All-magnetic extraction for cyclotron beam reacceleration

Abstract

The controlled simultaneous acceleration of two heavy ion beams in a cyclotron is made practical by an all-magnetic beam extractor. Use is made of the principle of double acceleration to increase the final energy of the ions extracted from the cyclotron.

Government Interests

BACKGROUND OF THE INVENTION

This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

Description

The present invention was conceived for use with isochronous cyclotrons such as the Oak Ridge Isochronous Cyclotron (ORIC) in use at the Oak Ridge National Laboratory. Details of the structure and operation of the ORIC system may be obtained from Nuclear Instruments and Methods, 18, 19, November 1962, pp. 46-61, 159-176, 303-308, and 601-605; from U.S. Pat. No. 3,624,527 issued Nov. 30, 1971; and from the Oak Ridge National Laboratory Electronuclear Division Annual Progress Report, No. ORNL-3630, dated June 1964, pp. 38-62.

For as long as twenty years, the reacceleration of ions in cyclotrons has been observed. It is a naturally occurring process wherein an ion with low charge is accelerated on a high harmonic (or mode) of the cyclotron frequency and, after undergoing increased ionization to a higher charge state, continues to be accelerated in the same cyclotron on a lower harmonic. The process is of continuing interest due to the fact that an ion twice accelerated according to this method experiences a great increase in energy. There have been some proposals of ways to utilize the effect, but prior to the present invention these other proposals have all been impractical to implement.

Thus there exists a need for a practical system for increasing the final energy of the extracted ions from a cyclotron. The present invention was conceived to meet this need in a manner to be described hereinbelow.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an improved system and method for substantially increasing the final energy of an extracted ion beam from a particle accelerator such as a cyclotron.

The above object has been accomplished in the present invention by providing in a cyclotron: an initial electron stripping stage; a complete acceleration of the stripped ions through the cyclotron to a first energy state; a means whereby the ions are returned to an intermediate cyclotron orbit; a second stripping stage; a second acceleration of the now higher stripped ions to their final energy; and final extraction of the ions from the cyclotron.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a recycling system for the ORIC device that incorporates the present invention.

FIG. 2 is a more detailed illustration of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, the smaller and larger generally circular dotted lines define the initial and final beam orbits, respectively, of the Oak Ridge Isochronous Cyclotron. The source of the primary ion beam can be either an internal ion source (not shown) or an external source such as a tandem Van de Graaff generator having an output beam as shown by the dashed line 7. If an external source is used, the injected beam passes through a stripping foil 8 to receive the required charge and orbit for acceleration.

An all-magnetic extraction system 1 pulls the primary beam from the cyclotron and, after its passage through an electrostatic velocity selector 2, the beam exits from the cyclotron along the path 3. A 180.degree. magnet 4 is used to turn the primary beam for reinjection into the cyclotron. A stripping foil 5, located between turns of the primary beam, achieves the proper charge on the ions for reacceleration.

The phase of the beam at the start of the second acceleration can be adjusted by changing the path length traveled outside the cyclotron, either by changing the position of the 180.degree. magnet, or by dividing it into three sectors and changing the magnetic fields in the end sections and center section independently. The location of the stripping foil 5 and other parameters can be adjusted so that the first pass and second pass beams both arrive at the entrance orbit with the correct phase, radius, and radial momentum. After passing through the magnetic extraction system, the final energy beam passes into the velocity selector again where it is electrostatically separated from the primary beam and exits along the path 6.

The all-magnetic extraction system 1 in the drawing is utilized to extract both the primary and reaccelerated (recycled) beam along the same path. By assuring identical paths for the two beams, the narrow aperture all-magnetic extractor makes possible beam separation outside the cyclotron while assuring close control of both beams while they are within the confines of the cyclotron. The existing ORIC magnetic extraction system includes first an electrostatic channel, then a coaxial magnetic channel, and finally a compensated iron channel as shown in FIG. 4.11, page 48, of the above-mentioned report, ORNL-3630. However, in the present invention, the electrostatic channel is replaced with a thin magnetic extractor such as the beam deflection septum magnet 11, as shown in FIG. 2 of the drawings, and as described in the above-mentioned U.S. Pat. No. 3,624,527. The unusually thin profile that the above septum presents to the circulating beam of the cyclotron makes ion beam reacceleration possible and practical for practicing the present invention. FIG. 2 illustrates schematically the other components of the extraction system 1 of FIG. 1. In addition to the septum magnet 11, there is also shown in FIG. 2 a coaxial magnetic channel 12, and a compensated iron channel 13. The components 12 and 13 are shown in greater detail in the above-mentioned report, ORNL-3630.

