US2615129A - Synchro-cyclotron - Google Patents

Description

Oct. 21, 1952 E. M. MCMILLAN sYNcHRo-CYCLOTRON 6 Sheets-Sheet 1 Filed May 16, 1947 Mdm f4 TTOR/VE K Ot. 21, 1952 E, M McMlLLAN SYNCHRO-CYCLOTRON 6 Sheets-Sheet 2 Filed May 16, 1947 N K R Z M NM. M m WM A W W E. M. MCMILLAN SYNCHRO-CYCLOTRON Oct 21, 1952 e sneetsheet 3 Filed May 16' 1947 /NVE/Vro@ N M M W M ,1W

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with a method and means for accelerating terium atoms.

Patented Oct. 2l, 1952 sYNcHRo-CYCLOTRON Edwin M. McMillan, Berkeley, alif.,assignor to theV United States of America aslrepresented by the United States Atomic Energy Commission Application May 16, 1947, serial No. 748,434

4 Claims.

This invention relates to the acceleration of It is particularly concerned charged particles to very great velocities. More specifically the invention relates to achieving the acceleration of ions to speeds approaching that of light by means of a magnetic resonance accelerator; a mechanism of the type known as a cyclotron.

A detailed description of a conventional cyclotron will be found in U. S. Patent No. 1,948,334

ygranted to E. O. Lawrence on February 20, 1934.

entitled IMethod and Apparatus for the Acceleration of Ions. The characteristics of cyclotrons lferred to as Deeswhich are coplanar radially and whose open diametrical edges are parallel and closely spaced. The particles to be accelerated are usually protons or deuterons formed by stripping the electrons from hydrogen or deu- The stripping is accomplished in an ion source at the center of the vacuum charnber and between the Dees and therein the hydrogen or deuterium gas is subjected to electron bombardment. Helium nuclei may also be accelerated in cyclotrons and are obtained by introducing helium gas in the ion source and subjecting it to electron bombardment.

A positive ion introduced between the Dees is pushed by the positive Dee and pulled by the negative one, thus being accelerated across the gap between them. Once the ion is urged into the cavity of a Dee it is no longer influenced by the electric field which has just accelerated it across the gap but is constrainedA by the magnetic field that is constantly applied, to travel in a circular' path. When the particle has progressed along the circular path about 180 it emerges into the region between the Dees where it once more is acted upon by both the magnetic and the electric fields. If the time which the ion has spent within the Dee is just half of the period of oscillation of the high-frequency circuit, then on emergence into the gap, the electric field will be reversed and the particle will once more be accelerated across the gap and into the `creases as the particle progresses.

2 electrically field-free region within the opposite Dee.` The length ofpath taken within theDees, between successive accelerations in thek gap,4v in- Thus, the particle spirals outwardly becausek as it'r gains speed, it has more, centrifugal force. l f

It 'has been demonstrated that within limits the speed of the particle increases at sucharate that the timeof travel between accelerations is constant.` Thus, it takes the same time to go around a small semicircle slowly as to go around a large semicircle rapidly. Therefore, the Dees may be energized byplus and minus alternations of constantfrequency. This becomes more lclear when one considers the motion of an ion rnathematically. For a charged particle 4nim/ing at right angles to a magnetic field one has the relation:

where H is the field intensity, e is the charge, v the velocity, and m the mass ofthe particle `and r is the radius of curvature of its path. Now

if the ion is allowed to traverse its semicircular path with an angular velocityequaltofW then:

U y Wr (2) and also:

WALIl (3) It is evident from this that the angular velocity of the ion is dependent of both its linear velocity and the radius of its path, or more precisely, of the energy of the ion. This is true only for particle velocities that are smallcompare'd'with the velocity of light. Furthermore it is evident that as the velocity of a particle increases, its energy or mass also increases.

Early in the development of the cyclotron it was recognized that the relativity increase of mass posed an obstacle in achieving very great particle energies therein. H. A. Bethe and M. E. Rose, in a letter Apublished in the December 1937 issue of The Physical Review stated that it was apparent that a cyclotron could'not be made to give much higher energies to particles than those which had been obtained at that time. The difiiculty is due to the relativistic change` of mass which has the effect of' destroying either resonance or focusing.

As is frequently the case in experimental physics it is theoretically possible to get around this 3 apparent practical limitation in several ways. The defocusing action which is inherent in a radially increasing magnetic eld required by the relativity condition can be overcome by providing suitable electrostatic focusing. Thomas has shown that an azimuthal adjustment of the niagnetic held will accomplish the same purpose. Each of these plans has inherent drawbacks and neither has been successiuly reduced to prac-- tice.

Again, Bethe has suggested that the only way to obtain higher particle energies in a cyclotron seems to be to increase the voltage on the Dees. Lawrence followed the suggestion when he built his 60" cyclotron in Crocker Laboratory to operate with about a quarter of a million volts on the Dees, to the end that when he was accelerating doubly charged helium ions to 32 million electron volts, the ions were required to circulate fewer than nity times. rhe advantage of such a scheme was that a rather large change in mass could take place without seriously impairing the synchronisin of the circulating ions. However when it is considered thatthe particle energy liniit increases only as the square root 01"' the Dee voltage, it is easy to see that even such a design as this has limitations beyond which it is not practical.

As previously stated, within limits the speed oi' a particle revolving in a uniform magnetic held and oscillating electric field increases at such a rate that the time of travel between accelerations is constant. ln other words, within liniits the particle stays in phase with the oscillator. As the particle velocity approaches that oi light, its energy increases and its mass becomes relatively greater. The increase in mass retards the particle and causes it to lag the oscillations oi the electric field. it is then out of synchronisin with the oscillator and will become increasingly out of phase until it enters a region of deceleration whereupon its energy will drop back to the original value. It is obvious then that there exists a range ci phase in which a particle will receive acceleration and beyond which it will not receive acceleration.

Pondering the prospect of completing a cyclotron larger than had ever been built before-not merely were the poles to have been three times the diameter of any previous model but the Dee potential was planned to be 1,000,000 volts and the oscillator power was set at 5000 kwhave devised a solut" n of this problem of phase stability as it applies to magnetic resonance accelerators. The term phase stability as here used refers to particles kept in phase with the oscillating electric field even after great mass increases have taken place therein. lr" one were to increase the magnetic held incrementally, or to decrease the elec 1ic reqency incrementL l 1J, the eduilibrium angular velocity ci the aiected particles would then be suihciently in excess of the value required to match their gain in energy that the instantaneous value oi the energy or the particle would oscillate 'cach and fort about the equilibrium value. e ene i the charged particles traveling in substantially orbits in a magnetic resonance accelerator would be phase stabilized about the equilibrium value.

The utilisation of the principle of phase stability or accelerating charged particles to high energy may be achieved in dilierent ways for light and heavy particles. For the acceleration of light particles, such as electrons, to high energy it is more convenient to vary the magnetic ield and hold the frequency of the electric cld constant, and this technique has been used in the synchrotron which is disclosed in the copending application of Edwin M. McMillan, Serial No. 656,908, for Method Of and Apparatus For Accelerating To High Energy Electrically Charged Particles. For the acceleration of heavy particles, suoli as protons, reuterons and particles, to high energy, it is more convenient to hold the magnetic field constant and vary the frequency of the oscillating electric field. This technique ernployed in the instant improvement in a cyclotron to meet the requirements ol fundamental researchers for an instrument which will supply charged particles at veryT much highe.` energy than has been obtainable heretofore.

