Laser-triggered Spark Gap Research Articles

Dynamics of Laser Triggered, Gas-Insulated Spark Gaps During Repetitive Operation

The Electra pulsed power system at the U.S. Naval Research Laboratory uses laser triggered SF6 gas-insulated spark gaps to transfer energy to a high power electron beam diode. These 1-MV spark gaps have a charge transfer of 10 mC per shot operating at 5 pules per second (pps) utilizing electrodes with the potential durability of 300 000 pulses. During continuous repetitive switch operation, after a few thousand shots intermittent nonfire events occur with greater frequency approaching 100 000 shots. The nonfire events and the lack of reproducibility and reliability reduce the potential applications for laser triggered spark gaps. Therefore, the temporal evolution and the spatial evolution of the switch dynamics have been examined. Gated camera images, streak camera images, and interferometry are used to characterize the dynamics of single shots to directly compare to the dynamics during continuous operation. The streak camera images indicate that the switch dynamics are dependent on the repetition rate. For single shots and 1 pps operation (9000 continuous shots), no misfires (mistimings or nonfires) are observed in the switch. This observation is concomitant with a consistent and reproducible breakdown position and breakdown dynamics. At 5 pps, the variations in the laser breakdown position (preferentially toward cathode) and the laser breakdown emission intensity are observed. After several thousands of continuous shots, nonfire events are detected in small groups, before normal operation resumes. A temperature differential of at least 22.2 K (40 F) between the SF6 gas at the top of the housing and the bottom of the spark gap housing is observed. The onset of nonfire events is correlated with the hydrodynamic changes, from increased temperature, of the SF6 gas flow within the spark gap. A redesign of the spark gap along with adequate cooling to reduce the hydrodynamic flow and forces of the insulating SF6 gas would mitigate if not eliminate nonfire events.

Laser Triggering of Spark Gap Switches

Switching time jitter is an important property to consider when selecting a closing switch for a pulsed-power system, and time-precise triggering may be achieved through the use of lasers. For a midgap laser-triggered spark gap, three different physical mechanisms can be used: nonresonant multiphoton ionization, resonant-enhanced multiphoton ionization, and electron tunneling. The first one is traditionally used, whereas the latter two are more exploratory. In this paper, the traditional method is employed to study the delay time and time jitter of a laser-triggered spark gap using an Nd:YAG laser at 1064 and 532 nm, where the laser pulse is guided via an optical fiber to the spark gap; the laser pulse energy and the applied voltage have been varied with nitrogen as the working gas. One draw back of the current laser triggering technology compared with other triggering techniques is that laser systems are more complex and prone to electromagnetic interference. Another downside is that the pulse-repetition rate is poor. A discussion about the development of lasers to overcome these issues is included, together with a deliberation about the pros and cons of the two exploratory methods of laser triggering.

Initial Performance of the Primary Test Stand

The primary test stand (PTS) is a multiterawatt facility with a current of 8-10 MA and rise time of 90 ns, which is being built for Z-pinch studies at the Institute of Fluid Physics, China Academy of Engineering Physics. The PTS consists of 24 modules that are connected in parallel. Each module is based on the Marx generator and water pulse forming line as well as laser-triggered spark gap switch working at 5-MV level. The nominal total stored energy of the PTS is 7.2 MJ. All the Marx generators and laser-triggered spark gap switches had been tested at full voltage level before the PTS was assembled. The design and installation of the PTS are described. In this paper, the most recent results of the PTS commissioning will be introduced.

Compensated Monte Carlo Collision Model for Particle-in-Cell Simulation in High-Pressure Plasmas

The Monte Carlo collision (MCC) model is widely adopted to simulate discharge plasmas using the particle-in-cell (PIC) method; however, it has low efficiency in high-pressure plasmas because of the small time steps required due to the constraint of high collision frequency. To relax this time step constraint, a compensated Monte Carlo collision model (CMCC) is proposed which considers multiple collisions in a time step as a series of single collisions to compensate for the neglected collisions. The electron motion in a high-pressure He gas for various reduced electric fields E/N and the streamer formation process in a laser-triggered spark gap were simulated using the CMCC model. Simulation results showed that the CMCC model with long time step obtained reasonable electron velocity distribution, temperature, drift velocity, plasma density, and space-charge field. It was demonstrated that the CMCC model had high accuracy and high efficiency, particularly for PIC simulation in high-pressure plasmas.

