|Scientific research and developments in “Accelerator” science and research establishment|
|Scientific research and developments in “Accelerator” science and research establishment|
1. Numerical simulations
The calculation of the electrodynamic characteristics of RF units and the simulation of particle dynamics plays the defining role for the electron linac development and construction. Analytical and numerical methods of studying particle dynamics are developed and extensively used in “Accelerator” S&R Establishment. They supplement each other permitting to carry out the research more effectively. For example, development of the method based on the model of coupling cavities , , , , , , , , allowed to develop methods of preliminary calculation and tuning of the inhomogeneous disk-loaded waveguides. The analytical calculations of particle dynamics that take into account non-synchronous spatial harmonics in the disk-loaded waveguides allowed to fully research a beam RF focusing as well as effects concerning radiation of electrons in such waveguides , , .
More detailed research of electron dynamics in the accelerating and bunching systems is carried out with both the world famous software (for example EGUN, SUPERFISH, PARMELA) and home-made software. Using this software the diode and RF guns were designed , , , , , , , , the effects of a back bombardment in thermionic RF guns were also researched as well as the features of beam formation in injectors and accelerators of S and K bands , , .
2. Electron sources and injectors
The electron source and injector system as a rule determine the beam characteristics at an accelerator output. Therefore, we put an emphasis on studying these devices.
Two types of injector systems - injectors based on RF guns and injectors with the use of diode electron sources and resonance bunching systems were designed.
For the last twenty years we carried out the researches of RF guns with different types of design . These devises can produce high quality beams; therefore they can be used as injector systems for accelerators. The thermionic RF guns with various types of resonance systems are theoretically studied and experimentally researched . The typical beam characteristics of these RF guns are as follows: electron energy of 0.7-0.9 MeV, pulse current of 1.5 A, phase bunch length is less than 50°, current pulse length of 0.7-1.5 µs, normalized emittance is not more than 12p mm×mrad. The beams with low pulse intensity but high repetition rate are needed to carry out the experiments on interaction of relativistic electrons with crystals. For this purpose a RF gun with thermionic metal cathode was designed . The high current RF guns with nanosecond pulse length based on photoemission photocathode have been designed and tested .
One of these RF guns was developed for use at the LIC accelerator (Fig.8). This gun can generate a beam in both the thermionic and photoemission modes while the cathode is irradiated with laser pulse.
The developed injectors using diode electron sources and resonance bunchers have different designs of the resonance system. The first injector of this type included a low-voltage (25 keV) diode gun and a buncher which consisted of two cavities: bunching and accelerating ones , . An electron beam with particle energy of 600 keV, pulse current of 1.5 A at pulse repetition rate up to 300 Hz was obtained at the injector output. The upgraded modifications of this injector are used in the technological KUT-30 linac .
The new type injector for use in different accelerators in order to produce high brightness accelerated beams consists of a chain of coupled cavities (Fig.9) with special amplitude and phase distribution along the axis . The main feature of the buncher is operation on the base of the evanescent mode.
The diode electron guns were studied and fabricated for injectors of different linacs. The hexaboride lanthanum (LaB6), impregnated and pressed BaNi cathodes are used as emitters. The developed electron sources are of two types: high voltage (80-120 kV) and low voltage (25 kV) [19, 20].
The operation principle of these guns is based on secondary-emission electron multiplication and their accumulation at the cathode surface under bombardment by electrons which are wrapped by magnetic field. The research showed that these guns generate tubular beams with high current density (up to 50 A/cm2), electron energies of 10-100 keV and pulse length up to 100 µs.
The experimental accelerating facility with this type of guns was constructed in “Accelerator” S&R Establishment of NSC KIPT to study the modification of surface properties in metals . These guns have a simple design, good stability, reliability and long life time, they do not require heating and do not lose the emission after atmosphere inlet during vacuum breakdown. The energy for guns can be supplied either from the impulse voltage generator assembled by the Marx scheme or from the impulse generator by the scheme with a full discharge of the pulse forming network.