It is therefore one object oi this invention to provide a method and for ,ccelerating charged particles to energies commensurate with substantial increa es in the mass of the particles.

Another object of the invention is to provide a method and nieans employing an oscillating electric iield and a magnetic field for controlling the energy of charged parti des by varying the ratio of the frequency oi oscillation to the magnetic neld strength.

Another object of the invention is to provide a method and means employing an oscillating electric held and a magnetic field for increasing the energy of charged particles by decreasing the frequency of scillaticn of the electric field.

A further object of the invention is to provide a method and means employing magnetic field and a varying oscillating electric field for accelerating charged particles along circular orbits in a cylindrical region and for directing the particles to a target in the outer portion of that region.

Other objects and advantages of the invention will become evident with reference to the following detailed description in conjunction with the accompanying drawings wherein:

Figure l is a diagrammatic plan, comparable to a cross section on a generally horizontal plane taken between the poles of the magnet.

Fig. 2 is a front elevation of a magnet structure and a forni of the invention ass ciated therewith but illustrating' primarily the envelope or tank or" the embodiment with portionsoi" various tank appurtenances omitted.

Fig. 3 is a view of the structure shown in Fig. 2, being generally a plan with portions of the magnet and tank broken away to increase the clarity oi disclosure.

Fig. 4 is a view of the structure disclosed in Fig. 3, the view being in part an elevation and the remainder a vertical cross section, and the several planes of the whole indicated by the line 1i4 of Fig. 3.

Fig. 5 is an end elevation partly in section on the line 5 5 of Fig. i showing to an enlarged scale the rotary condenser and its associated structures. with portions broken away to reduce the size of the figure and to improve the clarity of disclosure.

1Eig. 6 isa cross section on a vertical plane indicated by the line 5 5 of Fig. V, showing mechanical details of the rotary condenser with fragments broken away to increase the clarity of dis closure.

Fig. 7 is a frag.i entary View ci portions oi the rotary capacitor shaft showing, in section, details of the water cooling means.

Referring now to Fig. l there is illustrated schematically an electromagnetic accelerator enibodying the present invention and which I have y therefrom.

termed a synchro-cyclotron. The synchro-cyclotron II comprises a magnetic eld structure of which one pole I2 is shown, a tank I3 arranged `on a common vertical axis therewith, and a pair of electrodes I4 and I5 for establishing the required electric field. The electrode I4 as shown 'is a hollow, generally semipancake-shaped memtance equal to the distance between the sidewalls of electrode I4. The electrodes I4 and I5 are enclosed within the vacuum tight envelope or tank I3.

` The electrode I4 is insulated from the tank I3 being supported by stems I6 and I'I that are secured to the wall IB of electrode I4, and pass through insulators I9 and 2| which in turn are supported on a wall of the tank I3. The electrode I is electrically grounded through the pole I2, and since the poles may either support the tank I3 or be part of the tank, the electrode I5 is also electrically grounded through the tank I3.

Through a port 22 and a manifold. 23, two diffusion pumps 24 backed up by a motor driven pump 25 are used to maintain the necessary Vacuum within the tank I3.

A high frequency oscillatory electric lield is maintained between electrodes I4 and I5 by applying to electrode I4 a high frequency oscillating potential supplied from an electronic oscillator, the grounded electrode I5 being connected through the tank to one side of the oscillator. The oscillator circuit may be of any suitable type, such as the grounded grid circuit shown in Fig. l, comprising an oscillator tube 26 and suitable inductances and capacitances to produce an oscillator having a definite frequency. The input of the oscillator is -connected to a power supply which is fed from a 60 cycle line 2'I. The plate potential of about l5 kv. is fed in at the junction point 28. Grid bias is developed across the dropping resistor 29 Kand the radio frequency on the grid is by-passed to ground through capacitor 3I The phase of the excitation voltage between filament and grid is controlled by the capacitor 32 in series with the filament lines.

As a means of exciting the electrode I4 from the output of the oscillator, the filament lines are connected to a filament coupling loop 33, and the plate line is connected to a plate coupling loop 34. These loops 33 and 34 are arranged in parallel relationship adjacent the electrode stem I6 (Fig. 1) which constitutes a half-wave transmission line. It would of course be possible to use any convenient number of these transmission lines in parallel but for simplicity of illustration only two are shown in Fig. l. The filament and plate loops may be arranged adjacent any one or two of the transmission lines.

` The ends of the stems I6 and II, opposite the electrode I4, are supported by insulators 35 and 35 respectively and terminate in a rotary capacitor 31 enclosed in a vacuum tight tank 38. Secured to the stems are a number of spaced, fixed rings 39 with toothlike blades projecting inwardly Interposed between the iixed rings are rotatable disks 4I mounted on an insulator 42 which in turn is secured to a shaft 43 that is driven by an external motor 44 through a pulley 45. The disks 4I are also provided with peripheral blades, and 'their extent and spacing are substantially identical with those on, the xed ring 39. A second series of capacitor plates is located in the tank 38 and constitutes a coupling capacitor to ground. It comprises a rotating disk 46 mounted on the insulator 42 and positioned midway between two fixed rings 41 mounted on a wall of tank 38.

A positive bias of a few hundred volts is applied to the electrode I4 and the stator rings 39 through the choke 43 and the electrode stem I1. The radio-frequency potential on the electrode stem II is by-passed to ground through the'capacitor 49.

Ions may be provided in the apparatusr by any suitable means. Fig.- l illustrates ran ion source 5I positioned over the center of the` pole I2 and supported at the end of an arm 52midway between the electrodes I4 and I5. The arm 52 is sustained by an insulator 53 in the wall of the tank I3. The filament in the ion source 5I is energized through conductors 54 and a transformer 55 by a 60 cycle alternating current. Direct current is provided to the source 5I through a junction 56 by a pulsed supply 51 which can provide up to one thousand Volts.

Means are provided for sampling the ion current in any of the circulator orbits of the beam which are in a generally horizontal plane midway between the magnet poles. The supporting tube 58 is slidably arranged ina wall of the tank I3. As is common in cyclotron art the tube 58 is provided with an aperture l59 to receive the beam. Therein the beam falls on an electrode that is insulated from the shell of the tube and current fiows along an insulated conductor 6I through the current meter 62 to ground. The tube 58 may be moved axially through the several beam revolutions and therefore it may be positioned to intercept any single revolution of the beam from a point adjacent the source to the outermost revolution. Thus one may measure the ion current in the beam at any point along a radius of its circulatory orbits.

