The CIME cyclotron

  CIME (Cyclotron pour Ions de Moyenne Energie) is a room temperature, compact, medium energy cyclotron devoted to the acceleration of radioactive ions for the SPIRAL project. The table 1 gives its main characteristics. 

Table 1: CIME overall characteristics

   The CIME cyclotron: A first stable beam was injected in CIME on December 1997 while the commissioning started really in April 1998.

The magnetic system

   CIME is a compact cyclotron with two circular poles 3.5m in diameter. Nevertheless, the magnet has a relatively original structure with 4 return yokes made of thick slabs. The magnetic circuit was built by CREUSOT LOIRE and SFAR and the coils were manufactured by SIGMAPHI (main coils) and SEF (correcting coils). The table 2 gives some main characteristics of the magnet.

                       Table 2: the CIME magnet

   The correcting coils are made of circular pancakes closed in a stainless steel, vacuum tight box. They are located between the pole face and the sectors. All the magnetic calculations were made in 3D with TOSCA. The magnetic measurements took place in the spring 1997. They were done using a rotating arm with 93 hall probes 20 mm spaced in the acceleration zone and 10mm in the extraction region. Measurements were firstly done without the extraction elements and later-on with the extraction channels installed. The results of the measurements were very near the calculations.

Injection

Injection of the beam is done axially, through the lower yoke and pole. Two different geometries are used depending on the harmonic number (table 3).

The geometry of the central region was studied with special care. The electromagnetic fields were calculated in the whole injection space and multi-particle programs allowed to simulate the behavior of bunches of particles. The injection line allows to perfectly match the injected beam to the cyclotron acceptance. An injection efficiency higher than 50% (with buncher) has been measured.

Extraction

The beam is extracted at a mean radius of 1500 mm. Extraction is complete in less than half a turn. It is performed by one electrostatic deflector (splitted into two parts) and two magneto-static channels. Position and electric field of the electrostatic channel can be adjusted while only the positions of the magnetic channels are adjustable. The table 4 gives some details on these elements.

The magneto static channels were designed in order to minimize the perturbation of the field in the acceleration region. Nevertheless, in addition, image channels were added at 180° to suppress any possible first harmonic.

Acceleration, the RF Cavities

The beam is accelerated by two l/4, cantilever dees with a maximum peak voltage of 80/100 kV. The main characteristics of the CIME RF system are given in table 4.

Table 5: RF characteristics

The dee tips were carefully studied for injection. The angle is 60° for the first turns and 40° further. Posts help to get a better definition of the electric field in the first gaps.

Vacuum system

Pumping in the vacuum chamber is mainly done by two cryo-pannels installed in one valley. A temperature of 20K is obtained in the inner panel by circulation of liquid hydrogen. The hydrogen is liquefied in heat exchangers feeded by 4 cryo-generators. The pumping speed is of the order of 30000 l/s and the residual pressure is less than 5.10 -8 mbar after half a day of pumping.

Diagnostics

Due to the very large intensity range of the beams which can be accelerated by CIME (10 3 to 10 12 pps), two type of diagnostics are used :

- Classical diagnostics including :
- A main radial, intercepting probe, with several fingers, covering the whole radial range.
- A set of central non intercepting phase probes.
- A retractable plate in front of each magnetic channel.
- « Nuclear » type diagnostics :
- A plastic scintillator, followed by a photo multiplier is supported by the radial probe. It can be retracted behind the probe when the intensity is large.
- A silicon detector, installed in an other radial probe at 45° from the main one.

These detectors are sensitive to very low intensities (down to a few pps). They give a precise measurement of the phase and also, possibly of the energy. On the other hand, the life-time of these diagnostics is limited, they are easily destroyed by an excess of beam and their useful range is limited as they do not deliver signal if the energy is too low.