Periodic Reporting for period 1 - InnovaTron (Design study of an innovative high-intensity industrial cyclotron for production of Tc-99m and other frontier medical radioisotopes)
Período documentado: 2020-08-01 hasta 2022-07-31
Cyclotrons are widely used in modern medicine for cancer therapy and diagnosis. Current commercial cyclotrons (Energy = 15-70 MeV) achieve currents up to or just above 1 mA. Today, the requirement for high beam current is becoming more and more important. High proton current cyclotrons can be employed for production of new emerging radioisotopes with low production yield to be used for traditional PET/SPECT imaging and theragnosis. Another application is the production of Tc-99m, the most widely used radioisotope in nuclear medicine. Currently, it is distributed as Mo-99 => Tc-99m generators. Mo-99 is produced by neutron induced fission of highly enriched U-235 targets in a few ageing nuclear reactors worldwide. Direct production of Tc-99m by proton cyclotrons is the most promising route to prevent the global shortage of the radioisotope due to possible future shutdown of the few remaining reactors worldwide. However, a high-intensity cyclotron technology is required for a large-scale production of Tc-99m. Indeed, with the current technology, a very dense network of cyclotrons worldwide would be needed due to the small yield of the direct production method compared to the fission reactor route. The high-intensity self-extracting cyclotron is a promising tool for the above-mentioned purposes. The proof-of-principle of self-extraction was demonstrated by IBA in 2001 by extracting proton currents close to 2 mA from a prototype. In the self-extracting cyclotron, there is no dedicated extraction device as in the existing machines but instead a special shaping of the magnetic iron and the use of harmonic coils to create large turn-separation at extraction. The overall goal of InnovaTron was the improvement of the concept of self-extraction to achieve high extraction efficiency and beam current to be used for high-intensity industrial applications.
1) Optimization of the cyclotron magnet design
1a) Parametrized tools have been developed for automatic generation of FEM models of the magnet and its extraction magnetic elements.
1b) The magnet design has been optimized to ensure a perfect 2-fold rotational symmetry, an isochronous magnetic field during beam acceleration, a radial and axial (vertical) focusing of a beam during acceleration and the possibility to install all cyclotron subsystems.
1c) The extraction path in the magnet has been improved compared to the prototype: i) the groove acting like a sort of magnetic septum is now replaced by a ”plateau” to improve the extracted beam quality, ii) now the iso-gap contours follow equilibrium orbits.
2) Optimization of the central region design
2a) Parametrized tools have been developed for automatic generation of FEM models of the central region.
2b) The central region design has been optimized to ensure 2-fold rotational symmetry, good beam centering and vertical electric focusing, good RF phase acceptance and energy gain and beam collimation to remove unwanted particles.
3) Space charge simulations in the cyclotron central region
A quantitative self-consistent approach has been developed for simulation of a space charge dominated beam in the central region. This method consists of three steps:
- the central region model is solved by using the SCALA space charge solver of Opera3D to find the plasma meniscus and the beam properties on it.
- the central region model is solved again by using the TOSCA solver of Opera3D. In this model the meniscus surface is put at ground potential. This provides the 3D electric field map everywhere in the central region, including the source-puller gap.
- The beam extracted from the meniscus is simulated in the 3D field map using AOC, the IBA space-charge tracking code. This code has been extended to also simulate the bunch formation process in the first gap, including space charge.
4) Beam tracking simulations including space charge
Beam tracking in the simulated 3D electric field and magnetic field of the self-extracting cyclotron has been carried out for:
- Optimization of the magnet, extraction magnetic elements and central region design.
- Study of the dependence of the extraction efficiency and beam quality on the beam intensity.
- Study of the effect of the dee-voltage ripple on the extraction efficiency and beam energy spread.
5) Optimization of the extraction process
5a) A permanent magnet gradient corrector has been designed. It provides radial focusing to the extracted beam.
5b) Harmonic coils that are used to optimize the beam extraction have been included in the FEM model.
5c) Parametrized tools have been developed for automatic generation of FEM models of the harmonic coils.
5d) A dedicated program has been written to find the cyclotron settings that give the maximum extraction efficiency.
Data generated during the design study of the InnovaTron cyclotron has been shared with the worldwide specialized accelerator community by publications openly accessible on the Internet. The main simulation results have been presented at multiple accelerator conferences and the proceedings have been published on the public accessible JACOW website. They have also been presented at the annual congress of the Italian Physical Society, specialized workshops and seminars in high schools and universities.