At a definite point in the circulator path of the beam means are provided for withdrawing the ions from the magnetic iield. For this purpose there are shown, for example, electrodes 63 and 64 which delimit an electric field designed toreceive ions and deiiect them outwardly. The electrode 63 is a curved metal structure mounted within and electrically grounded to the tank I3, and extending from a point of tangency to the path of the ions in the magnetic eld but ared outwardly therefrom. Concentric with electrode 63 is an electrode 64 comprising a curved metal strap that is mounted on a pair of supporting members 65fand 66. The supports 65 and 66 are in turn respectively mounted on insulators 6l and 68 which are formed as bushings, secured to a wall of the tank I3. An adjustable external source of direct current supplies a suitable potential on the order of 200 kv., as an example, to the electrode 64 through the conductors 69 which pass through bushings 61 and 68, and are connected to the respective supports 65 and 66. When positively charged particles are being operated upon, the electrode 64 is maintained at a suitable negative potential to withdraw the particles from the magnetic eld. The grounded electrode 63 serves to prevent the eld set up around the electrode 64 from having any effect on the orbits of the beam. Thus the electrodes G3 and E4 constitute a deflector system which is capable of deecting the charged particles in the outermost revolution of the beam orbit outwardly from the region of the magnetic field.

The particles thus aiiected pass through the channel 1|, between the electrodes and E4, and emerge from the outlet opening i2 into a region of diminished magnetic eld wherein their centrifugal force causes them to traverse an ever straightening path 'i3 into the electrode if*i and through a window in the wall i4 thereof.

The high speed particles emerging from the Dee along the path '13 may be utilized in any suitable manner such as operations upon atomic structures, in medical physics research, etc. To facilitate such investigations the high speed particles may be withdrawn from the apparatus through a window 'l5 provided in a wall of the tank I3. The window may be made of any suitable material which will at once maintain the vacuum tight continuity of the tank structure and yet permit the passage oi the emergent particles therethrough without appreciable velocity loss or scattering.

Cyclotrons have been widely discussed in the literature. As an example, a comprehensive paper dealing with the prior art in such matters as magnet design, ion sources, deectors, target arrangements, vacuum systems and shielding appeared in the Journal ci Applied Physics in January 1944 (vol. l5, pages 249) and in February 1944 (vol. 15, pages 12S-247) under the title The Cyclotron, by M. S. Livingstone. Whereas the above disclosure is adequate as a general catalogue and identification of the apparatus composing the complete synchro-cyclotron, the following description of the present embodiment of the invention will aid in und rstanding its construction, operation and worth.

In the giant synchro-cyclotron as constructed (Figs. 2 and 3), the electromagnet includes an upper pole 8| having a generally horizontal, circular pole face 82 spaced in parallel relationship from a lower pole face 83 of a lower pole 8d. The surface of the upper pole face 82 while substantially planar is provided wit-h steps 85 arranged in the form of rings near the periphery of the upper pole Si (Fig. Il). The surface or" the lower pole face S3 is similar in area and conguration to the upper pole face, being provided with steps 85. Magnetic iluX between the pole faces is maintained by energized electric coil windings E? and SiS girdling the poles Si and 84 respectively and disposed within the encompassing yoke or frame 89 which constitutes a dual return path for the magnetic flux. ln operation the coil windings are effective in maintaining a substantially homogeneous and nonvarying magnetic field, the dui; direction or" which is perpendicular from the lower pole face 83 to the upper pole face 82. The steps 8B and S in the respective pole faces reduce the gap therebetween near their edges and are effective in maintaining a uniform magnetic field adjacent the marginal edges of the polesa regi-on wherein a magnetic eld maintained between poles lacking these characteristic contours would be distorted.

A vacuum envelope or tank 9| is positioned around and secured in vacuum tight relationship to the poles 8| and The tank 9i of the giant synchrocyclotron is about twenty feet square as viewed from above (Fig. 3) and is so oriented that one of its diagonale approximately coincides with the center line of the magnet frame 89. The upper wall 92 of the tank 9| is provided with an aperture 93 of suflicient size to fit around the upper pole 8i and permit the wall to be secured to a iiange 94 formed around the pole 8|, with a plurality of gaskets (not shown), clamping lugs 95, studs 95 and nuts 91. Similarly the lower wall 98 of the tank 9| is provided with an aperture 99 sized to encompass the lower pole 9F. and permit the wall to be secured to a flange i9! formed around the pole 84, with a plurality of gaskets (not shown), clamping lugs |02, studs 93 and nuts |94.

In addition to the support given the tank 9| by the clamping systems on the magnet poles 8| and 84, it is also held in place by a set of four stanchions |95; one under each of its corners (Fig. 3).

The opposing vertical Walls |06 and |91 in the tank 9| are each provided with a number of flanged ports and a third vertical wall is comprised of a cover plate |98 secured by bolts |09 and nuts to a ange |!2 with deformable d gasket material interposed between the flange and the plate to effect a vacuum seal. The iourth vertical sidewall of tank 9| is similarly closed with a face plate H3 secured by bolts ||4 and nuts ||5 to a fia-nge ||6, and with gaskets interposed between the flange and face plate to form a vacuum seal.

Secured to the flanged port |I`| in the side wall |99 is a manifold ||8 for junction with a pair of diffusion pumps ||9 and |2|. The diffusion pumps may be used in combination with a mechanical vacuum pump as shown diagrammatically in Fig. 1 to provide vacua on the order of l05 min. Hg in the tank 9i.

The ion source |22 for the synchro-cyclotron is essentially the same as those used in the early model cyclotrons to which reference has been made. It comprises a tubular arm |23 supporting at its inner extremity |24, in the region between the centers of the pole faces 92 and 83, means for maintaining an arc, and supported at its outer end by a face plate |25 secured to the hanged port 26 in the wall |96 of the tank 9|. The arc may be maintained by any suitable means such as a filament and an anode. A gas such as hydrogen is introduced into the region between the ilament and the anode through the supporting arm |23 and the arm also carries electric conductors to supply current from an exterior' source of D. C. to the nlarnent and anode. The face plate |29 also carries adjusting means |21 for orenting the extremity |24 of the arm |23 to the end that the ions produced will be made available at the center of the magnetic field.

The ion source appurtenances mounted on the face plate |25 also include means |28 for the introduction and regulation of the gas to be ionized and for introducing the electric conductors to the tubular arm |23.

.es have stated above in connection with the diagrammatic view in Fig. 1 the present invention employs but a single Dee, the other being replaced with a grounded electrode. The Dee |29, so-called because of the shape of a prototype, is supported by the face plate ||3 on a bulge |3| formed therein. Bulge |3| extends through the mid-portion of the plate H3, transversely, and consists of two fiat plates, the upper plate |32 forming an angle of about 30 with the main plate H3 and the lower plate |33 being parallel to the major portion of plate |3. The bulge |3| is enclcsed on the lower side and on the ends to make possible the maintenance of vacua within thetank 92.

The Dee |29 which in the giant synchro-cyclotron weighs on the order of a ton is carried by four hollow cylindrical ceramic insulators, two of which |34 and |35 are shown in Fig. 4. It will be understood from Fig. 4 that the two upper insulators are mounted on the plate |32 and the two lower insulators are similarly mounted on the plate |33. All four insulators are concentric with apertures in the bulge |3| and are secured in a vacuum tight manner to the latter by means of clamping rings |36. Other clamping rings |36 form vacuum tight seals against capping disks |31 at the outer ends of insulators |34 and |35. The disksI |31 are provided with re-entrant supports |38 which extend through said hollow insulators and terminate in seats upon which the frame |39 of the Dee |29 is fastened.

