The CERN 600 MeV synchro-cyclotron was recon structed during 1973/74 by introducing a hooded arc ion-source, a rotary condenser designed to raise the Dee-voltage and pulse repetition rate and an extrac tion channel using a current-bearing septum.The rebuilt accelerator has been in operation since Jan uary 1975• Beam Characteristics and recent perform ance data are presented and results of the accelera tion of ⅛e++ are briefly described.
A fast cycling RF-system glvlng high energy-gain per revolution was required to overcome the intensity limitations set by space-charge in the CERN SC. All other frequency modulation methods being discarded for power reasons, a high Q system tuned by a rotary capacitor was chosen.The rotary capacitor, constituing the most critical part, requires excellent vacuum, forced cooling on all RF electrodes and high mechanical precision. The mechanical problems which, in the early stage of the design, were feared most, proved to be solvable. However, considerable effort was necessary to obtain sufficient reliability of vacuum and cooling circuits. Specially shaped stator blades are used to furnish an optimized frequency modulation programme.The RF circuit design by means of models and computer programmes was initiated at first at CERN and continued later by industry. Detail hardware design and construction was performed by industry, further development and running-in by CERN.The system is performing satisfactorily and has permitted acceleration of the design proton intensity per machine pulse.1. History Excellent results of the new sector-focussed cyc 1 otrons were reported in 1963 at the "Conference on Sector-focussed Cyclotrons and Meson Factori es" concerning intensity, beam quality and extraction efficiency]}. The utilization of mid-plane hooded arc ion sources with high acceleration voltages was essential to achieve this result. The CERN SC magnet did not allow the conversion of the old SC into a sector-focussed fixed frequency machine while keeping the maximum energy unchanged. Therefore a first proposal for a conversion of the SC RF system into a fast cycling system with an acceleration time of 950 ~s, permitting the use of a hooded arc ion source in the SC was made in 1963 2 }3}. Detailed investigations lead to a "Proposal for the Improvement of the 600 MeV Synchrocyclotron,,4}. Because all other frequency modulation methods had to be discarded for power reasons, a high Q system, tuned by a rotary capacitor, was chosen. The design of frequency modulated RF systems has to take account of many limiting circumstances, which become particularly hampering when the maximum proton energy of the cyclotron exceeds several hundred MeV, because mechanical dimensions and necessary frequency swing increase at the same time. A careful system design was necessary to obtain the required frequency range, the frequency-time dependence, the variation range of the lower frequency end and last but not least to keep the RF power consumption and the electrical field strength between the capacitor electrodes within admissible limitsS}. (Fig. l}. Computations were checked at first on a 1:5 scale model, later on a 1:1 model. RF System Design
The essential features of the reconstructed CERN 600 MeV Synchro-Cyclotron are a hooded-arc mid-plane ion source, a radiofrequency system modulated between 30.4 and 16.7 MHz at about 450 cycles per second, and an electro-magnetic extraction channel designed to extract over 70% of a 10 ~A internal proton beam. Preparation of the material required for the reconstruction was started in 1967. The transformation of the synchro-cyclotron and of its associated equipment was carried out between June 1973 and September 1974. The principal phases of the reconstruction and recommissioning are described, and performance data of CERN SC2 are presented.
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