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Technical status of the first industrial unit of the 10 MeV, 100 kW Rhodotron
Radiat. Phys. Chem. Vol. 46, No. 4 - 6 , pp. 4 7 3 - 4 7 6 , 1995 Copyright © 1995 Elsevier Science Ltd 0969-806X(95)00197-2 Printed in Great Britain. All rights reserved 0969-806X/95 $9.50 + 0.00
Pergamon
TECHNICAL STATUS OF THE FIRST INDUSTRIAL UNIT OF THE 10 MeV, 100 kW RHODOTRON D. Defrise, M. Abs, F. Genin and Y. Jongen Ion Beam Applications, Chemin du Cyclotron, Rue J.E. Lenoir, 6 -1348 Louvain-La-Neuve Belgium
ABSTRACT The Rhodotron is a new type of industrial electron accelerator developed by IBA, Belgium, on the basis of the pioneering work performed by a French team at CEA, Saclay. The operating principle of this new technology makes it possible to easily produce high energy and high power electron beams. The combination of high energy and high current allows high product throughputs and/or the irradiation of thick structures in the electron mode. An X-ray target is also developed in such a way that the Rhodotron can also be operated in the X-ray mode while maintaining high dose rates. The construction of the first industrial unit (10 MeV, 100 kW) began in January 1992 and the first beam was obtained on October 29, 1993. This machine has now been tested at its full specifications. Technical performance data are presented in this paper. KEYWORDS Electron Accelerator, Irradiation facility, X-ray target, Power utilization INTRODUCTION The idea of using several times an electrical field in order to accelerate particle beams is not new, since this concept has been the basis for the development of electron accelerators such as synchrotrons or microtrons. However, in these machines, beam current is limited due to charge space effects and these types of accelerators are actually not used for the industrial irradiation processes. A few years ago, Pottier (1989) proposed to utilize a coaxial line short-circuited at both ends to accelerate particle beams. In this kind of cavity, the electrical field is radial and maximum at the median plane whereas the magnetic field, which is azimuthal, is equal to zero at this position. This plane is therefore convenient to accelerate particle beams crossing diametrically the cavity as there is no interfering magnetic field capable of deflecting the particles. By using appropriate bending devices located outside the cavity, successive accelerations of the beams are performed, using the same electrical field. As the beam describes a rose-shaped path in its acceleration, the machine has been called "Rhodotron", a name originating from the Greek name of the rose. The theory of the Rhodotron technology has been thoroughly described in previous papers (Pottier, 1989; Ngyuen et al., 1990; Bassaler et al., 1992) and it has been proven that this ~Pc46-~/6{~)-G
473
474
D. Defrise et al.
technology would be convenient to accelerate 20-500 kW electron beams to energies in the 120 MeV range. STATUS OF DEVELOPMENT OF THE RHODOTRON TECHNOLOGY In order to demonstrate the feasibility of the Rhodotron accelerating concept, a first prototype of 3.5 MeV and 20 kW has been built by the inventor's team at CEA in France (NGuyen, 1990). In December 1991, IBA (Louvain-La-Neuve) signed an agreement with the CEA, which gave the company the exclusive right to develop and market the Rhodotron technology. In January 1992, the construction of the first industrial Rhodotron, called Rhodotron TT200 began at IBA. The specifications of this machine, 10 MeV and 100 kW have been chosen in order to specifically address the market of the EB sterilization of medical devices (in-house or contact sterilization). The first beam has been accelerated on October 1993 and since April 1994, the machine has been tested at its full specifications (Jongen et al., 1994b). The Rhodotron TT200 is now more than a concept, it is a real operating machine that is available for demonstration in the IBA facility. Very high power beams (70-100 kW) have been accelerated up to 10 Fig. 1. The Rhodotron TT200, 10 MeV, 100 kW. MeV for extended periods of time. These experiments