Allardyce, Brian W

Beger, H

Galiana, Roger

Giannini, R

Michaelis, E G

Talas, S

Allardyce, B W

Galiana, R

Giannini,R

CERN Document Server

Performance & prospects of the reconstructed CERN 600 MeV synchro-cyclotron

1EEE ThanbaCtionb on NucReah Scie.nce, UoLNS-24, No.3, June 1977 PERFORMANCE & PROSPECTS OF THE RECONSTRUCTED CERN 600 MeV SYNCHRO-CYCLOTRON B. W. Allardyce, H. Bener. R. Gallana, R. Giannini, E. G. Michael& & S. Talak c ERN Geneva, Switzerland The CERN 600 MeV synchro-cyclotron was recon- structed during lY73/74 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 3He++ are briefly described. Introduction The reccnstruction of the CERN 600 MeV synchro- cyclotron, undertaken during 1973/74, was designed to raise the 1 pA internal beam and the 7% extrac- tion efficiency of the former accelerator (Xl) each by an order of magnitude. Details of the project I,2 and first results on the performance of SC2 have been published.3'7 This report updates this information to March 1977. Present Performance SC2 has been in scheduled operation since Jan- uary 1975, totalling 2757 hours for physics in 1975 and 4141 in 1976. Reliability was initially poor but improved gradually, averaging 95% between October 1976 and February 1977. A summary of recent performance data is given in Table I where SC1 is compared totheSC2 designand values achieved. The column "Optimum" refers to performance under specific test-conditions and the data do not form a consistent set. E.g. the highest extracted beam of 1.5 pih was produced with 'a 2 M , internal beam, while the best internal beam of 4.3 pA was not extracted. The success of the reconstruction is illus- trated by the extraction-efficiency of more than I 70$, unprecedented for a synchro-cyclotron, by the number of protons per pulse, which has exceeded the design value and by the high duty cycle in long- burst operation. The average internal beam, how- ever, is low since SC2 still operates at one out of N possible FM Cycles. At present a usual value is N = 4 chosen to limit the power dissipation in the rotary condenser, but modifications made,to the spare condenser in 1976 are intended to permit operation at N = 2. Secondary particle fluxes from internal and external targets are listed in Table II. They are normalised to 1 p.C of protons incident on the tar- get and agree satisfactorily with earlier estimates given in Ref. 2 except for the LEPC n- beam. Recent Progress With the aid of diagnostic targets which per- mit measurements of the internal beam characteris- tics at various radii the intensity of tQe accel- erated beam was measured as function of fo, the initial rate of frequency modulation. The results, shown in Fig. Ldisagree with an earlier calcula- tion which suggested a strong dependence of the beam current on f. below - 10 MHz/ms (see e.g. Fig. 8 of Ref. 7). The discrepancy was found to be due to a misinterpretation of the computed results, and Table I CEM SC Performance Data 1976/77 Proton kinetic ener Internal beam minimum (MHz) df/dt max. (MHz/ms) Pulse frequency (HZ) Oscillator power (kW) Energg gain per turn (keV) Protons per pulse Long burst, duty cycle ($) internal targets external targets 5a 5n 18n 6n 30.6 30.4 16.6 16.7 0.5 11 3 54.1 466 120 85 20 200 30 22 1.2 :':oll 50 1.3 x loll 2.2 4"lOll 1.1 :"lOll 25 30 70 1631 © 1977 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this materialfor advertising or promotional purposes or for creating new collective works for resale or redistribution to serversor lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Table II Observations of Secondary Particle Fluxes at SC2 Sign I &SC p LIE Flux Beam Beam Channel Field (P :w spot Composition Prediction (WV/c ) W1) (cm2 1 (%I W) x 1 e n IJEPC +1 156 8 2.5 x 105 100 40 30 30 105 II+ LEPC -t 156 0 2.1 x 105 100 44 30 26 5 x 10 5 II-* 125 -1 300 6 5.5 x lo5 27 3 x 105 n- P -1 127 14 5 x 10 4 50 1 90 9 5 x 104 p- B 01 -1200 2.1 x lo6 30 100 1.5 x lo6 n B 01 2 200 2.6 x lo8 380 100 C +1 250 6.6 x lo6 100 30 30 40 , c -1 300 I 5 1.2 x 106 5 -75 -5 20 1. 3 x 106 51- “1 at 180 MeV/c The duty-cycle of this mode of operation has I 5 i- _._., .-.---_--__~-~_ 0 5 '0 :5 2oJfNzirrsK) 25 Pig. 1 - Internal proton beam as function of initial rate of frequency-modulation there is now a satisfactory agreement surement and theory. Similarly the V3~zt~~~~n~~~~e of beam current on the Dee voltage VD shown in Fig. 6 of Ref. 3 is seen in Fig. 4 to hold good only over a limited range, while for VD ? 12 kV the dependence is linear in accordance with theoretical predictions.8 A study of the beam-characteristics close to extraction has revealed a resonance between a beam stored at an orbital frequency of 16.9 MHz and an RF of 17.45 MHz appearing on the Dee. The nature of this phenomenon is not understood, but it was shown to be influenced by the radial position of the beam regenerator. The time-distribution of the long-burst proton beam produced by the pulsed-field coils (PFC) (see Fig. 4 of Ref. 6) has been improved by using the Cee beam-stretching system instead of the Dee for the final stage of acceleration prior to ejection by the coils. Under present conditions of Dee operation at N = 4 this method also leads to a significant gain in duty-cycle. With both long- burst systems working in cascade at a repetition frequency of 560 Hz and the Dee system pulsed at 90 Hz an overall duty-cycle of 45% was observed, compared