WO1987005161A1

WO1987005161A1 - Control of large volume gaseous electric discharge system

Info

Publication number: WO1987005161A1
Application number: PCT/US1987/000301
Authority: WO
Inventors: Alan E. Hill
Original assignee: Hill Alan E
Priority date: 1986-02-18
Filing date: 1987-02-13
Publication date: 1987-08-27
Also published as: GB2194672A; FR2594605B1; GB2194672B; DE3790086T1; FR2594605A1; JPS63502471A; CA1273403A

Abstract

A control circuit for a laser system having a first and a second power supply (10, 26) connected in parallel. A mode selector (50) is available to the laser operator to change the operational mode of the laser system from continuous to pulse mode or vice versa, and any combination of the modes. In the pulse mode, a series of short-time voltage pulses is superimposed on a low power level laser mode using the circuit control characteristic of the thyratron tube (36) to sharply increase the laser output for the duration of the individual pulses.

Description

CONTROL OF LARGE VOLUME GASEOUS ELECTRIC DISCHARGE SYSTEM

TECHNICAL FIELD

One of the problems encountered when a laser system is designed and intended for use in an industrial environment is the compactness, size and reliability of the system. Lasing action in a system is obtained by subjecting a gas-filled vessel or channel to an electric discharge - the electrons provided by the discharge collide with active gas molecules thereby exciting them to higher energy levels, from which they descent to lower energy levels and emit excess energy in the form of photons, or light quanta. The population density of particles in the higher energy level must exceed that in the lower energy level to achieve optical gain. This population inversion is the opposite of the natural state.

A discharge, normally, has a very thin diameter because -the heat transfer rates in different parts of the discharge are not uniform, and result in lower press¬ ure and density at the inside of the plasma column, thus constricting the column.

My co-pending Patent Application S.N. 06/825,152, filed February 3, 1986 shows an electrical discharge hav¬ ing a large cross sectional area which will uniformly fill large volume cavities, regardless of size or shape, to provide reliable lasers suitable for industrial appli¬ cations.

BACKGROUND ART Lasers are now used to sharply reduce production costs in those industrial applications requiring cut- ting, welding, drillings heat treating and the like. However, according to industry publications, laser sales are only about three percent of the machine tool indus¬ try. The use of my invention with a laser will be most useful in such industrial applications as the rapid removal of paint, scale, rust or other unwanted coatings which must be removed from materials or vehicles such as ships. Applicant is aware of the following prior art patents :

Hill, U.S. Patent No. 3,491,309

Hill, U.S. Patent No. 3,581,146

Hill, U.S. Patent No. 3,735,284 Hill, U.S. Patent No. 3,795,838

The first listed patent to Hill reveals a high power C02 pulse laser wherein the required pulse voltage is in the order of 200 KV to 1 MV. The inven¬ tion to be later described herein provides a circuit for use with a high power C02 pulse laser and distinguish¬ es from the above identified patent by providing a low level ionization sustaining voltage to the laser elec¬ trodes and to superimpose short-time voltage pulse onto the low level ionization voltage. In the invention to be described, the pulse volt¬ age required to develop similar pulsed plasma current is much smaller than the above listed voltage range. This reduction in magnitude of voltage yields circuit param- eters which can easily be used in an industrial environ- ment. The uniformly distributed ionization provided by the co-pending patent application identified above provides a more uniform-pulsed plasma current distribu¬ tion and is stable over a longer pulse period. This results in greater achievable pulsed output power with a superior optical mode quality over that which could be achieved using the first listed patent.

The second listed patent to Hill relates to a method of ballasting a gaseous discharge-tube system wherein a plurality of tubes are excited from a single power source.

The third listed patent teaches the use of aerodynamic forces to control the spatial distribution of charge in a laser system to obtain a uniform plasma.

The fourth listed patent also shows the use of aerodynamic forces to obtain uniform plasma in a laser system.

None of the above patents appears to teach or even suggest the invention revealed and claimed herein.

Reference is made to the following works for those relationships and definitions which are used herein.

"An investigation of Ejector Design by Analysis "and Experiment"; Keenan, Neumann, and Lustwerk-Journal of Applied Mechanics, page 299 September 1950.

"Gaseous Conductors - Theory and Engineering Applications"; James D. Cobine PH.D., Dover Publica¬ tions, Inc. 1958 Edition.