have shown that there are very low beam losses in the machine (less than 10-3 from 1 MeV to 10 MeV). The figure 1 shows a picture of the Rhodotron TT200 as it stands now in the assembly hall in IBA. PRINCIPLE OF OPERATION The Rhodotron technology utilizes a new type of coaxial accelerating cavity whose dimensions have been optimized with regard to the specifications of a given accelerator. In the Rhodotron TT200, the accelerating cavity has a diameter of 2 meters. This dimension has been determined by the preliminary choice of the RF frequency at 107.5 MHz, where several power tetrodes of 200 kW are commercially available (Jongen et al., 1993, Jongen et al., 1994a). The electron gun, located at the outer wall of the cavity, is pulsed at the RF frequency. 10 mA of electrons are sent into the cavity where they undergo a first acceleration of 0.5 MeV towards the inner cylindrical conductor. They pass through opening in the center and when they emerge in the second part of the cavity, as the electrical field is reversed, they gain once more 0.5 MeV. In total, when the electrons complete an entire crossing of the cavity, they
9th International M e e t i n g on Radiation P r o c e s s i n g
475
gain 1 MeV. Around the cavity, window-frame magnets are bending the electrons and sending them back into the cavity for further acceleration steps. 10 successive crossing and 9 magnets are required to obtain 10 MeV beams with the Rhodotron TT200. At the exit of the accelerator, the beam is directed onto the products to be irradiated by means of a scanning magnet and scanning horn ADVANTAGES OF THE RHODOTRON TECHNOLOGY High Power Rating The Rhodotron TT200 is the first industrial EB accelerator able to produce very high power beams (100 kW) in the energy range > 5 MeV (Jongen et al., 1994b). An important feature of the Rhodotron TT200 is the fact that it can also produce 100 kW beam at 5 MeV. This allows to obtain X-ray beams which are still sufficiently powerful to be used for industrial processes. IBA is currently running a research program aiming at developing an optimized X-ray target in terms of product throughputs and product dosimetry. In these conditions, the Rhodotron TT200 (5 MeV, 100 kW) is equivalent to a cobalt-facility of 1 MCi. The Rhodotron technology opens therefore new areas in the field of radiation processing both in the electron mode and in the x-ray mode. RF Soecifications In the Rhodotron, unlike linacs, the energy gain per meter is relatively small (0.5 MeV/m) Therefore a moderate power output of 200 kW is required, allowing to operate the Rhodotron in a continuous mode (duty factor of 100 %). This is the best solution for industrial processes as this technology does not imply high peak current and therefore gains in reliability. Another advantage of this low power rating is the utilization of a tetrode as final stage of the amplifier. The tetrode technology has been extensively used in broadcasting transmitters and these devices are recognized as extremely reliable. Moreover, the tetrode used in the Rhodotron TT200 has a power utilization efficiency of 72 % and is relatively inexpensive as compared to the klystrons used in the linac technology. As a result of all these features combined with the efficient design of the whole RF system, very low power is required for the operation of the Rhodotron TT200. A total efficiency of 40% is attained at full beam power (see Table 1) Table 1. Power ~ q u i ~ m e n t o f t h e R h o d o t r o n TT200 Beam Power(kW) Power~quirement(kW) 50 <200 70 <225 100 <265 Reliability and Maintainability All sub-systems of the Rhodotron TT200 have been designed and manufactured in order to optimize the reliability of the machine and achieve a continuous operation of at least 6,000 hours per year. Standard and commercial assemblies have been used as much as possible to proceed with easy and cost-effective maintenance. For example, only a few minutes are required to change the cathode of the electron gun, and, as this operation does not involve any break of the vacuum of the Rhodotron system, the beam is available as soon as the new cathode is fitted.
