close orbit for large momentum offset

[yuehao]

Hi, Michael

Is there a way to calculate the path length difference of close orbit between the reference energy and with large energy deviation?
After the CO calculation, the parameter lengthError in the .clo file is always zero.

btw, I use MALIGN to change the energy.

Thanks a lot.
Yue

[yuehao]

HI, Michael

I have a updated question of the twiss parameter calculation for the large energy deviation case.

Is the twiss parameter calculated based on the close orbit of the deviated energy? I tried elegant as well as other tracking codes. Elegant gives large errors while several other codes agrees with each other. I am wondering if the energy deviation in twiss_output calculation is treated as small and is expended around 0?

Thanks again.
Yue

[michael_borland]

Yue,

If set up properly, elegant computes off-momentum twiss parameters using a matrix method with expansion about the closed orbit. Since the matrix is only available to second order for most elements, this has accuracy limitations for large energy deviations.

How large of a deviation are you using?

--Michael

[yuehao]

I am doing a FFAG calculation, with the energy deviation 72%.

Do I need to calculate close orbit before twiss command so that the program knows about the close orbit?

Thanks,
Yue

[michael_borland]

Yue,

You definitely need to compute the closed orbit first. Elegant won't do this automitically.

However, I don't expect the twiss parameters to be accurate for such a large energy deviation.

Tracking with CSBEND, KQUAD, KSEXT, and EDRIFT elements should be ok.

--Michael

[yuehao]

Yes, I am using EDRIFT and CSBEND, also MALIGN to change the energy. Elegant does gives the correct close orbit.

Could you give a hint on how to tell twiss_output to calculate based on the close orbit?

I am trying:
run_setup
closed_orbit
run_control
bunched_beam
track
run_setup
twiss_output

However, it gives the same answer as:
run_setup
twiss_output

Thanks,
Yue

[michael_borland]

Yue,

The sequence I recommend is
&run_setup
&closed_orbit
&twiss_output output_at_each_step=1
&run_control
&bunched_beam
&track

--Michael

[yuehao]

Hi, Michael

Thank you for the suggestion. The answer is still away from other codes.
I guess the discrepancy come from the treatment of 1/(1+delta). Do you think it is easy to change the code for the twiss parameter with large energy deviation?

Best,
Yue

[michael_borland]

Yue,

Sorry, I didn't see your reply.

Changing this wouldn't be easy, because the calculation makes use of the higher-order transport matrix. You'd have to add additional orders, which becomes increasingly difficult.

Do you understand how the other codes handle this?

--Michael

[yuehao]

Hi Michael,

I returned to this topic after a year . I managed to calculated the closed orbit correctly. How do I calculate the twiss parameters with respect to the calculated closed orbit? I used the sequence followed your recommendation:

&run_setup
&closed_orbit
&twiss_output # it seems the the flag output_at_each_step=1/0 won't change result.
&run_control
&bunched_beam
&track

The result is the same as if I don't calculate the closed orbit.

In the source code of twiss calculation twiss.cc , there is a flag called clorb. It seems it controls whether the twiss is calculated with respect to the close orbit or the reference orbit. However I did not find how to enable this flag.

Thank you very much.
Yue