X radiation from a laser-triggered spark gap

Laser triggering of spark gaps is now widely used for low-jitter firing of one or several spark gaps [1–5]. The most important advantage of laser-triggered high-voltage spark gaps is no galvanic coupling between the gap electrodes and the laser. This allows low-jitter firing of one or several spark gaps to which electrodes a dc or a pulsed voltage is applied. Conduction between the spark gap electrodes is initiated by the plasma produced at one of the electrodes or in the gas-filled gap. For a laser-triggered spark gap, the trigger delay and jitter depend on the gas mixture composition and pressure, laser beam focusing site, electric field strength, laser wavelength, radiation power density and pulse duration, and laser beam divergence. For low radiation pulse energies, the shortest trigger delays are achieved for systems with uv lasers, though in some cases, pulsed lasers operating in other spectral ranges are used. Elegas (SF6) or its mixtures with nitrogen are generally used for the working gas in high-voltage spark gaps. SF6 is electronegative gas possessing high electric strength. In optimum conditions (high electric fields, optical breakdown of the gas in a gap at the cathode) the delay time of the spark gap triggering after application of a laser pulse is a few nanoseconds, and the jitter is no more than 1 ns even for a rather short length of the laser spark (about ten gap spacings). Recently it has been shown [6] that the spark preionization systems that are widely used to initiate a volumetric discharge in pulsed lasers generate not only uv and vuv radiation, but also x rays. This makes preionization more uniform and improves the lasing performance for some lasers. There are many reports on x radiation and runaway electron beams in nanosecond discharges initiated in a nonuniform electric field (see, e.g., [7, 8–14]). Supershort avalanche electron beams (SAEBs) where produced in gas diodes at a pressure of 12 atm for helium, 4 atm for nitrogen, and 2 atm for xenon and elegas [8, 10, 14]. It was natural to suggest that runaway electrons and x radiation are generated in a laser-triggered discharge gap as well, and it is just the runaway electrons and x radiation that provide low-jitter laser triggering. However, we have found no information on this subject in the available literature. Note that in conditions of laser triggering, the threshold of optical breakdown for gases used in spark gaps is in the range 5⋅10– 5⋅10 W/cm [1, 15], which is several orders of magnitude lower than the threshold for the appearance of runaway electrons and x radiation in a laser plasma [16]. The goal of the work under discussion was to detect x radiation from a laser-triggered spark gap and to demonstrate that the generation of x radiation in the gap is due to the electric field enhancement by the laser-produced plasma.

Photoconductive Operation of a Laser Triggered Spark Gap

The operation of a photoconductive spark gap switch is reviewed. It is shown that it is possible to produce pulses far below the self-breakdown voltage of the gap. The resistance of the switch depends on the switched current through the switch. Electro-dynamic simulations show that the rise time of the pulse depends on the layout of the switch and is approximately 35 ps for the setup used here.

Particle-in-Cell Model of a Laser-Triggered Spark Gap

A particle-in-cell simulation of the streamer formation process in a laser-triggered spark gap has been developed to estimate streamer radius and conductivity under a variety of conditions. The motivation for developing the model was to understand the initial conditions of the switching arc in a laser-triggered spark gap, with the goal of accelerating the resistive decay of the arc. In a parallel plate switch configuration with dimensions of 0.6 mm times 0.3 mm, the electric field was varied from 2 to 5 MV/m, the gas pressure was varied from 1 to 1.5 atm, and the gas type was varied between Ar and Ne. The laser triggering parameters, including the laser ionization rate (from 5 to 20middot10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ), laser spot area (from 1200 to 4800 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), and laser spot orientation relative to the electric field, were also varied. The electric field had a large influence on radius, increasing it by 19% with increasing field, and on conductivity, resulting in a 285% increase with increasing field. Among the laser triggering parameters, a linear focal spot-oriented transverse relative to the electric field direction was observed to produce a very large increase in streamer radius