3. RF accelerating structures
For a long time homogeneous accelerating structures based on disk-loaded waveguides which had been developed in the early 60s were used in linear electron accelerators of NSC KIPT (Table 3, Type I).
At the end of 80s of the last century methods of fabrication and tuning of quasi-constant inhomogeneous accelerating structures were developed in “Accelerator” S&R Establishment. These structures consist of a number of homogeneous subsections connected by transient cavities. They have p/2 operating mode and were developed to increase the energy gain and pulse beam current at the LEA-2000 linac , , , (Table 3, Type II).
A number of these sections were fabricated and used during modernization of the LEA-40 accelerator with particle energies up to 100 MeV and during construction of a new injector accelerator in the “Nestor” storage ring. The accelerating section with a considerably (for that time) high accelerating gradient up to 20 MeV/m (Table 3, Type III) was designed and installed in a compact synchrotron radiation source which included the LEA-60 injector accelerator .
The main problem for production of quasi-constant inhomogeneous structures is the tuning of transient cavities. The novel mathematics model of coupled cavities and disc-loaded waveguides [1-7] allowed to create the base for development of the inhomogeneous waveguide tuning techniques . According to these techniques four inhomogeneous accelerating structures with variable geometry and 2p/3 operating mode were manufactured. Three of them have the quasi-constant law of coupling iris radius variation with a linear decrease of radii in transient cavities (Table 3, Type IV), while in the fourth one the coupling iris radius decreases linearly from the section entrance to exit (Table 3, Type V).
New modifications of accelerating sections with 4p/3 operating mode (Fig.11) were designed for the short-pulse high current accelerators , , . The acceleration in these structures is provided by the first spatial harmonic. Interaction of electrons with the field of fundamental harmonic (which is not synchronous in this case) results in radial focusing of a beam , , .
4. RF supply systems
The amplifying klystrons are used as RF supply sources in all electron linacs which have been developed in NSC KIPT. We have carried out research and developments of various modifications of RF supply systems for injector and acceleration units. As a result, powerful 10 cm band RF-stations for providing a pulse power not less than 10-12 MW with a pulse length of 4-5 µs and repetition rate of 150-300 Hz at the entrance of each accelerating section were developed , . The other types of RF-stations destined for supply of research accelerators with a high brightness beam were designed and manufactured.
The high-voltage pulse modulators intended for supply of klystrons (anode voltage up to 270 kV, pulse current up to 230 A, pulse length of 5 µs, repetition rate of 300-400 Hz) with pulse-forming efficiency ~85% and total efficiency ~75% were designed and optimized. They are successfully operated with industrial accelerators and display the necessary stability of the main characteristics.
It was shown that klystrons such as AURORA klystrons (pulse output power up to 20 MW, average output power of 2.6 kW, efficiency up to 30%), due to optimization of conditions of beam propagation and introduction of additional cooling systems, ensure the output average power which greatly exceeds a certified value. In particular, in this case the industrial serial klystron of the AURORA-type can operate at higher repetition rate and larger pulse length than it is acceptable by the manufacturer’s specifications and its average output power can be increased from 2.6 kW up to 24 kW.
Works on development of the technology for klystron rebuilt were completed. Several klystrons were rebuilted in accordance with the developed technology. The initial parameters of these rebuilt klystrons remained within operating limits (in some cases without sings of degradation after 14,000 hours of exploitation). The technology of necessary cathode production was developed within the framework of this programme.
Eight versions of the RF power supply systems were designed, constructed and installed. The results of high power supply stations operation showed that the total efficiency of modulators is ~70% and the total efficiency of RF-stations is ~22% at pulse output power of 12 MW, average output power up to 18 kW (400 Hz) and up to 13.5 kW (300 Hz).
5. Beam parameters measurement and control systems
Methods and instruments for measurement and control of beam parameters were designed. Some of them are used for research experiments and others are used for the metrological maintenance of the technological process.
