Pulse Forming Network Research Articles

Magnetic-spiker excitation of gas-discharge lasers

A review is presented on the use of magnetic-spiker circuits to electrically excite rare-gas-halide and CO2 gas-discharge lasers. The design and operation of the diode, switch, overshoot, and charge modes of magnetic-spiker excitation will be described. Special emphasis has been placed upon understanding the dynamical behaviour and saturation properties of the magnetic core material at high frequencies. The effect of voltage risetime, preionization, pulse-forming network design, and prefire on laser performance is also described. Experimental data are presented on a new and very simple magnetic-spiker circuit called the modified overshoot mode. A relatively long optical pulse magnetic-spiker KrF laser is also desribed which uses self-timed corona preionization initiated by the spiker circuit. The use of magnetic-spiker excitation to produce high output laser energies, high repetition rates, and long optical pulses is discussed. Applications of magnetic-spiker lasers in the field of medicine, microelectronics and materials processing will be reviewed.

A fast extraction instrument of a 1-GeV electron synchrotron

A kicker magnet and a modulator were developed as a fast extraction instrument of a 1-GeV electron synchrotron. A 1-kG pulsed magnetic field was generated by the kicker magnet with the rise time of 40 ns. The ripple of the flattop of the current wave form was less than ±1%, and the shot-to-shot fluctuation of the peak current was no more than 0.1%. The modulator consists of two Blumleins in series which operate at a quarter of the charging voltage as compared to a pulse forming network (PFN) type modulator. The damped oscillating wave form of the current could decrease the residual field to be less than 0.5 G. The effect of the residual field of the kicker magnet on the dynamic aperture of the low-energy electron beam was estimated by the tracking calculation. The result shows the dynamic aperture decreases as the residual field increases. To obtain enough accelerated current, the residual field must be less than 1 G. This restriction could be satisfied by the developed kicker magnet and the modulator. Successful extraction of eight bunches out of 15 bunches in the synchrotron were measured by a wall current monitor located in the extraction transport line.

Main Propulsion Power Take-Off Configuration for an ETC Gun Pulsed Power Generator

Electro-Thermal Chemical (ETC) Gun technology will increase the range and capabilities of existing CIWS and 5 inch guns. Because of their faster, yet more controllable acceleration, ETC guns will allow for the utilization of smart munitions and, in the case of the 5 inch guns, increase range to beyond 50 nautical miles. This paper examines five power take-off (PTO) gear configurations from a surface ship class main reduction gears (MRG's) which drive a nominal 10 Megawatt (MW) Pulsed Power (PP) generator. The PP generator is utilized to charge a capacitor-based pulsed forming network (PFN) which, in turn, provides an electrical pulse to the cartridge of a munition at the breech of an ETC gun upon demand. The PTO gear and resultant placement of the generator will be the major focus of this paper. Major issues that will be examined will include space constraints within the main engine and auxiliary machinery rooms, gear sizing, survivability, shock and vibration, the penetration of watertight bulkheads with high speed shafting and equipment accessibility. Also, design guidelines for the PP generator will be provided. Special emphasis will be placed on the connection of the PTO gear to the MRGs.

Optimization of the excitation characteristics of a flashlamp-pumped titanium sapphire laser

In this work, we attempted to optimize the flashlamp pumping of a titanium-doped sapphire rod. The dimensions of this rod are 8 mm diameter by 152.4 mm (6 inches) length. First, we realized a fast discharge circuit (2 μs FWHM) with six flashlamps. Afterwards, we used a four-flashlamp system with an enlarged discharge being shaped by a PFN (Pulse Forming Network). The first set-up yielded 530 mJ of output energy with an electrical-to-optical efficiency of 0.265%, whereas the second set-up yielded 3 J with an efficiency of 0.8% in the relaxed mode centred at 782 nm.

Pulse switching characteristics of MAGTs for pulsed power applications

AbstractFor pulsed power systems such as lasers and accelerators, semiconductor switches with their longer service life have actively been developed as replacements for thyratrons. The MOS‐driven thyristors are suitable for pulsed power applications because they have high‐power handling and fast turn‐on capabilities. The MOS‐assisted gate‐triggered thyristor (MAGT), designed especially for pulsed power, is a promising candidate in this field.This paper presents the results of an investigation into the performance of MAGTs. Using a pulse‐forming network (PEN), the pulse‐switching characteristics and the dynamic resistance characteristics during the current flow are investigated. A maximum current density of 21.8 kA/cm2 and di/dt of 106 kA/μs/cm2 with 1550‐V anode voltage on a single‐shot basis were obtained. Furthermore, a life test with 109 shots at a high repetition rate showed no degradation in the observed characteristics. Based on these experimental results, a carrier flow model of MAGT during turn‐on process is proposed and the turn‐on mechanism is considered.

