Elegant users forum

michael_borland wrote: ↑

14 Mar 2018, 16:25

Guohui,

I would need to see the lattice file to comment. Can you upload it?

--Michael

michael_borland wrote: ↑

15 Mar 2018, 09:34

Guohui,

The problem is that your lattice has x-y coupling, which twiss_output does not handle correctly. You can check the tunes by tracking, using the attached file. The result is postprocessed with sddsnaff, e.g.,

Code: Select all

sddsnaff model.w1 -col=Pass -pair=Cx,Cxp -pair=Cy,Cyp -pipe=out -term=freq=1 | sddsprintout -pipe -col=*Freq*
Printout for SDDS file stdin

  CxFrequency    CyFrequency  
------------------------------
  2.196534e-01   1.601147e-01 

These results are much closer to those given by MADX.

There is a coupled_twiss_output command available, but it doesn't give the tunes and I couldn't get it to work for your lattice. We'll look at this further, since it should work.

--Michael

michael_borland wrote: ↑

15 Mar 2018, 13:27

Guohui,

&twiss_output assumes that the lattice is uncoupled. The beta functions etc. are computed from the 2x2 submatrices for the x and y planes. So stritly speaking they are not correct anywhere in the presence of coupling.

You can compute the coupled twiss parameters for the four modes using the following command

Code: Select all

&coupled_twiss_output
    filename = %s.ctwi,
    emittance_ratio = 1e-02,
    emit_x = 1e-10,
    sigma_dp = 1e-10,
    calculate_3d_coupling = 1,
    concat_order = 2,
&end

If you compare betax1 from this command from betax from &twiss_output, you'll see differences throughout the lattice.

Unfortunately, &coupled_twiss_output doesn't provide the tunes, but we will add that in the next release.

--Michael

michael_borland wrote: ↑

15 Mar 2018, 13:27

Guohui,

&twiss_output assumes that the lattice is uncoupled. The beta functions etc. are computed from the 2x2 submatrices for the x and y planes. So stritly speaking they are not correct anywhere in the presence of coupling.

You can compute the coupled twiss parameters for the four modes using the following command

Code: Select all

&coupled_twiss_output
    filename = %s.ctwi,
    emittance_ratio = 1e-02,
    emit_x = 1e-10,
    sigma_dp = 1e-10,
    calculate_3d_coupling = 1,
    concat_order = 2,
&end

If you compare betax1 from this command from betax from &twiss_output, you'll see differences throughout the lattice.

Unfortunately, &coupled_twiss_output doesn't provide the tunes, but we will add that in the next release.

--Michael

gwei wrote: ↑

15 Mar 2018, 15:46

Hi Michael,

I add the your suggested lines of &coupled_twiss_output to the .ele file as attached.
But when I run it, it gives a error information as follow.
emittances_from_twiss_command was set but twiss calculations not seen

Thanks.

Guohui

michael_borland wrote: ↑

15 Mar 2018, 13:27

Guohui,

&twiss_output assumes that the lattice is uncoupled. The beta functions etc. are computed from the 2x2 submatrices for the x and y planes. So stritly speaking they are not correct anywhere in the presence of coupling.

You can compute the coupled twiss parameters for the four modes using the following command

Code: Select all

&coupled_twiss_output
    filename = %s.ctwi,
    emittance_ratio = 1e-02,
    emit_x = 1e-10,
    sigma_dp = 1e-10,
    calculate_3d_coupling = 1,
    concat_order = 2,
&end

If you compare betax1 from this command from betax from &twiss_output, you'll see differences throughout the lattice.

Unfortunately, &coupled_twiss_output doesn't provide the tunes, but we will add that in the next release.

--Michael