The Dee |29 has the appearance of a short Wide semicylinder supported on its curved side by insulators |34 and |35. The frame |39 is arranged with curved channels MI, on the sides of the cylinder, supporting a` number of small beams |62 and supported by a iiared truss structure (shown in Vertical section, Fig. 4) which is seated on the re-entrant supports |38. Thus the weight of the Dee is distributed in such a way that the two upper insulators are in tension and the two lower insulators are in compression. The frame |39 is fabricated of aluminum alloy to reduce its weight to a minimum and is substantially enclosed with a thin copper skin |43 to define the electric field. On the open diametral edge of the Dee the copper skin is formed into an upper lip |46. and a lower lip |65. These lips extend beyond the line between the ends of the channels |fl| and nearly to the center line of the poles of the magnet. The frame |39 also supports the weight of `a labyrinth of copper tubing (not shown) hard soldered to the inside of the Dee skin |43 to carry cooling water to all parts of the Dee as is common in the art. This serves to prevent the operating temperature of such an electrode from becoming excessive.

rlhe grounded electrode |46 is made up of a pair of thin copper skins, one mounted on a section of the upper pole face 62 and formed to fit over an arc of the steps 85 and the other mounted on a section of the lower pole face 83 and likewise formed to fit over the steps 86. The electrode |46 is similar in area and shape to the Dee |29 and is flared at the support end of the frame |96 to encompass the latter in a spaced relationship. The inside surface of the bulge [3| is also covered with a copper lining |41 designed to meet the pair of skinlike sheets comprising the electrode M6 when the face plate I3 is secured to flange H6. The lining |41 is, provided with apertures to admit the protruding re-entrant supports |36. The apertures are much larger in diameter than the supports |33 thus insuring against the grounding of these parts. diametral edges the two major constituents of electrode |36 are fashioned into lips |48 spaced apart vertically a distance equal to that between the lips |43 and |45 cf the Dee |29. This Vertical clearance between parts of the electrodes |29 and |36 in the giant cyclotron described is approxi" five inches and it has been found that this is ample to accommodate the oscillations of the charged particles about the median plane. In Fig. 3 I have shown how the lips of the electrodes On their 1o'- |29 and |46 are similarly contoured horizontally so that the interval` therebetween is uniform f throughout. f f

The insulators that are mounted on the face plate ||3 and referred to hereinbeforaserve as connecting links between the Dee and an oscillator (Fig. l). K p

The oscillator together with its associated equipment is preferably carried on a portable truck structure that may be rolled into place for operation or out of place for service on a pair of rails |49 imbedded in the floor of the synchrocyclotron building. The truck |5| consists of a lower main frame |52 made of structural steel and mounting a number of flanged wheels |53 engaging the rails |49, and an upper framework |54 adjustably anchored to the frame |52 by means of a plurality of brackets |55 and screwjacks |56. The truck |5| may be impelled in either direction by an electric motor |51 driving one of the wheels |53 through a gear train |58 and a chain belt |60. At the end ofthe frame-A work |54 that is adjacent the tank 9| a pair of braced, vertical arms |59 are provided to be at- H3. The framework |54 includes two `Sets of rails |6| and |6|a mounted on cross-members and each carrying a smaller truck.H The rails 6| carry truck |62 von a set of unflanged wheels |63 and a set of flanged wheels |64 while the rails |6|a carry truck |62a on a set of unilanged wheels |63a and a set of flanged wheels |64a. At that end of the truck |62a which is adjacent the face plate ||3 of the tank 9| there are a pair of rails |65 placed transverselyl thereon and provided to support another truckk |66 through engagement with the interposed wheels |61. Although the frame |66 is little more than a small rigid frame it is sufficiently sturdy to transportlthe weight of an oscillator house |63.

The shield |69 is supported on truck |62a and encloses a set of four transmission lines, two of which (Fig. 4) are coupled to the supporting insulators for the Dee. The `large copper tube |1|- which constitutes one of such lines is secured to the capping disk |31 on the insulator |35. The other one of the lower pair of lines is identically constructed, and is mounted on the second one cf the lower pair of insulators. per tube |12 is also identified as one of the lines which is secured to a capping disk |31 on the insulatoi` |34 through the medium of a transition member |13. upper pair of lines is identical. Shield 69 which encloses these four parallel transmission vlines is a thin walled metal shell stretched on small section channel ribs. It is preferably fabricated in sections for ease in assembly or disassembly. One of the walls of the shield |69 is common to the oscillator house |68 which is similar in construction thereto, and which houses the oscillator tube and its associated circuit components. It will be apparent that the complete shielding of the oscillator system herein provided insures a minimum of radiation therefrom.

The oscillator and its appurtenances dissipater a considerable amount of heat which it is desirable to remove from the confines of the oscillator house. To accomplish this I have mounted a pair of small, motor driven blowers |14 and |15 on the top of the oscillator house |68. These blowers do not cool the oscillator per se, since it is a water cooled triode, but serve to draw in filtered air and blow it down over the filament leads Aof f the triode and' also over the coupling loops in the The large cop-v Here again the other one of the' filament and plate lines. The cooling air leaves the oscillator house through screened windows in the walls thereof.

The four transmission lines comprising the tubes 111, 112 and their paired complements, together with the shield 100 are constituents of a resonant system whose effective length is that of one half wave. The system is so arranged that a voltage minimum exists near the insulators that support the Dee and voltage maxima exist at the gap between the lips of the Dee and of the grounded electrode, and inside the rotary capacitor. Four lines were used in this design both because of the great weight of the Dee and because the large electrical capacity of the system required either multiple lines or a single line of such low impedance as to be impracticable.

The coupling of the oscillator to the transmission lines is accomplished in a manner similar to that shown schematically in Fig. l. large tube 112 is split for a distance along the side and the edges of the split are rolled away from each other to form lips. The opening thus formed is large enough to admit a loop (Fig. 4) in the oscillator cathode line. Similarly the tube 111 is formed to admit a loop (not shown) in the oscillator plate line.

It was calculated that the mass increase of the accelerated particles at high velocities would require an 11% reduction in the applied frequency from the oscillator. A further frequency adjustment was necessitated by the fact that the magnetic field was allowed to decrease radially by 4% to improve focusing. Thus a frequency shift of the order of 15% is necessary theoretically. To accomplish this there is positioned at the ends of the transmission lines opposite the Dee, and affixed on the truck 162, a rotary capacitor 11B.

The moving parts 0f the rotary capacitor 116 are housed in a greatly reduced atmosphere. For such purpose there is provided a tank 111 to contain the environment of vacuum, and the system, of which the tank is a part, is separate and distinct from that provided for the main tank S1. The capacitor tank 111 is welded to a cradle 118 that is bolted to cross-members on the truck 162. A manifold 119 on the tank 111 communicates with the large diffusion pump 18|. A manifold 182 intercommunicates between the pump 181 and a small diffusion pump 103. The exhaust passage on the small diffusion pump 183 in turn is connected to the intake manifold 184 of a mechanical vacuum pump 185 that is driven by an electric motor through the enclosed pulley 186. The three pumps and the necessary plumbing and wiring are all carried on the truck 162 along with the rotary capacitor 118.