476
D. Defrise e t al.
Beam Characteristics The energy spread of the 10 MeV beam is very low (around 300 keV). This is due to phase focusing in the successive magnets bends. Consequently, all optic problems linked to large energy spread beams disappear, which facilitate the design and development of the beam transport systems. Another feature of the CW beam operation is the open choice of the scanning frequency that can be optimized to the given irradiation process. Comnactness The Rhodotron TT200 is a compact machine as its overall diameter is 3 meters, with a height of 3 meters. Moreover, as the RF system is directly connected to the accelerating cavity, there is no need for RF guide. No pressure vessel of any kind is required. This is of great importance since the compactness of the Rhodotron allows to reduce the building costs that are not negligible for an irradiation facility. Control System As all machines developed by IBA, the Rhodotron is controlled automatically through a industrial Programmable Logic Controller (PLC) which satisfies the highly automation and reliability standard. The control system interface is user-oriented as it consists of a multiwindow application, compatible with Windows-3 utilities. The system control is menu-driven through self-explanatory, color graphics displays representing essential aspects of the Rhodotron operation. All graphics use color codes to allow easy visualisation of the system status. This control system may be adapted to irradiation processes by pre-setting, for example, all parameters required to perform the irradiation of a specific product. ACKNOWLEDGEMENTS Special thanks to all engineers and technicians both from the CEA and the IBA company who contributed to the successful development of the Rhodotron technology. REFERENCES Bassaler, J.M., Capdevila, J.M., Gal, O., Lainr, F., NGuyen, A. Nicola'i, J.P. and Umiastowski, K. (1992), Rhodotron: an accelerator for industrial irradiation, Nuclear Instruments and Methods in Physics Research, B68, 92-95. Jongen, Y., Abs, M., Genin, F., NGuyen, A., Capdevila, J.M., Defrise, D. (1993), The Rhodotron, a new 10 MeV, 100 kW, CW metric wave electron accelerator, Nuclear Instruments and Methods in Physics Research, B79, 865-870. Jongen, Y., Abs, M., Capdevila, J.M., Defrise, D., Genin, F., NGuyen, A. (1994a), The Rhodotron, a new high-energy, high-power CW electron accelerator, Nuclear Instruments and Methods in Physics Research, B89, 60-64. Jongen, Y., Abs, M., Defrise, D., Genin, F., Capdevila, J.M., Gal, O., NGuyen, A. (1994b), First beam tests results of the 10 MeV, 100 kW Rhodotron, presented at EPAC 94, London. Nguyen, A., Umiastowski, K., Pottier, J., Capdevila, J.M., Lain6 F., Nicola'/, J.P. (1990), Rhodotron first operations, in Proc. European Particle Accelerators Conference, vol. 2, Nice, France, 1840-1841. Pottier, J. (1989), A new type of RF electron accelerator: the RHODOTRON, Nuclear Instruments and Methods in Physics Research, B 40/4 |, 943-945.
Pergamon
TECHNICAL STATUS OF THE FIRST INDUSTRIAL UNIT OF THE 10 MeV, 100 kW RHODOTRON D. Defrise, M. Abs, F. Genin and Y. Jongen Ion Beam Applications, Chemin du Cyclotron, Rue J.E. Lenoir, 6 -1348 Louvain-La-Neuve Belgium
ABSTRACT The Rhodotron is a new type of industrial electron accelerator developed by IBA, Belgium, on the basis of the pioneering work performed by a French team at CEA, Saclay. The operating principle of this new technology makes it possible to easily produce high energy and high power electron beams. The combination of high energy and high current allows high product throughputs and/or the irradiation of thick structures in the electron mode. An X-ray target is also developed in such a way that the Rhodotron can also be operated in the X-ray mode while maintaining high dose rates. The construction of the first industrial unit (10 MeV, 100 kW) began in January 1992 and the first beam was obtained on October 29, 1993. This machine has now been tested at its full specifications. Technical performance data are presented in this paper. KEYWORDS Electron Accelerator, Irradiation facility, X-ray target, Power utilization INTRODUCTION The idea of using several times an electrical field in order to accelerate particle beams is not new, since this concept has been the basis for the development of electron accelerators such as synchrotrons or microtrons. However, in these machines, beam current is limited due to charge space effects and these types of accelerators are actually not used for the industrial irradiation processes. A few years ago, Pottier (1989) proposed to utilize a coaxial line short-circuited at both ends to accelerate particle beams. In this kind of cavity, the electrical field is radial and maximum at the median plane whereas the magnetic field, which is azimuthal, is equal to zero at this position. This plane is therefore convenient to accelerate particle beams crossing diametrically the cavity as there is no interfering magnetic field capable of deflecting the particles. By using appropriate bending devices located outside the cavity, successive accelerations of the beams are performed, using the same electrical field. As the beam describes a rose-shaped path in its acceleration, the machine has been called "Rhodotron", a name originating from the Greek name of the rose. The theory of the Rhodotron technology has been thoroughly described in previous papers (Pottier, 1989; Ngyuen et al., 1990; Bassaler et al., 1992) and it has been proven that this ~Pc46-~/6{~)-G