Application of a phase conjugate Brillouin cavity for the generation of a train of short laser pulses

An oscillator and a regenerative amplifier have been realized using a phase conjugate mirror in the amplifier laser cavity. The phase conjugate mirror, via stimulated Brillouin scattering, acts as a modulator owing to its time-depending reflectivity. A pulse-forming network applied to the amplifier increases the discharging time which contributes to get an output beam formed by a train having four short pulses. The shortest pulse was 2 ns FWHM long.

X-ray diagnostics of a plasma-jet–liquid interaction in electrothermal guns

The penetration of a plasma jet into liquid water was observed at successive time intervals by means of x-ray shadowgraphy. The plasma jet was generated by producing in a polyethylene capillary tube a high-current pulsed discharge. The tube inner diameter and length were 0.64 and 22 cm, respectively. A pulse-forming network delivered 121–182 kJ of electrical energy in 500 μs. The plasma jet emerged from the open end of the tube and interacted with a column of water gel. Jet velocities around 250 m/s were measured by x-ray shadowgraphy. The water ablation rate, estimated by calculating the radiative energy flux emitted by the plasma and reaching the water surface, is approximately 0.0155 kg/s cm2. This rate is significantly lower than the measured average flux of mass lost by the water: 3.5 kg/s cm2. It is proposed, but not proven conclusively, that the main mechanism for water loss is in the form of droplets pulled apart by shear forces impressed by the plasma-water direct interaction. These droplets are entrained in the back-flowing part of the plasma jet, which streams out of the interaction chamber. Peak pressures up to 3.5×108 Pa were measured during the process.

A 50 MW pulse-forming network with a voltage stability within 0.03%

Requirements on electron energy stability are extremely tight for operation of a free-electron laser. In case a radio-frequency field is used to accelerate the electrons, this leads to stringent requirements on the stability of the klystron output power and, hence, on the stability of the source producing the cathode voltage. A pulse-forming network (PFN) capable of delivering a 19 kV pulse with a stability of 0.03%, during 20 μs, is presented. The PFN consists of four parallel lines, each containing 19 cells of a 20 nF capacitor and a 20 μH solenoid. The solenoids are equipped with taps and a plunge tuner. An innovative line switch is used, namely a stack of 32 thyristors in series. Numerical simulations of the pulse shape show that, in order to obtain the best possible pulse flatness, it is essential to arrange the capacitors such that the difference between adjacent capacitances is minimized.

A pulse forming network and test fixture for screening electrothermal chemical candidate propellants

A pulse power supply has been designed to support the development and evaluation of candidate propellants for an electrothermal chemical gun program at the Army Research, Development and Engineering Center. The flexibility needed for this evaluation is achieved through the modular design of the test fixture and power supply. The test fixture is composed of a plasma chamber and an instrumented combustion chamber interconnected by a convergent-divergent nozzle. The combustion chamber is terminated by an exhaust nozzle and overall thrust is measured. The volume of both chambers and nozzle configurations can be varied to optimize performance. A 10 mm gun tube is an option. The power supply is readily reconfigured to provide either an RLC or PFN (pulse forming network) discharge. Either configuration can store up to 34 kJ and provide a pulse between 0.5 and 2 ms. Load behavior and the effects of parasitic energy losses on circuit performance are presented. The results are compared with PSpice circuit models.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Propulsion powered electric guns-a comparison of power system architectures

Alternative power system architectures and interface technologies for driving high-energy pulsed loads from the propulsion equipment of naval surface combatants are compared. Gas turbine mechanical and gas turbine integrated electric drive propulsion trains are considered as prime power sources. Focus is on the energy path between the ship's prime power and the pulsed load, comparing system candidates on a size, weight, and component cost basis. A capacitor-based pulse forming network (PFN) supplying an electrothermal chemical gun is utilized as the common load for all configurations. Results indicate that dedicated auxiliary generators driven from propulsion gas turbine engines are the preferred method of powering electric gun PFNs from ship drive-train equipment. Auxiliary generator-based systems are compact and lightweight over a wide power range, are equally attractive with both electric and mechanical drive trains, and demonstrate retrofit potential to the current fleet. >

