The present invention reiates to high power pulse generation, being more oarπcuiariy concerned with the generation of relatively wide d.c. pulses with soiid-state circuitry that, πoπe-the-less, have the extremely fast πse time attainable with the generation of much narrower pulses, gac ground

The generation by soiid-state systems of narrow puises of the orαer of about one or a few microseconds with Eracnoπai microsecond πse times has eariier oeen accomt-iished and widely used ;n many applications including radar, pulse communications, stroboscopy. Sash photography ana other uses. The combining, overlapping and sequencing operation of such pulses to generate high power radio* frequency pulses as for Loran-C navigation and other purposes has been successfully implemented and widely used, (as. for example, in US Patents Nos. 3,786,334; 3,889,263, and 4,001.598 to Paul R. Johanessen, and eariier US Patent No. 2,786, 132 to Robert H. Rines. of common assignee herewith. ,

There are speαai applications and problems, however, where such techniques are not directiy transferable or useabie to achieve required d.c. puises of the order of say, 10- 15 microseconds width (or more), That still require the very fast πse time (of the order of 0.5 microseconds) that have only heretofore been achieved in one or a few microsecond pulsing circuits, and, moreover, have the added function of amplitude and pulse width, variability during operation.

Among such special applications is, for example, the soiid-state circuit generation of about 10- IS microsecond d.c. puises having about 0.5 microsecond rise and fail times, and pulse repetition rates of the order of about five to ten thousand per second, with an rms power of up to 1 megawatt or so, for such uses as a plasma source for ion implantation (PSH) on various surfaces and substrates, or the like.

Such plasma source ion implantation processes alter the surface composition of many different materials. These processes are somewhat similar to the concept of electro¬ plating, but the working fluid is an ionized gas, as described, for example, in U.S. Patent No. 4,764,394. Ion implantation, however, requires a voltage that may be lOOkv or more, as compared with the small voltage required for electroplating. All surfaces, moreover, may be simultaneously implanted, with the implant forming a covalent bond with the surface or base material— the covalent bond insuring that the coating is well adhered to the surface. Such bonded coatings may improve surface hardness and/or reduce friction and wear, and corrosion resistance may be achieved. The process may aiso be athermal, presenting alteration of the material temper, stress relief or dimensional change — factors of important implication for many different types of industrial manufacture. Desired results and processes may depend upon the particular "reapes," but may, for example, include diamond films, nitriding and carbiding, among other applications.

In such processes, moreover, the plasma has deleterious effects, causing arcing, spitting and sputtering, putting additional stringent demands upon the pulse generating modulator not found in other pulsing applications.

It is to the solution of these difficulties in providing suitable solid state pulsing circuits and techniques that can meet these requirements and obviate the plasma ion

implantation problems, or similar problems in other applications, that the present invention is primarily directed. Objects of Invention

It is a primary object of the invention, accordingly, to provide a new and improved

solid-state pulse generating apparatus and method that are particularly, though not

exclusively, adapted to produce the relatively wide d.c. pulses with very short rise times and high voltages and often amplitude envelope shaping required for plasma ion

implantation and similar processes, and that, in addition, are tolerant to the arcing and

other insults inherent in the above-mentioned and other deleterious effects of such plasma ion implantation procedures

Other and further objects will be explained hereinafter and are more paπicuiariy

delineated in the appended claims.

Sμmmary

In summary, however, from one of its viewpoints, the invention embraces

apparatus for generating relatively wide d.c. pulses having the fast rise time of much narrower d.c. pulses and having, in combination, a plurality of sequentially operated d.c.

pulse generating modules each having SCR-controlled capacitor means initially charged

from a d.c. power supply through a first inductor and a first SCR switch, and then discharged through a further inductor, a further SCR switch and a saturable magnetic switch upon turn-on of the further SCR switch; the magnetic switch, in turn, being connected to a pulse forming network and a load, the network discharging into the load upon the closing of the magnetic switch; and a square-ioop-core inductor connected across the ioad and of unsaturated high impedance during the discharge of the network

into the load and of saturated low impedance upon the turning on of the magnetic switch, thereby providing the combination of zero current SCR switching and rapid magnetic switching; and, where needed, means for shaping the amplitude envelope of the resulting pulses to correspond substantially to a predetermined contour.

Preferred and best mode designs and operational techniques including for overall puise shaping, where desired, will be hereinafter described in detail.

Drawings

The invention will now be described in connection with the accompanying

drawing, Fig. 1 of which is a wave-form diagram illustrating the development of the c. puises of the invention;

Fig. 2 is a combined block and schematic circuii diagram of the preferred type of circuit for the purposes of the invention; and

Fig. 3 are graphs of the various voltages and currents generated in the circuit of Fig. 2 as a function of time. Preferred Embodiment

Underlying the present invention, for the purposes above-described, is a novel combination of pulse generator modules, as of the before-mentioned Loran-C type generators of some of the before-cited patents, appropriately modified and operating with zero current switching for SCR or similar soiid-state switching devices, together with a fast magnetic switch, providing very high reliability and efficiency with capability for continuous pulse width variation.

