Laboratori Nazionali del Sud

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Accelerators

Superconducting Cyclotron

The Superconducting Cyclotron (CS) K800 is a cyclical compact three-sector accelerator capable of accelerating ion beams from protons to uranium at energy levels of up to 80 MeV/A. The pole has a 90-centimetre radius and the magnetic field inside can reach a value of 4.8T. In order to achieve such intense magnetic field values, the superconducting cyclotron is equipped with two series of superconductive bobbins at nb-Ti (α and β) immersed in a liquid helium bath (LHe) at a working temperature of 4.2K.

The positive ions produced in an ECR (Electron Cyclotron resonance) source are injected along the vertical injection line of the acceleration chamber and travel through spiral orbits with a revolution frequency that depends on the state charge, on the magnetic field and on their mass. Acceleration is achieved thanks to the presence of a high-frequency electrical field in the accelerating gaps between electrodes (called dees), which oscillate at a radio frequency between 15 and 48 MHz, equal to double the revolution frequency of the particles in as much as the CS works almost exclusively in so-called harmonic h=2, even if theoretically the harmonics allowed are h=1, 2, 3 ,4. The time sequence of the flow of particles is structured in bunches, distanced by a period given by the inverse of the radiofrequency.

When the beam energy reaches its peak, the particles find themselves in maximum radius orbit and are extracted through an electrostatic deflector and sent along the extraction line to then be transported into an experimental room. Following the bending limits Kb=800 and focusing limits Kf=200, the top energy values that can be obtained are 80MeV/A for light ions completely ionised and 20 MeV/A for heavier ions, like I’238U38+

   

The CS was designed by Francesco Resmini in the early 1980s at LASA in Milan, where the main macro components were built and where the first cooling of the cryostat at 4.2K was successfully carried out. At the start of the 1990s the CS was transferred to LNS, all its macro components were assembled and in 1994 it was finally accelerated and a particle beam of 58Ni at 30MeV/A was extracted for the first time.

Until 1999 the CS used the Tandem electrostatic accelerator as the radial beam injector. This was then replaced by the current system of axial injection based on the use of ECR source, which produces ions to be accelerated with a very high charge state. Once they are extracted from the source, the ions are transported along the so-called axial line up to the accelerating chamber of the CS. This configuration means the CS is totally independent from the Tandem, making the coexistence of particle beams produced with two accelerators possible.

One of the more critical aspects of the CS concerns the beam extraction from the acceleration chamber, which requires two electrostatic deflectors, seven magnetic channels and two bars of magnetic compensation. The position and the working parameters of all these elements depend on the ions (mass and charge state) and on their energy. The two deflectors have a 6 mm gap, inside which the maximum value of electrical field applied to extract the most energetic ions is 120 kV/cm.

More than 20 years of cyclotron activity shows an extremely versatility and reliability of this particles accelerator. A wide range of beams can be produced by the k-800 superconducting cyclotron. It allows, hundreds of experiments in a large bandwidth of different research and technology fields, with lot of various applied implications.

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