Elegant users forum

Good Morning,
I am facing an unexpected difficulty in calculating residual dispersion, I hope you can help me solve it.

I have two elegant runs (let's call them run0 and run1) which are identical but for a detail: in the second run, the first RF cavity of the LINAC is changed to have a different voltage changed with &alter_elements as follows:

&alter_elements
name = RF_C_TW_0,
item = VOLT,
value = 0.9,
multiplicative = 1,
&end

I track the same beam (same initial conditions) in both runs.

I want to calculate now the difference in the centroids along the LINAC: I use the run0.cen and run1.cen files and calculate it as

CxDiff=Cx0-Cx1

Now this difference should be equal, to first order, to the dispersion times the energy difference i.e.

CxDiff=-etax*(pCentral0-pCentral1)/pCentral0

where pCentral0 is the value of the pCentral parameter read from the run0.fin file, and analogously for pCentral1.

Unfortunately when comparing the data this is not the result I get...

In attachment you find the .ele file of the run1.ele (run0.ele is identical but for the absence of &alter_elements) and the result of the postprocessing of the data done with MATLAB.
Can you help me understand where is the problem?

Many thanks,
best regards,
Giovanna

Dear all,
after some deeper reading of the elegant manual I realised I had to set the first_is_fiducial=1 flag , I only now realised it was a fiducialization issue.

Best regards,
Giovanna