In 1938, L.H. Thomas proposed to provide axial focus by using the magnetic field adjusted along the azimuth.The magnetic field initially proposed by Thomas has quadruple symmetry (N=4), which has been shelved for more than ten years and has not been put into practice, partly because the theory of particle dynamics in periodic fields was not yet mature at that time, and the shape of the magnetic pole that produced Thomas field was too complex to process.
Results According to the principle of automatic phase stabilization discovered in 1944, the synchronous cyclotron was built before the Thomas accelerator.
In the 1950sfocusingThe discovery of the principle, the progress of computer and particle orbital dynamics have made people have a new understanding and impetus to the implementation of Thomas accelerator.It is on this basis that people have successfully developed a sector focusing cyclotron that uses various planar sector blades on the magnetic pole to provide the focusing force, including the radial sector and curled spiral sector accelerators, as well as the split sector accelerator composed of several independent sectors.Their magnetic pole shape is simple, easy to process and their focusing performance is better than that of Thomas accelerator.
So in the mid-1960s, a worldwide upsurge was set off to build a fan-shaped focusing cyclotron.The sector focused accelerator not only opens up new fields in the middle energy region, but also completely replaces the classical cyclotron in the low energy region.
Of the seven large isochronous accelerators built in the mid-1980s, five were superconducting, with the largest energy constant K=1200 (Oak Ridge National Laboratory).It seems that the potential of superconducting cyclotron is great in the future development.
In the past decade, the development of superconducting isochronous cyclotron has attracted more and more attention.So far, the built superconducting cyclotron includes the isochronous accelerator of Canada's Chalk River Nuclear Research Institute, the cyclotron of Milan University in Italy, and the two-stage superconducting cyclotron system of Michigan State University (MSU) in the United States.
principle
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The main obstacle to limit the increase of cyclotron energy is the sliding phase of ions.In order to break this barrier, people have considered a variety of ways to eliminate slip phase. One of the more direct methods is to use high-frequency electrodes with curved boundaries.It can shorten and adjust the distance between adjacent secondary accelerations of ions, offset the sliding phase caused by the relativistic effect or magnetic field drop, and thus may be used to increase the energy of ions.However, this approach has considerable limitations.As the ion energy increases, the curvature of the electrode boundary will continue to increase. As a result, the acceleration component of the electric field in the gap, that is, the azimuthal electric field, rapidly decays, while the radial electric field that cannot be used to accelerate the ion becomes larger and larger.Finally, the electric field turns to radial direction, and the acceleration of ions stops completely.It can be seen that such a method cannot accelerate ions to higher energy.[1]
So far, the successful way to overcome slip phase is to accelerate ions in the so-called isochronous magnetic field.The strength of this magnetic field increases synchronously with the energy of the ions along the radius direction, so that the rotation period of the ions remains constant throughout the acceleration process and does not change with the energy.
structure
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The structure of isochronous cyclotron is very similar to that of ordinary cyclotron.The main difference is that, in order to form an isochronous magnetic field that is adjusted along the azimuth, there are special shaped sizing blocks and coils on the surface of the magnetic pole to pad the magnetic field. The annular sizing blocks and coils can be used to pad the distribution of the average magnetic field along the radius;The sector magnet and coil can be used to supplement the adjustment of the magnetic field along the azimuth.
Some isochronous cyclotron magnets are not whole, but divided into thousands of segments, such as four, six or eight segments, which are determined by the number of periods of magnetic field modulation along the orbit.D-shaped electrodes or particles can be installed between magnets.
Some accelerators, especially those that accelerate negative ions, polarized ions and heavy ions, often place ion sources outside the accelerator. Ions are formed in the ion source and incident into the cyclotron through an appropriate ion optical system.There are two methods of incidence: one is to pierce a hole in the center of the magnetic pole along the axis direction, along which particles are injected into the vacuum chamber. Near the central plane of the vacuum chamber, there is a pair of deflection electrode plates. The plane of the electrode plate forms an angle of 45 ° with the central plane of the accelerator
The particles first pass through the grid grid electrode plate, which is grounded.The other electrode plate is connected to the high-voltage power supply, and its polarity is the same as the charge of the incident particles. A deflection electric field is formed between it and the grid electrode. The particles pass through the decayed grid and enter the deflection electric field. Under the effect of the electric field, they deflect 90 °, and then pass through the grid in the direction parallel to the central plane, and enter the D-shaped electrode for acceleration.
purpose
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Isochronous cyclotron is a medium energy accelerator with high current intensity, which can produce 100 or hundreds of microamps of particle flow.It plays an important role in the research of intermediate energy nuclear physics.
It can produce strong meson flow, also known as meson factory.In addition to meson physics and nuclear physics, it can also be used for research on radiochemistry, radiobiology, solid physics, etc. It can also be used for research on the treatment of cancer with wild mesons. In addition, strong proton flow and uranium target interaction can produce strong fast neutron flow, which can be used for research work such as neutron physics and material tests required for atomic energy.
