US2970273A

US2970273A - Method of producing large circular currents

Info

Publication number: US2970273A
Application number: US841655A
Authority: US
Inventors: Willard H Bennett
Original assignee: Individual
Current assignee: Individual
Priority date: 1959-09-22
Filing date: 1959-09-22
Publication date: 1961-01-31
Anticipated expiration: 1978-01-31

Description

Jan. 31, 1961 w. H. BENNETT METHOD OF PRODUCING LARGE CIRCULAR CURRENTS Original Filed Nov. 22, 1957 151.]. 1:25:14 25 26 I? ,26 25 27 0 0 SOURCE SOURCE D G SOURCE STEADY COMPONENT PULSE INTER PULSE I INTERVAL TIME WILLARD H. BENN ETT.

INVENTOR ATTORNEY berwbich illustrates the METHOD OF PRODUCING LARGE CIRCULAR C URREN TS Willard H. Bennett, 174 Chesapeake St. SW., Washington, D.C.

'(Gi'anted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States i of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention is a continuing application of Serial No. 698,315, filed November 22, 1957, now Patent No. 2,905,842, and relates to electron accelerating devices and more particularly to a method for producing sustained magnetic self-focusing streams of relativistic electrons in a closed loop.

'Herctofo're devices have been used for accelerating electrically charged particles such as electrons to high velocity and hence a high potential by means of magnetic induction effects. The electrons are then diverted from their circular orbit to produce a desired result such as bombarding a target to produce X-rays. It has also been proposed that a self-focusing stream be produced in a closed loop in a betatron, wherein motions of the electr'o'ns in a self-focusing stream in directions transverse to the ads of the stream would produce radiation which would damp such motion and that such damping would oppose the thermal dispersion of the stream. Such a stream wouldinvolve the simultaneous acceleration of all of the electrons in the stream and have one major difficulty. The combined action of the accelerating electric field and the self-magnetic field of the current in the "stream drives the electrons toward the axis. The continued application of the accelerating electric force produces a continually increasing density of electrons with their associated space-charge neutralizing ions .near the axis, and the rate at which energy must be supplied to compensate the loss of energy of the electrons due to their collisions with the ions increases correspondingly. Such a rate may amount to thousands of kilowatts.

It is accordingly, an object of the present invention to provide a method for producing self-focusing streams of relativistic electronsin a closed loop.

Another object is to provide a method which is ca pable of holding electrons in a closed loop orbit sufiiciently long to prevent the electrons from escaping from the guide field.

Still another object is to provide a method which is capable of intensifying the current within a closed loop sufficientto produce self-focusing streams.

Other objects and advantages of the present invention will hereinafter become more fully apparent from the following description of the annexed drawings, which illustrates the preferred embodiments, and whereini Fig. 1 schematically illustrates an end view of the deviceofthe present invention; V H

- Fig. 2 illustrates a section 'throughthe dischargechami ber ofthe device; I

Fig. 3' illustrates another section through the chamelectron injection tube and-the electronpath;

Fig. 4 is another section through th e chamber'which illustrates 'soft iron fmembers on the outer surface l i l "Q 2,970,273" l Prensa Jan. 31, 1951 Fig. 5 represents the steady magnetic guide-field with additional sawtooth pulses added thereto; and

Fig. 6 illustrates a modification of the device of Fig. 1.

The device of the present invention used to carry out the method comprises a cylindrical or pill box shaped evacuated discharge chamber positioned within a magnetic guide-field similar in form to that of a cyclotron. Electrons from an electron linear accelerator are injected in puses at full energy into the chamber within the guide-field. The electrons injected in each pulse are held by an increase in the guide-field lasting long enough for the electrons to radiate energy and shrink in loop radius sufficient to prevent the electrons from escaping from the chamber. By successively injecting electrons into the chamber, the cycling current is built up to the minimum critical value for self-focusing and held there by the guide-field.

Now referring to the drawings wherein like reference characters represent like by illustration in Fig. l, a device according to the present invention. The device includes similar to that of a cyclotron rate. The magnetic field contour which includes a

pair

15 and 16 se arated by an air Yoke member 18 completes the magnetic circuit for the flux set up in the cylindrical pole pieces. The

cylindrical poe pieces

15 and 16 are surrounded by an annular winding preferably split into two coils 19 which are Wound in the same relative direction and series for energization from a direct current source 21 to maintain the poles steadily magnetized.