"Basic Data of Plasma Physics"; Sanborn C. Brown, M.I.T. Press 1959 Edition.

DISCLOSURE OF INVENTION The embodiment of my co-pending Patent Applica¬ tion identified above, relates to a high power laser, but this invention encompasses a circuit to gate the operation of the laser to change its operation from continuous to intermittant, or pulse or vice versa or any combination thereof. The pulse operation is useful in an industrial environment performing duty as clean¬ ing, paint removal, rust removal and the like.

It is accordingly a broad object of the invention to provide a control circuit for a laser. It is another object of the invention to provide a control circuit for a laser which will selectively change the operation from continuous mode to pulse mode or vice versa.

It is yet another object of the invention to provide a laser control circuit which will selectively connect parallel power supplies to a laser to switch from one operating mode to another or to combine the operating modes.

It is yet still another object of the invention to provide a laser pulse circuit which superimposes a series of short-time voltage pulses onto a low power level ionization producing direct current to provide high power lasers pulses.

BRIEF DESCRIPTION OF DRAWING The single figure shows a schematic circuit for the control of a laser system including a mode selector, whereby the laser operator can selectively operate the laser in the continuous mode* or the pulse mode or any combination. A second power supply is connected in para¬ llel with a first power supply to superimpose a series of shor -time pulses on a relatively low level DC laser voltage which results in a short-time repetitive peak power laser output pulse many times greater than the normal continuous power output. The first power supply is infinitely variable so that pulse power may be super- imposed upon continuous power.

BEST MODE OF CARRYING OUT THE INVENTION Referring now to the drawing wherein reference character 10 indicates a first power supply for multi- electrode laser 12, as shown schematically. A plurality of laser anodes 14 are connected through ballast resis¬ tors 16, current regulator 18, and pulse blocking diode 20 to the positive terminal of variable voltage power supply 10. A- plurality of cathodes 22 are connected through ballast resistors 24 to the negative terminal of power supply. The positive terminal of auxiliary power supply 26 is connected to the negative terminal of power supply 10 and to ground. The purpose of power supply 26 is to insure that the cathodes 22 float negative with respect to ground to avoid backstreaming. This completes the first power supply.

A second power supply is connected in parallel with the above described first power supply and com¬ prises a pulse forming network shown generally as refer¬ ence character 28 connected to the primary of step-up transformer 30. The pulse forming circuit comprises a plurality of parallel connected capacitors 32 and series connected inductance coils 34 connected between thyra- tron switch 36 and resonant charging system 38. The secondary of transformer 30 is connected at one end through blocking diodes 40 to the laser anodes 14 and at its other end through blocking diodes 42 to the laser cathodes 22 to complete the circuit. A laser operation mode selector, shown generally as referenced character 50, is connected mechanically or electrically to control the infinitely variable voltage first power supply 10 which is symbolically represented by the arrow appearing thereon. The selector is electrically connected through trigger transformer 60 to the grid of thyratron 36 for a purpose that will later be explained.

In the operation of the circuit to change the laser from continuous phase to pulse phase, the mode selector is actuated by the laser operator to reduce the voltage output of first power supply 10 to a low level sufficient to maintain ionization in the laser at about the voltage glow state, and a voltage pulse is applied to the grid of thyratron 36 through transformer 60. In the actual reduction to practice of the system, the oper¬ ating characteristics of the thyratron tube was found useful to control the high energy short-time duration pulse needed. Gas-filled tubes generate their charge carriers from ionizing electron-molecule collisions which produce an electrically neutral plasma with electrons and posi¬ tive ions moving in opposite directions. Conduction may be held off by a control grid in the absence of current flow, but once switched on, the ions form a space charge around the grid which tries to go negative so that its controlling electric field is cancelled. This results in an out of control arc. The gas filled tubes provide on-switching capability of almost unlimited currents, but must be externally turned off until recombination dissipates the plasma. Only then may the control grid hold-off function be re-established. Statistical ioniza¬ tion processes can also be important when jitter require¬ ments are severe, or when trigger-to-breakdown times must be 0.5 nanosecond or less. Recent use of hydrogen and grounded-grid designs have shortened conventional thyratron ionization and deionization times, but hydro¬ gen clean up presents a problem, and heated reservoirs are needed to attempt to maintain a suitable equilibrium operating pressure over the tube's lifetime. When the thyratron 36 is fired by the voltage pulse to its control grid from mode selector 50, the pulse forming network 28 having been charged from reso¬ nant charging system 38 generates a square wave output pulse which is applied to the primary of transformer 30. The secondary of transformer 30 superimposes _.the short-time high voltage pulse on the before-mentioned low level DC voltage existing across the electrodes of laser 12 thus sharply increasing its output peak beam power. Stated differently, when the laser's plasma is controlled or adjusted to operate at a low power level sufficient to maintain ionization at the voltage glow state and wherein the ion density is high enough to support conduction, the system can be gated by superim¬ posing a short-time voltage pulse through the thyratron tube circuit onto the low level voltage thus increasing the current and the output power many times for the duration of the pulse. The duration of the high current state is not long enough to develop instabilities in the discharge. For example, ?■_ 5 KW maximum continuous wave laser operated in this manner yields from zero to 5 KW average power; however, the peak power is many times greater than the above-mentioned continuous power. The pulse width, as an example, can be about 30 microseconds at 1000 pulses per second (1000 PPS) yielding a peak power of perhaps 300 times the continuous power level. Pulse forming network 28 having discharged itself through transformer 30, the grid of thyratron 36 now regains control and the pulse dies. Adroit design of circuit parameters permits a pulse repetition rate as a function of pulse forming circuit charging time and thyratron grid pulse-application rate.

