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Therefore metrological data about TOD ... (week TOD, Fig.2), blue line – case of a full reflection (strong TOD, Fig. 3), red and green lines – intermediate cases.
Dynamics of the THz Optical Discharge V.V. Kubarev1,2 1

Budker Institute of Nuclear Physics, Novosibirsk, 630090, Russia 2 Novosibirsk State University, Novosibirsk, 630090, Russia

Abstract— Different regimes of the quasi-continuous THz optical discharge in air and argon produced by powerful radiation of the Novosibirsk terahertz free-electron laser were investigated. Breakdown threshold and dynamics of the week discharge well agree with a classical theory. Unusual dynamics with different oscillation frequencies of optical light intensity was observed for the strong optical discharge.

I. INTRODUCTION

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HZ optical discharge (TOD) was obtained early near output window of NovoFEL as one of the visual demonstrations of its high power [1]. Now powerful TOD is obtained in the safety user hall where a study of this phenomenon is possible in more details. Investigation of possible TOD’s applications is also started. It can be specific source of optical and UV radiations, plasma generator of second THz harmonic, medium for a creation of nanoparticles and nanotubes in the joint laser ablation and TOD process, source of the different gas-dynamic waves, etc. Laser optical discharge in the other side is one of the forms of gas discharge [2]. Therefore metrological data about TOD are important for gas discharge science. II. RESULTS Instrumental equipment of the experiment includes both full diagnostics of the input THz NovoFEL beam and optical system for measurement of intensity and spectrum of visual light. Measuring of the separate NovoFEL pulses and a pumpprobe THz experiment were made by our ultrafast Schottky diode detectors [3]. 8

We found that the cut-off plasma frequency in a typical TOD was close to the frequency of the NovoFEL radiation. Therefore reflection of the THz radiation depended on a diameter of plasma ball with above-critical density (Fig.1). We found that the week optical discharge good agreed with our theoretical predictions [4]. Optical radiation in the case had of the form of picks at frequency of THz NovoFEL pulses with decaying tail of a plasma relaxation (Fig. 2). Really each pick has three character decay times due to different plasma processes. Measured breakdown threshold intensity of 130 mcm radiation was 0.8 GW/cm2 for Ar and 1 GW/cm2 for air that were matching to a rough theoretical estimation of the values [4]. Much more rich dynamics of the discharge was at a higher THz power (Fig.3). When it was higher by 20-30 % than the threshold value, we found many character gas-plasma frequencies in the optical signal, which had a practically 100% modulation depth. The lowest frequencies lay in a sound range, the middle frequencies in 100-200 kHz range and the high frequencies were close to 0.6 and 1.2 MHz. Lowfrequency modulation was caused by technical reason (modulation of NovoFEL power) and will be damped in near future. Middle-frequency modulation has probably a fundamental gas-dynamic nature. High-frequency modulation in Fig.3c) is caused by the NovoFEL pulse excitation. Depth of the modulation strongly decreases with increasing of the NovoFEL power. It is obvious that increasing of input laser power transforms the phenomenon from gas-plasma breakdown to some continuous plasma discharge.

Measuring detector signal (a.u.)

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Fig. 1. Ultrafast pump-probe THz experiment: first pulse is transmitted part of THz radiation exciting the TOD, second pulse is part of the THz radiation reflected from the TOD (points): black line – case of a full passing (week TOD, Fig.2), blue line – case of a full reflection (strong TOD, Fig. 3), red and green lines – intermediate cases.

Fig. 2. Signal of the optical radiation, its Fourier spectrum, and photo of a week THz optical discharge. Periodicity of the signal coincides well with periodicity of the THz NovoFEL pulses (5.6 MHz).

Optical light intensity (a.u.)

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When THz power was higher by 50-60 % than the threshold value we found sometime a gas-plasma oscillation at frequency which exceeded exciting THz frequency (Fig.4). We propose some nonlinear gas-plasma-radiation interaction in the case. Study of the interaction is a subject of our investigation now.

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III. SUMMARY

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Different regimes of the quasi-continuous THz optical discharge in air and argon produced by powerful radiation of the Novosibirsk terahertz free-electron laser were investigated. We found a rich dynamics of the discharge. Additional investigations of the phenomenon are planned.

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REFERENCES

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Fig. 3. Typical dynamics of the THz optical discharge: low-frequency technical and middle-frequency gas-dynamic oscillations (a), middlefrequency gas-dynamic and high-frequency laser pulse modulations (b), and high-frequency laser pulse modulations in a strong discharge (c).

Fig. 4. Signal of the optical radiation, its Fourier spectrum, and photo of a strong THz optical discharge. High frequency of the signal (7.3 MHz) essentially exceeds the repetition frequency of the THz NovoFEL pulses (5.6 MHz).

[1]. V.P. Bolotin et al., “Status of the Novosibirsk High Power Free Electron Laser,” IRMMW-THz 2004, September 27 – October 1, 2004, Karlsruhe, Germany, p. 55. [2]. Yu. P. Raizer, “Gas Discharge Physics”, Springer, 1991. [3]. V.V. Kubarev et al., “Ultra-fast Terahertz Schottky Diode Detector”, IRMMW-THz 2009, Busan, Korea, 21–25 Sept., 2009, report 09030439. [4]. V.V. Kubarev et al., “Quasi-continuous sub-millimeter optical discharge on Novosibirsk free electron laser: experiments and elementary theory”, IRMMW-THz 2005, 19-23 Sept., 2005, Williamsburg, USA, p. 126-127.