Considering the tank 111 in detail, the circular cylindrical wall i81 is welded to a rear flange 188 and a forward fiange 180. A plurality of tubes 191 are welded between the flanges at uniform intervals coaxially with holes therein and serve as sleeves through which long bolts 192 are passed to engage the cover plates of the tank. The bolts engage the rear cover plates 193 in threaded holes and they pass through the forward cover plate in clearance holes. Interspersed between the long bolts in the plate 184 are a number of short bolts 105 to hold this plate to the forward flange. Thus it is possible by removing the long bolts 192 and leaving the short bolts 195 undisturbed to break the tank 111 at the joint 196 for servicing operations.

The rear cover |93 is weakened by five aper- The tures and to prevent it from sagging inwardly when the tank 111 is under vacuum, crossed stiffeners |91 are welded to its outer surface in the general shape of a plus sign. In section the stiffeners are T-shaped with a web 198 and a ange 199. In the region where the stiifeners cross they are interrupted by and welded to a sleeve 201.

The four transmission lines are supported at their ends opposite the Dee on four insulators 202 that are secured to the plate 193 of the tank 111. Inasmuch as the mcuntings at their end of the transmission lines are all similar a description of one will suffice for all.

Referring then to Fig. 6, the tube 111 is hard soldered to one side of a mounting flange 203. An insulator 202 is seated in a recess milled in the opposing surface, with a gasket in the seat to prevent damage under pressure. The insulator is of thin walled ceramic construction and is formed with a small lip 204 at each end. Adjacent to and coaxial with the flange 203 is a mounting collar 205 fabricated as a metal annulus peripherally pierced to receive a plurality of machine screws anchored in the flange 203. The annular collar 205 is stepped on its inner face to cooperate with a split ring 206 in bringing pressure to bear on a clamp ring 201 which in turn bears on and distorts the gasket 203 against the lip 204, the flange 203, and the mounting collar 205 to form a vacuum-tight joint. A plurality of screws hold the mounting collar 205 and the split ring 206 together once they have been assembled around the insulator, and a soft copper ring is interposed between the split ring 206 and the lip 204 to safely urge the insulator against the flange.

The other end of the insulator 202 is similarly fastened with an annular collar 200, a split ring 211, a clamping ring 212, and a gasket 213; all cooperating to form a gas tight seal when constrained by the plurality of machine screws in the collar 200 toward the insulator lip 204 and the plate 214. A soft copper ring is seated between the split ring 21 1 and the insulator` lip 204 to transmit pressure safely from the former to the latter. The plate 214 is recessed to receive a gasket upon which the end of the insulator 202 bears. Thus damage to the insulator in the clamping process is avoided.

The plate 214 possesses an aperture 215 that is concentric with the insulator and is mounted in a fixed position concentrically over an aperture 215 of greater diameter in the cover plate i53 on the tank |11. A copper sleeve 211 with a copper flange 218 hard soldered around it somewhat nearer one extremity than the other is secured to the inner surface of the plate 214 and inside of the aperture 216 with a number of fasteners through the flange 210. A groove 219 in the periphery of the plate 214, covered with a band 221 serves as a means of circulating cooling water around this plate. The end wall 222 of the shield 163 is affixed to the outside surface of the cover plate 193.

Each of the transmission lines is provided with an electric current carrying structure that is connected to it at the point where it is supported on the insulator 202 and that extends through its insulator and sleeve 211 well into the tank |11. Each of these structures includes a disk 223 positioned inside and concentric with the insulator 202 and held to the flange 203 with several machine screws 224. The extension portion of each structure is a pair of tubes 225 and 226 coaxial with each other and with the surrounding insulator 202 4and sleeve l2|1. These tubes are hard soldered to the disk 223 for support, and at the opposite ends they are individually sealed.

A uniform space between the tubes is maintained by a helix 221 which forms ahelical passage for cooling water. The water is introduced by suitable means through a tube 228 and after flowing through the passage leaves via a hole in the disk 223, a fitting 229, and a tube 23| to circulate through the Dee |29. Although not deemed necessary to specifically illustrate, it will be understood that ther water then 'leaves the Dee through a tube 232 that passes through the seals at 'the ends of the tubes 225 and 226.

The circulation of cooling water inside the in# sulators is necessitated by the voltage maximum that exists in the system near this region. This high voltage causes heating in the insulators,

and the heat must be removed to prevent their ultimate failure. As a further means of remov- -ing heat from the insulators, air ducts convey cool air to them.

As shown in Fig. 5, a suitable bracket surmounting the vertical stifener |91 bears an yelectric motor 233 that drives a blower 234. The latter draws in filtered air and forces it through the four ducts 235. Each duct conveys air to a single insulator through an opening in the sectional transmission line shield |69. Thus, the interiors of the insulators, which share the vacuum that is maintained in the rotary capacitor tank, are water cooled and the exteriors of the insulators are air cooled.

The four extensions of the transmission lines provide support for the static members of the rotary capacitor (Fig. 6). The outer tube 225 of each of the upper two supports is notched to receive either end of a barV 236 which is secured therein. Midway the bar is drilled to receive a stud 231. Each of the lower two supports mounts a stud 238 fixed in the center of its end. The studs 231 and 238 with the aid of stop nuts as shown hold a thick metal plate 239 at three points in an adjustable vertical position. heavy ring 24| welded to the plate 239 serves to make it rigid.

The upper corners of the plate 239 are trimmed ofi' immediately outside the ring 24|, and a large circular portion of its center has also been cut out. Whereas the bar 236 and the stud 231 are adequate as an adjustable support for the plate 239, they do not provide an adequate electrical path for R. F. currents' that appear on the extensions. Therefore a thin copper section 242, shaped somewhat like the plate 239 and secured to its back surface, is provided with ears formed to overlay the ends of the upper extensions; the ears are held thereto by rings 243 and fasteners 244 and thereby provide supplementary paths for the current. The lower corners of thesection 242 are similarly formed and are clamped against the ends of the lower extensions with rings 249 and fasteners 244.

The plate 239 is provided with a plurality of holes whose centers are equidistant on a circle of somewhat lesser radius than that of the ring 24|. A series of metal fingers or blades 245 are held in uniform alignment on a bolt 246 in each of the holes in the circle on the plate 239. A spacing washer 241 is mounted between each of the blades on each bolt; in a blade, washer, blade sequence, with a washer and a nut 248 on top of each assembly. The blades each taper to a re- 14 duced dimension toward their tips which point toward a common center.

The rotating members in this capacitor comprise'a series of spiders 249 each formed as an annulus with radiating coplanar blades 25| projecting from its periphery at uniform intervals. The kblades on each spider are the same in number as are the bolts 246 that hold the blades 245 of the stator, and the number of spiders is one less than the number of blades on each bolt 246. The annular portions of the spiders together with interposed rings 252 are held rigidly secured to a hub 253 with a number of bolts 254.

The hub 253 is seated on a pair of heavy supporting annuli 255 and 256 that in turn are borne by and keyed to an elaborate arbor or shaft 251. The supporting annuli are made of a material, for example a ceramic, that has good electrical insulating characteristics and high mechanical strength. The annulus 255 is locked ina seat in the hub 253 with a clamping ring 258 and a plurality of screws 259 that engage the hub. In a similar manner the annulus 256 is locked in a seat in the hub 2-53 with a pressure ring 26| that is urged against it by several set screws 262 through a threaded ring 263. The screws 262 are retained by jam nuts 264. External threads in the ring 263 engage internal threads in the hub 253. Gaskets are interposed between the annulus 256 and the ring 26| and also between the annulus 255 and the ring 258 to effect water tight joints.

