Electric lamp and discharge devices: systems – High energy particle accelerator tube – Magnetic field acceleration means
Patent
1998-04-03
2000-05-02
Westin, Edward P.
Electric lamp and discharge devices: systems
High energy particle accelerator tube
Magnetic field acceleration means
315507, 313 62, 3133591, H05H 1300
Patent
active
060576553
DESCRIPTION:
BRIEF SUMMARY
SUBJECT OF THE INVENTION
The present invention relates to a method of extracting charged particles from an isochronous cyclotron in which the particle beam is focused by sectors.
The present invention also relates to the said isochronous cyclotron which applies this method of extracting charged particles.
The present invention relates both to compact isochronous cyclotrons and to cyclotrons focused by sectors. Similarly, the present invention relates to isochronous cyclotrons referred to as superconducting or non-superconducting.
Prior art
Cyclotrons are particle accelerators used, in particular, for the production of radioactive isotopes. These cyclotrons are usually composed of two distinct main assemblies, consisting on the one hand of the electromagnet and on the other hand of the radiofrequency resonator.
The electromagnet guides the charged particles on a path approximately representing a spiral whose radius increases around the acceleration. In modern cyclotrons of the isochronous type, the electromagnet poles are divided into sectors which alternately have a reduced air gap and a larger air gap. The azimuthal variation in the magnetic field which results therefrom has the effect of focusing the beam vertically and horizontally during the acceleration.
Among isochronous cyclotrons, distinction should be made between cyclotrons of the compact type, which are excited by at least one main circular coil, and cyclotrons referred to as having separate sectors, in which the magnetic structure is divided into fully self-contained separate units.
The second assembly consists of the accelerating electrodes, frequently referred to as "dees" for historical reasons. An alternating voltage of several tens of kilovolts is thus applied to the electrodes at the frequency of rotation of the particles in the magnet, or alternately at a frequency which is an exact multiple of the frequency of rotation of the particles in the magnet. This has the effect of accelerating the particles of the beam circuiting in the cyclotron.
For a number of applications which use a cyclotron, it is necessary to extract the beam of accelerated particles from the cyclotron and guide it to a target where it is intended to be used. This beam extraction operation is considered by the person skilled in the art to be the most difficult step in the production of a beam of accelerated particles using a cyclotron. This operation consists in bringing the beam from the part of the magnetic field where it is accelerated to the point where the magnetic field is no longer capable of holding the beam. In this case, the beam is free to escape from the influence of the field and is extracted from the cyclotron.
In the case of cyclotrons which accelerate positively charged particles, it is known to use an electrostatic deflector, the purpose of which is to pull the particles out of the magnetic field in the manner of an extraction device. In order to obtain an effect of this type, it is necessary for an electrode, which is referred to as the septum and will intersect a fraction of the particles, to be interposed on the path of these particles. For this reason, the extraction efficiency is relatively limited, and the loss of particles in the septum will contribute, in particular, to making the cyclotron highly radioactive.
It is also known to extract negatively charged particles by converting the negative ions into positive ions by passing them through a sheet whose function is to strip the electrons from the negative ions. This technique makes it possible to obtain extraction efficiencies close to 100% and also makes it possible to use a device which is must less complex than the one described above. Nevertheless, for its part, the acceleration of the negative particles presents major difficulties. The main drawback resides in the fact that the negative ions are fragile, and are therefore readily dissociated by residual gas molecules or excessive magnetic fields which are present in the cyclotron and through which the ions pass at high energy. The transmis
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Wolber, Gerd et al, "A Kicker Magnet for Sweeping Ion Beams from a Medical Cyclotron", Nuclear Instr. and Methods in Physics Research A256 (1987) 434-438.
Ion Beam Applications S.A.
Wells Nikita
Westin Edward P.
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