Victor Hasson

Pulsed corona excitation of high‐power uv nitrogen lasers at pressures of 0–3 bar

Stabilized pulsed corona‐type discharges can be used to pump uv laser efficiently at high pressures and on nanosecond time scales. This letter deals with the adaptation of this principle to the excitation of molecular nitrogen lasers at pressures ranging from 0 to 3 bar. Magewatt pulses of nanosecond duration may be obtained from a simple 25‐cm atmospheric‐pressure device. The electrical excitation is effected by a flat‐plate Blumlein circuit.

Transverse double‐discharge high‐pressure glow excitation of uv lasing action in molecular nitrogen

Pulsed high‐pressure photostabilized glow discharges can be used to pump ultraviolet lasers on nanosecond time scales. This capability is demonstrated by efficient glow excitation of ultraviolet lasing action in molecular nitrogen. The glow can be generated by simultaneously initiating many electron avalanches over the cathode surface. This glow mode of operation was achieved using a transverse double‐discharge excitation principle.

Traveling‐wave corona excitation of high‐power uv nitrogen lasers operating at gas pressures ranging from 0 to 3 bar

The authors use a novel 60‐cm traveling‐wave pulsing system to investigate the properties of corona‐excited uv nitrogen lasers operating at pressures ranging from 0 to 3 bar. The device will provide subnanosecond pulses at megawatt power levels. The lasing output at 337 nm can be enhanced by the addition of sulphur hexafluoride at total pressures of ≲1.5 bar.

The importance of corona generation and leader formation during laser filament guided discharges in air

Images taken with an intensified CCD camera show the dynamics during filament guided discharge events. The images reveal that filament initiated corona plays a role in the presented results. Furthermore, the images show the formation of leaders, propagating and eventually bridging the gap between the high voltage (HV) electrodes. Analysis of the images and comparison to oscilloscope traces of voltage and current dynamics reveal the origin of the delay between the filament and HV discharge and allows for a probability of discharge analysis.

Dependence of single-shot pulse durations on near-infrared filamentation-guided breakdown in air

We present results of an experimental investigation of laser pulsewidth dependence of filamentation-guided high voltage breakdown in air. The experiments are conducted at laser peak power levels of 1 TW and pulse durations of 0.7 to 10 ps with a discharge gap separation of 10 cm. Synchronized electrical and optical diagnostic techniques were used to determine the pulsewidth dependence on the breakdown mechanism, threshold levels, time delays and associated jitter. The results indicate that longer pulses provide greater than 30% reduction in breakdown threshold voltage.

Laser plasmas from picosecond laser filamentation in the atmosphere and its application on guided high voltage discharges

Summary form only given. Femtosecond laser filamentation in the atmosphere produces plasmas with densities on the order of 1016 cm-3 or less than 1% of air ionized [1]. Filamentation is a nonlinear process where a short laser pulse is propagating through a nonlinear medium while a dynamic balance between non-linear focusing, diffraction and plasma defocusing is established. The short lifetime and relative low temperature of the laser-generated plasma results in a high resistivity, making femtosecond filaments difficult to utilize for some applications including guiding high voltage electric discharges [2]. To achieve guided discharges with femtosecond filaments over several meters requires voltages larger than 100 kV between the electrodes. Alternatively nanosecond lasers have been used to create plasma sparks and trigger short discharges. The nanosecond laser plasma is denser and hotter in comparison to the femtosecond filament plasma. These properties result in a high conductivity and a fast trigger of the high voltage discharge [3]. However, the spatial extent of these plasmas is limited and only short discharges can be initiated. A recent experiment on picosecond laser filamentation [4] indicates the possibility of combining the properties of the two plasmas described above: an extended plasma channel with high conductivity. Here, we present experimental results that show the effect of picosecond laser filamentation on guiding high voltage discharges. The results indicate that higher conductivity plasma is produced in comparison to a femtosecond pulse. We achieved a reduction of threshold break down voltage by 20% with our experimental setup. The experiments are conducted using the COMET laser, at Jupiter Laser facility, Lawrence Livermore Laboratory.

Advanced concepts for high-power, short-pulse CO2 laser development

Ultra-short pulse lasers are dominated by solid-state technology, which typically operates in the near-infrared. Efforts to extend this technology to longer wavelengths are meeting with some success, but the trend remains that longer wavelengths correlate with greatly reduced power. The carbon dioxide (CO2) laser is capable of delivering high energy, 10 micron wavelength pulses, but the gain structure makes operating in the ultra-short pulse regime difficult. The Naval Research Laboratory and Air Force Research Laboratory are developing a novel CO2 laser designed to deliver ~1 Joule, ~1 picosecond pulses, from a compact gain volume (~2x2x80 cm). The design is based on injection seeding an unstable resonator, in order to achieve high energy extraction efficiency, and to take advantage of power broadening. The unstable resonator is seeded by a solid state front end, pumped by a custom built titanium sapphire laser matched to the CO2 laser bandwidth. In order to access a broader range of mid infrared wavelengths using CO2 lasers, one must consider nonlinear frequency multiplication, which is non-trivial due to the bandwidth of the 10 micron radiation.

The role of corona and space charges during femtosecond laser pulse filament guided high voltage discharges in air

Summary form only given. The plasma column left behind by ultrashort laser pulse filamentation is utilized to guide high voltage (HV) discharges in air. Many experiments have been carried out where the filament plasma is placed between two HV electrodes and a discharge is guided across the large gap (for example Ref [1]). Researchers still believe that one application could be potentially guiding lightning in the atmosphere, like a lightning rod, due to the longitudinal extent of the plasma ranging over several 10s of meters [2]. However lab experiments can only demonstrate discharges over a few meters. The reason for this is still not very clear due to the overall physics of the process not being fully understood. Here, we present experimental results in air on the role of space charges and corona during the filament guided HV discharge. Our conclusion is that the main driver of the breakdown is an enhancement of the corona between the electrodes and that the electric fields available limit the discharge distance achievable.Filamentation is a non-linear process where non-linear selffocusing (due to the Kerr effect), diffraction, and plasma defocusing create a dynamic balance [3]. The laser pulse propagates in a small beam diameter longer than the usual Rayleigh length. The plasma left behind the filament has a density on the order of 1016 cm-3 and a low conductivity. Initially it was believed that the plasma could directly guide HV discharges. However, a delay between the actual discharge and the filament placement is observed, ruling out the direct wire-like guiding mechanism. Previously, the reason for this delay was not completely understood. Our experimental results show that an enhancement of the formation of corona at the HV probes can be observed. From there, the dynamics rely on the electric field as a driver for developing leaders across the gap until a conducting connection is formed between the electrodes. These results show that the discharge distance is limited by the electric field and therefore limits the usefulness for the applications originally intended.