Tests in CSF laboratories have given evidence for the excellent quality of the bunching in the first section. Tests on the site starting in january 1968 were performed during all the year. Operations with both electron and positron beams give many results, namely maximum electron energy 640 MeV with zero current and 540 MeV with 40 mA peak current and an energy distribution with FWHM 3 to 4/1000. The maximum peak current obtained was 60 mA and no beam break-up does appear.Operations were performed at duty cycles of I % and 2 % with electron beam power of the order of 100 kW during many shifts of 30 hours ; energy stability was better than 1 % for 30 hours. Special devices helping to handle the beam inside and outside the linac at such a power level are described.First measurements of e /e conversion efficiency give an intensity ratio of positron beam relatively to electron beam, above 400 MeV positron energy, of the order of 10-3 and improvements are expected.
L'exposé donne les performances actuellement obtenues lors des essais de cet accélérateur fonctionnant à grand cycle utile (2 % maximum), grande intensité (jusqu'à 60 mA crête) et une énergie maximale de l'ordre de 600 MeV à courant nul. L'accent est porté sur les divers facteurs contribuant à la finesse du spectre en énergie (4 1 %) et à la grande stabilité des caractéristiques du faisceau de puissance moyenne élevée (ordre de 100 kW).
To achieve A.L.S. expected performances an excellent bunching was indispensable. The adjonction of a chopper and a prebunching cavityto buncher gives results conform to those given by computers. Slight modifications in RF structures and magnetic focusing were succesful as tests showed no trace of BBU.PART I -BUNCHERIn order to obtain 50 % of current in an 1 % energy spread, elementary bu4ch phase extension must be less than 5°at first section end. The chopper gives 900 phase width bunches. After prebunching cavity phase extension is reduced to 200 and at first section end the bunch is only 50 wide. A low voltage injector was chosen solution, already selected by CSF for its previous constructions. A.L.S. buncher main parts are -a 2.998 Mcs chopper. -a prebunching cavity and -A.L.S. first section. - ChopperTwo RF cavities are powered at accelerator frequency. They are oscillating in phase opposition on TM 011 mode. The chopper is put in the accelerator magnetic field B. An electron entering first cavity sees RF electric field E and magnetic field B ; under their action he is going through a cyclotron resonance. In the XY plane equations of motion are the same as for the cyclotron. Along Z axis there is no force applied and velocity remains constant. Calculation shows that impact trajectory of electrons on median wall is nearly a circle. If an aperture of angle f is cut in this wall only electrons with right phase shall go in the second cavity where due to the opposite electric field they will enter the prebunching cavity on Z axis. Relation between RF pulsation and cyclotron pulsation at resonance is as follows : (I) it QIt -__ n 8T _~x U y,F Q = e B Im C) = RF pulsation ; = electron charge 1r). = electron mass; velocitytu , A= RF wave length in chopper guide, B = magnetic field ; ', IL odd or even numbers. Cavity dimensions were the following: 155 mm x 20 mm x 70,5 mm with a Q. = 1600Computed resonances were in accordances with those observed. ALS operating values were : injection THT 33,5 KV, magnetic field : 1200 G. RF peak power 2,5 KW.Section S 1 only It was tested in function of RF frequency at different power levels. For each parameters values a spectrum was measured with a 10-3 width slit. Energy variation from 5,25 MeV to 6,25 MeV was obtained with a RF peak power varying between 700 KW and 1,45 MW. It was observed that spectrum is nearly independant of power level and injection tension as computer calculations showed it. Measured mean spectrum was about 1,6 % with a 1,4 % minimum at 6MeV -Section S 1 with chopper and prebunching cavity Without chopper spectrum shows that 10 % -20 % of electrons are not in the main bunch and have a wide energy spread. In the same conditions but with chopper operating these electrons are wiped out and spectrum bettered. The best energy spectrum thus measured with a 10
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