The shaft 251 passes through a housing 265 that is welded to a flange 266. The housing 265 intrudes the tank |11 through an aperture 261 in the center of the cover |94. The flange 266 is secured to a collar 268 that is held to the cover |94 with a number of studs and nuts. Gaskets between the flange 266 and the collar 268, and more gaskets between the latter and the cover |94 are effective in maintaining vacuum tight joints.

The flange 265 is integral with a casting 269. Bearings 21|, 212, and 213 set in the casting support and align the shaft 251 atthis end.` A short sleeve 214 brazed in the casting 269 contains a conventional chevron seal that when urged against the shaft 251 by a gland 215 forms a vacuum seal. The chevron seal is bathed in oil supplied from the sight glasses 216.

A .family of pulleys 211 on the shaft 251 is positioned to receive a belt 238 driven by an elec tric motor 219 and mounted on a bracket 28| that is bolted on the cover plate |94.

The shaft 251 is hollow at this end from the region near the annulus 256 substantially to the bearing 213. This tubular portion of the shaft may be provided with any desired means of sup plying cooling water to the rotating parts of the rotary capacitor |16; for example, a squirt tube 282 (Fig. 7), and said tubular portion originates in that section of the shaft that is within a hollow protuberance 283 in the casting 269. A water fitting 284 in the wall of the protuberance supplies cooling Water to an annular chamber 285 therein, and the latter communicates with the input orifice of the squirt tube 262 through small radial bores 286. Water iicvs the length of the tube 282 and emerges through radial passages 281 drilled in the shaft 267, into the space enclosed by the annuli 255 and 256, and the hub 253.

The volume so defined is divided by a Water circulating baffle 288 made of a ceramic material and formed as a fiat wheel with a Wide flange. The web of this ceramic wheel is held against a shoulder S on the shaft 257 by a pressure ring 25! whose inner surface threadably engages the shaft 25's.

After water emerges from the inner tube through the radial passages 23': it is constrained to ow over the surface of the baille flange where it is in intimate contact with the inner surface of the hub 253, after which i., passes through the toroidal chamber 292 and 're-enters the shaft 25? via a second set of radial passages 253. An outer water passage 251i, defined by the outer surface of the squirt tube 282 and the inner surface of the shaft 25?, terminates in the region between the two sets of radial passages where the tube and shaft are sealed together. Thus water must flow outwardly over the inner tube to a point where it is once more within the protuberance 233 and where this passage again terminates. The water leaves the passage here Via more radial bores 295, a companion annular chamber 255, and a second water tting 29?. rlhe two annular chambers are separated by a common end wall 25S and share the fixed shell of the protuberance 283 for their outer walls and the rotatable shaft 5l for their inner walls. The opposite end walls are formed by packing glands 252.

he hub 253 is amply proportioned to carry the rotating members of a coupling section designed to by-pass the voltage on the rotating parts to ground. On a portion of the hub with a reduced diameter', a number of annular' plates are mounted alternately with spacer rings l The assembly is held firmly against a shouldered portion of the hub by a plurality of fasteners S02. Interleaved with the plates are the fixed plates S53 which for ease of assembly are semiannular in shape. These latter plates are positioned so that each pair forms a complete annular disk. rlhis arrangement provides alternate fixed and movable plates for the coupling section of the capacitor with a fixed member at each end of the assembly. The fixed plates are precisely spaced by spacing rings 325i in such a way that each rotating plate is stationed midway between two fixed plates. The assembly of plates 353 and rings 3635 is secured to a heavy flanged ring 355 by a number of bolts 396 and nuts .'l'l.

A plurality of standoff studs 358 and stop nuts 355 provide support for the ring 355 and its load, and the adjustability of the support makes it possible to accurately space the rotor and stator plates of the coupling section of the variable capacitor. The standoff studs are based in the cover E93 of the tank i'll.

Although the studs 353 are adequate as supports for the ring 3% they do not provide adequate paths for the currents that this coupling structure is required to carry. For this purpose there is added a thin copper cylinder 35i that is held in contact with the ring 355 by a band 3l2 and a plurality of screws SiS. rhe cylinde 3H extends to the inside of the tank cover l53 Where it is contiguous with a disk-like portion 3M and forms part of the ground path for the current. A good electrical contact with the cover l33 is assured by a ruggedly proportioned ring 3l5 and a great number of studs SiS and acorn nuts 3 l'l. rfhe cylinder is rendered somewhat flexible by a deeply spun, outwardly projecting, annular wrinkle 3 l 8 midway on its side, thereby facilitating the adjustment of the position of the ring 355 on the standoff studs 308.

ri'he other end of the shaft 25'! is cradled in a set of bearings Sie positioned within an inner sleeve 32| that in turn is enclosed by the coaxial outer sleeve 322. rEhe latter ends in, and is supported by a flange 323that is fastened to the cover plate l53 with studs 324 and nuts 325. The inner sleeve may be adjusted axially in either direction within the fixed outer sleeve with the aid of a plurality of adjusting screws 325 and their cooperating jam nuts 32?. The screws 325 threadably engage the sleeve 322, piercing the opposing beveled sides of a pair of shoulders 323 and 325. rI'he tips of the screws 325 bear against surfaces provided for the purpose on the inner nonrotating sleeve 32 l. Axial adjustment of the arbor or shaft 25'! may be accomplished by loosening the screws 325 in the beveled surface of shoulder 32S and then tightening the screws 326 in the beveled shoulder 529. Adjustment in the opposite direction may be accomplished by loosening the screws in shoulder' 329 and tightening those in shoulder 323. By making the appropriate adjustment of these screws in the sleeve 322, the shaft 251, and hence the hub 255 and the rotating plates carried thereon, may be translated so that the blades 25l can rotate in paths that are equidistant from consecutive fingers 245 on any bolt 246.

To enhance the flexibility of the rotary capacitor the shaft 257 has been extended through the plate ISS well into the sleeve 201 where it terminates in a section of reduced diameter on which is milled a flat 33 i. This is intended as a seat for a possible future addition. Surrounding the end of the shaft is a tube 32 with a ring 333 welded to it at one end and a disk 334 welded to it at the other end. Between the ring S33 and the plate l93 are two rubber gaskets. A plurality of bolts 335 urge the disk, tube and ring toward the plate l93 thus squeezing the gaskets to form a vacuum tight joint.

A viewing port 33S is situated on the front cover plate l94 of the rotary capacitor ll'. The flanged pipe 33T, welded around the aperture 333, serves as a base for a thick glass window A clamp ring 3M drawn toward the iange by a number of fasteners 342 depresses gaskets on either surface of the window and seals the vacuum in the tank lll.

Motor 215 for driving the shaft 25'! is remotely controlled by a suitable speed varying device, preferably of the .electronic type. The hub 253, upon which the rotating parts of the capacitor are mounted, is carried on the arbor by the insulating disks 255 and 25E. The latter are preferably made of Zircon porcelain. Disks 255 and 255 serve to prevent the R. ll. currents from flowing in the arbor bearings eliminating the necessity of sliding contacts.

