US3459988A - Cyclotron having charged particle and electron beams - Google Patents

Description

1969 F. M. RUSSELL 3,459,988

CYCLOTRON HAVING CHARGED PARTICLE AND ELECTRON BEAMS Filed Jan. 5, 1967 4 Sheets-Sheet l A g- 5, 1969 F. M. RUSSELL 3,459,988

CYCLOTRON HAVING CHARGED PARTICLE AND ELECTRON BEAMS Filed Jan. 5, 1967 4 Sheets-Sheet 2 5, 1969 F. M. RUSSELL 3,459,988

CYCLOTRON HAVING CHARGED PARTICLE AND ELECTRON BEAMS Filed Jan. 5, 1967 4 Sheets-Sheet 5 Aug. 5, 1969 F. M. RUSSELL 3,459,938

CYCLOTRON HAVING CHARGED PARTICLE AND ELECTRON BEAMS Filed Jan. 5, 1967 4 Sheets-Sheet 4 tates Patent Ofice 3,459,988 Patented Aug. 5, 1969 US. Cl. 315-542 13 Claims ABSTRACT OF THE DISCLOSURE Energy is supplied to the cavities of a separated orbit cyclotron by passing a pulsed beam of electrons through the cavities in such a phase relationship with the pulsed beam of charged particles being accelerated that power is coupled from the electron beam to the cavities and thence to the charged particles.

BACKGROUND OF THE INVENTION This invention relates to cyclotrons, and more particularly to separated orbit cyclotrons.

The basic concept of separated orbit cyclotrons is referred to, for example, in Nuclear Instruments and Methods, 23, pp. 229-230 (1963).

In a separated orbit cyclotron a beam of charged particles moves in a spiral path under the influence of a steady magnetic field and an electric field of unvarying, radio-frequency, these fields focussing, accelerating and confining the beam. The magnetic and electric fields are normally provided by an alternating series of magnets and excited cavities which are disposed in a closed ring and which are of such dimensions that all the turns of the spiral path pass between the poles of the magnets and through the cavities.

Normally the radio-frequency energy to excite the cavities and hence accelerate the beam of charged particles is provided by a power amplifier using thermionic valves, and is supplied to the cavities over transmission lines. The efiiciency with which such amplifiers generate the required large quantities of radio-frequency power is, however, not very great.

It is therefore an object of the present invention to provide an alternative method of exciting the cavities.

SUMMARY OF THE INVENTION According to one aspect of the present invention, a method of exciting the cavities of a separated orbit cyclotron comprises passing a pulsed beam of electrons through the cavities in such a phase relationship with the pulsed beam of charged particles being accelerated that energy is coupled from the electron beam to the cavities and thence to the charged particles, the energy of the electrons being such that they circulate in an orbit outside the outermost turn of the spiral path followed by the beam of charged particles.

According to another aspect of the present invention, a separated orbit cyclotron comprises a series of magnets and cavities disposed in a closed ring, a beam tube within which a pulsed beam of charged particles is arranged to be focussed, accelerated and confined by the magnetic and electric fields associated with the magnets and cavities, respectively, the beam following a spiral path the radius of which increases as the energy of the charged particles increases, and means to excite the cavities with radio-frequency energy by passing a pulsed beam of high energy electrons through the cavities in such a phase relationship with the beam of charged particles being accelerated that energy is coupled from the electron beam to the cavities so that the charged particles are accelerated, the energy of the electrons being such that they circulate in an orbit outside the outermost turn of the spiral path followed by the beam of charged particles.

It may be preferable to use several such electron beams which enter the cyclotron at symmetrically spaced points around the periphery. Each such electron beam then passes through a fraction of the total number of cavities and is diverted out of the cyclotron at a point in the periphery just prior to the point where the next electron beam enters.

