2
INTRODUCTION................................................................................ 3
Cyclotrons..................................................................................3
Ion Sources................................................................................ 5
Neutral Beam Injection......................................................................6
MEDIAN PLANE ION INJECTION..................................................................7
Heucal axial injection......................................................................8
Axial Injection systems.....................................................................8
Fundamentals of beam transport..............................................................9
The U-400M Cyclotron.......................................................................14-
Motivation of This study...................................................................17
Outline of Thesis..........................................................................17
CHAPTER 1.....................................................................................19
BEAM TRANSMISSION LINE AND THE OPTICAL ELEMENTS................................................19
The main concept............................................................................19
The beam line.............................................................................. 20
Coil Design............................................................................... 33
Solenoid lens...............................................................................35
Charge Analysis of the BEAM and the bending magnet..........................................39
Solenoids....................................................................................4“
The Fringing magnetic field.................................................................50
Steering magnet.............................................................................52
Buncher......................................................................................56
CHAPTER 2......................................................................................65
AN INVESTIGATION OF THE PRESSURE DISTRIBUTION AND THE TRANSMISSION FACTOR DUE TO THE CHARGE EXCHANGE CROSS SECTION IN THE AXIAL INJECTION SYSTEM OF
THE U-400M CYCLOTRON.........................................................................65
Introduction...............................................................................65
Pressure distribution......................................................................66
Program versions...........................................................................78
Vacuum system..............................................................................80
Transmission factor....................................................................... 85
Results....................................................................................90
CHAPTER 3.................................................................................. 92
INFLECTOR AND CENTRAL REGION...................................................................92
Inflector Types.............................................................................92
The Electrostatic Mirror..................................................................93
Spiral Injlector......................................................................... 93
The Hyperboloid Injlector.................................................................93
The Parabolic Injlector...................................................................94
The inflector of the .axial injection system................................................94
The central region..........................................................................97
ACKNOWLEDGEMENTS.............................................................................105
REFERENCES
106
2
INTRODUCTION...................................................................................3
Cyclotrons...................................................................................3
Ion Sources..................................................................................3
Neutral Beam Injection.......................................................................6
Median plane ion injection...................................................................7
Helical axial injection......................................................................8
Axial Injection systems......................................................................8
Fundamentals of beam transport............................................................. 9
The U-400M Cyclotron.........................................................................U
Motivation of This study....................................................................11
Outline of Thesis...........................................................................I"
CHAPTER I.................................................................................... 19
BEAM TRANSMISSION LINE AND THE OPTICAL ELEMENTS................................................19
The main concept............................................................................19
The be.am line..............................................................................20
Coil Design.................................................................................33
Solenoid lens...............................................................................35
CHARGE ANALYSIS OF THE BEAM AND THE BENDING MAGNET..........................................39
Solenoids...................................................................................47
The Fringing magnetic field.................................................................50
Steering magnet.............................................................................52
Buncher.....................................................................................56
CHAPTER 2......................................................................................65
AN INVESTIGATION OF THE PRESSURE DISTRIBUTION AND THE TRANSMISSION FACTOR DUE TO THE CHARGE EXCHANGE CROSS SECTION IN THE AXL\L LNJF.CTION SYSTEM OF THE U-400M CYCLOTRON ......................................................................65
Introduction................................................................................65
Pressure distribution.......................................................................66
Program versions............................................................................78
Vacuum system...............................................................................80
Transmission factor.........................................................................85
Results.....................................................................................90
CHAPTER 3.................................................................................... 92
LNFLECTOR AND CENTRAL REGION...................................................................92
Inflf.ctor Typf.s...........................................................................92
The Electrostatic Mirror..................................................................93
Spiral hiflector..........................................................................93
The Hyperboloid In/lector.................................................................93
The Parabolic Inflector...................................................................94
The inflector of the axial injection system.................................................94
The central region..........................................................................97
ACKNOWLEDGEMENTS.............................................................................105
REFERENCES
106
3
INTRODUCTION
Cyclotrons
The development of nuclear physics lead to the construction of many huge machines that enabled us to achieve more developed and complicated investigation in the field. The types of cyclic charged particle accelerators played a significant role in the development of the experimental set-up possibilities in the nuclear research. The cyclotron is one of these machines. Cyclotrons were subjected to many stages of improvement. The whole process of the cyclotron improvement can be summarised in five stages. The first stage began in 1929 when conceived the idea of non-relativistic isochronous cyclotron. The first cyclotron was built in 1931 at the university of California in Berkeley-USA by E.O. Lowrence and M.S. Livingstone. This kind of the cyclotron was capable of accelerating high current beam to non-relativistic energies (v/c ^ 0.1 or a kinetic energy' of about 12 MeV for protons).