Experimental investigation of an atmospheric photoconductively switched high-voltage spark gap

We report on the experimental investigation of the photoconductively switched gas-filled spark gap. When the laser intensity of a femtosecond laser is high enough (around 1018Wm−2), a plasma can be created that spans the complete distance between the electrodes. The gas-filled spark gap is then closed on a femtosecond time scale, similar to photoconductive switching of a semiconductor switch. Stochastic breakdown processes, such as avalanche and streamer formation that cause the breakdown in laser triggered spark gaps, are passed over, which results in faster rise time and less jitter. Measurements of the switched pulses as a function of laser energy were performed in a 1-mm gap at an applied voltage of 4.5 kV. A clear transition from triggering to switching was measured with increased laser energy. Measurements of the output pulses with the gap filled with nitrogen at 1 atm showed results very similar to measurements in air in the same gap. In the switching regime, the amplitude of the switched pulse did not depend strongly on the laser energy. Measurements at lower applied voltages but with the same gap distance showed that it was possible to switch voltages as low as 10% of the self-breakdown voltage. At low applied voltages, a significant difference between the applied voltage and the output voltage is measured. A possible explanation is given based on the dynamic behavior of the laser-created plasma. The measured rise time and jitter of the switched pulses were both below the resolution of the measurement equipment, i.e., better than 100 and 15 ps, respectively.

Electrodynamic simulations of a photoconductively switched high voltage spark gap

We present a full three-dimensional electrodynamic model to simulate a photoconductively switched high voltage spark gap. This model describes the electromagnetic field-propagation in a coaxial spark gap set-up, which determines the rise time of the switched pulse and reveals the influence of discontinuities, such as view ports, on the pulse shape and the rise time. Existing inductive lumped element and transmission line models, used to model laser-triggered spark gaps, are compared with our electrodynamic model. The rise time of the switched pulses in the different models does not differ significantly. In the electrodynamic simulation, a curvature of the electric field wave front is visible, resulting from the presence of non-TEM modes near the gap. Furthermore, oscillations on the output signal are revealed. These oscillations are caused by internal reflections on the inner and outer conductors. Our electrodynamic model is able to visualize the rise time evolution by monitoring the electric field-propagation in the gap region. The presence of view ports in the set-up increases the rise time at the output significantly and induces, owing to internal reflections, extra oscillations in the signal.

A 2.5-MV Subnanosecond Pulser With Laser-Triggered Spark Gap for the Generation of High-Brightness Electron Bunches

High-intensity, low-emittance, short electron bunches are needed to attain self-amplified spontaneous emission (SASE) in X-ray free electron lasers (X-FEL) and for injection in future laser-Wakefield accelerators (LWFA). At such a high density, conventional techniques using magnetic compression of long bunches suffer from collective effects such as coherent synchrotron radiation, limiting the brightness achievable. On the other hand, when creating very short electron bunches using a high intensity femtosecond laser the internal space charge forces rapidly decrease the brightness of the bunch. However, these space charge forces become negligibly small at relativistic energies due to the Lorentz force and contraction. Simulations have shown that acceleration to relativistic energies in a field of 1 GV/m would preserve enough of the phase-space density to meet the requirements for SASE and bunch length sufficiently small for injection into a LWFA. Conventional (RF) accelerators have maximum fields of approximately 100 MV/m and are usually operated in the 10-30 MV/m range. A novel approach was proposed by Srinivasan-Rao et al. [T. Srinivasan-Rao and J. Smedley, 1996] in which a short high-voltage pulse is applied to a simple diode configuration to generate a GV/m electric field.

Design of a laser-triggered driver for fast capillary discharge

We are developing a new upgraded capillary discharge device. The assembled set-up consists of a Marx generator, a pulse forming line, a gas-filled laser-triggered spark gap and a ceramic capillary. The special attention has been paid to the design of the spark gap. The laser triggering of this spark gap ensures a very low jitter in comparison with the present capillary discharge device CAPEX, where the main spark gap works in a self-breakdown regime. The description of the assembled apparatus, the main predicted parameters of the designed device (such as capillary current, capillary current rise-rte, and/or voltage at the end of pulse forming line) are presented.