BRIEF DESCRIPTION OF THE DRAWINGS A cyclotron in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 shows a first diagrammatic plan view of the cyclotron,

FIGURE 2 shows a second diagrammatic plan view of the cyclotron,

FIGURES 3 to 6 show diagrammatically parts of the arrangement for generating radio-frequency electric energy which is to be supplied to the cyclotron.

DESCRIPTION OF THE PREFERRED EMBODIMENT The cyclotron to be described is a separated orbit cyclotron for accelerating intense pulsed beams of protons to high energies.

Referring to FIGURE 1, the cyclotron comprises an alternating series of electro-magnets 1 and radio-frequency excited cavities 2 disposed in a closed ring. Theproton beam 3 is focussed, accelerated and confined by a steady magnetic field associated with themagnets 1 and an electric field of unvarying, radio-frequency associated with the cavities 2, such that it moves in a spiral path the radius of which increases as the energy of the protons increases. The spiral path lies within an evacuated beam tube (not indicated) which passes between the poles of themagnets 1 and through the cavities 2.

The manner in which the radio-frequency energy is supplied to the cavities 2 will now be described with reference to FIGURE 2. This figure shows the magnets I and cavities 2 making up half the cyclotron, and part of the path of thebeam 3, the confines of which are indicated by the broken lines 4. At symmetrically spaced points around the periphery of the cyclotron pulsed beams 5 of high energy electrons enter the beam tube and circulate through several cavities 2 before being diverted out of the cyclotron at points in the periphery just prior to points where the next electron beam 5 enters.

The energy of the electron beams 5 is such that the orbit in which they move is outside the outermost turn of the spiral path followed by theproton beam 3. To achieve this the electrons will normally have to be relativistic initially, having an energy of say 4.2 million electron volts. In passing through the cavities 2 the electron beams 5 couple energy to the cavities 2. Some of this energy is lost in the metallic walls of the cavities 2, but assuming the correct phase relationship is maintained between the electron beams 5 and theproton beam 3, the remainder of the energy is absorbed by theproton beam 3, so bringing about the desired acceleration.

For eificiency it is desirable to couple the electron beams 5 into the cavities 2 at zero phase angle relative to the electric field. As however theproton beam 3 crosses the cavities 2 at a finite phase angle relative to the electric field, the cavities 2 are detuned slightly to accommodate the reactance introduced by theproton beam 3.

To reduce the effects of the finite cavity transit time of the electron beams 5, each cavity 2 has a portion 6 of restricted width in the region where the electron bear 5 is to pass through the cavity 2. The width of each portion 6 is the same, so that the resonant frequency and voltage distribution with radius are substantially independent of the exact location in azimuth of the portion 6. With the deceleration of an electron beam 5 in each cavity 2 the average velocity of the electrons in each pulse decreases, and it is necessary to vary the flight path between each adjacent pair of cavities 2. This is achieved by changing the relative locations in azimuth of the portions 6, and by simultaneously guiding the electron beam by means of magnets 7 to circulate at slightly less than the synchronous radius. In this way the pulses of electrons pass through each cavity 2 at the same radius from the centre of the cyclotron and the energy loss to each cavity 2 is identical.

Although the required azimuthal position of each portion 6 can be calculated, it is preferable to arrange that some adjustment of the positions can be made during setting-up.

It can be shown that the interaction of the electron bears 5 with the cavities 2 is self-regulating. Thus the rate at which energy is transferred to the electric fields in the cavities 2 is proportional to the product of the cavity voltage V and theelectron beam current 1 whereas the rate of loss of energy to the walls of the cavities 2 is proportional to the square of the cavity voltage. Thus the magnitude of the cavity voltage in steady-state conditions is directly proportional to the electron beam current. Additional losses caused by proton beam loading are proportional to the product of cavity voltage and accelerated beam current, I Under steady conditions this gives:

V l /C1 V02 +16 V l where k andk are constants.