The compact structure cyclotron of small size was initially launched by the American Cyclotron Company for radioisotope production or neutron therapy. It can produce 26 MeV protons, 15 MeV deuterium, 38 MeV He-3 ions and 30 MeV alpha particles. The internal beam intensity is 90-300 μ A, the external beam intensity is 40-100 μ A, and the ion energy divergence is 1%,The emittance is 50mm mrad, the accelerator is 2.13m high, and the floor area is only 3.3mX2.41m.
In recent years, Belgium IBA Company has launched a new "Cyclone30" accelerator, which can produce 30MeV protons, and its energy can continuously change in the range of 15-30MeV. The external beam strength can reach 350-500 μ A, which is suitable for productionelevenC,thirteenN，fifteenO，eighteenF，sixty-sevenGa，one hundred and elevenIn,one hundred and twenty-threeI andtwo hundred and oneTI and other isotopes used in nuclear medicine, one of its distinctive features is that it can simultaneously draw out two beams for the use of two different radioisotopes produced at the same time.The emittance of the external beam is 5-10 π mm-m rad, and the beam quality is obviously better than that of the general cyclotron.
The cyclotron accelerator adopts the technology of negative ion acceleration. At the end of acceleration, the carbon foil is used to peel off the target to convert H-ion into proton, and then the hydrogen ion is extracted outside the extractor.This is the most basic feature of this type of accelerator, which brings a series of advantages.For example, by adjusting the radial position of the stripping target, the energy of the ion beam can be continuously adjusted within 15-30 MeV without changing the magnetic induction intensity of the main magnetic field and the frequency of the accelerating electric field.Another example is that two carbon foils partially blocking the beam can lead out two beams in two directions at the same time.In addition, since the extraction efficiency of negative ions is close to 100%, even if the extracted beam is up to 500 μ A, the residual radioactivity of the accelerator is within the allowable range.
The negative ions accelerated by "Swirl 30" are generated by a high-power arc source outside the accelerator, and the H - yield can reach 2mA.The negative ions are guided by an axial injection system and injected into the accelerator through the yoke, with an injection efficiency of 35%.The accelerator magnet has four sectors with an opening angle of 54 º - 58 º (increasing with the radius). Because the acceleration electrode is placed in the valley, the magnet has small peak field gap, strong vertical focusing and good optical performance of the extracted beam.The magnetic induction intensity in the peak field area is 1.7T, and 0.12T in the valley area.The axial free vibration frequency z ν=0.54-0.63, the whole magnetic weight is 49 t, and the excitation power is 7.2 kW.
The opening angle of the two accelerating electrodes is 30 º, which are placed in the valley area of the magnet, supported by the resonance line vertically inserted into the valley area from top to bottom. This structure has a small capacitance to ground, and only 5.5 kW power is needed to generate 50 kV voltage during no-load.The frequency of the accelerating electric field is 65.5 MHz, working on the fourth harmonic of the H-ion cyclotron frequency.The drive amplifier power of the high-frequency system is 2 kW, the power of the final power amplifier is 26 kW, and the beam acceleration power available is 15 kW.
2. Lanzhou Heavy Ion Acceleration System
This is the largest isochronous cyclotron system in China, which includes a large separation fan accelerator with an energy constant of K=450 and a sector focused isochronous cyclotron with an energy constant of K=69 as an injector.This system can accelerate the ion from carbon to xenon on the periodic table to 100 MeV (light ion) or 5 MeV/A (heavy ion) per nucleon, and the beam intensity is 10twelve~10fourteenParticles/s, beam energy divergence 5 x 10-3, emittance 10 π cm · mrad.
System diagram
The magnet system of the main accelerator consists of four straight sided sector magnets with an opening angle of 52o.Each piece weighs 500 t and consists of 12 low-carbon steel forgings.The air gap of each magnet is 10 cm, and the maximum magnetic induction strength in the gap can reach 1.6 T.The average radius of the ion implantation magnetic field is ri=1m, and the extraction radius is re=3.21m.The highest average magnetic field B=0.99 T.36 pairs of compensation coils are installed on the surface of the upper and lower magnetic poles, including 25 pairs of compensation coils for isochronous fields, 5 pairs of local defect compensation coils, and 6 pairs of compensation coils for the disturbance of the injection element to the main magnetic field.After these coils are padded, the actual field distribution is equal to the time of various accelerating ions
The deviation of the property field is ≤ 1 × 10-3。Excitation current of main magnetic field of magnet 4000 A, current stability 5x 10-6, total power 550 kW.Excitation current of compensation adjusting coil 100-300 A, stability 1 x 10-4, total power 110kW.
The high frequency system of the main accelerator consists of two swallow resonators, each of which is composed of an accelerating electrode with an opening angle of 30 º, two inclined inner rods, a corrugated tuning plate and an outer casing.The working frequency range of the resonant cavity is 6-15MHz, the corresponding Q value is 6000-10000, and the feeding power of each cavity is 120 kW. At 12.8 MHz, the voltage amplitude at the small radius of the accelerating electrode is Va=220 kV, and the voltage amplitude at the large radius is Va=245 kV.It can be seen that the acceleration voltage increases with the radius.The system is equipped with automatic frequency tuning, amplitude stabilization and phase locking devices.The tuning accuracy of the cavity during operation Δ f/f=± 5x10-6, amplitude stability Δ V/V=5x10-4, phase stability Δφ ≤ 1 º.
