Electromagnetic emissions in the MHz and GHz frequency ranges driven by the streamer formation processes

We provide comprehensive data on the spectral and temporal characteristics of low-frequency (LF) (MHz) and high-frequency (HF) (GHz) radio emissions and investigate their correlation with the streamer formation. We show that the propagation of streamers from the cathode is accompanied only by the LF radio emission (10–150 MHz). In contrast, the HF radio emission (1–4 GHz) arises during the travel of counterstreamers from the anode, which is also indicated by radio interferometric measurements. The power of the LF radio emission sharply increases almost synchronously with that of the HF radio emission. We find that the HF radio emission has a complex spectral and temporal structure and appears as multiple short (less than 1 ns) bursts characterized by various frequency components, existing in subnanosecond time intervals.

Figure 1

Experimental setup and devices employed for registering electromagnetic emissions and imaging the discharge. Inset 1 presents the simulated voltage standing-wave ratio of the ultrawideband antenna. Inset 2 shows the integral image of the discharge in the gap, formed by a needle-inside-cone-type cathode and a hemispherical wire-mesh anode.

Figure 2

Images are obtained in different shots and illustrate streamers propagating towards the anode (frame 1), counterstreamers traveling to the cathode (frame 2), and a complex net of numerous plasma channels resulting from the intense development of streamers in the discharge bulk (frame 3). Here exp denotes the exposure time.

Figure 3

Voltage waveform, microwave signals, and their spectra (insets 1 and 2) obtained in a single discharge event. Digits 1–3 in the top panel indicate the time intervals $\mathrm{\Delta }{t}_1$, $\mathrm{\Delta }{t}_2$, and $\mathrm{\Delta }{t}_3$ associated with the discharge images in Fig. 2.

Figure 4

Discharge images recorded in different shots with the simultaneous use of two sCMOS cameras, both having an exposure time (Exp.) approximately equal to 51 ns and imaging the discharge with $\mathrm{\Delta }t\approx 5$ ns delay between the frames.

Figure 5

(a) Scheme of a radio interferometric system, (b) side view of the discharge gap, and (c) view of the gap from the anode side. Superimposed spheres in (b) and (c) are the zones related to the onsets of the intense HF radio emissions localized in eight single shots.

Figure 6

(a) Weighted moving average (over 100 ns) $P_{\text{WMA}}\left(t\right)$ of the instantaneous power calculated for the monopole antenna signal in Fig. 3 within 120–550 ns. (b) Map illustrating the evolution of the microwave spectrum in the frequency range of 10–150 MHz. The map intensity characterizes the values of the average instantaneous power ${\overline{P}}_f\left(t\right)$ in the time and frequency domain $(t,f)$.

Figure 7

(a) Weighted moving average (over 1 ns) $P_{\text{WMA}}\left(t\right)$ of the instantaneous power calculated for the ultrawideband antenna signal in Fig. 3 within 325–550 ns. (b) Map illustrating the evolution of the microwave spectrum in the frequency range of 1–6 GHz. The map intensity characterizes the values of the average instantaneous power ${\overline{P}}_f\left(t\right)$ in the time and frequency domain $(t,f)$.

Figure 8

(a) Certain time period of the instantaneous power $P\left(t\right)$ of a typical HF radio signal. (b) Average sum ${\overline{P}}_{\text{WMA}}\left(t\right)$ of WMA functions obtained over 1 ns. (c) Bar chart of the burst duration $\mathrm{\Delta }{t}_{\text{FWHM}}$. (d) Asymmetric bar chart of the delay $\mathrm{\Delta }{t}_{\text{delay}}$ between the maxima of neighboring bursts. (e) Burst maxima. (f) Scatter plot of the burst energy $Q_i$ depending on the burst duration. Here ${\overline{\mathrm{\Delta }t}}_{\text{FWHM}}$, ${\overline{\mathrm{\Delta }t}}_{\text{delay}}$, and ${\overline{Q}}_i$ are mean values; ${\delta }_{\text{FWHM}}$, ${\delta }_{\text{delay}}$, and ${\delta }_Q$ are mean deviations; ${\sigma }_{\text{FWHM}}$, ${\sigma }_{\text{delay}}$, and ${\sigma }_Q$ are standard deviations; $\mathrm{\Delta }{t}_{\text{delay}}^{\text{mode}}$ is the mode; $\mathrm{\Delta }{t}_{\text{delay}}^{\text{median}}$ is the median; and $N_{\text{total}}$ is the total number of observations. The data are obtained in 20 discharge events.

Figure 9

(a) Illustrative spectrum map of a HF radio signal subjected to tracing in the frequency range of $\mathrm{\Delta }{f}^{*}$, (b) distribution of the average instantaneous power ${\overline{P}}_f\left(t\right)$ extracted for $\mathrm{\Delta }{f}^{*}$, and (c) number of observations $N$ (in 20 discharge events) of the time intervals $\mathrm{\Delta }{t}^{*}$ during which the HF radio emission is described by the frequencies in the spectral interval $\mathrm{\Delta }{f}^{*}$ (GHz): (1) 1–1.5, (2) 1.5–2, (3) 2–2.5, (4) 2.5–3, (5) 3–3.5, and (6) 3.5–4. The inset shows the average profile ${\overline{P}}_f^{*}$ of the burst power depending on frequency $f$.