If the electron current is made proportional to (k l plus an additional current i then:

If the electron current is related to the accelerated current in this manner the system is stable, any transient departures, such as caused by sparking, decaying rapidly with a characteristic time of Q /arf where Q =21r (the energy stored in cavity 2)/ (the energy lost from a cavity 2 per cycle of the electric field), and f is the frequency of the electric field. An additional feature is that if there is no loss of electron current during the process of deceleration then the magnitudes of the cavity voltages are automatically made equal.

The principal effect of a finite cavity transit time on the electron beams 5 is to introduce a dispersion in the energy of the pulses following deceleration. Such dispersion is of little consequence if the electrons remain relativistic in energy, but tends to dominate the behaviour of the pulses during the final stages of deceleration. As loss of an electron beam 5 in the last cavity 2 must be avoided if the cavity voltages are to be maintained contant, then a minimum requirement is that all electrons in a pulse should leave the last cavity 2 with finite energy. If there is an appreciable energy dispersion then the overall efiiciency of the system is reduced, because the energy of the electron beam 5 emerging from the last cavity 2 is lost. Added disadvantages of dispersion are a progressive increase in emittance of the electron beams 5 and a tendency to redistribute particles within apulse.

To minimise such dispersion the electron beams 5 are coupled to the cavities 2 at zero phase angle relative to the electric field. The residual dispersion for an elec- Cal tron beam 5 of reasonable phase-spread is still unacceptably large, and so the electron beams 5 are arranged to pass through asmall cavity resonator 8 following each deceleration. Thesecorrection cavities 8 are operated at a high harmonic (say the eighth) of the frequency of the main cavities 2, and reduce the dispersion in energy to an acceptable amount. The high frequency and low amplitude of the signal needed in thecorrection cavities 8 leads to quite modest power requirements for their excitation. Further, most of the energy given to the electron beams 5 in thecavities 8 is usefully coupled into the main cavities 2. In this manner the average energy of the electron beam 5 emerging from the last cavity 2 before being diverted out of the cyclotron can be as low as 0.2 million electron volts, and with a phase spread of In FIGURE 2 the electron beams 5 are shown as being absorbed by blocks 9 of metal after leaving the cyclotron. Clearly, however, these electron beams 5, although of insufficient energy to do further useful work in accelerating theproton beam 3, have considerable energy and the overall efiiciency of the cyclotron can therefore by improved by using this energy for some other purpose, and not merely dumping it.

It is here convenient to consider the energies which might be involved. Suppose it is desired to accelerate aproton beam 3 having a mean intensity of about milliamps to about 1000 million electron volts. It is not convenient to do this in a single cyclotron, so it might be done in four stages as follows:

(l) protonlinear accelerator 0 to 10 million electron volts (2) first cyclotronlO to 70 million electron volts (3) second cyclotron 70 to 350 million electron volts (4) third cyclotron 350 to 1000 million electron volts Each of the three cyclotrons would be generally similar to the one described above and suitable beam transport systems would be provided to convey theproton beam 3 from the linear accelerator to the first cyclotron and from one cyclotron to the next. To avoid making the lower energy cyclotrons unnecessarily large the electron beams 5 in those cyclotrons are arranged to circulate at half the synchronous radius, and the cavities are spaced such that theproton beam 3 is accelerated once every two cycles of the radio-frequency.

It is now necessary to consider the way in which the electron beams 5 are generated, as it will be appreciated that they are of large intensity and high energy; approximately 1 amp and 4.2 million electron volts respectively in the example quoted above.

Referring to FIGURE 3, the electron beams 5 are generated by a device comprising the combination of electron sources 10, a high voltage generator 11 andlinear accelerators 12. The whole device is housed in a containingvessel 13 filled with a gas, such as sulphur hexafiuoride, at high pressure.