A pulse forming network design for blocked-bore plasma armature experiments

Recent experimental studies on plasma armatures at the Applied Physical Electronics Research Center at the University of Texas at Arlington require the delivery of a single high-current pulse. The railgun diagnostic program now in progress requires a ramping current waveform with a continuously positive di/dt until peak current is delivered approximately 1 ms after circuit closure. This pulse forming network (PFN) is designed to power blocked-bore plasma armature studies being conducted on a section of the THUNDERBOLT prototype, SUVAC. The PFN design has an operating voltage range from 300 to 1400 V and is capable of delivering peak currents up to 200 kA.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pulse Switching Characteristics of MAGTs for Pulsed Power Applications.

For pulsed power systems such as lasers and accelerators, semiconductor switches with their longer service life have been actively developed as replacements for thyratrons. The MOS-driven thyristors are suitable for pulsed power applications because they have the high-power handling and fast turn-on capabilities. The MOS Assisted Gate-triggered Thyristor (MAGT), especially designed for pulsed power, is a promising candidate in this field.This paper presents the results of an investigation into the performance of MAGTs. Using a pulse forming network (PFN), the pulse switching characteristics and the dynamic resistance characteristics during the current flow were investigated. A maximum current density of 21.8kA/cm2 and di/dt of 106kA/ μs/cm2 with 1, 550 V anode voltage on a single shot basis were obtained. Furthermore, a life test with 109 shots at a high repetition rate showed no degradation in the observed characteristics. Based on these experimental results, a carrier flow model of MAGT during turn-on process was proposed and the turn-on mechanism was considered.

Mathematical formulation of 2D and 3D finite and boundary element model for transient electric fields in high performance capacitors

Capacitors and their pulse-forming networks deliver very high currents in the millisecond time frame to electromagnetic and electrothermal chemical guns. The authors outline the principles of 2-D and 3-D finite and boundary element method codes for calculating electric field strength distribution in high-voltage capacitor insulation systems in arrangements with complex boundaries. A very accurate representation of the space distribution of electric fields, varying in time during the discharge as the combined effect of the circuit parameters, modulated by diffusion and time-changing parameters as properties of the dielectric materials, corona effects, and accelerated aging, can be the basis for a global performance criterion for the system. This criterion should be large enough to represent the complexity of the phenomena in heavy-duty capacitors but still simple enough to be meaningful.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Use of an SCR switch for a long pulse high duty line type modulator

An SCR switch for a long pulse, high duty, line type modulator, which had been fabricated as a prototype for the development of an rf source for the JHP 1 GeV proton linac, has been constructed and tested. The switch was designed to be used for discharging a pulse-forming network charged at 40 kV and producing pulses with MW of peak power (20 kV × 750 A) and 450 kW of average power (15 MW × 600 μs × 50 pps). This switch comprises of 30 general use type SCRs, arranged in series so as to withstand high voltage. After installing this switch in the modulator as a replacement of the thyratron switch, a test was carried out up to 12.5 MW peak and 250 kW average output power, which was limited by the maximum capabilities of the present modulator (400 μs) and a klystron, used as a load through a pulse transformer (18.6 kV at primary, 130 kV at secondary). The design considerations and the test results of the SCR switch are described, along with an outline of the prototype modulator.

Propulsion Powered Electric Guns: A Comparison of Power System Architectures

ABSTRACTAlternative power system architectures and interface technologies for driving high energy pulsed loads from the propulsion equipment of surface combatants are compared. Gas turbine mechanical and gas turbine Integrated Electric Drive (IED) propulsion trains are considered as prime power sources. Focus is on the energy path between the ship's prime power and the pulsed load, comparing system candidates on a size, weight and component cost basis. A capacitor based Pulse Forming Network (PFN) supplying an Electro‐Thermal (ET) Gun is utilized as the common load for all configurations. Results indicate that dedicated auxiliary generators driven from propulsion gas turbine engines is the preferred method of powering electric gun PFNs from ship drivetrain equipment. Auxiliary generator‐based systems are compact and lightweight over a wide power range, are equally attractive with both electric and mechanical drive trains and demonstrate retrofit potential to the current fleet.

