Laser-triggered Spark Gap Research Articles

Repetitive Pulse Accelerator Technology for Light Ion Inertial Confinement Fusion

Successful ignition of an inertial confinement fusion (ICF) pellet is calculated to require that several megajoules of energy be deposited in the pellet's centimeter-sized shell within 10 ns. This implies a driver power of several hundreds of terawatts and power density around 100 TW/cm2 . The Sandia ICF approach is to deposit the energy with beams of 30 MV lithium ions. The first accelerator capable of producing these beams (PBFA II, 100 TW) will be used to study beam formation and target physics on a single pulse basis. To utilize this technology for power production, repetitive pulsing at rates that may be as high as 10 Hz will be required. This paper will overview the technologies being studied for a repetitively pulsed ICF accelerator. As presently conceived, power is supplied by rotating machinery providing 16 MJ in 1 ms. The generator output is transformed to 3 MV, then switched into a pulse compression system using laser triggered spark gaps. These must be synchronized to about 1 ns. Pulse compression is performed with saturable inductor switches, the output being 40 ns, 1.5 MV pulses. These are transformed to 30 MV in a self-magnetically insulated cavity adder structure. Space charge limited ion beams are drawn from anode plasmas with electron counter streaming being magnetically inhibited. The ions are ballistically focused into the entrances of guiding discharge channels for transport to the pellet. The status of component development from the prime power to the ion source will be reviewed.

Simple ruby laser producing variable pulse lengths from 800-psec to 80-nsec duration

Remote control of ruby laser pulse length could be achieved over 2 orders of magnitude. The technique for generating such a variability of pulse lengths with the same laser makes use of well-known cavity dumping and mechanical or simple electronic modifications. Short pulses could be lengthened by increasing the parallel capacity of a laser-triggered spark gap.

Breakdown mechanism of a laser triggered spark gap in a non‐uniform field (III). laser irradiation under negative polarity

Breakdown mechanism of a laser triggered spark gap in a non‐uniform field (III). laser irradiation under negative polarity

KrF Laser-Triggered SF6 Spark Gap for Low Jitter Timing

KrF laser-triggered spark gaps exploit the high dc-dielectric strength and low ultraviolet (UV) breakdown threshold of SF6 gas. Detailed measurements using a dc-charged pulser demonstrate subnanosecond jitter for switching a 0.5-cm gap operated at 80 kV with 7 mJ in 20 ns of 248-nm KrF radiation. A 200-kV pulse-charged 0.7-cm gap gives similar performance.

Sweep devices for picosecond image-converter streak cameras

Four different sweep devices based on microwave tubes, avalanche transistors, krytrons, and laser-triggered spark gaps are treated in detail. These control circuits are developed for picosecond image-converter cameras and generate sweep pulses providing streak speeds in the range of 107 to 5×1010 cm/s with maximum time resolution better than 10−12 s. Special low-jitter triggering schemes reduce the jitter to less than 5×10−11 s. Some problems arising in the construction and matching of the sweep devices and image-streak tube are discussed. Comparative parameters of nanosecond switching elements are presented. The results described can be used by other authors involved in streak camera development.

Low jitter, low inductance solid dielectric switches

It has been shown that the use of graded solid dielectric sandwiches in laser-triggered spark gaps (LTS) can lead to highly desirable multichannel operations while maintaining the low delay and jitter performance characteristics of LTS. As many as ten separate breakdown channels were observed when small circular or hexagonal aluminum inserts were inserted between two Mylar dielectric sheets stressed at 4.1 kV/mil. A reduction in rise time was noted for these multichannel switching events.

The production of variable-length subnanosecond light pulses in a laser system

A system for shaping the output pulse of a Q-spoiled laser is described. With this system, light pulses having a half-peak width no greater than 250 ps have been produced. Equally as important, there is only a marginal decrease, due to the additional optical circuitry, in the peak intensity value between the original Q-switched pulse and the final shaped pulse. A laser-triggered spark gap is used to generate the controlling electrical pulses which are generated with a rise time of less than 180 ps. The design of a Pockels cell is described which has an optical aperture of 5 mm and an electrical rise time of 74 ps.

Pressure dependence of the rise time of laser-triggered spark gaps

The laser-triggered spark gap (LTSG) has found uses in many applications where ultra-fast voltage transitions are required, primarily because of its reliability of operation, simplicity of construction and speed of response. The LTSG can be incorporated into most coaxial systems with minimum change of impedance, a necessary condition to achieve fast electrical switching. The authors report on practical measurements of the rise times generated by the LTSG which show that, for a certain gap setting, there is a minimum rise time that may be achieved and that the parameters, voltage and pressure, which are determined theoretically to give this minimum rise time are in reasonable agreement with the practically measured values.

A multichannel laser triggered spark gap with subnanosecond risetime for electro-optic switching

The construction and some of the operating characteristics of a multichannel, laser triggered spark gap (LTSG) are described. Simultaneous subnanosecond switching in five channels of up to 10 kV is possible, making the LTSG an ideal master switch for pulsing Pockel cells to control the pulse shape and isolation necessary in a large laser oscillator-amplifier system. Operated under N2 at a pressure of approximately 10 kTorr the LTSG had a risetime <300 ps and used in conjunction with a fast Pockel cell switch could shutter reliably optical pulses of <or approximately=500 ps duration from the output of a Q-switched laser. The results indicate that at higher pressures the risetime of the LTSG should be further reduced allowing switching in approximately 100 ps to be achieved.

