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Considerations on the SuperB injection system

Considerations on the SuperB injection system. M. A. Preger LNF/INFN. Injection requirements. “Topping up” injection is mandatory to keep the average luminosity close to the maximum one

sarah-jacobs
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Considerations on the SuperB injection system

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  1. Considerations on the SuperB injection system M. A. Preger LNF/INFN

  2. Injection requirements • “Topping up” injection is mandatory to keep the average luminosity close to the maximum one • Assuming an exponential decay, keeping currents > 90% of the maximum, with a luminosity variation within 20% below the maximum (10% loss in each beam current) • Each bunch in the low energy ring must be refilled each 22 sec • In order to fill both beams at this rate in the single bunch mode the injection repetition must be at least 160 Hz

  3. DAFNE Linac • Scaling from DAFNE injection system, where Linac pulse length is ≈10 ns, we can inject a train of 3÷4 bunches at each injection pulse, being the bunch separation in the SuperB rings 2.1 ns. • The injection repetition in this case can drops below 50 Hz • Partial recirculation of the beam to save on the Linac total length is possible • Drawbacks of injecting >1 bunch: • slight loss in injection efficiency due the limited longitudinal acceptance of the ring • necessity of a sufficiently flat Linac pulse to obtain a constant current bunch pattern

  4. Injection from Linac • Required n. of particles to refill 10% current: 6.2x109 in LER possible with same Linac as in DAFNE • In the DAFNE Linac: • Electron beam parameters: = 1 mmxmrad, energy spread of ±0.5% at 500 MeV. BUT: e- recent measurement at the end of the Linac on has given e almost an order of magnitude smaller.

  5. Injection from Linac • Direct acceleration to 7 GeV into the Linac, yields a final e due to adiabatic damping = 0.07 mmxmrad, corresponding to a beam size ≈ 6 times larger than the stored beam one. • With an energy spread of 0.43% coming from prebunching within 15° in the injector section, the injected beam is half the size of the foreseen dynamic aperture.

  6. Injection from Linac, Conclusions • Direct acceleration in the Linac to 4 GeV would yield an emittance of 0.6 mm.mrad, corresponding to about 20 times the stored beam size, which is almost twice the dynamic aperture. This makes the use of a DR for e+ mandatory. • e- can be safely injected into the HER, making simpler the design of a DR to be used only for positrons.

  7. Injection scheme, summary • Positrons accelerated to 4 GeV after conversion at 2 GeV, preaccelerated to 1 GeV, stored into a DR at 1 GeV and finally accelerated to 4 GeV. • It is possible to inject 7 GeV e- directly from Linac • This scheme fully exploits the total acceleration length of the Linac without any recirculation of the beam.

  8. 1 GeV Damping Ring • For 50 Hz injection of short bunch trains the damping time should be a small fraction of 20 ms and RF frequency same as the main ring one. • Therefore length of the DR must be a rational fraction of the main ring one, and also a multiple of the Linac bunch spacing • Example: 40 m ring, transverse damping times ≈6.5 ms and synchrotron d. t. ≈3.3 ms. At 50 Hz, this is > 3 damping times for betatron oscillations and > 6 for the synchrotron ones. • Assuming conversion at 2 GeV, the number of e+ in a 10 ns pulse from the Linac should be ≈5x1010, i.e. 8 times larger than the single bunch requirement for topping up injection, but injection of a small train of 3÷4 bunches at the same time will leave a safety factor of 2 on the single bunch charge.

  9. Equilibrium emittance in DR is 0.80 mmxmrad. With the beam extracted after 3 betatron damping times, extraction e(e+) is 2% larger than the equilibrium one, about 3 times smaller than what can be obtained with direct acceleration in the Linac . • Reacceleration in the Linac to 4 GeV yields an emittance of 0.20 mmxmrad, corresponding to a beam size 11 times larger than the stored beam one. • The dynamic aperture is 12-13 times larger than the stored beam, so that injection from the damping ring is possible, but without any tolerance. • A different design for the damping ring, to be used only for positrons, yielding at least a factor 2 smaller emittance is therefore recommended.

  10. Energy spread • The equilibrium energy spread in DR is 5.8x10-4 • Energy spread at injection into the main rings depends on the bunch length in the DR. • With a RF at 476 MHz and 0.5 MV, the equilibrium bunch length is ≈6mm. • With Linac at 3 GHz, this means that ±2s of the DR bunch occupies ≈85° on crest, which would give a very large energy spread at the end of the Linac. • In order to come back to the 15° achievable with direct injection from the Linac, the bunch length must be reduced to ≈1 mm by means of a rather strong bunch compressing system.

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