Longitudinal behavior- Bendings_on/off

[samira.fatehi]

Dear Michael,

I have a question regarding the longitudinal behavior I observe in ELEGANT simulations.

I compared two versions of the same lattice:

Case 1: Four dipoles active (first dipole with a 5° bend) → R56 ≈ 1.9×10⁻⁶, bunch length ≈ 149 µm.

Case 2: All dipole bending angles set to 0°, but the septum and kicker remain active → R56 ≈ 9.8×10⁻³, bunch length ≈ 96 µm.

The energy spread (Δp/p = 10⁻³) is identical in both cases, so I would expect a longer bunch for the larger R56 (three orders of magnitude higher). However, the opposite occurs. Could you please help me understand why this happens?

In addition, I noticed that even when varying the beam energy from 90 MeV up to 1 GeV, the final bunch length remains almost unchanged. It seems that ELEGANT may be assuming β ≈ 1 (particle velocity equal to the speed of light).

Is this β≈1 assumption still valid at 90 MeV, or could it introduce a noticeable error in the longitudinal dynamics, such as in the bunch length or R56 effects?

Thank you very much in advance for your time and clarification.

Best regards,
Samira Fatehi