レーザトリガによる不平等電界ギャップの絶縁破壊[I

レーザトリガによる不平等電界ギャップの絶縁破壊[I

A dye laser triggered spark gap

Performance of a dye laser triggered spark gap is described with a triggering threshold of only 2 kW peak power and 5 μJ pulse energy. This spark gap is used as a switching element in a nitrogen laser resulting in a delay between dye laser and nitrogen laser emission of only 20 nsec and a subnanosecond jitter.

Production of single subnanosecond multi-line 10.6 μm laser pulses by fast optical gating

An electro-optic shutter, activated by a pulse generator switched with a laser triggered spark gap having a risetime of ∼120 psec, has been used to produce optical pulses of variable duration, as short as ∼600 psec, from the output of a uv photo-preionized CO 2 laser, which simultaneously oscillates on several rotational transitions.

Single Nanosecond Pulse Generation at 10.6 μ Using a Brewster Angle Modulator

A method is described for generating single millijoule pulses of nanosecond duration through use of a CO2 laser mode-locked with a Brewster angle, acousto-optic modulator coupled with an electro-optic switch (extinction ratio of 50 000:1) driven by a laser-triggered spark gap. The Brewster angle geometry eliminates the damage problems associated with the use of AR-coated normal incidence modulators.

レーザトリガギャップの諸特性

レーザトリガギャップの諸特性

Operation of a Grid-Shuttered Image Converter Tube in the Picosecond Region

Experiments have been carried out to investigate the performance of a standard, commercial, image converter tube (RCA C73435) in the picosecond region. Using a multikilovolt subnanosecond-rise time pulse generator, switched by a laser-triggered spark gap, to generate the deflection pulses, streak velocities of 1.6×1010 cm/sec and a streak-limited resolution of &amp;lt;5 psec have been obtained. Photocathode saturation and space charge effects are avoided with the use of low cathode currents, and, in order that photographic records can be obtained on high-speed film, a high-gain (106), four-stage magnetically focused image intensifier (EMI 9694) is optically coupled to the screen of the image converter tube. The over-all response time of the camera is also dependent on the transit time spread of the photoelectrons between the cathode and the screen and this is reduced by the application of accelerating voltages of 2500 V to the ICT grid. Evaluation of the camera system has been undertaken with the 1.06 μ fundamental output of a mode-locked neodymium glass laser. Mode-locked pulses of ∼6 psec duration were resolved.

Switching Jitter in Spark Gap Triggered by a TEM00-Mode Mode-Locked Ruby Laser

The first study of jitter in a laser triggered spark gap switched by pulse trains from a mode-locked ruby laser is described. The spark gap was fired by producing gas breakdown in a high pressure argon-nitrogen mixture between the pole pieces. Jitter was measured as a function of the position of the lens used to focus the laser beam and as a function of the ratio of the applied voltage to the self-breakdown voltage of the gap. Jitter values of less than 2 nsec were obtained under optimum conditions provided that the gap was fired by the early part of the pulse train.

A Transmission Line Circuit for Reliable Spark Gap Operation

The common transmission line circuits employed to enable a fast laser triggered spark gap to switch an optical cell are discussed. A modification is suggested that leads to a more reliable operation of the gap.

Laser-Triggered Avalanche-Transistor Voltage Generator for a Picosecond Streak Camera

Direct optical triggering of an avalanche transistor with a short laser pulse has been demonstrated. In some applications this system provides a compact low-jitter replacement for a laser-triggered spark gap. The technique has been applied to generating the gating and sweep voltages in a picosecond streak camera for laser pulse diagnostics where it eliminates the need for multiple beam splitters and long delays. The ``trigger'' avalanche transistor was placed as one of a series string of avalanche transistors. A portion of the switched-out laser pulse to be diagnosed was focused onto the trigger transistor chip. Nanosecond-rise kilovolt waveforms are thus generated with time jitter of the entire system being less than 100 psec.

Investigation of Laser Pulse Forming by LTSG and Pockels Cell

The properties of laser triggered spark gap (LTSG) and the new technique of laser pulse forming by two LTSG were investigated. The risetime and jitter of LTSG were less than 1 ns and 1.5 ns respectively. The threshold laser power of the operation was 5 kW (35 ns).Variable laser pulsewidth from 1 to 30 ns were obtained by controlling the gas pressure of LTSG and/or the electrode spacing. The risetime of laser pulse obtained by this method was 1.5 ns.

Picosecond triggering of a laser-triggered spark gap

The characteristics of laser-triggered spark gaps, initiated by a switched-out pulse of about 7 ps from a model-locked Nd: glass laser system, are investigated. The formation time delay of breakdown across the gap can be of the order of 1 ns and is found to increase as the gap voltage is reduced. Comparison with spark gaps triggered by Q-spoiled nanosecond laser pulses is made. Finally, a comparison of the agreement between the experimental results and theory suggests that the initiation of gap breakdown is due to the production of only a few free electrons from the electrode surface on which the laser beam is focused.