produce a stabilizing her to substantially The chamber 17 material such as g ass, sillimanite, porcelain, etc., with side walls 22 supported by a support rod 23 across thecenter of the chamberwhich prevents the chamber from collapsing when under a high vacuum. The chamber may be held in position by any means familiar in theart, one ofwhich is illustrated by two pairs of supports 24 on the outer circumferential surface and adapted to be positioned within the magnetic guide-field with the lines of force symmetrical with the axis of the chamber. The circumferential surface of the chamber is surrounded by two pairs of

coils

25 and 26 positioned on opposite sides of holders 24 and each of which are wound in the same relative directions and connected in series for ehergization respectively from variable direct

current sources

27 and 28 which may be the same or different and which can be steeply increased in pulses. The pulsating current in coils25 and 26 is for the purpose of increasing the flux inside the coils in approximately a sawtooth manner as illustrated by Fig. 5. A pair of coils 31 are provided along the side walls of the chamber and a pulsating current from a source 32 is passed therethrough for the purpose of holding electron orbits in a plane within the chamber and'to prevent wobbling aong the axis during operation of the device. An electron linear accelerator 33 of any well. known type which periodically injects electrons into the chamber through an electron injection tube 34 is located at the midplane of the chamberand arranged tangentially to the inner periphery of the chamher. The tangential arrangement permits the electrons to be injected into the chamber suchthat on entering the chamberthe magnetic guide-field will force the electrons predetermined circular orbits.

into an orbit about the axis ofthechamber.

parts throughout, there is showna magnetic guide-field and which is made to decrease in magnitude with increasing radius at a slow structure is made up from steel" laminations or any other suitable material of appropriate of cylindrical oole pieces gap within which a cylindrical or pill box shaped discharge chamber 17 is placed...

connected in.-

The spacing between the poles rovides a space distribution such as to field on charged particles within a chamber 17 as to confine the particles within said chamis made of any suitable insulating" The electron injection tube 34 is surrounded by a magnetic shielding tube 35 which magnetically shields the beam of electrons from the guide-field until the beam emerges from the injection tube and enters the magnetic guide-field which crosses the chamber. The magnetic shield is made of soft iron or like magnetic material and consequently weakens the field at that point to aid in preventing the electrons in the initial loops from hitting the end of the injection tube at 36 while the electrons are rotating within the chamber. The chamber is also provided with two pieces of soft iron 37 connected with the magnetic pole pieces and extending over to the center of the chamber at a point approximately 270 degrees from the injection tube. These pieces of soft iron increase the magnetic field nearby and deflect the electron orbits away from the electron injection tube. The combination of the locally increased magnetic field near the soft iron pieces 37 and the locally reduced magnetic field near the magnetic shielding tube operates to set up a field which forces'the electrons in orbits toward the center of the chamber such that the electrons do not hit the end of the electron injection tube. The field set up by the soft iron pieces 37 and the magnetic shielding tube 35 will be referred as thepiler field.

Fig. 6 illustrates a modification of the coil arrangement about the chamber 17. As shown in Fig. 6, the

coils

25 and 26 are placcd on the inside of the chamber and the leads brought out through the chamber to the current source. This modification operates in the same manner as that of the coil-chamber arrangement of Figs. 17-4. 7 j

- In operation of the device to carry out the method of this invention the. cylindrical discharge chamber is positioned in the, radially decreasing magnetic field such that the lines of: force of the magnetic guide-field are symmetrical with the axis of the chamber. The chamber is evacuated and high energy electrons from the electron linear accelerator are. injected in pulses into the chamber at an energy large compared to the rest-energy of an electron wherein they are directed in orbits by the guidefield. Beginning near the end of each electron injection pulse a steeply increasing current is passed through the

coils

25 and 26 and then returned slowly to zero at the beginning of the next injection pulse to provide a somewhat sawtooth-shaped increase in the guide-field. The field produced by the steeply increased current w'illbe referred to as the gripper field. The'steep rise in the guide-field produced by the gripper field reduces the radii of the oribits of the electrons that'were just injected into the chamber, thus holding the electrons and moving them away from the injector and toward the axis of the r n e V. if q j: rent i for self-focusing which can be determined from i the relation where c is the speed of light; e is the charge on the electron and y is the average energy of an'electr'on duefto momenta transverse to the direction of the stream as seen in the laboratory system of coordinates.- The mean transverse energy y is due' principally to the injected stream. As these electrons decrease in energy due to radiation, the value y increases in the ratio x /x where x is the energy of a freshly injected electronand x is the energy of the electron after a time, t. Representing the mean angular divergence at-injection as w, the critical current i" after the electrons have decreased in energy to x from the value at injection of x;,, is determined by stream under the effects of the self-magnetic field produced by the electron travel, and the acceleration of the electrons by the self-magnetic field of the stream make the electrons radiate energy of motion transversetothe direction of the stream. This is to be distinguished from the radiation by the electrons due to their acceleration in the guide-field.