The signal that is induced in the transmission lines by the filament and plate coupling loops of the oscillator flows through the tubes 225 inside the insulators 262, through the thin copper section 242, to the plate 23), and the stator fingers 245.

The blades 25| on the spiders 249 modulate the R. F. signal from the oscillator as they rotate between the stator fingers 245. The degree or percentage of modulation is a function of the speed of rotation of the spiders 249. The R. F. signal is by-passed to ground through the coupling section of the capacitor. The rotatable plates 30D and the stationary plates 303 that form the coupling section are sufficient in number and area to provide a capacitance of the order of ten times the maximum capacitance of all the blades in the modulating section.

All of the inner surfaces of the tank l'l'l are heavily copper-plated to provide a better path for the E; F. curr-'ents than the steelr tank alone could provide. This serves to reduce lheating effects andininin'nz'e the need for-'cooling mediums.- For like reasons, allof the major par-ts in the rotary capacitor, that are not made of copper because of strength requirements, are also` heavily copper plated.

The flange-like copper disk 3|4` has four circular areas'- cut out to receive the four sleeves' 2I`| with a Snug't. The-joints are preferably hard soldered. Eachsleeve 2-|-'| is made-to extend Well into its insulator! in order to prot-ect the insulator from-the fieldr that otherwise` would exist between the tube 2-'25 andthe split ring 21H in theclarnping mechanism. In that case the field would in'- duceheatingin thei insulator, and because of the unequal Wallthickness at the flangeA 2ll4vthe heat would cause' it to fail after brief operation.

"Th-us the return-path for R. F. current includes the' sleeves 2|"| and their mounting iianges 218, tlie plates 21M-,the end Wall 22-2' o'f' the transmission line shield |69, and the shield |69. At its tankfend,l the shield is in contact with the -face plate I |3. The' inside of the bulge I3 is covered with the lining |41` that contacts the uppery and lower skins of rthe grounded electrode |46 to" com# `plete the'v ground path;

A large flanged port 343'., onthe wall |01 of the synchro-'cyclotron tank- `9|, is used' as a point of ventry'iora breec'h loading probe 344. A coverplat'e' 345 that may be' demfountably secured to the nange in any convenient' manner has' a smaller rectangular section 345 weldedlin' its inidportion. A plate'c'losure' 34'1, removably mounted on the section 34B, carries' a' short sleeve welded normal toits exterior surf-ace; the sleeve is of suona diameter that it will just accommodate a long. multi-Walled' tube' 348. 'A- chevron seal and gland' within thesleeve' are relied upon to-retaln the vacuum at this point.

- A` saddle 349y around the tube 348'` near its outer end straps itl to a small truck vcarried on four' wheels. The Wheel-s engage a pair-.of rails 35| 'beneath' the tube. The truck is linked to an end-'- less chain=belt1352 that-travels' over a sprocket at either end ofthey rails. B'y linking a reversible motor 353 to the outermost sprocket -354 with a beltr't'he truck' may be operated either toward or awayffr'om' the tank 9|. As this is done the tube 368 is respectively urged into or withdrawnv from the tank'. .Thus with appropriate switch gear' the probe tube may be caused to' `enter the tank any desired distance from'- the wall |01 -to the center. I-n"` practi'oe-f-itfhas" been found sui-'licient tol limitthe probe travel from a point 20.5 inchesout on'- probe 344i rolled away from 'the tankv on tirera-ils Cooling Water is circulated between' the .wallsv of, the tube 348"r to help' dissipateY the, heat that develops inv thev probe.' .Forl this purpose ail-rose 355: is coupled to" the saddle-1349-.by-n1eans of` a` pipe fitting, anda .passage iii/theVv saddle conclu-cts water to the'space? between the Walls ottubel 3484. Thehose: is ample in length to extendirom a pipe' source near-the outer end ofthefr'ailsato-the'region near .theftank wall and therefore does not-hamperthe travel 0f the probe. A trough 356" adjacent the rails is provided to receive thefs'lac-k hosei" A similar hose is provided to carry off' the Water Adischarged frofnthe tube 348. t

0nthe inner end of the tube 348 `a..vtai-'get structure is' mounted. Because of the experimental nature of the Work done-'with a cyclotron the forni of th target I'nay be Var-led to suit the inlined-iste problem.` Ingenei'al it includes a target vane',- partially or Wholly enclosed' in an igloo or housing;v The housing may incorporate a narroii'v slot to Areceive the beari oifcharged particles and allvv tl'ien to iinping'e ontlie col-l letoi" van.- Or the-housingmay be 'a thin vacuumtig 4r'n'et'allic' membrane inside-of which the vane is inunted on a base.- Inthis latter case the base and vane may be withdrawn through the tube' without disturbing the: vacuum in the tank-,- a new vane installed, and the base and newfi'la-n replaced in `the igloo through the tube 348'. Stich` a xnechanisrn permits Vfurtlier measurements tobe made at an old setting of the probe." In ithe base, the' errent'collect'ing vane niay beconneeted to some external current circuitas is! indicated schematically in A great many ,usefulA investigations may'be carried out v With-- a` cyclotron using a 'probeas above" desribed'v'vithin the 'vacuum environment in thetank; l lotvever for ce1-'tain other types'of research it is advantageous to -x'feniovel the beamof'char'gedparticlesfron--the tank. t

If a;- cy'clotron isV to bef flexible enough forrboth types 'efe'xperixii t some mechanism is requiredy that canV de'ileo'tI the'bear'n outwardly from the magnetic field.y Accordingly'provision-'is' made for mounting a deflector on-v thec'o've'r plate |08 that constitutesi'the tankv vall opposite the Dee. For-tais; purpose a pair' of bushing-insulators 351 are mounted over apertures in the plate fyns'with cjlarxipingV rings 358 and fasteners'of suitable type (nots'hovv'n). A portion ofa bushing extends tlirl'iigh-l 'each aperture; and a metal conductor 359is' sealed lnfar hole along the ani-a1 cen-tei' of each bushing. On these supports a deflector',`of the general cha-racter s'hovvrrfscheina-isi'zally` in Fig'. l, ymay be mounted and used to deneot the: be'ani'` from the regionL between the magnet pole faces f The ceriteflfiine of the Dee' P29 r` deesnctpass t `h"i"oug"11`'the' line between the centers of the pole' mees; Therefore, the vertical 4wail;` se l, which is section of? ythe curved wan ofthe nee, seiner what closer to the wan we' of the-tank' all than is thek vertiear wa1f1-3ez, another sectionof the' Deefscurved'waii; t'q the* wau- |01y or the tank gel.- This arrangement of` centers positions` theDee inrsuch a Way that it can receive the dellected orbitfof. the beam as it commences to straighten out. A`Win`d'0vf 3`63i'r`rthewall `362 of the Dee' provides an. avenue @rescap for 'the beam, y anrith'ujs it domes' to' triejw'iridw rsa in the tasks/ran' m1- The werdowxss has the sansL character;

istics as the window vl5-in Fig'. 1 inr that it canre- .tain the vacuum vthat Aexists in 'tite-tank wht-'1e permitting/I the high speed particles to pass on..

through without serious loss of energy and withf out appreciable scattering. l l

When a magnetic resonancev accelerator ofthe characterF and magnitudeembodied in the-instanti invention is being operated. it develops a radiation level in. its vimmediate surroundings that makes it advisable to keep all personnel at a, distance. With thisfiri mind' all' controls for 19 the synchro-cyclotron are operable from a remote control room.

