Elegant users forum

Dear Michael,

How are you? Can you help me to figure out what happens when i calculate the chromaticities? The situation is following.

First of all, I have to claim that I use elegant for a proton ring with energy 60GeV. Therefore, I give "&change_particle name=proton &end" in the beginning of .ele file.
No sextupoles at this moment. I have an .lte file which bends are set as "sben", quads are "quad". I used the default_order=3 and concat_order=3. So I could get the natural chromacities from up to the 3rd order matrices, am I right? What I get was (-319.6, -396.9). However, occasionally, I set "sben" to "csbend" and "quad" to "kquad" in the .lte file (using the canonical elements definition). I was still using the same .ele file, it came out chromaticities (-319.6,-393.3), a 0.75% discrepancy in the vertical direction. You may say such a trivial off is acceptable. The problem is when I use two sextupoles families to correct this first order chromaticities, they make (-319.6, -396.9) to (0,0) exactly, but canonically get (0,+3). Actually, I do find this off-center when I plot nuy vs. dp/p after canonically tracking particles a few thousands turns. Therefore, this chromaticies were not corrected for off momentum particle, in stead of on momentum particle.

I compare this calculated chromaticities from MADX by using both matrices and PTC twiss. Both of them show (-319, -396) for a ring without sextupoles. I also find some discrepancy for an electron ring lattice. So could you please explain to me where I made mistake and how elegant obtain the natural chromaticies?

Thank you so much!

Best,

Fanglei

Fanglei,

My guess is that the discrepancy results from the default values for the order of the edge matrix in CSBEND vs SBEN. In SBEN, it defaults to the default_order value given in &run_setup. In CSBEND, it defaults to 1, because this is symplectic. You can try setting EDGE_ORDER=2 on the CSBEND elements. Also, check that you are using sufficiently large values of N_KICKS for the KQUAD, KSEXT, and CSBEND elements when tracking.

--Michael