The electron bunch length measurements are very important for research in the field of accelerator physics and accelerator applications. In this connection, the bunch length measurement method based on measuring the characteristics of bunch coherent diffraction radiation at its motion over a metallic grid was developed and tested . The new method based on temporal scanning of the optical radiation of an electron bunch was also proposed .
The technological measurement channels of accelerators are based on the transit sensors (Rogowski coils of different modifications , radiation-acoustic string , thin-wall ionization chambers ). Some of these methods were computer simulated using GEANT software.
The special system has been developed for linac control . It controls the current and beam location , , , , electron energy , protects the accelerating and scanning systems from the damage caused by the beam, blocks the modulator and klystron in case of intolerable operation modes. This system controls the phase and power of RF signals which are supplied to injector elements, and regulates the current in elements of the magneto-optical system. In addition, the radiation absorbed dose is monitored.
The accelerator control unit consists of the PC with CAMAC crate (or measuring channels in PC standards), synchronization system, microprocessor which provides the klystron operation, temperature monitoring system, control system of magnetic elements and target devices.
6. Projects of new accelerators
The results of the research and developments are used for construction of new accelerators. Each new accelerator is equipped with new devices or systems which undergo testing at the commissioning stage, and their defects are revealed while in service and then corrected. Several accelerators the construction of which will promote the development of both the fundamental and applied physics in NSC KIPT are now designed. One of them is the superconducting SALO accelerator.
In 2003 the work on development of the Accelerating Complex SALO Project (Fig.12) was begun in NSC KIPT in cooperation with the Eindhoven University of Technology (Kingdom of the Netherlands). This project is based on the layout of the recirculator with three-time passing of a beam through the superconducting accelerating structure TESLA , , . It is possible to receive quasicontinuous beams of non-polarized and polarized electrons with energy up to 730 MeV and current up to 100 µA at the beam extraction channels to the experimental chambers . For this purpose it is supposed to use the RF gun with a superconducting accelerating structure and polarized electron source on the basis of gallium arsenide. The operating mode of the accelerator with energy up to 130 MeV and current of 1 mA is provided for a number of applied research , . Draft designs of all dipole and quadrupole magnets and draft design of the recirculator magnetic system are developed. The detailed design is developed and the prototype of a dipole magnet for the recirculator injection system is fabricated. The first ring of the recirculator can be completely started up with the use of dipole and quadrupole magnets delivered by the Eindhoven University of Technology.
In 2009 the project on construction of the powerful neutron generator was begun. The generator is based on the direct action accelerator; the transformer with isolated cores is used as a high voltage source. The accelerator has been designed and constructed in 2011. This accelerator is designed for obtaining the proton and deuteron beams with energy up to 2.5 MeV and current up to 10-20 mA. It is planned to use the generator for production of short-living isotopes and for neutron and neutron capture cancer therapy in the future. The project has been supported by the US Department of Energy.
7. Research in the field of nuclear physics
More than half of isotopes which are applied in medicine and activation analysis are isomers. Among nuclear constructional materials there are many elements the nuclei of which have isomeric levels. Therefore, research of their properties is not only of fundamental but also of applied value. Research on studying the population and depopulation of nuclei isomeric states in reactions with photons and electrons is carried out in “Accelerator” S&R Establishment. Studying the processes of interaction of photons and electrons with nuclei at low excitation energies is the urgent question in connection with the problem of isomer energy liberation and acquisition of new spectroscopic data. During the experiments on studying the laws of isomer excitation and de-excitation a number of applied problems are solved in addition to fundamental ones.
Recently the considerable attention is paid to creation of subcritical assemblies and neutron sources in which materials of middle-heavy nuclei such as lead and bismuth are used as neutron-making targets. Therefore, experiments on studying the interaction of braking γ-quanta and electrons with energy up to 100 MeV with these nuclei are carried out in “Accelerator” S&R Establishment. In particular, mass distributions of the photofission fragments and multinucleon reactions are studied.
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