moments_output

[n.milas]

Hi Michel

I having been using Elegant and now I'm trying to get a idea of the coupling introduced when I put alignment error in my lattice. I understand that for that I need to use the "moments_output" command but I'm not really able to understand what is coming out of it.
After doing a lot of simplifications on my input file I need you just with the plain lattice (no errors at all) and even then the horizontal emittance is a lot different form the one calculated from the twiss output.

twiss output = 2.8e-10 (ex)
moments output = 3.7e-10 (also ex)

I really don't understand why is this happening. I even included the rf_setup and the closed_orbit calculation (although I think they were not really needed in this case).

Maybe I'm doing something very silly but I really cannot think about anything else to do. Here is my .ele file, I can also send you the .let file.

Thanks!

&run_setup
lattice = Sirius_v601_ac10.lte,
use_beamline = anelrf,
p_central_mev = 3e3,
default_order = 2,
random_number_seed = 98777521.0,
rootname = calc_moments,
tracking_updates = 0
parameters = %s.param
&end

&twiss_output
filename = %s.twi
matched = 1
radiation_integrals = 1
&end

&rf_setup
filename = %s.rf
name = RFC
harmonic = 864
over_voltage = 5.062
&end

&run_control
n_steps = 10
&end

&closed_orbit
output = %s.clo
&end

&moments_output
filename = %s.mom
output_at_each_step = 1
matched = 1
equilibrium = 1
radiation = 1
verbosity = 1
&end

&bunched_beam &end

&track &end

[michael_borland]

I don't see anything obviously wrong here. Is it possible you have non-zero dispersion in the straight sections? The "ex" column in the moments output gives the effective emittance, whereas the "ex0" parameter in the twiss output file gives the natural emittance. If that's not it, please send me the lattice file and I'll have a look.

--Michael

[n.milas]

Hi Michel

Here is the .lte file I'm using. Could you also explain me something, what's the difference between ex, exbeta and e1 in the moments output file?

Thanks a lot for the help!

Natalia


PS: I also tested the n_slices parameters and, as described on the manual, at around 10 slices the values for ex (or the other remittances) don't change very much.

[michael_borland]

Natalia,

The problem seems to be that synchrotron radiation was not turned on for the CSBEND elements. Probably the program should issue a warning about this. I've attached a command file that you can use to test this.

Code: Select all

% elegant run.ele
...
% sddsprintout -param=ex0 run.twi 
Printout for SDDS file run.twi

ex0 ($gp$rm) =  2.806679e-10

% sddsprintout -param=e1 run.mom 
Printout for SDDS file run.mom

e1 (m) =  2.801644e-10

To explain the different quantities: Parameter ex0 in the .twi file is the natural emittance, ignoring coupling. Parameters e1, e2, and e3 in the .mom file are the emittances of the three modes; if coupling is small, e1=ex, e2=ey, and e3=ez, but as coupling increases and x-y tilts become larger this easy interpretation doesn't exist. Columns ex and ey are the effective emittances in the x and y planes, while exbeta and eybeta are the effective emittances ignoring the dispersive terms. When coupling is small, exbeta and eybeta tend to be constant vs s, while ex and ey are not because of the variation in dispersion.

--Michael

[n.milas]

Thanks for the help and very fast replies!!

Natalia