VACUUM SYSTEM OF THE RCNP RING CYCLOTRON
Akira Shimizu and Iwao Miura
Research Center for Nuclear Physics, Osaka University
Ibaraki, Osaka 567,
Abstract
The object of the vacuum system of the RCNP ring cyclotron
is to evacuate the acceleration chamber to reach a pressure
region where the accelerated ions are transmitted without
significant beam loss due to charge state exchanges. The
design goal is to attain to 1 x l0- 7 Torr for 60 m 3 chamberwithin 10 hours after the pumping has been started from air.
vacuum Requirements
The ring cyclotron is a booster accelerator for protons
through heavy ions following an injector cyclotronl). The
acceleration chamber consists of 8 demountable sections; 4separate sector magnets, 2 RF cavities and 2 valley sections
where beam injection and extraction systems and beam diagnostics should be equipped (Fig. 1). The RF cavities should be
drawn out radially with ease for maintenances.
The attenuation of a beam dn in traveling a distance dl ina vacuum chamber having Ni atoms/cm 3 , where i denotes a specificspecies, is;dn = n Z N. Z a.
i1m(q,q±m)dl1

where q denotes the ion charge and m=l, 2, ·•·• • In theacceleration process, charge exchange cross section a(q,q±m) isa function of ion velocity and charge state, an order ofmagnitude of 10- 16 cm 2 roughly.
In our case, a residual pressure of around 2 xlo- 7 Torr isestimated to transmit 95% Ar ions for 1.5 km traveling length
in the ring cyclotron.
Surfaces Exposed to vacuum
These surfaces include;
Stainless steel walls of the chamber
Copper-clad steel of RF cavities
Copper plates of cavity electrodes
Mild steel of the poles of 4 sector magnets
Stainless steel and copper assemblies of the trim coils
Electric and magnetic elements for beam injection andextraction
Beam monitoring and diagnostic instrumentations
Elastmer seals for demountable sections
The diameter of the chamber is about 8 m, the height of RF
cavity is about 4 m and total volume to evacuate is around 60
m3 • The surface area exposed to vacuum amounts to about 800m 2
and the estimated degassing rate from well treated metal walls-205---------- - - - -after 10 hours of pumping is around 4 xl0- 3 Torr 1/sec andanother same amount of those from elastmer seals and miscellaneous components equipped under vacuum. Sub-vacuum system covering trim coil assemblies will be examined in case of apprehensions of noticeable degassing source from coil insulator
materials and feedthroughes.
Pumping System
To attain a design goal of 1 x1o- 7 Torr, the evacuation
system should have high pumping speeds and conductances at 10- 7
Torr region. Several choices of pumping system are available
based upon the present art of technology. Cryo-condensation
panel circulating liq. He mounted on the magnet gap or inside
dee aperture which offers all advantages namely extreme cleanness, high pumping speeds and conductances, low ultimate
pressure and complete regeneration is one of the most challenging technique if the other obstacles are overcome.
The installation of the 20 K cryopumps with cryo-absorbing
charcoal units are the realistic choice at present stage. The
auxiliary pumping units --- turbomolecular and/or baffled
diffusion pumps --- will be added for rare gasses and forpumping down the chamber to the operating pressure of thecryopumps. These pumping units, the total speeds of around1 xl0 5 1/sec for N2, are distributed on R.F. cavities andvalleys. The influences of ambient magnetic field which
remains at cyclotron periphery on pumps should be taken into
account. Generally, 100 Gauss of axial magnetic field is
maximum permissible field level for the turbomolecular pumprotors whereas the cryopumps withstand up to several times ofthe field level.

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Fig. 1.

I -·An aspect of the ring cyclotron

Reference
1)
Miura, et al., Proceeding of 9th International conference
on Cyclotron and their Application, p. 89, CAEN, 1981.
Miura et al. Design Study of the RCNP Ring Cyclotron,
these conference.

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