473
474
D. Defrise et al.
technology would be convenient to accelerate 20-500 kW electron beams to energies in the 120 MeV range. STATUS OF DEVELOPMENT OF THE RHODOTRON TECHNOLOGY In order to demonstrate the feasibility of the Rhodotron accelerating concept, a first prototype of 3.5 MeV and 20 kW has been built by the inventor's team at CEA in France (NGuyen, 1990). In December 1991, IBA (Louvain-La-Neuve) signed an agreement with the CEA, which gave the company the exclusive right to develop and market the Rhodotron technology. In January 1992, the construction of the first industrial Rhodotron, called Rhodotron TT200 began at IBA. The specifications of this machine, 10 MeV and 100 kW have been chosen in order to specifically address the market of the EB sterilization of medical devices (in-house or contact sterilization). The first beam has been accelerated on October 1993 and since April 1994, the machine has been tested at its full specifications (Jongen et al., 1994b). The Rhodotron TT200 is now more than a concept, it is a real operating machine that is available for demonstration in the IBA facility. Very high power beams (70-100 kW) have been accelerated up to 10 Fig. 1. The Rhodotron TT200, 10 MeV, 100 kW. MeV for extended periods of time. These experiments have shown that there are very low beam losses in the machine (less than 10-3 from 1 MeV to 10 MeV). The figure 1 shows a picture of the Rhodotron TT200 as it stands now in the assembly hall in IBA. PRINCIPLE OF OPERATION The Rhodotron technology utilizes a new type of coaxial accelerating cavity whose dimensions have been optimized with regard to the specifications of a given accelerator. In the Rhodotron TT200, the accelerating cavity has a diameter of 2 meters. This dimension has been determined by the preliminary choice of the RF frequency at 107.5 MHz, where several power tetrodes of 200 kW are commercially available (Jongen et al., 1993, Jongen et al., 1994a). The electron gun, located at the outer wall of the cavity, is pulsed at the RF frequency. 10 mA of electrons are sent into the cavity where they undergo a first acceleration of 0.5 MeV towards the inner cylindrical conductor. They pass through opening in the center and when they emerge in the second part of the cavity, as the electrical field is reversed, they gain once more 0.5 MeV. In total, when the electrons complete an entire crossing of the cavity, they
9th International M e e t i n g on Radiation P r o c e s s i n g
475
gain 1 MeV. Around the cavity, window-frame magnets are bending the electrons and sending them back into the cavity for further acceleration steps. 10 successive crossing and 9 magnets are required to obtain 10 MeV beams with the Rhodotron TT200. At the exit of the accelerator, the beam is directed onto the products to be irradiated by means of a scanning magnet and scanning horn ADVANTAGES OF THE RHODOTRON TECHNOLOGY High Power Rating The Rhodotron TT200 is the first industrial EB accelerator able to produce very high power beams (100 kW) in the energy range > 5 MeV (Jongen et al., 1994b). An important feature of the Rhodotron TT200 is the fact that it can also produce 100 kW beam at 5 MeV. This allows to obtain X-ray beams which are still sufficiently powerful to be used for industrial processes. IBA is currently running a research program aiming at developing an optimized X-ray target in terms of product throughputs and product dosimetry. In these conditions, the Rhodotron TT200 (5 MeV, 100 kW) is equivalent to a cobalt-facility of 1 MCi. The Rhodotron technology opens therefore new areas in the field of radiation processing both in the electron mode and in the x-ray mode. RF Soecifications In the Rhodotron, unlike linacs, the energy gain per meter is relatively small (0.5 MeV/m) Therefore a moderate power output of 200 kW is required, allowing to operate the Rhodotron in a continuous mode (duty factor of 100 %). This is the best solution for industrial processes as this technology does not imply high peak current and therefore gains in reliability. Another advantage of this low power rating is the utilization of a tetrode as final stage of the amplifier. The tetrode technology has been extensively used in broadcasting transmitters and these devices are recognized as extremely reliable. Moreover, the tetrode used in the Rhodotron TT200 has a power utilization efficiency of 72 % and is relatively inexpensive as compared to the klystrons used in the linac technology. As a result of all these features combined with the efficient design of the whole RF system, very low power is required for the operation of the Rhodotron TT200. A total efficiency of 40% is attained at full beam power (see Table 1) Table 1. Power ~ q u i ~ m e n t o f t h e R h o d o t r o n TT200 Beam Power(kW) Power~quirement(kW) 50 <200 70 <225 100 <265 Reliability and Maintainability All sub-systems of the Rhodotron TT200 have been designed and manufactured in order to optimize the reliability of the machine and achieve a continuous operation of at least 6,000 hours per year. Standard and commercial assemblies have been used as much as possible to proceed with easy and cost-effective maintenance. For example, only a few minutes are required to change the cathode of the electron gun, and, as this operation does not involve any break of the vacuum of the Rhodotron system, the beam is available as soon as the new cathode is fitted.