Theoretical modeling of the interior ballistic processes in an electrothermal chemical gun

A comprehensive theoretical model has been developed to describe the interior ballistic processes in an electrothermal chemical (ETC) gun. The model, which takes into account the plasma/working fluid interaction, predicts transient and spatial variations of the gun chamber pressure, instantaneous projectile motion, and gun muzzle velocity. A complete set of governing equations is derived from first principles of physics, and the governing system is solved by an implicit finite difference scheme. Two sets of pulse-forming network (PFN) discharge curves, with various peak power times and total discharge times; are employed to study the ETC gun performance. The numerical results show that a ballistic efficiency of more than 13% can be achieved for an exothermic working fluid (C8H18/H2O2). The breech pressure and projectile-base pressure exhibit double peaks during the ballistic cycle. The first peak is due to the initial discharge of a plasma jet into the gun chamber, whereas the second peak is the result of vigorous interactions between the plasma jet and the working fluid. The calculated maximum breech pressure is on the order of 400 MPa, and a typical muzzle velocity is on the order of 2 km/s. Comparison of present numerical results with limited available experimental data is made and found to be good.

Small volume coaxial discharge as precision testbed for 0D-models of XeCl lasers

In order to check the predictions of 0D-models experimentally, a small coaxial discharge configuration for the generation of homogeneous high pressure glow discharges (diameter 11 mm, length 20 mm) in rare gas halogen excimer laser gas mixtures under accurately controlled conditions has been developed. It uses X-ray preionization and a special pulse-forming network (PFN) delivering fast rising (8 ns) single square pulses (U 0=25 kV; I=300 A; ∝=100 ns). Discharge current and voltage are measured precisely by a capacitive voltage divider and a shunt integrated into the discharge chamber. All circuit data needed for the model calculations have been evaluated. Interferometric and spectroscopic diagnostics of the bulk of the discharge and of the cathode sheath have been performed. First results for Ne/Xe/HCl mixtures are compared with model calculations.

Transversely excited CO 2 laser with long pulse duration discharge

A transversely excited CO 2 laser has been constructed using a pulse-forming network source. Reliable, stable operation was demonstrated for pulselengths up to 30 μs at a pressure of 0.65 atm. Observations suggest that the discharge runs in a pulser- sustainer mode. The multimode discharge-optical efficiency was found to reach 13%.

A high-voltage modulator for high-power RF source research

The design, construction, and operating results of a high-voltage modulator system capable of generating 700-kV, 2.5- mu s pulses at 5 p.p.s. into a load of 900 Omega are presented. The modular is used to energize a variety of high power microwave devices requiring voltage stability and reproducibility. Voltage ripple is less than 0.2% during the 1.0- mu s flat top, with a shot-to-shot voltage variation of less than 0.1%. The primary circuit consists of two seven-stage tunable Rayleigh-type pulse-forming networks (PFNs) connected in parallel with a total impedance of 2.25 Omega , a total capacitance of 0.56 mu F, and a total inductance of 2.8 mu H. The PFN is charged by a highly stable 80-kV capacitor charging power supply (0.1% RMS voltage ripple) at a rate of 10 KJ/s. The total energy stored (1.5 kJ) is released through an ITT F-187 thyratron into a 20:1 pulse transformer, which generates 700-kV, 2.5- mu s pulses. By changing the transformer, it was possible to obtain 250-kV, 1.70-kA pulses for driving low-impedance relativistic magnetron diodes. The flat-top voltage generated by the modulator is highly desirable for driving RF sources requiring high-quality electron beams, such as free-electron lasers (FELs) and cyclotron autoresonance masers (CARMs). The modulator performance in the relativistic magnetron and CARM experiments is described. >

Optical control of semiconductor closing and opening switches

Recent progress in the research of high-power photoconductive semiconductor switches is reviewed. Material issues and switch design considerations are discussed. High-power ultra-wide-band microwave generation using these switches and a pulse-forming network is presented. The application of the photoconductive switch both as a closing and and as an opening switch in an inductive energy storage system has been demonstrated. Electric pulse compression with a peak power gain of 30 has been observed. Future prospects in this area of research are discussed. >