with 159 when using the PFC alone.g been further im roved current supplylg by the use of a transistorized instead of the thyristor supply described in Ref. 2. By suitably modulating the current the overall duty-cycle of the extracted beam was raised to 82%, the beam intensity being about 6C$ of that obtained in short burst. Problems In spite of three years' development work the rotary condenser has continued to be the principal cause of breakdowns. While RF, drive and control problems no longer affect operation at the present level of RF - power vacuum and mechanical troubles have limited the performance of the system during the past year and caused two prolonged suspension5 of operation. Intermittent leaks and a progressive misalign- ment of the rotor of condenser No. 1 necessitated a reduction of the Dee-voltage during several months of 1976 but these defects were remedied during the past year by minor modifications. Condenser No. 2, on the other hand, required a major intervention when, after eighteen months of successful operation and a subsequent overhaul, there was a sudden in- crease in the vibration-level of the rotor. This defect was traced to a cracked insulating bushing protecting the ball-bearings. A dimensional check during dismantling showed various manufactur- ing faults and the 300 mm diameter bushing had to be replaced. To avoid a recurrence of the defect and to achieve greater reliability the alumina rings constituting the bushing were replaced by an assem- bly of 20 mm diameter rods shrunk between two steel rings. The new design is shown in section in Fig. 2. Its construction involved the remachining of the 340 mm diameter bore holding the assembly to a tolerance of 15 pm and a subsequent rebalancing of the rotor. This work has now been completed and the rotor has performed satisfactorily on test. The 220 mm diameter rotary face seal, which separates the ball-bearing space from the high vac- uum region of the condenser, is another weak ele- ment, many seals having shown excessive oil leakage without an identifiable reason. It is likely that the condensers still contain other hidden defects, whose identification will require much time and effort. 1632 B 3d I - ------~ ~ Pig. 2 - Rotor bushing with insulating rods Prospects There is nevertheless good reason to believe that the limitation of the beam current to 2 WA will gradually be lifted with the improving per- formance of the condensers. A current of about 4 PA, achieved under test with N = 3, should be available for operation in 1977 unless new defects come to light. The achievement of the full design current will still require much work, but meanwhile SC2 is able to supply protons for challenging phys- ics programme ranging from particle-physics via the study of unstable nuclei to solid-state and bio- physics. . To facilitate beam changes around the external pion production target and to reduce doses to per- sonnel a new beam-layout has been designed. This will permit directing the extracted proton beam either onto the external pion target or into the underground experimental area without moving any beam elements or vacuum connections and will result in a gain of physics time. The prospects for research are further en- hanced by a modification designed to adapt SC2 to the acceleration of 3He++ ions. The elements re- quired for tuning the RF system to a frequency- range of 20-l$1MH.i have been designed and are being manufactured. 9 An attempt to test the produc- tion of 3He++ ions in the present proton-source and their acceleration to 400 MeV was made in November 1976. It showed that it was possible to produce 3He++ in adequate quantity and to accelerate them by using the low-frequency end of the proton cycle between 20.2 and 16.7 MHe.13 By raising the arc-current of the source from 1 A used for protons to 20 A, a 2 PA beam of 3He++ was obtained at 58 cm orbit radius. With 17 kV Dee voltage 50 to 80% of this beam reached 400 MeV. The beam intensity as function of arc-current Is shown in Fig. 3 and as function of Dee voltage in Fig. 4. The irradiation of a carbon target at 1.4 m radius showed a beam height of about 2 mm. The captured charg per pulse agrees with a theoretical predic- tion. f4 One may therefore 'assume that the modifica- tion of the radio-frequency resonator w-l.11 suffice to produce a 3He++ current of at least 1 uA at 914 MeV. Fig. 3 - 'He++ current as function of ion-source arc current PA la Protons ‘0 45~~ 1 VRF 0 2 4 0 0 10 12 14 16 kr Fig. 4 - Proton & 3He++ current8 as function of Dee voltage 2 3 10 11 12 13 14 References MSC Staff, Proc. 5th Internat. Cyclotron Conf., Oxford, 1969, p.719 E. G. Michaelis, A.I.P. Conf. Proc. NO. 9, Cyclotron8 1972, p.141 H. Beger et al., Proc. 7th Internat. Cyclotron Conf., Zurich, 19'75, p.49 H. Beger et al., ibid., p.146 A. Fiebig & R. Hohbach, ibid., p.175 B. W.'Allardyce et al., ibid., p.287 R. Galiana et al., ibid., p-371 8. Comiti & R. Giannini, MSC 72-3, 1972 B. Y. Allardyce et al., SC Tech. Dev. Note No. 17, 4.3.76 A. Zanada, private communication, 1977 R. Hohbach, MSC 73-2, 5.2.73 H. Beger & A. Fiebig, C.17.1/692, 1.7.75 R. Galiana, SC Tech. Dev. Note No. 44, 1.3.77 R. Giannini, PS/CD/183, 15.10.76 1633 

Performance and prospects of the reconstructed CERN 600 MeV synchro-cyclotron

Performance and prospects of the reconstructed CERN 600 MeV synchro-cyclotron

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