Method of reducing axial beam focusing

Details Method of reducing axial beam focusing Method of reducing axial beam focusing

1999-11-02

2002-09-03

Anderson, Bruce (Department: 2881)

Electric lamp and discharge devices: systems

High energy particle accelerator tube

Magnetic field acceleration means

C315S507000, C313S062000, C313S359100

Reexamination Certificate

active

06445146

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method and system for minimising the magnet size in a cyclotron.
BACKGROUND OF THE INVENTION
Production of radioisotopes normally takes place by means of a suitable particle accelerator, for instance a cyclotron, in which an ion beam (i.e., a beam of charged particles) is accelerated. The radioisotopes are formed via nuclear reactions between an incident ion beam and a target medium, which can be a pressurised gas, a liquid or a solid.
Cyclotrons make use of a magnetic field for deflection of accelerated ions into circular orbits. The ion beam will pick up energy successively in the acceleration process and the ion beam trace will become a multi-turn spiral until the ions have reached their final energy at the edge of the magnet poles. The relatively long spiral beam path in the magnet field calls for ion beam focusing properties of the magnet field in order to keep the ion beam concentrated. Modern cyclotrons make use of so called “sector focusing” by means of shaping sectors in the magnet poles for obtaining an improved ion beam axial focusing. This is achieved by dividing the pole surface of the magnet into sectors normally three or four per pole, i.e., 6 or 8 totally. The regions presenting a larger distance between the poles are then referred to as “valleys”.
The acceleration of ions in a cyclotron is performed via a so called RF electrode system maintained at a high radio frequency (RF) voltage, which oscillates with a period time (or a multiple thereof) corresponding to the orbit revolution time of the beam in the cyclotron as given by the average magnetic field of the cyclotron magnet system and the mass/charge ratio of the accelerated ions. Originally the shape of the RF electrodes was like two opposite “D”-formed hollow electrodes in which an accelerated ion beam orbits dependent of the applied magnetic field and the energy of the ions. Every time the beam enters and leaves one electrode, it gains energy and then increases the radius of its orbit.
An ion beam make many orbit revolutions in the acceleration vacuum space between the magnet's poles while increasing its orbit radius. Finally the beam will be extracted from its orbit at the edge of the magnet pole to be incident onto the specific target material. The magnetic field is stronger in the sector regions than in the valley regions due to the different pole gaps. The bigger the difference in magnetic field strength between sectors and valleys, the stronger the axial beam focusing will be, but as a result the average magnetic field will of course be less, which demands a larger diameter of the magnet to ensure its desired energy.
In order to make the cyclotron as compact as possible (i.e., having a small pole diameter) the average magnetic field must be kept high. This implies that the magnet pole gap should be kept as small as possible. This in turn keeps electrical power consumption low, but directly two undesirable effects arise:
Firstly, there will be a reduced conductance in the pole gap for vacuum pumping and secondly there will be very little space for the RF acceleration electrodes.
The nature of the first effect refers to the fact that reduced opening areas has a negative effect on the vacuum pumping conductance leading to deterioration of the vacuum. The accelerated ions in the case of an isotope production facility for PET (Positron Emission Tomography) have a negative charge created by an additional electron bound to the atom. The binding force of the additional electron is weak and the electron will easily be “knocked off” in interactions between the accelerated ions and vacuum rest gas elements. The “hit” ion will be irreversibly neutralised, loosing its sensitivity for electrical and magnetic fields and get lost. A lower vacuum conductance leads to higher amounts of rest gasses, thus resulting in higher beam losses and vice versa. This is a very important factor particularly in the case of a radioactive tracer production system for PET demanding acceleration of negative hydrogen ions.
The second problem can to some extent be compensated for by placing the RF acceleration electrodes in the valleys where the magnet gap is the largest, thereby also keeping the loading capacitance down for the RF acceleration electrodes which is advantageous from the RF power consumption point of view. The obvious solution should be to keep the distance between the sectors small in order to keep the high magnetic field in sector areas and to expand the valley gap in some extent to create a better environment for the RF acceleration electrodes and at the same time get a better pumping conductance.
However, as already noted above, if the valley gap gets too large, the magnetic field strength in the valley gets too small relative to the sector field strength and the axial beam focusing as expressed by v
z
(number of axial ion beam oscillations per orbit revolution) will increase and eventually get into the v
z
=½ resonance which prohibits stable beam acceleration.
Some modern cyclotrons (<20 MeV proton energy) are based on the so called “deep valley” design, where the pole consists of large (thick) sector plates fixed directly onto the magnet yoke, yielding very large valley gaps suitable for the RF electrodes, and in this type of cyclotrons the value of v
z
stays well above the resonance value v
z
=½. Such cyclotrons will have a lower magnetic average field depending on the large valley gaps resulting in a larger pole radius for any given ion energy and, hence, such cyclotrons will be physically larger than a design based on a v
z
value below the v
z
=½ resonance. More extensive information on this is for example to be found in “Principles of cyclic particle accelerators”, by John J. Livingood (D. Van Nostrand Company, Inc., Princeton, N.J., USA).
Consequently, there are two alternatives available in designing a compact cyclotron magnet, namely to either choose a value of v
z
well below 0.5 or well above 0.5 to stay away from the mentioned critical v
z
=½ resonance.
The first choice results in a compact magnet but a design with too small valley gaps to satisfy the demands of a low power RF system and a satisfactory vacuum conductance while the other choice results in too large a magnet in order to fulfil the size requirements. The best average design option for a compact cyclotron magnet seems to be obsolete due to the restrictions related to axial focusing.
Therefore there is a demand of a method for cyclotron design for optimising the size of a cyclotron device applicable for a PET Isotope Production facility which takes into account the opposing parameters to allow a very compact device suitable, for instance, for installation at a local hospital where limited space is the normal case. The compactness of the cyclotron itself will also then promote small overall size of the system including the integrated radiation shield, which could be the golden standard for such equipment in the future. There is also a demand for a system taking advantage of such a method.
SHORT DESCRIPTION OF THE INVENTION
A method is disclosed for minimising the size of the magnet system and especially the diameter of the magnet poles of a cyclotron system for production of radioactive tracers. The method and a cyclotron according to the method make use of an operation mode having v
z
well below the critical resonance value of v
z
=½. Firstly, the sector gap is fixed at a small value (typically 15-30 mm) giving relatively few ampere-turns. Secondly, the valley pole gap is fixed at a value large enough to give good vacuum pumping conductance and to house a narrow spaced RF electrode system with acceptable capacitance and power consumption. For medium field strengths the value of v
z
will now be lower than v
z
=½ but still too close. The method now involves the step of raising the ampere-turns/coil current such that the sector field becomes greater than the saturation value for soft steel, which is ap

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