Laser Triggering Research Articles

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-I: experimental results

Using a plasma channel produced by an ultrashort laser pulse, we have studied the laser triggering and guiding of a positive leader from the tip of a 2-m vertical rod standing on the bottom plane of a 7-m plane-plane gap. The purpose of this setup was to reproduce in the laboratory the electric field conditions leading to the onset of a positive upward leader from a ground rod as a downward negative leader is approaching during a thunderstorm, in order to demonstrate the working principle of a possible future laser lightning rod. The leader triggering properties of the laser-created plasma channel have been studied as a function of the synchronization of the laser pulse with the voltage impulse applied to the gap. We show that the laser pulse reduces the inception voltage of the leader compared to its normal value and that the laser plasma channel guides the propagation of the upward leader at a velocity ten times higher than that of an ordinary leader, with a significantly lower charge per unit length. We show that laser guiding of the leader significantly reduces the breakdown voltage of the gap and that the effect of the laser channel at the end of a lightning rod can be compared quite favorably with the effect of an additional metal rod of the same length.

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse-II: modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.

C109 超高速ビデオカメラ用画像トリガーの開発

An ultra high-speed video camera whose frame rate is 1 million frames/second was developed by a cooperative R and D project of Kinki University and Shimadzu Corporation in September, 2001. In this ultra high-speed video camera, the continuous overwriting system is used. Number of recorded frame is 103. In case of high-speed image capturing, a method of sending a trigger signal for stopping continuous overwriting becomes important. Currently available trigger systems are mainly a trigger by sound and a trigger by laser. In the case of image capturing of a bursting balloon, we can use the sound and/or laser trigger. However, it is difficult to capture images at an ultra-high frame rate under a microscope. A video trigger system, which can be applied to microscopic ultra high-speed image capturing, is under development. In this paper, an algorithm for detection of a sudden change of brightness in an image is presented.

Prototype IVA module

A prospective module has been designed for a future large inductive voltage adder (IVA) machine that would be suitable for the most demanding radiography carried out at AWE Aldermaston, Berkshire, UK. This machine would operate at 13-15 MV. The prototype IVA module (PIM) machine has been built to test a single such module and may also provide the basis for other, lower voltage machines in the future. It consists of a single inductive cavity pulsed by a 10-/spl Omega/ water dielectric Blumlein pulse forming line which is charged by a Marx generator. The Blumlein switch initially installed was a coaxial two stage Rimfire and Trigatron switch. A laser triggering configuration utilizing two radial switches, designed by Titan PSI, is also to be tested. The Blumlein PFL and its switch were tested using a copper sulphate resistive load and achieved a peak output voltage of 1.7 MV. The inductive cavity has been added and a pulse of 1.5-MV peak voltage has been successfully applied to it.

Effective laser field influence on the transverse triggering of a spark gap in air

Nd : YAG laser triggering of a centimetric air gap in a homogeneous electric field is studied for pressures ranged from 0.5 to 5 bar. Breakdown transient voltage and current are measured and the formative time lag dependence on reduced field and laser intensity is analysed. It is quantitatively shown that the laser field strongly influences the time lag to breakdown through the effective reduced field.

Increase of Trigger Efficiency by Using an Artificial Aerosol Flow Produced by Gun Powder in Laser Triggered Lightning Experiment.

Increase of Trigger Efficiency by Using an Artificial Aerosol Flow Produced by Gun Powder in Laser Triggered Lightning Experiment.

Electric Properties and Irradiation of a Pulse-Laser-Excited Miniature Magnetic-Dipole Antenna

The electric properties and frequency band of irradiation of time-domain excited miniature magnetic-dipole antennas were investigated. An ultrafast switch of superconductor (SPC) or semiconductor (SMC) material, incorporated into a current-biased contour of metal-printed antenna and triggered by 14 ps laser pulses was used as time-domain generator. Amplitude and duration of the electric pulses, generated by laser triggering antennas were investigated as a function of the voltage amplitude feeding these antennas and limits of SPC and SMC switches, applied in magnetic dipoles for generation of high-power electromagnetic pulses, are discussed.

