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Modification of the vacuum chamber in ATLAS Effect on impedance and related quantities

Modification of the vacuum chamber in ATLAS Effect on impedance and related quantities. E. M étral, N. Mounet and B. Salvant with the help of R. Veness and M. A. Gallilee. Impedance contributions. Two different contributions are to be distinguished:

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Modification of the vacuum chamber in ATLAS Effect on impedance and related quantities

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  1. Modification of the vacuum chamber in ATLASEffect on impedance and related quantities E. Métral, N. Mounet and B. Salvant with the help ofR. Veness and M. A. Gallilee

  2. Impedance contributions • Two different contributions are to be distinguished: • Resistive wall impedance, due to the resistivity of the beam pipe. This will be increased when the radius is reduced (29 mm → 22.5 mm). • Geometrical impedance, due to the new conical transitions (tapering, angle 3.7 deg) in the modified situation. Note: this contribution is much smaller than the one coming from the unshielded bellows (in the same area).

  3. Power loss due to the impedance • Resistive wall part, in the most critical case (up to now), i.e. short bunches, and two beams of 2808 bunches each: • Geometrical part: negligible (≈10-7 W/m) → about 30% increase in the power loss (obtained with two different approaches). Becomes ~1.9 W/m.

  4. Impact on beam stability: Transverse effective impedance • Sacherer transverse effective impedance: → Transverse effective impedances for both current and future situations are estimated to be negligible with respect to the rest of the ring.

  5. Conclusion • Power loss (from resistive-wall): at most 1.9 W/m (with bunches of 0.8 ns at 3.5 TeV), increased by 30% with respect to the previous configuration. • With our current understanding, there is very little effect of the vacuum pipe modification on the beam stability (longitudinal and transverse).

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