To illustrate the principles of the invention, the same will be described in connection with the before-discovered plasma ion implementation application, it being understood that illustrative circuit parameter the values are exemplary only

Through the sequential combination of; say, five microsecond d.c. rectangular pulses (1), (2) and (3), Fig. 1, each having steep rise (and fall) times of the order of, say, half a microsecond or so, a much wider d.c. pulse (4) is attained — in this case, of fifteen

microseconds width, but having the same half microsecond or so rise time of the narrower puises. This is achieved, in accordance with the invention, by converting say, five microsecond wide generated sinusoidal pulses through use of a puise forming network into corresponding five microsecond d.c. rectangular puises as later explained, with, for example, three pulse generating modules combined sequentially for the performance shown in Fig. 1 As further later detailed, the third puise generating module embodies a magnetic switch across the output load, providing the capability to vary the puise width continuously — say from 10 to 15 microseconds.

Again for illustrative purposes, consider the following specifications:

Rise Time: .5 usec

Ripple: 3 to 6%

Pulse Rep. Rate: 5 to 10 thousand per sec

Peak Voltage: 20 to 100 kV

Current: 20 to 50 amps

Pulse Width: 10 - 15 usec

From these specifications, a peak pulse energy of: 75 Joules is required: Up-n- V p« sl p«k. .To...- 100,000 x 50 x 15 x 1(T- 75 Joules

Maximum output peak power is:

(P otrt)-V pβ_.xI pβ-_c- 5 Million watts

Average output power is:

(P <«)„ - 75 x 10,000 - 750,000 wans If an efficiency of 75% is postulated, then the required input power is:

Pi. - 750.000 ^» 1 Million watts

75

Present - day solid - state puise generators used in the Loraπ -C transmitter of Megapuise. Inc., the assignee of the present invention, are described in "Current Developments in Loran-C, R. . Frank, Proceedings of the IEEE, Vol. 71, October. 1983 and "Loran-C Solid State Transmitter Operation ^' ', Wild Goose Associauon International Loran Radioπavigaπon Forum Preliminary Proceedings, August 24-27, 1992. These are modular in construction, with later-described soft failure characteristics, and deliver 9 joules per puise to the load, operating at a puise repetition rate of 1000 pps. To obtain a puise energy of 75 joules, rune such pulse generators are required to form each puise. To obtain the 10,000 pps of the illustrative specification, ten groups of 9 pulse generating modules are needed, triggered sequenήaliy. Thus, a total of 90 modules is required. The combinations of output voltages and currents that can be obtained with 90 or less number of modules axe shown in Table 1

Ta l

A schematic circuit diagram of the preferred modified pulse generating module for these purposes is shown in Fig. 2, and the operational voltages and current waveforms are shown in Fig. 3.

A capacitor C ι, is charged from a DC Power Supply, so-labeied. through an inductor L _t and thvristor SCR ι during the time-interval t _« .t _t . Fig. 3. In the time interval t ₍ - 1 ₂ , SCR i recovers. At time tt the second thvristor SCR ₂ , in series with inductor Lj, is turned on. The energy on d is transferred through transformer T ₍ (primary N _t and secondary N ₂ ) to a Pulse Forming Network (PFN in the time interval t* - tj.

The transformer Ti is the final switching element, being designed to saturate at time .3 or siightiy later, as shown in Fig. 3. hen this occurs, the transformer magnettring impedance becomes very low (closing magnetic switch) and the Pulse Forming Network (PFN) is discharged into the load (LOAD) in the form of an almost rectangular d.c. puise, Fig. 1.

During the PFN discharge time-interval, the inductor L _} across the LOAD is in the

unsaturated high impedance state. Thus, only a very small percentage of the discharge current, i.. Fig. 3 , flows through L ₃ , indicated as Δ I ₅ in Fig. 2.

This magnetic switch is used across the LOAD because it has no restriction on di/dt as is the case for thyristors. Furthermore, it is a nondestructive element in the most critical switching action. The turn-on of this switch is controlled by setting the flux level of the square-loop magnetic core material in L , with the output voltage driving the core into saturation, turning on the magnetic switch. The time it takes to saturate is a function

of the flux level settings, thus providing a time-controlled magnetic switch capability, enabling varying of the puise width continuously, say from 10 to 15 microseconds.

A circuit referred to as "Priming and Recovery" is preferably connected in series with each thvristor SCRi and SCR ₂ through respective inductors L _v and L ₂ , Fig. 2. The

priming part of this circuit allows the thvristor junction to be fully turned on before the start of the main thyristor cu eπt, thereby greatly increasing the di/dt capability of the thyristor and the recovery part of this circuit prevents high thyristor reverse voltage during the reverse-recovery interval as described, for example, in assignee's U.S. Patents Nos 4,230,955; 4, 191,922 and 4,674,022.

During the puise interval, the amplitude of the puise can be changed in steps by assigning a different number of modules to each of the five microsecond time intervals. Since the ion plasma sheet has a finite thickness, the ions close to the target or substrate get less energy than those ions farther away from the target. Thus, the depth of processing penetration of the ions and the profile contour of the depth penetration over the substrate are not uniform. By varying the overall pulse amplitude to shape the envelope to follow or correspond substantially to such process depth penetration profile or contour, control of the depth of the ion implantation processing may be obtained.

Clearly the invention is also useful, as before explained, for other processing applications and further modifications will occur to those skilled in this art, such being considered to fall within the spirit and scope of the invention as defined in the appended claims.