Laser-Triggered Spark Gap for High-Energy KrF Laser Amplifiers

The design and experimental results of laser-triggered spark gaps for switching pulse forming lines in a 3 kJ class KrF laser amplifier is reported. Through the electric-field optimization in the spark-gap vessel, electrical stress on the insulator surface is alleviated and the gap inductance is reduced. With the use of this spark gap, fast rise-time (10 ns) high-voltage pulses (600 kV/270 ns) are obtained with a small jitter (5 ns/1σ).

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.

Development of an advanced voltage control technique to improve voltage and reactive power characteristics of a large‐scale power cable system

AbstractThe power supply system for a metropolitan area consists of multiple 275‐kV power cable systems to supply power to a large number of consumers. Each system has a large charging capacity (capacitive reactive power). When 275‐kV power cable systems experience a voltage drop, their charging capacity decreases. However, the reactive power losses increase in the 275‐kV overhead transmission lines, which supply large power to power cable systems, thus causing the performance of the power supply system to deteriorate.In this paper, the following three points are reported: When the secondary power system of the trunk substation is a large‐scale power cable system, new techniques can greatly improve the voltage and reactive power characteristics by controlling the rise of the sending‐end voltage at the secondary side of the trunk substations and maintaining the voltage of power cable systems at a constant level. In the use of this control technique, it has been demonstrated that controlling by the direct‐detecting method of a voltage drop in the primary power system is superior to one that controls by the indirect‐detecting method, which increases load power in the cable system. The results of simulations obtained by using a 275‐kV cable system model and a performance power system model show that the use of this control technique can produce a good effect despite the allowances made for the opposite effect, i.e., a decrease in the capacity of power capacitors resulting from voltage drop at the tertiary voltage of a 500‐kV transformer. This study is carried out to examine the possibility of lightning control using a photoionized plasma produced by a UV laser without an optical air breakdown. As the fundamental experiment, the characteristics of a laser‐triggered spark gap (LTSG) are examined, where a laser beam is not irradiated on the surface of the electrodes. In this experiment, the KrF excimer laser is very effective in increasing the plasma density ne and the reduction ratio p of 50 percent breakdown voltage against the self‐breakdown voltage. The relation between ne and p is increased by generating a long and high‐density plasma parallel to the discharge axis.On the other hand, the abnormal discharge process in triggered lightning using a rocket is examined, and it is indicated that the most important condition to trigger a lightning discharge is the production of a plasma channel of length 200 m and density about 1019 m−3. It is suggested that such a plasma channel could be produced by a KrF excimer laser with an energy of only about 3.67 J.

Optical free induction decay (OFID) 10 μm co 2 laser systems

This paper describes new unproved and reliable optical free induction decay (OFID) picosecond-pulse CO 2-10 μm laser systems, differing mainly in the pump laser and in the peak power of the generated pulses. In this context a single-mode TEA-CO 2-10 μm laser preionized by a novel surfacecorona discharge was developed. Our improved plasma shutters operate as optical switches at variable gas pressures. They are triggered either by a laser-produced surface plasma or by an electrical high voltage discharge. With respect to the first trigger scheme we found new target materials for the surface plasma generation, i.e. metallized graphites. In the second scheme the electrical discharge is initiated by the TEA-laser pulse via a new laser-triggered spark gap. Single-mode TEA-CO 2-10 μm laser, plasma shutter plus a spectral filter form the OFID laser system, which generates the picosecond pulses. An electronic control completes the system as reliable working tool. Each of the OFID laser systems developed is capable to produce 10 μm pulses with a duration of 30–300 ps. Numerical calculations are performed to estimate the influence of various parameters on pulse shape and duration. Since a direct measurement of the time variation of the short pulses by fast IR detectors is not possible, we measure the picosecond pulses by means of an optical autocorrelation method with optical second harmonic generation. For the first time this is performed with polycrystalline material, e.g. GaAs or ZnSe, instead of a single crystal, e.g. GaAs or Te.