Since voltage supply 10 is infinitely variable, any combination of operational modes is available at the selection of the operation. Thus, pulse power can be superimposed upon continuous power to perform any given industrial operation.

An alternative and perhaps preferred switching means is revealed in my Patent No. 4,442,383 issued April 10, 1984 and entitled "Plasma Switch" which is incorporated herein by reference. The acoustic shock must be considered in this system because an acoustic wave enters the gas stream. The acoustic wave may be powerful enough to damage parts of the laser assembly by repeated stresses and would degrade the optical .quality of the laser beam. Sound absorbing structure can easily be incorporated in the gas return flow cavity. For example, an acoustic trap (not shown) can be effectively utilized.

It will be seen that I have provided a laser control circuit which greatly increases the flexibility and usefulness of a high power laser. It will be appar¬ ent to those skilled in the art that many changes may be made in the construction and arrangement of parts with¬ out necessarily departing from the scope of the inven¬ tion as defined in the claims.

Claims

1. A circuit to control the operation of a laser having at least one pair of electrodes and to select¬ ively convert laser operation from one operational mode to another comprising in combination: (a) a laser first power supply circuit having means to vary the output voltage connected to the laser electrodes,

(b) a laser second power supply circuit connected in parallel with said first power supply circuit and to the laser electrodes, a d

(c) switching means connected to said first power supply circuit and to the second power supply circuit to simultaneously reduce the first power supply output voltage to a level which barely sustains plasma ionization in the glow state and to apply a short-time high voltage pulse to the laser electrodes to sharply increase the plasma current, resulting in a short burst of output beam power.

2. The invention according to claim 1 including a plurality of blocking diodes connected between the said first power supply circuit and said second power supply circuit to prevent voltage feedback between the. two circuits.

3. The invention according to claim 2 wherein the said laser second power supply circuit includes a pulse forming network connected to a switching means whereby actuation thereof reduces the first power supply output voltage, and yields repeated high voltage laser output pulses.

4. The invention according to claim 3 wherein th_= laser output pulses are superimposed on laser contin- ious power output.

5. The invention according to claim 4 wherein said switching means comprises a thyratron tube.

6. The invention according to claim 5 including sound absorbing means in the laser cavity to reduce the effect of high levels acoustic waves generated by the laser pulses.

7. For use with a high power gas laser, a circuit to operate the laser in the pulse mode compris¬ ing in combination:

(a) a first power supply having control means to operate the laser at a low /olt ge level suffi- cient to maintain ionization where the ion density is sufficient to support conduction connected to the laser electrodes,

(b) a second power supply connected in parallel with said first power supply through blocking diodes to superimpose a series of short-time voltage pulses onto the low level ionization sustaining voltage to yield a series of laser pulses.

8. The invention according to claim 7 wherein said second power supply includes a pulse forming net- work connected to a switching means whereby actuation of said switching means superimposes the series of short- time voltage pulses onto the ionization sustaining voltage.

9. The invention according to claim 8 wherein said switching means comprises a plasma switch.

10. The invention according to claim 9 wherein said switching means is a thyratron.