The U-300 heavy ion accelerator is a cyclotron of a classical type. It is designed for acceleration of heave ions from B to Zn to the energies of about 5 up to 10 MeV/nucleon. The cyclotron was started in I960 at the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research (Dubna, USSR). The accelerator was designed and built at the enterprises of the USSR. The U-300 cyclotron is equipped with a powerful pulsed cyclotron source of an arc type with a heated cathode.
The second stage was required when, the quest for a solution to reach a higher energies was strongly stimulated. The development of the relativistic frequency modulated (FM) synchrocyclotron has began when two proposals was independently published by V. Veksler [I] arid E.M. McMillan [2]. These cyclotrons do have a radially decreasing magnetic field in order to enhance axial focusing, but in order to reach relativistic energies the frequency has to be lowered while the particles are accelerated. The 84-inch’" synchrocyclotron in
4
Berkeley was the first one to be built, again by E.O. Lawrence [3]. The highest beam energy from a frequency modulated cyclotron was achieved in Gatchina (St. Petersburg Russia). These machines can accelerate beam to relativistic energy of I GeV for protons; however they have a low duty cycle and the current produced are much smaller than those in an isochronous cyclotron.
The cyclotron went through a third stage of evolution when Thomas suggested that axial stability could be obtained if the magnetic field had an azimuthal variation but it was not taken up at that time [4]. It was not until the early 1950s that his work received experimental verification, hi 1957 the first AVF cyclotron in Delft (The Netherlands) produced a beam. With the discovery of the sector focusing principle, it became possible to construct isochronous cyclotron high duty cycle, high current cyclotron operating at relativistic energy’.
At the Laboratory of the Nuclear Reaction has become a traditional one and has received further development. In 1968 in Dubna, a new ion accelerator -isochronous cyclotron with the pole diameter of 2 m (U-200) was built. The U-200 cyclotron is the first isochronous cyclotron that accelerates heavy ions in the USSR. On the U-200 cyclotron a wide range of ions from helium up to .Argon can be accelerated. A high level of the average magnet field (20 kG) made it possible to accelerate ions on this cyclotron up to the energy' of 5-16 MeV/nucleon [5]
One of the greatest improvement in the operation of the AVF cyclotrons over the last decades has been the use of the external ion source, especially for the acceleration of heavy ions (ECR) [6,7], negative ions [8,9] and polarised ions, as shown in this chapter further.
The fourth stage in cyclotron design from sector-shaped pole shims to separated-sector magnets, as proposed by H.A. W'illax [10], was just a logical expansion of the AVF principle. The free space between separated sectors or magnets allows the installation of powerful an efficient high RF cavities or resonators instead of Dees squeezed in between the magnet poles. This gives
5
considerably higher acceleration voltage which results in good turn separation.
The beam can easily be extracted with little beam loss, which is a mandatory
condition for high beam intensity cyclotrons.
* *
With the development of superconductivity the question had to come up whether superconductivity would to make a contribution to cyclotrons and the answer was the fifth stage. The first was built by H.G. Blosser [11] at the National Superconducting Cyclotron Laboratory in East Lansing (USA) producing beam in 1982. It is a K>500 superconducting cyclotron.
Ion Sources.