476
D. Defrise e t al.
Beam Characteristics The energy spread of the 10 MeV beam is very low (around 300 keV). This is due to phase focusing in the successive magnets bends. Consequently, all optic problems linked to large energy spread beams disappear, which facilitate the design and development of the beam transport systems. Another feature of the CW beam operation is the open choice of the scanning frequency that can be optimized to the given irradiation process. Comnactness The Rhodotron TT200 is a compact machine as its overall diameter is 3 meters, with a height of 3 meters. Moreover, as the RF system is directly connected to the accelerating cavity, there is no need for RF guide. No pressure vessel of any kind is required. This is of great importance since the compactness of the Rhodotron allows to reduce the building costs that are not negligible for an irradiation facility. Control System As all machines developed by IBA, the Rhodotron is controlled automatically through a industrial Programmable Logic Controller (PLC) which satisfies the highly automation and reliability standard. The control system interface is user-oriented as it consists of a multiwindow application, compatible with Windows-3 utilities. The system control is menu-driven through self-explanatory, color graphics displays representing essential aspects of the Rhodotron operation. All graphics use color codes to allow easy visualisation of the system status. This control system may be adapted to irradiation processes by pre-setting, for example, all parameters required to perform the irradiation of a specific product. ACKNOWLEDGEMENTS Special thanks to all engineers and technicians both from the CEA and the IBA company who contributed to the successful development of the Rhodotron technology. REFERENCES Bassaler, J.M., Capdevila, J.M., Gal, O., Lainr, F., NGuyen, A. Nicola'i, J.P. and Umiastowski, K. (1992), Rhodotron: an accelerator for industrial irradiation, Nuclear Instruments and Methods in Physics Research, B68, 92-95. Jongen, Y., Abs, M., Genin, F., NGuyen, A., Capdevila, J.M., Defrise, D. (1993), The Rhodotron, a new 10 MeV, 100 kW, CW metric wave electron accelerator, Nuclear Instruments and Methods in Physics Research, B79, 865-870. Jongen, Y., Abs, M., Capdevila, J.M., Defrise, D., Genin, F., NGuyen, A. (1994a), The Rhodotron, a new high-energy, high-power CW electron accelerator, Nuclear Instruments and Methods in Physics Research, B89, 60-64. Jongen, Y., Abs, M., Defrise, D., Genin, F., Capdevila, J.M., Gal, O., NGuyen, A. (1994b), First beam tests results of the 10 MeV, 100 kW Rhodotron, presented at EPAC 94, London. Nguyen, A., Umiastowski, K., Pottier, J., Capdevila, J.M., Lain6 F., Nicola'/, J.P. (1990), Rhodotron first operations, in Proc. European Particle Accelerators Conference, vol. 2, Nice, France, 1840-1841. Pottier, J. (1989), A new type of RF electron accelerator: the RHODOTRON, Nuclear Instruments and Methods in Physics Research, B 40/4 |, 943-945.