The electron sources 10 are mounted near the top of thevessel 13, and each one is formed by anelectron gun 14 and amodulating grid 15. In operation large numbers of electrons are emitted by theguns 14 and the velocity of these electrons is modulated by sinusoidally-varying radio-frequency signals supplied to thegrids 15 such that the electrons emerge from the guns 10 in bunches or pulses. The bunched electrons pass into theaccelerators 12 where they are accelerated to 4.2 million electron volts and the resulting high energy pulsed electron beams 5 are passed by way of suitable beam transport systems including the bendingmagnets 16 to the desired points on the periphery of the cyclotron (FIGURE 2).

The radio-frequency source energising thegrids 15 determines the frequency of the power supplied to the cavities 2 (FIGURE 2) of the cyclotron. 'In addition, the required phase relationship between the electron beams 5 and the proton beam 3 (FIGURE 2) is achieved by adjustment of the phase of the signal supplied by the radio frequency source.

The high voltage generator 11 comprises a verticallymounted electrically-insulatingshaft 17 held by an electrically-insulatingbearing support 18 at its upper end and driven by amotor 19 at its lower end. Theshaft 17 is some 2 metres in diameter and is driven by themotor 19 at a speed of some 2000 revolutions per minute.

Referring also to FIGURES 4 and 5,shaft 17 is made up of a series of generally flat,permanent magnet sections 20, each of which has 18pole pieces 21 of alternate polarity projecting from its periphery. The outer face of eachpole piece 21 is some 15 cms. square and the gap betweenadjacent pole pieces 21 is also some 15 cms. To connect thesections 20 together and to transmit the torque along theshaft 17, discs 22 having alternately castellated edges are interposed betweeneach adjacent pair ofsections 20, a portion of a disc 22 being shown in FIGURE 6. The discs 22 are made of a non-magnetic metal such as titanium or stainless steel. Theselections 20 and the discs 22 are bonded together but maintained electrically-insulated from one another by being embedded in epoxy resin, the outer surface of the resin being smoothly cylindrical to minimise viscosity losses as theshaft 17 rotates.

Referring again to FIGURES 3 and 4, theshaft 17 rotates within astator 23 formed by a number ofstator sections 24 equal in number to, and adjacent in position to, themagnet sections 20. Eachstator section 24 has a number of inwardly-projecting projections 25, equal in number to thepole pieces 21 on amagnet section 20 and similarly disposed. On each projection 25 is wound acoil 26, only two of which are shown in FIGURE 4. Across eachcoil 26 is connected a bridge rectifier 27 and a smoothing capacitor 28. As theshaft 17 is rotated, therefore, alternating currents are induced in thecoils 26 and these currents are rectified and smoothed to produce direct current potentials across the capacitors 28. These potentials are summed in series around thestator section 24, one end of the series connected arrangement being connected to ametal disc 29 which surrounds thestator section 24 and forms an accelerating electrode of theaccelerators 12, and the other end being connected to thedisc 29 associated with the nextadjacent stator section 24 and so on throughout the length of thestator 23.

Thediscs 29 have aligned apertures through which pass evacuatedglass tubes 30 down which the electron beams 5 pass. The successively increasing potentials of thediscs 29 which may amount to 100 kilo volts perdisc 29 over the range to 4.2 megavolts results in the required acceleration of the electron beams 5. Some at least of thediscs 29 havequadrapo'le focusing magnets 31.

As shown the generator 11 has associated with it 6accelerator 12, so that six electron beams can be produced simultaneously. Depending on the total energy requirements stillmore accelerators 12 may be provided, either associated with the same generator 11 or associated with a further, similar generator.

I claim:

1. A method of exciting the cavities of a separated orbit cyclotron comprising passing a pulsed beam of electrons through the cavities in such a phase relationship with the pulsed beam of charged particles being accelerated that power is coupled from the electron beam to the cavities and thence to the charged particles, guiding the beam of electrons so that it passes through the cavities, the electrons circulating in an orbit outside the outermost turn of the spiral path followed by the beam of charged particles.

