Coherent light generators – Particular pumping means – Electrical
Reexamination Certificate
1993-06-08
2004-04-27
Moskowitz, Nelson (Department: 2202)
Coherent light generators
Particular pumping means
Electrical
C359S345000
Reexamination Certificate
active
06728284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high average power magnetic modulator for metal vapor lasers and the like.
2. Description of the Related Art
Many different types of lasers are energized by pulsed electrical discharges. Where higher average output power is desired from a pulsed laser system, more power per pulse, shorter pulse lengths and less time between pulses is generally required. Extensive experience with thyratron driven capacitor inversion circuits for driving moderate power metal vapor lasers such as copper vapor lasers (about 10 kW input) teaches that in such circuits the thyratrons do not exhibit the lifetime required for the new higher power output copper vapor lasers. The operating constraints that moderate power modulators impose on a thyratron with regard to di/dt, peak current amplitudes, and repetition rates often cause premature thyratron failure due to cathode depletion, gas depletion, and anode erosion; a significant increase in the power switched by the thyratron would only exacerbate this problem. An examination of other known types of high voltage modulator switches, technologies, and circuit topologies has not revealed a simple, inexpensive, or proven alternative. The search for longer lifetimes has led to the use of magnetic compression circuits even though they are a more complex and more expensive technological approach as compared with thyratron switched capacitor inversion circuits. While the use of magnetic compression circuits reduces the electrical stress on the thyratrons (often used as the primary switching device) and thereby increases the thyratrons' operating lifetime, experience with such systems still indicates insufficient thyratron lifetime. Although thyratrons are designed as high peak power, high repetition rate commutation devices, thyratrons by their construction are plasma devices and have infinite lifetimes due to gas cleanup, cathode depletion and filament lifetime limits. This leads to the conclusion that other types of commutation switches must be used in conjunction with magnetic compression circuits to achieve the required power levels, reliabilities and lifetimes.
In an attempt to overcome the short device lifetimes and other limitations of thyratron driven modulators, solid state devices such as thyristors and Silicon Controlled Rectifiers (SCRs) have been used in the past as commutators in magnetic compression circuits for driving high power lasers. Such solid state devices have a potentially very long lifetime (more than tens of thousands of hours) if their performance ratings are not exceeded. However, solid state switches are relatively slow devices (conduction times are usually in the tens of microseconds and longer as compared with thyratrons that can have conduction times measured in tens of nanoseconds and longer) designed for high average power but not capable of switching high peak power. Accordingly, they must be coupled with a magnetic compression circuit in order to drive high peak power loads such as a gas discharge load.
Solid state devices are low voltage devices; most of the high average power devices designed for short conduction times (inverter grade devices) have maximum voltage ratings of less than 1500 volts. Consequently, to drive high voltage loads, solid state devices have to be stacked in series to increase the overall switching voltage and/or a step-up pulse transformer must be incorporated into the modulator circuit. If devices are not stacked in series, several devices operating in parallel are usually required and a very high step-up ratio, pulse transformer is required. Either series operation or parallel operation of solid state devices have both advantages and disadvantages.
A series stack can be designed to be equivalent to a thyratron in voltage rating and can replace the thryatron in a magnetic compression circuit if the switch conduction time is long enough; thyristors and SCRs do not switch or turn on as fast as thyratrons and cannot handle large rates of change in current (the maximum di/dt for most available devices is less than 1000 amperes per microsecond). The advantages of a series stack include: additional devices can be used to provide redundancy and increase the operating lifetime (one or more devices can fail (short) but the stack and modulator can continue operating); at high voltages the peak current requirements through switches and other components at the modulator input are reduced; and a pulse transformer, if required, will have a lower step-up ratio and low leakage inductance (required for an efficient pulse transformer) will not be difficult to attain. The disadvantages include: a series stack is expensive because, with a safety factor, twenty or more devices (at a 1200 volt rating) are required to make up a 20 kV thyratron replacement; voltage grading and overvoltage protection devices must be placed across every device; devices must be reasonably matched as to turn-on times to insure simultaneous or near simultaneous stack turn-on time (although external devices such as a magnetic assist can alleviate this problem); and individual trigger circuits with high voltage isolation are required for each device.
A switching circuit consisting of parallel solid state devices limits the input voltage of the modulator to the maximum operating voltage of the devices. Advantages of parallel operation include: the number of switching devices decreases as the energy transfer time increases; using few devices reduces the cost and volume; and the devices and their trigger circuits only need circuit isolation for 1 kV. Disadvantages include: a single device failure (short) results in modulator failure; the modulator circuit must be designed to insure current sharing between devices; operation at lower voltages implies high peak and rms currents so capacitors and conductors are required to have small series resistances in order to keep power losses small; and the required high step-up ratio pulse transformer (1:60 to 1:80) with very low leakage inductance can be inexpensive and difficult to manufacture.
Magnetic compression circuits are well-known in the art for having the capability of generating high peak power, short time duration voltage pulses by time compression of energy. Being composed of passive circuit elements (capacitors and non-linear inductors) they are very robust and can be very reliable. In application, magnetic compression circuits usually serve as an interface between a controllable switching device and a power load that usually requires high voltage, high peak power, short time duration, and often high repetition rate pulses. The controllable switching device is usually incapable of driving the load directly with any reasonable reliability or lifetime. The complexity and cost of magnetic compression circuits usually restrict their use to high average power, high repetition rate systems; metal vapor laser systems have these requirements and have utilized magnetic compression circuits. Such pulsed lasers are utilized in many applications such as medical diagnostics, laser isotope separation of an atomic vapor (known as an AVLIS (Atomic Vapor Laser Isotope Separation) process), and many other applications.
The basic principle underlying magnetic pulse compression operation involves a saturable inductor, often referred to as a magnetic switch, which consists of a winding around a saturating magnetic core. In operation, the inductance of the magnetic switch will change from a large value (unsaturated core) to a small value (saturated core) when a voltage is applied across the switch for a specified length of time. The gain of a magnetic switch is defined as the ratio of two time periods; the time that the magnetic switch can hold-off an applied voltage (prior to core saturation) and the time required to transfer energy through the switch (after core saturation). A typical magnetic switch has a gain of between 3 to 10. The resonant circuit consisting of the saturable inductor and two capacitors of approximately
Ball Don G.
Birx Daniel L.
Cook Edward G.
Daubenspeck William C.
Gottlieb Paul A.
Moskowitz Nelson
The United States of America as represented by the United States
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