A hot filament ion source was placed within the first cyclotron. Sources of polarised protons, deuterons and high charge state ions are too large to fit inside a cyclotron. In the unrestricted exterior space very large and powerful ion sources can be built; it is only necessary to inject the ion beams into the cyclotron in such a way that panicles will be accelerated just as through they come from an internal source. The ECR ion sources have become the most frequently used as external heavy ion source for cyclotrons. The Grenoble group developed the first high charge state ECR source in 1974. The main advantages of an external source are listed below:-
1. With an external source differential pumping may be used between the source and the median plane of the cyclotron. This improves the vacuum of the central region and the beam loss will be decreased consequently.
2. If an external source is used, a bunching system can be placed between the ion source and the central region to increase the intensity of the accelerated ions.
3. If a polarised beam have to be accelerated, a polarised source must be used. These sources are too large and must be shielded from stray magnetic field
5
then, die unique method is to locate the ion source away from the cyclotron magnet.
4. All ions with wrong charge to mass ratio are removed before acceleration so. the space charge effect will be less and the sputtering of the central region components.
The main disadvantages of an external source is that:-
1. It requires an injection system which cost additional money.
2. In an injection system there is excessive beam loss due to charge exchange with the residual gases.
Before beginning these studies, we shall briefly review some of the work being done on the cyclotron injection system.
Neutral Beam Injection
One of the idea in designing an injection system for use with an external
source is to inject neutral particles which may be ionised by means of stripping foil or an electric arc once they have reached the centre.
The first published proposal for neutral hydrogen injection was made by Keller at CERN [12]. Kellers group proposed to inject neutral beam from a polarised source in the cyclotron median plane. The atoms were ionised in the centre by electron bombardment. The group tested this method with a 4.5 MeV model cyclotron.
More recently, neutral thermal ion injection has been used at Saclay to accelerate the beam of 0.5 nA of polarised protons to an energy of 22 MeV. The current were at least a factor of ten below the currents which could be obtained using an external source combined with an external ionizer. A thermal ion neutral
7
beam injection system has also been used with the Lyon 28 MeV synchrocyclotron; however, the injected current is again quite low [13]. This method has the advantage that it does not ionise residual gas. Another advantage is that the beam can be focused after the ion source, before the neutralisation. A neutral beam injection scheme using 30 keV hydrogen atoms has also been investigated by Pils at Dubna [14].
Median plane ion injection
This type of injection has been usually used in the separated sector cyclotrons.
In median plane injection systems a beam of ions is injected into the cyclotron by travelling along the radial path of the beam from the outer edge of the cyclotron. Once the beam has reached the centre of the cyclotron, steering inflectors are used to centre the beam for injection.
One particularly simple form of radial injection is the trochoidal method used by Lebedev Institute in Moscow [15]. In this injection system the ions loop inward by travelling along a hill-valley interface, and focusing of the incoming beam is achieved by means of the hill-valley gradient. Using tins method approximately 20% of the injected beam has been accelerated in a small 300 keV cyclotron. An alternative median plane injection system has been used for injection of polarised protons at Sacaly [16,17]. Here the effect of the magnetic field were compensated by placing electrostatic deflectors along the path of the incoming ions. Electrostatic quadmpoles were installed along the injection path to provide focusing. Also, median plane injection was installed for the 580 MeV SIN cyclotron and for the 200 MeV Indiana University cyclotron, both of which have separated sector magnets. Both of these cyclotrons are operated in conjunction with small injector cyclotrons and the injection energies are 70 MeV and 15 MeV respectively.
8
Helical axial injection
A concept for injection of high energy beams into a very high field cyclotron
has been presented by Hudson [18]. In the helical injection system, the incoming beam is in a plane parallel to the median plane. Using the long solenoid coil, the beam is deflected into a helical path, transported to the median plane and inflected into an accelerated orbit as shown in Fig.l. The field along the axis is about twice the median plane field.
Fig. 1 The helical axial injection.
Axial Injection systems
In Axial injection systems the beam is injected through an axial hole in the
magnet yoke. Once the beam has reached the median plane, it is bent through 90 degree by means of an inflector. This method was first developed by Powel for use on the Birmingham ragial ridge cyclotron [19]. Since then many groups have worked on the problem of axial injection. The hardware for these injection systems differs in the type of focusing elements and inflector used.
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