The electrostatic velocity selector 2 of FIG. 1 includes three components as more clearly illustrated in FIG. 2 of the drawings, and these components include a beam channeling unit 19, and vertical positioning magnets 18 and 15. The 180.degree. magnet 4 is also shown in detail in FIG. 2 and is shown in three sectors as a means for adjusting the phase of the beam at the start of the second acceleration as mentioned hereinabove.

If an internal ion source, illustrated schematically at 10 in FIG. 2, adjacent to the dee 17, is not utilized, then the beam 7 from a tandem Van de Graaff generator is utilized and is guided by suitable and conventional means and by a vertical positioning magnet 16 so as to pass through the electron stripping foil 8. The beam 7 exits from the ORIC as the beam 3, and after passing through a vertical positioning magnet 20, the 180.degree. magnetic unit 4, the magnet 16, and a stripping foil 5, the beam is further accelerated by the ORIC and is finally exited along the path 6, and if desired may be passed through a final positioning magnet 14.

The stripping foils 5 and 8 are simply very thin carbon sheets, for example, held by any suitable holders.

In the operation of the system illustrated in the drawing, the following steps or stages are achieved thereby:

1. An initial electron stripping stage;

2. A complete acceleration of the stripped ions through the cyclotron to a first energy state;

3. Extraction plus means for returning the ions to an intermediate orbit;

4. A second electron stripping stage;

5. A second acceleration of the now higher energy stripped ions to their final energy; and

6. Extraction of the ions from the cyclotron.

It should be noted that ions can be accelerated on the fundamental mode and all odd-multiple modes of the cyclotron frequency according to the equation: ##EQU1## where F is the oscillator frequency; h, the mode number, is the ratio of the oscillator frequency to the rotational frequency of the ion; e is the charge of the ion; H is the magnetic field; m is the mass; and c is the velocity of light. Since F does not change in an isochronous cyclotron such as ORNL's ORIC machine, an ion may start on a high mode with low charge, become more highly ionized, and continue to be accelerated in a lower mode.

If a suitable number of electrons are stripped from the ions after one acceleration to full cyclotron energy on a harmonic h.sub.1, the ions can be reaccelerated in the same cyclotron on a different harmonic h.sub.2. The energy gain in the second acceleration is given by the square of the harmonic ratio, E.sub.2 /E.sub.1 = (h.sub.1 /h.sub.2).sup.2. Using, for example, the third and the first harmonics, respectively, the energy gain of an ion is (3/1).sup.2, or nine times the energy of a single pass through the cyclotron.

Ion reacceleration should be very valuable in providing a number of opportunities to increase the heavy-ion energy of many existing cyclotrons and for integrating them into systems with higher energy. These systems include, but are not limited to, simple recycling in an existing cyclotron, injecting into a cyclotron from an external source such as a Van de Graaff and then recycling, and injection from an existing cyclotron into a new recycling machine. It should be understood that there are other possible combinations.

As an example of what the present invention can achieve, the energy of a .sup.58 Ni beam from ORIC could be raised to 9 MeV/u with a simple recycling. If a 20 MV tandem Van de Graaff were used as an injector for ORIC, and the beam were then recycled, a .sup.208 Pb beam of about 6 MeV/u could be obtained. Also, utilizing the present invention, a calcium beam might be accelerated to the energy necessary to begin forming the superheavy elements.

This invention has been described by way of illustration rather than by limitation and it should be apparent that it is equally applicable in fields other than those described.

Claims

What is claimed is:

1. In an isochronous cyclotron provided with a magnetic field and a beam deflection assembly for separating a first extracted beam from the circulating beam of said cyclotron, said assembly including a thin current-carrying sheet arcuate conductor as the septum of said beam deflection assembly, means for adjustably holding fast the end points of said sheet conductor, means for passing a desired and selected amount of current through said sheet conductor, and means for providing a compensating magnetic field in the vicinity of said sheet conductor, said conductor assuming the same shape as the path of a charged particle in said extracted beam as a result of forces induced in said current-carrying conductor by the magnetic field of said cyclotron, the improvement comprising a 180.degree. bending magnet positioned for receiving said extracted beam and thus turning it around, said bending magnet's position also being such as to direct said beam for reinjection back into said cyclotron for a second pass therethrough, thereby substantially increasing the final energy thereof upon its second extraction from said cyclotron by said deflection assembly.

2. The cyclotron set forth in claim 1, and further including a first stripping foil positioned within said cyclotron, and external means for providing a primary ion beam for injection into said cyclotron and through said stripping foil.

3. The cyclotron set forth in claim 2, and further including a second stripping foil positioned within said cyclotron for receiving and stripping said reinjected beam prior to its second pass through said cyclotron.

4. The cyclotron set forth in claim 3, wherein said means for providing a primary ion beam comprises a tandem Van de Graaff.