1a) Parametrized tools have been developed for automatic generation of FEM models of the magnet and its extraction magnetic elements.
1b) The magnet design has been optimized to ensure a perfect 2-fold rotational symmetry, an isochronous magnetic field during beam acceleration, a radial and axial (vertical) focusing of a beam during acceleration and the possibility to install all cyclotron subsystems.
1c) The extraction path in the magnet has been improved compared to the prototype: i) the groove acting like a sort of magnetic septum is now replaced by a ”plateau” to improve the extracted beam quality, ii) now the iso-gap contours follow equilibrium orbits.
2) Optimization of the central region design
2a) Parametrized tools have been developed for automatic generation of FEM models of the central region.
2b) The central region design has been optimized to ensure 2-fold rotational symmetry, good beam centering and vertical electric focusing, good RF phase acceptance and energy gain and beam collimation to remove unwanted particles.
3) Space charge simulations in the cyclotron central region
A quantitative self-consistent approach has been developed for simulation of a space charge dominated beam in the central region. This method consists of three steps:
- the central region model is solved by using the SCALA space charge solver of Opera3D to find the plasma meniscus and the beam properties on it.
- the central region model is solved again by using the TOSCA solver of Opera3D. In this model the meniscus surface is put at ground potential. This provides the 3D electric field map everywhere in the central region, including the source-puller gap.
- The beam extracted from the meniscus is simulated in the 3D field map using AOC, the IBA space-charge tracking code. This code has been extended to also simulate the bunch formation process in the first gap, including space charge.
4) Beam tracking simulations including space charge
Beam tracking in the simulated 3D electric field and magnetic field of the self-extracting cyclotron has been carried out for:
- Optimization of the magnet, extraction magnetic elements and central region design.
- Study of the dependence of the extraction efficiency and beam quality on the beam intensity.
- Study of the effect of the dee-voltage ripple on the extraction efficiency and beam energy spread.
5) Optimization of the extraction process
5a) A permanent magnet gradient corrector has been designed. It provides radial focusing to the extracted beam.
5b) Harmonic coils that are used to optimize the beam extraction have been included in the FEM model.
5c) Parametrized tools have been developed for automatic generation of FEM models of the harmonic coils.
5d) A dedicated program has been written to find the cyclotron settings that give the maximum extraction efficiency.
Data generated during the design study of the InnovaTron cyclotron has been shared with the worldwide specialized accelerator community by publications openly accessible on the Internet. The main simulation results have been presented at multiple accelerator conferences and the proceedings have been published on the public accessible JACOW website. They have also been presented at the annual congress of the Italian Physical Society, specialized workshops and seminars in high schools and universities.
The action has allowed for threefold progress beyond the state of the art in the field. The cyclotron community can benefit from the knowledge created by this project and the development of design procedures and tools. First, the project enabled the improvement of the magnet design and the beam-optics of the prototype. It has been realized using high-level computer-aided design and beam physics studies. Second, the action pioneered in developing a self-consistent method for simulation of a space charge dominated beam in the central region. Central region design simulations for cyclotrons with an internal ion source are often complicated by the fact that the initial particle phase space distribution is not well known. Our effort consisted in developing tools and procedures that allowed us to better predict the beam injected in the cyclotron and the beam dynamics in the full cyclotron under space charge conditions. Third, an optimization program has been written for fast optimization of the extraction efficiency, but it can be used in a wide range of applications. This program uses standard optimization routines to optimize a task (project). The task is defined by a user-defined script which is executed by the program in an iterative process. It reads new values of independent variables as suggested by the program, executes the task, and writes its results (new values of the objectives) to a file. The program then resumes and compares the results of the script with the (user defined) objectives to calculate the fitting error and suggest new values for the variables. This process is repeated until the fitting error is smaller than a given tolerance.
The results of simulations have shown that it is possible to achieve an extraction efficiency up to 91% and emittances a factor 3 lower as compared to the original design. Further optimization of the magnet profile at the extraction may allow for the improvement of the extraction efficiency: the creation of a region close to extraction where νr < 1 and decreases gradually in few turns may be used to create a coherent beam oscillation, enabling the increase of the turn separation (precessional extraction).
The results of simulations have shown that it is possible to achieve an extraction efficiency up to 91% and emittances a factor 3 lower as compared to the original design. Further optimization of the magnet profile at the extraction may allow for the improvement of the extraction efficiency: the creation of a region close to extraction where νr < 1 and decreases gradually in few turns may be used to create a coherent beam oscillation, enabling the increase of the turn separation (precessional extraction).