The PBX-M Thomson scattering system

The PBX-M Thomson scattering system is reviewed after its first 9 months of operation. The system measures Te(R) and ne(R) at 55 radial points on the horizontal midplane with a spatial resolution of ≤1.1 cm. The scattered light is collected by a Bouwers concentric mirror system and imaged onto 12-m-long fiber bundles. A composite entrance slit is used to optimize the system performance. The synchronization between the scattered light and detector gate is done via a laser-triggered spark gap. A penetration in the indentation coil allows the (ruby) laser beam to be dumped away from the plasma chamber. Other aspects of the system will be discussed.

Characteristics of a laser triggered spark gap using air, Ar, CH4, H2, He, N2, SF6, and Xe

A KrF discharge laser (248 nm) has been used to laser trigger, by volume preionization, a spark gap switch (38–65 kV, &amp;gt;10 kA, 100 ns pulse duration) filled with 20 different gas mixtures using various combinations of air, Ar, CH4, H2, He, N2 SF6, and Xe. A pulsed laser interferometer is used to probe the spark column. Characteristics studied include the internal structure of the column, the arc expansion rate, and evidence of any photoionization precursor effect. Our results show that the rate of arc expansion varies depending on the average molecular weight of the mixtures. In this experiment, pure H2 has the highest rate (≊9.5×105 cm/s) and air has one of the lowest (≊7×105 cm/s) for the same hold-off voltage. A computer model of the spark column formation is able to predict most of the structure observed in the arcs, including the effect of mixing gases with widely different molecular weights. The work suggests that, under proper circumstances, the spark gap switch performance may be improved by using gases lighter than conventional switch gases such as SF6.

Breakdown mechanism of a laser triggered spark gap in a uniform field gap

Breakdown mechanism of a laser triggered spark gap was studied in a uniform field gap. The laser beam was focused on the electrode with positive and negative polarities, and vaporized metal plasma produced by the laser irradiation was used as a trigger. Our investigation shows that the breakdown characteristics in the uniform field are essentially different from those of a needle-to-plane gap, and also shows different characteristics under positive and negative polarities.

Low-inductance capacitive probe for spark gap voltage measurements

A novel high-voltage (&amp;gt;50 kV) capacitive probe has been developed to measure the voltage drop across a laser-triggered spark gap. The capacitors which comprise the voltage probe consist of three flat, annular rings that are housed within the spark gap chamber. The rings are oriented perpendicular to the spark column axis such that the column is formed in the open center of the rings. Polyethylene and Kapton foil are employed as dielectrics. The resistive portion of the divider is housed in a shielded enclosure external to the switch chamber. The inherent simplicity of the probe design ensures low inductance while minimizing stray capacitance; thus, the probe has excellent response characteristics (≊1-ns theoretical rise time), and does not interfere with the performance of the switch. The probe has also been designed to permit access for laser triggering and interferometric measurements of the spark column formation. The voltage, current, and resistance characteristics of a laser-triggered spark gap for various gas mixtures are also discussed.

Arc resistance of laser-triggered spark gaps

In the use of spark gaps as switching devices, it is desirable to maximize the power delivered to the load and to minimize the power deposited in the switch; that is, it is desirable for the resistance of the switch to be negligible as compared to the load. The hydrodynamic time scale for expansion of the arc in a spark gap and hence for the reduction in its resistance to a small value is tens to hundreds of nanoseconds. Therefore, with current pulses of duration of a few hundred nanoseconds or less, the resistance of the spark gap may be a significant fraction of that of the load. In this paper, we report on measurements that determine the resistance of the arc in a fully diagnosed laser-triggered spark gap. The spark gap switches a 100-ns, 1.5-Ω waterline into a 1.5-Ω load resistor. A capacitive voltage divider housed within the switch enclosure measures the voltage drop across the switch, a current-viewing resistor measures the current, and an interferometer measures the diameter of the plasma column, a value required to calculate its inductance. The resistance of the arc is found to remain in excess of 0.1–0.2 Ω for the duration of the current pulse for a variety of switch gas mixtures. The resistance decreases with increasing charging voltage on the waterline at the time of triggering and decreases with decreasing average molecular weight of the gas mixture in which the arc is sustained.