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AdA at Frascati

AdA at Frascati.

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AdA at Frascati

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  1. AdA at Frascati

  2. The discovery of the Touschek effectThe prototype AdA (1960) was a single ring in which counter-rotating electrons and positrons might annihilate in the cms.The most important parameter is the Luminosity(L): The number of events poduced per unit time at the collision points was n = Lwhere  is the relevant cross section

  3. La LuminositàL depends on particle’s numbers, N1 e N2 ineach beam on the rotation frequency,  = c/R where R is the orbit radius raggio, on thr number of bunches(that is on the harmonic of RF) and on S, the transverse area of the beam sections. S was computed after the root mean square widths x e z asproduced by the competition between radiation damping and scattering. The “magic” formulas we were using were those classical in the Molière and Rossi theory concerning multiple scattering compensated by radiation damping. I realized that this calculation was partially wrong in our special case.

  4. Touschek’s Notebook: cover and first page February 16, 1960

  5. Vertical dimensions of the beamsWhere the radial dimensions are due both to scattering and radiation fluctuations, the vertical ones are only due to scattering.However, the time between two collisions at AdA’s energies is an order of magnitude larger than the damping time constant corresponding to the radiative dissipation of transversal betatron oscillation energy by.Therefore, the actual regime for the vertical case was single and not multiple scattering: the effect of every collision was immediately quenched by radiation. A simple calculatio did show that the vertical size zwas nearly 1000 times smaller than previously computed(about 1 micron instead of 1 mm!). The luminosity L ~ N 1 N 2 /S is larger!

  6. Beam storageThe storage of beami (= 1,2) proceeds according to dNi/dt = c0 - Niwere c0is the injection rate and  is the time constant for losses. Therefore, the maximum number of stored particles will depend on the injector intensity and on the vacuum (c0 and , respectively)and it is simply è Nimax = c0 / ; we called it “the natural”limit charge preventing unlimited luminosity in a given ring.

  7. However…The longitudinal oscillations, the so called synchrotron oscillations, have a strong non-linearity introducing a limit Cycle in phase space, beyond which particles spiral to the internal wall of the donut and get lost.An energy transfer from the tansversal betatron harmonic oscillations to the longitudinal ones will produce a loss by this mechanism .

  8. However…(follows)The transverse oscillations can be viewed as relative motions of the particles in a bunch if we assume the synchronous particle as the origin of coordinates. Two electrons (or positrons?who knows?) in the same bunch can undergo elettronic “Moeller scattering” and transfer by this mechanism energy from the transversal to the non linear longitudinal mode. The probability of this event depends on the density of particles in the bunch. If you work in the synchronous particle reference system the calculations become simple (after some Lorents transformations).

  9. TOUSCHEK’S ROLE It was winter 1963. We were at Lab. in Orsay as every week-end. The Linac was working very well, the first beam in AdA was growing in intensity. Hours were passing without surprises. After 4 or 5 hours of steady charging, we saw, plotting the data that some kind of saturation was beginning; “as if the beam lifetime depended on the stored current”: this was the immediate perception, confirmed by a simple plot. Bruno went crazy. It was nearly 4 a.m. in a cold morning. Bruno left and went to the Café de la Gare, as usual when he was proccupied.

  10. THE BATH Corazza and myself had immediately offered a possible explanation: the syncrotron radiation extracts residual gas from the walls of the donut. The intensity of radiation depends on particle’s number, therefore the scattering lifetime depends too. Looked plausible; but no vacuometer signal was registered, even if the sensitivity was adequate. Bruno said: I will think; try to make some measurements at different energies of AdA. And left. He came back a couple of hours later, very excited: THE BEAMS ARE IN A BATH! cryed

  11. A VERY PECULIAR BATH A bath? Which bath? Actually, meanwhilewe had checked the very unusual enery dependence of the effect. Touschek said: consudera long bath with three orthogonal kinds of lateral walls: two are transversal and infinite, one is longitudinal and finite. If a swimmer is swimming in this pool, where the water can splash off the bath? From the “bordelli” he said in italian to mean the lower borders. Well, mister Moeller is swimming in AdA! “What?” We asked with one voice. “Look”, he answered, “here is the calculation of the coefficient between inverse life and number of electrons according to Moeller scattering”

  12. Touschek Effect

  13. The original calculation The data were already there; we got more in the next few days. There was an astonishing agreement between measurements and the touschek plot on the tablecloth of Café de la Gare. Bruno had computed by hand all details but asked me to check the scale factor, wich I did immediately in the late morning. Scale, exponents etc. were perfectly working. The reason why Bruno was more calm was that the energy dependence did not anticipate a disaster for higher energy rings like Adone: this was la low energy effect. But we had the opportunity to see it with AdA just because the vertical size of the beam was much smaller than predicted with multiple scattering.

  14. THE LUMINOSITY We lost the opportunity to reach a luminosity adequate to the cross sectin of muon or pion pairs. Still, did we have the possibility of measuring a luminosity? This was the end goal of AdA: to prove that the beams actually met. Since we had a Lead Cerenkov counter aligned at a crossing point of the beams, we decided to look at single photon production (bremstrahlung, not an annihilatio product!) since it had a very large and well known cross section. We saturated the beams in the ring and measured the rate per particle in a beam of gamma rays as a function of the number of particles in the opposite beam. The result was as expected!

  15. Luminosity of AdA  December run • January run ° February run April run C. Bernardini et al., “Measurements of the Rate of Interaction between Stored Electrons and Positrons”, Il Nuovo Cimento 34 (December 16, 1964 - Received July 16)

  16. Orsay Frascati 1960 Bruno Touschek Carlo Bernardini Gianfranco Corazza Giorgio Ghigo (Giancarlo Sacerdoti: magnete; Antonio Massarotti e Mario Puglisi: RF Cavity) 1961 (joined in:) (Ubaldo Bizzarri) Giuseppe Di Giugno Ruggero Querzoli 1962 (joined in:) (François Lacoste) Pierre Marin Jacques Haïssinski 1963 (final group:) Carlo Bernardini Gianfranco Corazza Giuseppe Di Giugno (Giorgio Ghigo) Jacques Haïssinski Pierre Marin Ruggero Querzoli Bruno Touschek

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