Switching characteristics of vacuum gap switch employing UV‐laser triggering

The switching time and arc-ignition probability of a sustained main discharge in a laser trigger vacuum gap were measured. The third harmonic beam of an Nd:YAG pulse laser (wavelength 355 nm, energy 5 mJ/pulse, power density 4 × 107 W/cm2) was used to trigger the gap. The main electrodes 85 mm in diameter were made of oxygen-free copper. The gap length was 1.6 mm and the experimental chamber was evacuated to about 1.3 × 104 Pa by a turbomolecular pump. The UV beam from the laser was focused at normal incidence onto the grounded cathode. The switching time decreased and the arc-ignition probability increased with increasing applied voltage. The switching time was 326 ns at 15 kV with a circuit inductance of 107 μH. Its jitter was 24.8 ns. The advantageous results obtained with the UV beam are compared to the previously reported data on triggered vacuum gaps. © 2000 Scripta Technica, Electr Eng Jpn, 132(3): 8–13, 2000

Synchronous Trigger Detection for Pulse Laser Readout on Super Resolution Erasable Optical Disks

The laser pulse readout scheme is advantageous in reading thermal super resolution disks, but the light modulation on the RF signal confuses the detection channel. A synchronous trigger detection scheme, using the pulse laser trigger signal to pick up the desired signal, is proposed to suppress the adverse effects of the light modulation. A 5-dB increase of the signal on an magneto-optical super resolution center aperture detection disk by the synchronous detection system was demonstrated experimentally.

A213高速度カメラによる小形モータの整流火花の可視化

Universal motors are used in mainly cleaners. The commutating processes take place by sliding contact of commutator segment and brush, and sparks occur when a segment separates from the brush. The commutation sparks cause wear on the brush and electromagnetic interference to other electronic machines. We visualized images of sparks on high-revolving. motors by a high-speed camera, and analyzed the mechanism of the spark generation. We devised external laser triggering using laser light for shuttering of the camera. This method make it easier to analyze images because it provides with images of the same point of the same segment every time. By photographing spark generation with a high-speed camera by external laser triggering we probed that sparks occurred on every other segment and also visualized that commutation spark occurred on one segment.

Photographing the Commutation Spark from Universal Motor by External Laser Trigger Method.

We have attempted to clarify the mechanism of generating commutation spark by means of the images photographed with a high-speed camera. In the photographing system with the high-speed camera, we have contrived a new external laser trigger method that uses a laser beam as trigger signal for shutting of camera. In the external laser trigger method, we provide the motor fan with a 1-mm hole. When the laser beam passes through this hole, a trigger signal is generated, and this trigger signal performs shuttering of the high-speed camera via an optical detector and a pulse generator. The new external laser trigger method has enabled us to photograph spark of any commutator segment of the motor in high-speed rotation. Moreover, it has also become possible to photograph the process from generation to extinction of commutation spark, making us capable of analyzing the commutation spark from photographed images.

Switching characteristics of vacuum gap switch employing UV-laser triggering

The switching time and arc-ignition probability of a sustained main discharge in a laser trigger vacuum gap were measured. The third harmonic beam of an Nd:YAG pulse laser (wavelength 355 nm, energy 5 mJ/pulse, power density 4 × 107 W/cm2) was used to trigger the gap. The main electrodes 85 mm in diameter were made of oxygen-free copper. The gap length was 1.6 mm and the experimental chamber was evacuated to about 1.3 × 104 Pa by a turbomolecular pump. The UV beam from the laser was focused at normal incidence onto the grounded cathode. The switching time decreased and the arc-ignition probability increased with increasing applied voltage. The switching time was 326 ns at 15 kV with a circuit inductance of 107 μH. Its jitter was 24.8 ns. The advantageous results obtained with the UV beam are compared to the previously reported data on triggered vacuum gaps. © 2000 Scripta Technica, Electr Eng Jpn, 132(3): 8–13, 2000

Compact laser molecular beam epitaxy system using laser heating of substrate for oxide film growth

A high-temperature, oxygen compatible, and compact laser molecular beam epitaxy (laser MBE) system has been developed. The 1.06 μm infrared light from a continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to achieve a wide range and rapid control of substrate temperature in ultrahigh vacuum and at up to 1 atm oxygen pressure. The maximum usable temperature was limited to 1453 °C by the melting point of the nickel sample holder. To our knowledge, this is the highest temperature reported for pulsed laser deposition of oxide films. The efficient laser heating combined with temperature monitoring by a pyrometer and feedback control of the Nd:YAG laser power by a personal computer made it possible to regulate the substrate temperature accurately and to achieve high sample heating and cooling rates. The oxygen pressure and ablation laser triggering were also controlled by the computer. The accurate growth parameter control was combined with real-time in situ surface structure monitoring by reflection high energy electron diffraction to investigate oxide thin film growth in detail over a wide range of temperatures, oxygen partial pressures, and deposition rates. We have demonstrated the performance of this system by the fabrication of homoepitaxial SrTiO3 films as well as heteroepitaxial Sr2RuO4, and SrRuO3 films on SrTiO3 substrates at temperatures of up to 1300 °C. This temperature was high enough to change the film growth mode from layer by layer to step flow.