chamber. The motion of the electrons in approximately I circular. loops results in the electrons radiating some of their energy, and the reduction insenergy produces a corresponding reduction in loop radius. The reduction in gripper field which follows thesteep rise in current is made slow enough for the correspondingincreasein loop radius of the electrons to remain less than the concurrent decrease in loop radius caused by the radiation of electron energy. The injection of pulses offelectrons with subsequent application of the gripper field is constantly repeated for continued use and operation of the device. The coils along the side Walls of the chamber act on the orbital travel of the electrons to maintain the orbits in a plane such that the electron path does not wobble back and forth along the axis too-much. The

magnetic 'field' incombination 'with-the gripper field holds the electrons in orbital paths about the axis of the chamber such thatthey are'held in space away from the walls of the'chamber.

The first phase build-up of current in the orbits spii'alljng inward toward the center of the guide-field continues until the total current, exceeds the minimum critical c tr- V The electrons are injected into the chamber at full energy and accumulate in the stream without being further energized before the critical current for self-focusing is reached. An application of steady-state self-focusing stream is the use of the stream as a strongmagnetic guidefield for ions while the ions are being accelerated to very high energies by means with which the electrons in the stream are not resonant. a V Ions can be injected into the stream to run in the opposite direction around the loop. The ions cannotbe stored in the stream by any practical kind of process using radiation like that which was used for storing electrons because the radiation rate from ions is too small. If-the ions are injected so that a part of the first few loops .lies inside the concentrated self-focusing stream, coulomb collisionsbetween ions'and electrons can deflect a few of the ions through the small angle needed to put those ions in the stream. Those ions will stay in the stream until their transverse energy has become much greater than the ions which have little motion in the direction of the stream. This kind of ion injection can be maderapid enough to keep thestream filled with high energyfions of ,the species being injected and thus prevent the ions formed by ionization of residual gas from remainingin the stream and forming any important part of thestream.

Obviously many modifications and variations -of the present invention are possible in the .light of the above.

teachings. It is therefore to be understood that within the scope ofnthe appended claims the invention may be said particle injection pulses and more slowly decreasing said increased magnetic field to zero prior to the begin ningiof each successive particle injectionpulse.

f l A method of producing large circularcurrents which comprises applying a'radially decreasing magnetic guide field symmetrically across an evacuated cylindrical chamber, injecting pulses of a narrow beam of high speed electrons into said chamber near the inner surface thereof, applying a steeply increasing current through coils along the surface of said chamber to increase the magnetic guide-field across said chamber near the end of each of said electron injection pulses for a period less than the time interval between successive electron pulses and more slowly decreasing said increased current through said coils to zero prior to the beginning of each successive electron injection pulse.

3. A method of producing large circular currents which comprises applying a magnetic guide-field symmetrically across a cylindrical chamber, injecting pulses of particles into said chamber near the inner surface thereof, delaying successive injection pulses long enough that losses in energy by radiation of the previously injected particles is suflicient to enable those previously injected particles to avoid being ejected from said chamber by successive increases in magnetic guide-field, and increasing the magnetic field across said chamber for each of said particle injection pulses and more slowly decreasing said increased magnetic field to zero prior to the beginning of each successive particle injection pulse.

4. A method of producing large circular currents which comprises applying a radially decreasing magnetic guidefield symmetrically across an evacuated cylindrical chamber, injecting pulses of a narrow beam of high speed electrons into said chamber near the inner surface thereof, applying a steeply increasing current through coils along the surface of said chamber to increase the magnetic G guide-field across said chamber near the end of each of said electron injection pulses for a period less than the time interval between successive electron pulses and more slowly decreasing said increased current through said coils to zero prior to the beginning of each successive electron injection pulse and delaying subsequent injection pulses long enough such that losses in energy by radiation of the previously injected electron pulses is sufficient to enable previously injected electrons to avoid being ejected from said chamber by successive increases in the magnetic guide-field across said chamber.

5. A method of producing large circular currents which comprises applying a magnetic guide-field symmetrically across a cylindrical chamber, injecting pulses of high energy electrons into said chamber near the inner surface thereof, directing said injected electrons into orbital loops near the inner surface thereof, forcing the initial orbits of said injected electrons into orbital loops of smaller radius by applying a steeply rising magnetic field across at least a portion of said chamber near the end of each of said injection pulses, decreasing said increased magnetic field so slowly that the loop radius of previously injected electrons remain smaller than the loop radius at injection, and continuously decreasing said increased magnetic field to zero prior to the beginning of each successive injection pulse.

References Cited in the file of this patent UNITED STATES PATENTS 2,905,842 Bennett Sept. 22, 1959