The magnet windings are energized by generators that supply 1500 amperes at 500 volts. At this operating power a substantially uniform field of about 15,000 gausses is maintained With the aid of the steps on the pole faces as shims.

For most operating conditions it has been found desirable to use a low voltage arc discharge type of ion source with a 100 mil D. C. filament enclosed by a graphite cone containing a small rectangular opening. The source |22, mounted at'the inner extremity |24 of the arm |23, may be adjusted plus or minus three degrees in any direction in the horizontal plane by appropriate operation of the adjusting means |21 on the plate |25.

The single Dee |29 and the capacitorllfi are mounted at either end of a shielded line forming a resonant system whose frequency is variable between 12.6 and 9.0 megacycles. The system is excited by means of the grounded grid self-excited oscillator inductively coupled to a resonant circuit as described. A power input of 18 kilowatts to the plate circuit is sufficient to produce a Dee voltage of l kilovolts peak averaged over a modulation cycle.

It has been found desirable to enclose the Dee with grounded shields to reduce the Volume available for glow discharges. Experience has also shown that a residual ionization is associated with low Dee voltages and tends to load the oscillator so that the Dee voltage cannot build up. To overcome this difficulty the usual procedure is to sweep the field free of ions by applying a few hundred volts of D. C. bias to the Dee system at the start of operations.

The frequency modulation of the resonant system depends directly upon the rotating mechanical vacuum capacitor from which frequencies up to 2000 C. P. S. are obtainable. Deuterons have been observed when the Dee voltage was as low as 8 kilovolts and the associated optimum capacitor speed was 120 R. P. M. This speed corresponds to a frequency of 48 C. P. S. Normal operation conditions are considered to include a Dee voltage of kilovolts and a modulation frequency rate of 120 C. P. S. These are quite remarkable values when it is considered that original plans called for a Dee voltage of one million volts. With normal operating conditions prevailing the particles receive an average energy gain per revolution of only one half the maximum possible, and traverse a total number of turns of the order of 104. The time of fiight of particles from the ion source to a probe target at 80 inches radius has been measured at 890 microseconds. 4

For some types of study, particularly those in which a Wilson cloud chamber is used in conjunction with the cyclotron, it is advantageous to' use a pulsed arc rather than a continuous one. During cloud chamber experiments both the arc voltage and the oscillator plate voltage may be triggered by the cloud chamber.

In operating the new and improved synchrocyclotron under certain conditions and using deuterium in the source, the final venergy of the particles has been found to be about 200 m. e. v. By the time the particles have reached their outer orbits they have revolved about 1300 times and traveled about 6 miles. Their speed at that time is nearly 90,000 miles per second or about 43% of the speed of light.

If after a thorough fiushing of thegas lines to the ion source, helium is introduced therein and the resultant alpha particles accelerated in the synchro-cyclotron, the final particle energies will be circa 400 m. e. v. These final particle energies that are obtainable with the synchrocyclotron represent a threefold increase over the most optimistic predictions and are a vast improvement over anything achieved with heavy particles before.

In the synchro-cyclotrons hereinabove described, the energy of the accelerated ions is increased by decreasing the frequency of the oscillating electric field during the time that the ions are under the inliuence of the magnetic field of constant strength. By reference to Equation 3 it may be noted that the equilibrium angular velocity of charged particles can be increased in accordance with the present invention 4by increasing the ratio of the magnetic field strength to the frequency of oscillation of the electric field during the acceleration of the charged particles. In the preferred form of this inventionl positively charged ions are accelerated to high energy by reducing the frequency of oscilltaion while keeping the magnetic field strength constant.

'While I have described the salient features of this invention in detail with respect to one embodiment, it will, of course, be apparent that numerous modifications may be made within the spirit and scope of this invention; I do not, therefore, desire to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is:

1. A rotating mechanical vacuum capacitor. for changing the resonant frequency of a, cyclotron, including accelerating electrodes, comprising an evacuated envelope; fixed, toothed members mounted within said envelope and electrically connected to one of the accelerating electrodes of said cyclotron; rotatable, toothed members positioned between said fixed, toothed members; mea-ns for rotating said rotatable, toothed members including a supporting structure extending through a wall of said envelope, and an external source of power; insulating means mounted between said rotating means and said rotatable, toothed members; cooling means for said rotatable, toothed members and said insulator means provided within said rotating means; and capacitive coupling means between said insulator means and the other 0f said accelerating electrodes of said cyclotron.

2. A cyclotron having in combination a pair of electrodes, a source of alternating electric potential, and a rotary mechanical vacuum capacitor comprising an evacuated envelope, modulating means including fixed, toothed members mounted within said envelope and electrically connected to one of said electrodes, rotatable, toothed members positioned between said fixed toothed members, means for rotating said rotatable, toothed members including a supporting structure extending through a wall of said envelope, insulating means mounted between said rotating means and said rotatable toothed members; and coupling means `comprising rotatable continuous members mounted on said insulating means, fixed continuous members .positioned between said rotatable continuous members and electrically connected to the other of said electrodes.

. 3. In apparatus for accelerating charged particles, the combination comprising an evacuated tank, means for injecting charged particles centrally within said tank, mea-ns for establishing a 21 magnetic field of substantially constant value through said tank to force said particles to follow a curved path, a first electrode disposed Within said tank and enclosing substantially half of the path of said particles, a second electrode surrounding said first electrode and having a di-ametral slot .positioned normal to the path of said particles, a transmission line connected to said rst and second electrodes and extended externally of said tank, a motor-driven variable capacitor connected at the extended end of said line, and an oscillator tube having its anode and cathode circuits ycoupled to said transmission line at spaced-apart positions.

4. In apparatus for accelerating charged particles, the combination comprising an evacuated tank, means for injecting charged particles centrally Within said tank, means for establishing a magnetic field of substantially constant value through said tank to force said particles to follow a curved path, a first electrode disposed within said tank and enclosing substantially half of the path of said particles, a second electrode surrounding said rst electrode and having a diametral slot positioned normal to the Ipath of Said particles, a rigid conductor connected at one end to said first electrode and extended at the other end externally of said tank, a plurality of spacedapart parallel plates mounted on the extended end of said conductor, a rotor having a plurality of parallel plates disposed so that such plates interleave with the rst-mentioned plates, driving 22 means attached to said rotor to provide rotation thereof, the plates of said rotor being coupled to said second electrode, and an oscillator tube having its anode and cathode circuits coupled to said conductor at spaced-apart positions.

EDWIN M. MCMILLAN.

REFERENCES CITED The following references are of record in the Smith, Kilgore, Donal, Pro. I. R. E. vol. 35, No. 7, July 1947, pages 644 and 668.

The Synchrotron, etc., McMillan, Physical Re- View, vol. 68, September 1945, pages 143 and 144.

Theory of the Synchrotron, Bohm and Foldy, Physical Review, vol. 70, September 15, 1946, pages 249 to 258.

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