasma can be generated through vibrational relaxation at the detonation front, adiabatic compression of gas inclusions, and dissociation of detonation products within the chemical reaction zone,” explains Zhai. “As detonation products expand and propagate, their asymmetric motion and pulsation further influence plasma generation. When the products interact with the surrounding air, shock-induced ionization can generate additional plasma.”
Notably, the simulated pressure evolution of the detonation products exhibits multiple peaks that closely follow the temporal evolution of the electron density. “The results therefore indicate that pulsation of the detonation products is the primary factor responsible for the multi-peak structure of the electron-density evolution . As the detonation products subsequently expand, cool, and dissipate, the plasma rapidly decays and nearly disappears by approximately 3.3 μs,” notes Zhai.
###
Contact author details:
Zhe Zhai, School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, China b Micro-Nano Energetic Devices Key Laboratory, Ministry of Industry and Information Technology, Nanjing, 210094, China c China North Industries Group Corporation Limited, Beijing, 100070, China, E-mail addresses: Zhangren0433@njust.edu.cn (R. Zhang), zhaizhe2020@126.com (Z. Zhai)
The publisher KeAi was established by Elsevier and China Science Publishing & Media Ltd to unfold quality research globally. In 2013, our focus shifted to open access publishing. We now proudly publish more than 300 world-class, open access, English language journals, spanning all scientific disciplines. Many of these are titles we publish in partnership with prestigious societies and academic institutions, such as the National Natural Science Foundation of China (NSFC).
Energetic Materials Frontiers
Time-resolved microwave rayleigh scattering diagnostics of electron density in plasma generated by detonation of microscale explosive charges
The author is an Editorial Board Member/Editor-in-Chief/Associate Editor/Guest Editor for this journal and was not involved in the editorial review or the decision to publish this article. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests.