US2988642A - Particle accelerating system - Google Patents

Description

June 13, 1961 s, SQLQWAY PARTICLE ACCELERATING SYSTEM Filed June 50, 1958 FIG-I A a a rm f 5 .M 2/ mwm w M MW. aw

%4 w L5 LEE h v1 m H 6 m w W a U W 6 n WWW Y B 3 m f 95 M, t. f MA v uw ww 1 3 I Z Q A A K H n F Patented June 13, 1961 2,988,642 PARTICLE ACCELERATING SYSTEM Sidney Soloway, Norwalk, Conn., assignor to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Texas Filed June 30, 1958, Ser. No. 745,387 11 Claims. (Cl. 250-845) This invention relates to particle accelerating systems and, more particularly, pertains to new and improved particle accelerating systems adapted to utilize an undulating potential.

There are in present use various devices for deriving radioactive radiation which, in general, comprise a source of electrically-charged particles and a charged-particle accelerator for directing the particles at a high velocity against a reactive target. By properly selecting the type of particles, the target material and the particle velocity at the instant of bombardment, a particular type of nuclear reaction may be obtained at the target.

To energize the particle accelerator in apparatus of the foregoing type, ordinarily a unidirectional potential is employed. For example, to energize one type of apparatus in which ions of deuterium are accelerated toward a target containing tritium to produce nuclear reactions from which neutrons are derived, a potential in a range from 50-100 kilovolts is needed. Of course, there are many other reactions in which even higher voltages are required.

Various forms of generators and power supplies have been proposed to provide the high voltages required for particle accelerators of the foregoing type. For instance, van de Graaf machines have been employed in which an insulating conveyor carries charges deposited at a relatively low potential to a relatively high potential where they are removed. These generators, while satisfactory for laboratory use, are usually quite bulky and are not well suited for applications in which the source of radioactive radiation must be portable, such as in apparatus for use in a borehole drilled into the earth.

High voltages may also be obtained through the use of a step-up transformer and associated voltage-multiplying rectifier and a smoothing filter. This type of power supply, however, may be relatively complex in design where a large number of stages of multiplication is required. If fewer such stages are used, the design requirements on the step-up transformer become quite severe to maintain a given DC. output voltage.

It is an object of the present invention, therefore, to provide a new and improved A.C. operated particle accelerating system.

Another object of the present invention is to provide a new and improved particle accelerating system Wherein the effective accelerating potential is equal to the peakto-peak voltage of an applied alternating voltage.

Yet another object of the present invention is to provide a new and improved particle accelerating system and associated power source relatively small in size and thereby compatible with the space requirements of well logging service.

A particle accelerating system in accordance with the present invention is adapted to be energized by an alternating potential and includes an envelope containing an ionizable gas, discharge-producing means for selectively producing ions and electrons in the envelope, and particle accelerating means having at least one electrode. Means including a series capacitance is provided for applying the alternating potential between the electrode of the particle accelerating means and a reference point within the envelope. The system further comprises means synchronous with the applied alternating potential for operatively conditioning the discharge-producing means during a relatively short interval of time in each of alternate halfeycles of the applied alternating potential.

The novel features of the present invention are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a schematic diagram of a radiant energy source including a particle accelerating system featuring the present invention;

FIG. 2 includes a series of curves representing various operating conditions in the apparatus illustrated in FIG. 1 plotted as a function of time and useful in explaining its operation;

FIG. 3 is a schematic diagram of a-power supply arrangement which may be used with the apparatus shown in FIG. 1; and I FIGS. 4, 5 and 6 represent various modifications which may be made to the apparatus illustrated in FIG. 1.

As shown in FIG. 1 of the drawing, a particle accelerating system embodying the present invention may employ a cylindrical, elongated envelope 10 which may be manufactured, for example, of out-gassed glass, or other electrically insulating material. The envelope is evacuated, and filled with an ionizable gas such as deuterium at a pressure in a range from 0.1-10 microns of mercury.

Envelope 10 includes an ion source section 11 having a cylindrical anode 12 and disc-like cathodes sup ported at the open ends of the anode. One of the cathodes 14 is provided with an ion-admitting aperture 15. A cylindrical magnet 16 encloses the section of envelope 10 containing the ion source and is essentially coextensive with cylindrical electrode 12. The spacing between the anode and cathode electrodes of the ion source section 11 is selected in a known manner so that an ionic discharge occurs when a given potential is applied.

Another disc-like electrode 17 is supported in parallel relation to electrode 14 and is provided with a central aperture 18 axially aligned with aperture 15. Electrode 17 is spaced from electrode 14 at appropriate distance to minimize the possibility of electrical break-down at a given potential. This spacing is selected in a known manner. Supported adjacent electrode 17, on the side thereof opposite electrode 14-, is a target 19 which may, for example, be composed of zirconium in which another isotope of hydrogen, tritium, is occluded. If desired, however, a deuterium containing target may be employed.

Suitable electrical connections may be made to the electrodes within envelope 10' by various leads which pass through the envelope through appropriate fluid-tight seals.

To energize the device thus-far described, a source of alternating potential 20 is coupled to and powers a conventional direct current pulser 21. The 'pulser is arranged in-a conventional manner to provide pulses in synchronism with the undulations of the applied voltage and having a magnitude sufficient to energize the ion source 11. These pulses occur during respective half-cycles of the applied alternating potential. They have a relatively short duration compared to such half-cycles, may alternate in polarity and are applied between anode 12 and cathodes 13, 14 over leads 22 and 23.

Source 20 is also coupled to a primary winding 24 of a step-up transformer 25. The transformer has a high voltage secondary winding 26, one terminal 27 of which is connected to cathodes 13, 14. The opposite terminal '28 is connected via a charging condenser 29 to electrode 17. A tap 30 on winding 26, relatively close to terminal 28, is connected by a lead 31 to target 19. Transformer 25 is constructed in a known manner so that the, voltage de- 3 rived at secondary 26 has a peak-to-peak value of, for example, 65 kilovolts supplied at terminals 27 and 28 at acurrent of one milliampere.

In describing-the operation of the apparatus illustrated in FIG. 1, occasional reference will be made to FIG. 2 in which the wave forms of the-voltages at various points in the system are plotted to a common time scale.

Let it be assumed that ion source section 11 of the system is energized by a suitable voltage at leads 22 and 23, and that initially anode 12 is positive relative to cathodes 13 and 14. Any stray electrons within the envelope are attracted toward the positively charged electrode 12, but because of the presence of the field produced'by magnet 16 these electrons travel in spiral paths and have a good chance of experiencing ionizing collisions with molecules of deuterium gas. Of course, from each such collisiona plurality of electrons are derived all opposite polarity is applied to the electrodes. However,

it is evident that the positively charged ions would then be attracted toward electrode 12 while electrons would be attracted toward cathodes 13 and 14.

As seen in FIG. 2A, the voltage between terminals 27 and 28 alternates in polarity so that during successive half-cycles the voltage between electrodes 14 and 17 is correspondingly positive and negative. During the portion a, b, of a positive half-cycle represented in FIG. 2A, the voltage at terminal 28 and thus at electrode 18 becomes increasingly positive relative to the voltage at terminal 27 (and electrode 14). At the instant this volt age is a maximum, a negative pulse is initiated by pulser 21 and supplied to ion source 11, as represented by the pulse pin FIG. 2B. Thus, the voltage at electrode 12 is driven sharply negative relative to electrodes 13 and 14 to a value at which a discharge occurs in ion source section 11 and, as mentioned hereinbefore, ions and electrons are derived within the ion source. Since electrode 17 is positive relative to electrode 14, electrons are drawn from within the plasma through aperture and impinge on electrode 17. Those of the electron which pass through aperture 18, of course, are collected at anode 19. Ac-

cordingly, electrons from ion source section 11 effectively short circuit electrodes 14 and 17, and condenser 29 is effectively connected across secondary winding 26. This permits condenser 29 to charge to a potential equal to the peak value, E, of the voltage at secondary 26. Pulse ppersists long enough for the condenser to charge completely, however, this interval is usually short compared to the period of the alternating wave represented in FIG. 2A.

After the termination of pulse p the discharge in ion source 11 ceases and the circuit to condenser 29 is effectively opened. The condenser thus retains the charge it derives during the occurrence of pulse pwhile the voltage at terminal 28 changes from the maximum positive to-peak value of the applied alternating potential, represented as -2E in FIG. 2C. It is at this instant that pulser 21 supplies a positive pulse p+ (FIG. 2B) to ion source'll to'produce a discharge. Positive ions derived and 46 are electrically insulated from one another.

in this discharge, namely deuterons, are accelerated by the potential between electrodes 14 and 17, and many pass through aperture 18 and impinge on target 19.

Because of its connection at tap 39, the target is at a potential slightly more positive than electrode 17, tap 30 and secondary electron acceleration is minimized.

Since deuterons are accelerated through a potential of 65 kilovolts before impinging on target 19, they reach velocities sufiicient to enter into interactions with tritium in the target to produce neutrons of an energy of 14 million electron volts. This occurs during each pulse p+, i.e., a burst of neutrons is produced.

During the occurrence of pulse p+, electrodes 14 and 17 are effectively short circuited by the ion beam and condenser 29 charges to the polarity opposite to that represented in FIG. 1. This charge remains until the succeeding half-cycle when pulse poccurs and the condenser is discharged before it is charged in the opposite direction in a manner explained hereinbefore.

It is thus evident that by coupling the source of alternating potential to the accelerating means through a capacitor, and effectively short circuiting the capacitor at the peaks of the applied voltage, the excursions of the electrodes 14 and 18 extend through a voltage 2E, where E is the peak value of the applied alternating potential.

By pulsing the ion source 11, the accelerator section 14, 17 itself performs as the switch for effectively short circuiting the condenser 29. Accordingly, there is no need to provide external voltage multipliers.

Because the effective voltage is equal to the peak-topeak voltage of the secondary 26, rather than the average voltage, as would occur in a typical rectifier system, the design requirements for transformer 25 are much less stringent than heretofore required. As a result, the transformer may be small in size and compatible with the space requirements of well logging service.

If desired, a system ground may be connected at any desired point in the circuit. For example, terminal 27 may be grounded. In this case terminal 28 would exhibit a voltage relative to ground equal to the peak-to-peak value of the developed alternating voltage at secondary 26. If, for example, a center tap for winding 26 is grounded, the voltage between terminal 27 and the system ground would be equal to an appropriate fraction of the developed peak-to-peak voltage.

An arrangement for achieving a high alternating potential in a narrow diameter is represented in FIG. 3. Source 20 is coupled to primary winding 35 of a first transformer having a rectangular core 36. This transformer has a secondary winding 37 having a turns ratio to primary winding 35 of 1:1, and a high voltage secondary winding 38 whose center tap 39 is connected to core 36.

Secondary winding- 37 is connected to primary winding 40 of another transformer having a rectangular core 41 and a low voltage secondary winding 42 similar to winding 37. On core 41 is a high voltage winding 43 like winding 38 provided with a center tap connected to core 41.

Secondary 42 is connected to primary winding 45 of a third transformer having a rectangular core 48 and a high voltage secondary winding 46 similar to the other high voltage secondary windings. Winding 46 has a center tap 47 connected to core 48.

The several secondary windings 38, 43 and 46 are connected in series-aiding relationship and the cores 36, 41 In this way the transformers act as isolation as well as stepup transformers. The cores may be of conventional construction, for example, they may be fabricated in the usual way of laminated iron. High voltages and small size maybe achieved by using cores made of ferro-magnetic particles bound together by an electrical insulator. One core material of this general class is known as ferroxcube and an operating frequency of 50 kilocycles per second may be employed for source 20.

In FIG. 4, the invention is illustrated as applied to a device in which ions are derived by the use of a radiofrequency field. Elements which are similar to ones represented in FIG. 1 are denoted by the same reference numerals.

Here, a portion of envelope 10, designated as an ion source section 50, is provided with a pair of opposed electrodes 51 and 52, the latter of which is provided with an ion-admitting aperture 53. Enclosing the envelope in a zone between electrodes 51 and 52 is a radio-frequency coil 54 connected by leads 55 and 56 to a pulser 57 driven by a radio-frequency generator 58. If desired, an appropriately energized suppression electrode may be positioned adjacent to target 19.

An alternating potential source 59 is connected to a power supply 60 which provides a unidirectional potential for energizing generator 58. Source 59 is also connected to primary winding 24 of a step-up transformer 25 whose secondary winding 26 develops the required high voltage. Terminal 27 of winding 26 is connected to electrodes 51 and 52, and terminal 28 is connected via condenser 29 to target electrode 19. Connections from source 59 extend to pulser 57 for synchronizing its operation.

The operation of the system of FIG. 4 is similar to the one illustrated in FIG. 1, however, during each of the pulsespand 12+ of FIG. 23, a burst of radio-frequency energy is supplied to coil 54. As is well known, if the radio-frequency field thus created is of sufficient intensity, an ionic discharge is produced in which both electrons and ions are derived. During one portion of an operating cycle, electrode 52 operates as a probe for electrons which pass through aperture 53. During alternate halfcycles it acts as a probe for deuterium ions which also pass through the aperture. It should thus be evident that condenser 29 attains a charge of the polarity represented during half-cycles of one polarity and then its voltage is added to the developed A.C. voltage during the other half-cycle as described in connection with FIG. 1.

In selecting a value for the capacitance of condenser 29 in either of FIGS. 1 or 4, it has been found that a capacitance of, for example, 25 micromicrofarads may be employed with an operating potential of 6S kilovolts. Under these conditions, the condenser accumulates a charge of 1.63 coulombs. At an operating frequency of 1,000 cycles per second, of the energy is delivered by the condenser and thus an average current of 0.33 milliampere is obtained. This is entirely satisfactory for the proper operation of apparatus embodying the invention, as a neutron producer.

A convenient way of constructing condenser 29 is illustrated in FIG. 5. Here, glass envelope 10 includes a lower semi-cylindrical section 65 and a modified target 66 is conformed to and disposed in interfitting relation with the inner wall of envelope portion 65. A metallic plate 67 is conformed to the outer wall of envelope portion 65 and is substantially coextensive with target 66. Plate 67 is provided with a central terminal 68 to facilitate electrical connection to it. In this way, glass envelope 10 operates as a dielectric for a condenser constituted by target 66 and plate 67. The surface areas of the target and of the plate are selected in view of the thickness of envelope portion 65 to obtain a desired capacitance in a known manner.

This arrangement of FIG. 5 may be modified in the manner illustrated in FIG. 6 where the target is shown to be constituted of a plurality of small, discrete metal hydride plates 661, 66-2, 66-3, 66n-1, and 66n electrically insulated from one another. Each of these plates forms a capacitance with plate 67 and a charge is built up in the same manner described hereinbefore. However, when the ion beam comes into action, it discharges the capacitance formed by the central plates, while charges retained on adjacent plates tend to pull the beam apart. In this way, the beam is defocussed and the life of the target is improved.

In the arrangement of FIG. 6 it may be desirable to provide a suitable electrode between targets 66 and the other member of the accelerating gap and operated in such a way so that during the portions of operating cycles when electrons are accelerated, i.e., electrons effectively short circuit the accelerating gap, the beam is defocussed so that all of the several capacitors are charged. During the portion of each operating cycle when the ion beam is produced, this defocussing is disabled so that the ions impinge on the target plates in the form of a beam which is appropriately defocussed in the manner described.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broad aspects, and therefore the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. A particle accelerating system comprising: an envelope containing an ionizable gas; discharge-producing means associated With a portion of said envelope for selectively producing ions and electrons therein; a pair of accelerating electrodes within said envelope, one of said electrodes being in the vincinity of said discharge-producing means; a source of alternating potential having a pair of terminals connected to respective ones of said pair of electrodes; a capacitor in series circuit relation with one of the connections between said source and said electrodes; and means coupled to said discharge-producing means and operative synchronously with the alternating potential of said source for energizing said dischargeproducing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

2. A particle accelerating system comprising: an envelope containing an ionizable gas; discharge-producing and ion accelerating means associated with said envelope; a source of alternating potential coupled to the ion accelerating portion of the aforesaid means; a capacitor in series circuit relation with said source; and means coupled to the discharge-producing portion of said means and operative synchronously with the alternating potential of said source for energizing said discharge-producing portion during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

3. A particle accelerating system comprising: an envelope containing an ionizable gas; discharge-producing means including at least one electrode disposed within said envelope; at least one accelerating electrode disposed within said envelope in the vicinity of said discharge-pro ducing means; a source of alternating potential coupled to said accelerating electrode and to a reference point; a capacitor in series circuit relation with said source; and means coupled to said discharge-producing means and operative synchronously with the alternating potential of said source for energizing said discharge-producing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

4. A particle accelerating system comprising: an envelope containing an ionizable gas; discharge-producing means associated with said envelope for selectively producing ions and electrons therein; a first electrode supported within said envelope and associated with said discharge-producing means; a second electrode supported within said envelope and operatively associated with said first electrode to constitute an ion accelerating gap; a target supported within said envelope in the vicinity of said second electrode; a source of alternating potential having a pair of terminals connected to respective ones of said first and second electrodes; a capacitor in series circuit relation with one of the connections between said source and said electrodes; means coupling said target to said source of alternating potential; and means coupled to said discharge-producing means and operative synchronously with the alternating .potential of said source forenergizing said discharge-producing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

5. A particle accelerating system comprising: an envelope containing an ionizable gas; discharge-producing means associated with a portion of said envelope for selectively producing ions and electrons therein; an electrode supported within said envelope and associated with said discharge-producing means; a target electrode sup ported Within said envelope and operatively associated with said first-mentioned electrode to constitute an accelerating gap; a source of alternating potential having a pair of terminals connected to respective ones of said first-mentioned electrode and said target; a capacitor in series circuit relation with one of the connections between said source and said electrode; and means coupled to said discharge-producing means and operative synchronously with the alternating potential of said source for energizing said discharge-producing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

6. A particle accelerating system comprising: an envelope containing an ionizable gas and having a wall portion constructed of an electrically insulating material; discharge-producing means associated with a portion of said envelope for selectively producing ions and electrons therein; an electrode supported within said envelope and associated with said discharge producing means; a target electrode supported within said envelope on said wall portion thereof; a conductive member supported outside said wall portion of said envelope adjacent said target and defining therewith a capacitor; a source of alternating potential having one terminal connected to said first-mentioned electrode and another terminal connected to said conductive member; and means coupled to said dischargeproducing means and operative synchronously with the alternating potential of said source for energizing said discharge-producing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

7. A particle accelerating system comprising: an envelope containing an ionizable gas and having a wall portion constructed of an electrically insulating material; discharge-producing means associated with a portion of said envelope for selectively producing ions and electrons therein; an electrode supported within said envelope and associated with said discharge-producing means; target means supported within said envelope and including a plurality of conductive elements disposed adjacent to said wall portion of said envelope and electrically insulated from one another; a conductive member supported outside said wall portion of said envelope adjacent said conductive elements and defining capacitors with said elements; a source of alternating potential having one terminal connected to said first-mentioned electrode and another terminal connected to said conductive member; and means coupled to said discharge-producing means and operative synchronously with the alternating potential of said source for energizing said discharge-producing means during relatively short intervals of time in the vicinity of maximum negative and positive excursions of the alternating potential.

8. In a particle accelerating system including an envelope containing an ionizable gas, discharge-producing 8 means for selectively producing ions and electrons in sai envelope and particle accelerating means having at least one electrode, an energizing system comprising: means including a series-capacitance for applying an alternating potential between said electrode of said particle accelerating means and a reference point within said envelope; and means synchronous with the applied alternating potential ,for operatively conditioning said discharge-producing means during a relatively short interval of time in each of alternate half-cycles of the alternating potential in the vicinity of maximum negative and positive excursions of the alternating potential.

9. In a particle accelerating system including an envelope containing an ionizable gas, discharge-producing means for selectively producing ions and electrons in said envelope and particle accelerating means having at least one electrode, an energizing system comprising: means including a series-capacitance for applying an alternating potential between said electrode of said particle accelerating means and a reference point within said envelope; and means synchronous with the applied alternating potential for operatively conditioning said discharge-producing means during a relatively short interval of time in each of alternate half-cycles of the alternating potential.

10. In a particle accelerating system including an envelope containing an ionizable gas, discharge-producing means for selectively producing ions and electrons in said envelope and particle accelerating means having at least one electrode, an energizing system comprising: means including a series-capacitance for applying an alternating potential between said electrode of said particle accelerating means and a reference point within said envelope; a pulse generator for deriving a pulse signal synchronous with the applied alternating potential, each pulse in said signal occurring during a relatively short interval of time in a respective one of alternate half-cycles of the alternating potential; and means for supplying said pulse signal to said discharge-producing means.

11. In a neutron generator, an envelope containing an ionizable gas, an electrode and a target spaced apart in said envelope to define an ion accelerating gap, dischargeproducing means responsive to a pulse signal for producing electrons and ions of said gas in a region of said envelope outside said gap during the continuance of said pulse signal, means including a capacitor for seriesconnection with an AC. source to produce a peaked alternating potential accelerating field between said target and said electrode, and means for applying a pulse signal to said discharge-producing means synchronously with the alternate peaks of said potential field to produce said electrons and ions for acceleration in said gap during respective alternate intervals and for alternate charging of said capacitor, said gaseou ions and said target being reactive to produce neutrons during alternate intervals when said ions are accelerated by-the peaked gap potential supplied by said A.C. source and charged capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,163,740 Wales June 27, 1939 2,214,871 Westendorp Sept. 17, 1940 2,504,231 Smith Apr. 18, 1950 2,570,158 Schissel Oct. 2, 1951 FOREIGN PATENTS 145,084 Great Britain Sept. 19, 1921

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