Plasma pinch for final focus and transport

Adiabatic plasma lens and Z-pinch channel transport represent attractive alternatives for the final transport of Heavy-Ion Fusion (HIF) beams to target. Production of stable, reliable, precisely timed Z-pinches is a first step to establish this method of channel transport for HIF. Stable Z-pinches at currents greater than 30 kA over a distance of 400 mm were routinely produced at 3–10 Torr. The technique uses four distinct steps: (1) laser ionization; (2) laser initiated resistor; (3) prepulse Z-pinch; and (4) high-current Z-pinch. The time interval between laser trigger and main-bank spark-gap trigger is controllable to less than 10 ns, limited by the firing characteristics of the spark gap. Optical channel diameters measured with a gated TV camera and correlated with a current probe are approximately those needed for transport in fusion chamber. No large or small scale instabilities were observed before the first current peak. A symmetric two-sided return path had a stable target interaction, even with inductively unbalanced currents. As expected, a single return path, 90° to a single-pinch channel had unstable alignment on the target, due to higher magnetic pressure on the inside of the 90° bend.

Starting Process of Laser-Triggered Vacuum Arc Ion Source

A laser-triggering method was applied to a vacuum arc ion source to improve triggering reliability. A frequency-doubled Nd-YAG laser was focused onto the arc cathode at a laser power density of (4.5–6.1)×108 W/cm2. Time-resolved observations revealed that a cathode spot was initially formed within the laser focusing area and the distribution of the cathode spots expanded circularly with time because of the retrograde motion of each cathode spot. The mean square displacement of the retrograde cathode spot was found to be proportional to the discharge time. Moreover, arc ignition probability with the laser trigger was measured under various conditions. It was found that the ignition probability strongly depends on the laser intensity and the gap spacing. This paper discusses the starting process of the laser-triggered vacuum arc discharge on the basis of the experimental results.

`Triggerless' triggering of vacuum arcs

Vacuum arcs can be initiated without external means of power enhancement at the cathode surface (by using a high-voltage trigger, laser triggering, and so on). Arc initiation by simply applying the relatively low voltage of the arc power supply is possible if the cathode-anode-separating insulator is coated with a conducting layer and the current at the layer-cathode interface is concentrated at one or a few contact points. The local power density at these contact points can exceed which is sufficient for plasma production and thus arc initiation. This `triggerless' principle has been tested successfully with a large number of cathode materials. One extended test was performed with titanium and more than pulsed vacuum arc initiations have been obtained.

Photographing of Commutation Spark by External Laser Triggering.

The single-phase AC commutator motor (universal motor) is a compact, lightweight and high-revolution motor that is widely used in such domestic electric appliances as cleaners, juicers, electric drills. Some problems generated by commutation sparks are wear on the brush and noise impediments. We have therefore attempted to use a high-speed camera to elucidate the commutation spark mechanism visually. We proposed an external trigger method that an argon laser is used for the external trigger signal of high-speed camera. This method enabled us to photograph the motor's commutator segment from any position.

レーザトリガ合成試験始動システムの研究

レーザトリガ合成試験始動システムの研究

Instrumentation for laser-induced annihilation spectroscopy of metastable antiprotonic helium atoms

We have established a technique for laser resonance spectroscopy of metastable antiprotonic helium atoms (p̄He +) by successfully observing laser-induced annihilation. In this paper, we describe the full instrumentation of the experiment in detail, namely, the particle detectors, the laser system, the cryostat and the helium gas target, the data acquisition and analysis method. For an effective laser triggering selective to 3% metastable antiprotons, a highly efficient detection system for antiproton annihilation was required, with a suppressed inefficiency of less than a percent. Guided by simulations, we designed a system of seven lead-scintillator sandwich counters to detect the charged pions and π 0-decay gamma rays from antiproton annihilation, and achieved 99.7±0.1% efficiency for the detection of annihilation. We employed two identical sets of excimer-pumped dye laser systems with 2–5 mJ output energy per pulse, which were triggered by every metastable antiproton formed in a helium gas target of 0.3–1.0 bar at 4.5–10 k. The resonant deexcitation of the metastable states was detected as a spike-like forced annihilation in the time spectrum, thus revealing the level structure of this exotic atom. This powerful technique enabled us to study also the lifetimes and populations of specific metastable states, by changing the timing and the wavelengths of the laser pulses.

A large aperture laser triggered intensified charge coupled device using second-harmonic laser light triggering

For application to a ruby laser Thomson scattering system, we have developed a laser triggered intensified charge coupled device (CCD) with 80 mm aperture, two stages of intensification, and 80 ns gating. To improve the dynamic range, the CCD is cooled and read out slowly (1 s). To obtain a good extinction ratio (>1.1×107), the zoom electrode of the first intensifier is gated using a ∼10 kV laser triggered spark gap. The stability of this spark gap has been greatly improved by frequency doubling the laser trigger light.