The vacuum chamber of the separation fan accelerator is made of 316L stainless steel with μ<1.01.This is an overall structure formed by welding octagonal stiffened folded plates, with the maximum transverse width of~10 m, the maximum vertical height of~4.5 m, and the volume of 100 mthree, net weight 65 t, working vacuum of vacuum chamber 1.3 x 10-5Pa(1 x 10-7mmHg)，A set of effective pumping speed of 140 m is configured for this purposethree/S extraction system.It includes 8 RKP-800 cryogenic pumps as main pumping pumps and 4 TPH-500 turbomolecular pumps as auxiliary pumps.
The injector system uses two kinds of ion sources, one is the ordinary Penning arc source, which is used to generate the ions of various elements from carbon to xenon on the periodic table, and the other is the microwave electron cyclotron resonance source (ECR source), which can generate various highly charged ions, so it can expand the ion range of the injector from xenon to tantalum.The injector itself is a fan focusing accelerator reconstructed from a 1.5m classic cyclotron. The ions accelerated by it enter the injection system of the main accelerator through a 65m long transport line. The latter includes two bent magnets, two magnetic channels and an electrostatic deflection electrode. The optical performance of the inlet end of the injection system matches the beam from the transport line,The exit end matches the initial acceleration orbit on the injection radius.
Layout plan
The extraction system of the main accelerator is also composed of an electrostatic deflection electrode, two magnetic channels and two bending magnets.The inlet end of the system is matched with the beam on the extraction radius, and the outlet end is matched with the beam on the rear transport line. In order to improve the extraction efficiency, the extraction area is equipped with a first harmonic coil to stimulate the precession of the track center and expand the beam coil spacing.
One of the characteristics of this accelerator combination system is that it is fully equipped with various beam probes to monitor the beam status at various stages.For example, there are four movable radial probes along the centerline of four sector magnets on the main accelerator, which can measure the size, radial orbit distribution and vertical distribution of the beam from the injection radius to the extraction radius, and there are 15 capacitive probes along the radial direction, which can monitor the central phase of each circle of beam,The probe placed in the extraction area can measure the beam orbit distribution more than ten circles before extraction.These probes enable operators to easily judge the beam shape of each part under the condition of beam carrying operation of the accelerator system, so as to optimize the operating parameters of various components, improve the efficiency of beam injection, acceleration and extraction, and improve the beam quality, which is very important for the commissioning and operation of the accelerator.
3. Superconducting isochronous cyclotron
So far, the built superconducting cyclotron includes the isochronous accelerator of Canada's Chalk River Institute for Nuclear Research, the cyclotron of Milan University in Italy, and the two-stage superconducting cyclotron system of Michigan State University (MSU) in the United States.The first two use a cascade electrostatic accelerator as an injector, while the latter is operated by a combination of two superconducting cyclotrons with energy constants K=500 and K=800.
Choke River Superconducting Cyclotron
The energy constant of the Choke River accelerator is K=520, which is used to accelerate the energy of ions of various elements on the periodic table from Li3+to U33+to 10 MeV per nucleon.The magnetic field of this accelerator is a four sector structure, which is excited by superconducting coils made of Nb Ti and Cu alloys. At a low temperature of 4.5K, the highest magnetic induction intensity can reach about 5.0T.The entire excitation winding is contained in a large yoke with a diameter of about 3m and a height of 3m. There are some holes around the yoke, at the top and bottom for injection and extraction of ion beams or insertion of magnetic field compensation rods and detection targets.The cooling of the superconducting coil is completed by a liquid helium machine with a cooling power of 100 W, and the cooling time from room temperature to 4.5K is about 150 h. In order to make full contact between the liquid helium and the surface of the superconductor, each layer of the coil is provided with slots along the radial direction to guide the liquid helium through the conductor. Except for the superconducting coil, other parts of the accelerator are still at room temperature during operation.
The ions pre accelerated by the BMV cascade electrostatic accelerator enter the superconducting magnetic field along the central symmetry plane through the injection channel on the yoke. The incident ion beam enters the initial acceleration orbit after electron stripping on the injection radius. Four pairs of 1/4 wavelength acceleration structures are set along the orbit. The total acceleration voltage of each cycle is 0.6-0.8 MV, and the frequency range of the high-frequency acceleration electric field is 33-62 MHz,The particles accelerated to the final energy are led out of the system extractor by deflection set at radius r=0.65m.
The magnet of this accelerator is equipped with four curled fan-shaped iron blocks to provide axial focus.Under the high magnetic field of 5.0T, the sector magnetic poles are fully saturated, and the degree of modulation they produce is limited. Under such conditions, the maximum energy that the ions can reach may be limited due to insufficient axial focusing force.[2]
