Electric lamp and discharge devices: systems – High energy particle accelerator tube – Magnetic field acceleration means
Reexamination Certificate
2002-01-14
2004-01-27
Lee, John R. (Department: 2881)
Electric lamp and discharge devices: systems
High energy particle accelerator tube
Magnetic field acceleration means
C315S500000, C315S501000
Reexamination Certificate
active
06683426
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to an isochronous cyclotron that can be a compact isochronous cyclotron as well as a separate sector cyclotron.
The present invention applies both to super-conducting and non-super-conducting cyclotrons.
The present invention is also related to a new method to extract charged particles from an isochronous sector-focused cyclotron.
STATE OF THE ART
A cyclotron is a circular particle accelerator which is used to accelerate positive or negative ions up to energies of a few MeV or more. Cyclotrons can be used for medical applications (production of radioisotopes or for proton therapy) but also for industrial applications, as injector into another accelerator, or for fundamental research.
A cyclotron consists of several sub-systems of which the most important are mainly the magnetic circuit; the RF acceleration system, the vacuum system, the injection system and the extraction system.
The most important is the magnetic circuit by which a magnetic field is created. This magnetic field guides the accelerated particles from the centre of the machine towards the outer radius of the machine in such a way that the orbits of the particles describe a spiral. In the earliest cyclotrons the magnetic field was created in a vertical gap between two cylindrically shaped poles by two solenoid coils wound around these poles. In more recent isochronous cyclotrons these poles no longer consist of one solid cylinder, but are divided into sectors such that the circulating beam alternately experiences a high magnetic field created in a hill sector where the gap between the poles is small, followed by a lower magnetic field in a valley sector where the gap between the poles is large. This azimuthal magnetic field variation, when properly designed, provides radial as well as vertical focusing and at the same time allows the particle revolution frequency to be constant throughout the machine.
Two types of isochronous cyclotrons exist: the first type is the compact cyclotron where the magnetic field is created by one set of circular coils wound around the total pole; the second type is the separate sector cyclotron where each sector is provided with its own set of coils.
Document EP-A-0222786 describes a compact sector-focused isochronous cyclotron, called “deep-valley cyclotron”, which has a very low electrical power consumption in the coils. This is achieved by a specific magnetic structure having a strongly reduced pole gap in the hill sectors and a very large pole gap in the valley sectors, combined with one circular shaped return yoke placed around the coils which serves to close the magnetic circuit.
Document WO93/10651 describes a compact sector-focused isochronous cyclotron having the special feature of an elliptically or quasi-elliptically shaped pole gap in the hill sectors which tends to close towards the outer radius of the hill sector and which allows to accelerate the particles very close to the outer radius of the hill sector without losing the focusing action and the isochronism of the magnetic field. This will facilitate the extraction of the beam as is pointed out later.
The second main sub-system of a cyclotron is the RF accelerating system which consists of resonating radio-frequency cavities which are terminated by the accelerating electrodes, usually called the “dees”. The RF system creates an alternating voltage of several tenths of kilovolts on the dees at a frequency which is equal to the revolution frequency of the particle or a higher harmonic thereof. This alternating voltage is used to accelerate the particle when it is spiralling outwards to the edge of the pole. Another main advantage of the deep-valley cyclotron is that the RF-cavities and dees can be placed in the valleys, allowing for a very compact design of the cyclotron.
The third main sub-system of a cyclotron is the vacuum system. The purpose of the vacuum system is to evacuate the air in the gap where the particles are moving in order to avoid too much scattering of the accelerating particles by the rest-gas in the vacuum tank and also to prevent electrical sparks and discharges created by the RF system.
The fourth sub-system is the injection system which consists basically of an ion source in which the charged particles are created before starting the accelerating process. The ion source can be mounted internally in the centre of the cyclotron or it can be installed outside of the machine. In the latter case the injection system also includes the means to guide the particles from the ion source to the centre of the cyclotron where they start the acceleration process.
When the particles have completed the acceleration and have reached the outer radius of the pole sectors they can be extracted from the machine, or they can be used in the machine itself. In the latter case an isotope production target is mounted in the vacuum chamber. The main disadvantage of this is however, that the particles partly scatter away from the target and then become lost in an uncontrolled manner all over the vacuum tank. This may cause a strong radio-activation of the machine.
In many applications it is wished to bring the beam outside of the machine and guide it to a target where it can be used. In this case an extraction system is installed near the outer radius in the machine. The beam extraction is considered as one of the most difficult processes in generating a cyclotron beam. It basically consists in bringing the beam in a controlled manner from the acceleration region to an outer radius where the magnetic field is low enough so that the beam can freely exit the machine.
For extracting positively charged particles the common method is to use an electrostatic deflector which produces on outward electric field which pulls the particles out of the confining influence of the magnetic field. To achieve this action, a very thin electrode called septum is placed between the last internal orbit in the machine and the orbit that will be extracted. However, this septum always intercepts a certain fraction of the beam and therefore this extraction method has two main drawbacks. The first one is that the extraction efficiency is limited, thereby limiting the maximum beam intensity that can be extracted due to thermal heating of the septum by the intercepted beam. The second is that interception of particles by the septum contributes strongly to the radio-activation of the cyclotron.
Another well known extraction method concerns negatively charged particles. Here the extraction is obtained by passing the beam through a thin foil wherein the negative ions are stripped from their electrons and are converted into positive ions. This technique allows for an extraction efficiency close to 100% and furthermore an extraction system which is considerably simpler then the previous one. However, also here there is a main disadvantage caused by the fact that the negative ions are not very stable and therefore easily get lost by collisions with the rest gas in the vacuum tank or by too large magnetic forces acting on the ion. This beam loss again causes unwanted radio-activation of the cyclotron. Furthermore, cyclotrons accelerating positive ions allow to produce higher beam intensities with a higher reliability of the accelerator and at the same time allow a strong reduction in size and weight of the machine.
Also known from the publication “The Review of Scientific Instruments, 27 (1956), No. 7” and from the publication “Nuclear Instruments and Methods 18, 19 (1962), pp. 41-45e by J. Reginald Richardson, is a claim of a method where the beam could be extracted from the cyclotron without the use of an extraction system. The conditions needed for this auto-extraction are certain resonance conditions of the particle orbits in the magnetic field. However, this method will be difficult to realise and also would give such a bad extracted optical beam quality that in practice it will never be applied.
Also known is the document U.S. Pat. No. 3024379 which reports on a cyclotron system in which the magnet
El-Shammaa Mary
Ion Beam Applications S.A.
Lee John R.
Merchant & Gould P.C.
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