2. A separated orbit cyclotron comprising a series of magnets and cavities disposed in a closed ring, a beam tube within which a pulsed beam of charged particles is focussed, accelerated and confined by the magnetic and electric fields associated with the magnets and cavities, respectively, the beam following a spiral path the radius of which increases as the energy of the charged particles increases, an electron beam source for producing a pulsed beam of high energy electrons, and guide means for guiding the beam of electrons so that it passes through the cavities in such a phase relationship with the beam of charged particles being accelerated that power is coupled from the electron beam to the cavities so that the charged particles are accelerated, the energy of the electrons being such that they circulate in a orbit outside the outermost turn of the spiral path followed by the beam of charged particles.

3. A cyclotron in accordance with claim 2 wherein each cavity has a portion of restricted width formed by closing together parts of two opposite walls of the cavity, the electron beam crossing the cavity in this portion of restricted width so that the cavity transit time of the electron beam is reduced.

4. A cyclotron in accordance withclaim 3 wherein the position of said portion of restricted width is varied in azimuth from cavity to cavity so as to maintain the required phase relationship between the electron beam and the beam of charged particles being accelerated.

5. A cyclotron in accordance with claim 4 wherein magnets are provided between each adjacent pair of cavities such that the electron beam passes through each cavity at the same radius, this radius being less than the synchronous radius for the electron beam.

6. A cyclotron in accordance with claim 2 wherein means is provided between adjacent pair of cavities to reduce the energy dispersion of pulses of electrons in the electron beam.

7. A cyclotron in accordance with claim 6 wherein said means comprises cavity resonators through which the electron beam passes, each such cavity resonator being excited at a high harmonic of the frequency of said electric field associated with the cavities.

8. A cyclotron in accordance with claim 7 wherein said harmonic is the eighth or higher harmonic of the frequency of the electric field associated with the cavities.

9. A cyclotron in accordance with claim 2 wherein the electrons of the electron beam are initially relativistic.

10. A cyclotron in accordance with claim 6 wherein more than one such beam of electrons supplied to the cyclotron, the electron beams entering the cyclotron at symmetrically disposed points around the periphery, and each electron beam passing through a plurality of consecutive cavities before being diverted out of the cyclotron at a point in the periphery just prior to the point where the next electron beam enters.

11. A cyclotron in accordance with claim 6 wherein the electron beam is generated by a device comprising an electron source, means to modulate the electron source, a high voltage generator and a linear accelerator.

12. A cyclotron in accordance with claim 11 wherein the high voltage generator comprises a rotor formed by an electrically insulating shaft made up of a plurality of magnet sections having outwardly projecting pole pieces, adjacent pole pieces having opposite magnetic polarity, a stator having a similar number of correspondingly disposed inwardly-projecting projections on each of which is wound a coil, means to drive the rotor, means to rectify the alternating currents developed in the coils as the rotor rotates, and means to apply the resulting direct current potentials to the electrodes of said accelerator to produce the required acceleration of the electron beam.

13. An accelerator for accelerating a beam of charged particles comprising at least one electrically resonant cavity for providing an electric field to act upon the beam of charged particles, an electron beam source for producing a pulsed beam of high energy electrons, and guide means for guiding the beam of electrons so that it passes through the cavity or cavities in such a phase relationship with the beam of charged particles being accelerated that power is coupled from the electron beam to the cavities so that the charged particles are accelerated, the

7 8 energy of the electron beam and the action of the guide 2,979,635 4/1961 Burleigh 313-62 X 3,328,708 6/1967 Smith et al. 3l362 X means being such that the electron beam path is spaced apart from the path limits of the beam of charged JAMES W. LAWRENCE, Primary Examiner particles.

References Clted 5 c. R. CAMPBELL, Assistant Examiner UNITED STATES PATENTS 2,890,348 6/1959 Ohkawa 31362 X CL 2,882,396 4/1959 Courant et al 313--62 X

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