Differece between the envlope plots and the phase-space plots

[Emily]

Dear Didier,
I found differece between the envlope plots and the phase-space plots, I don't know why?
As shown in the following picture, there are two phase-space plots, the upper one is the input distribution, and the lower one is the output distribution. For the output distribution, the beam beta are different with those from the envelope plots.
Best regards,
Emily

[Didier]

Dear Emily,

I am really unable to have an opinion on what you are telling me.
I have very few information and without your project files I can not really help you.

Regards,

Didier

[Emily]

Dear Didier,
Sorry, this is a general problem. I tried the project DTL_2.ini(from TraceWin examples) and found the same results. At the end of the lattice, the beam beta from the envelope plot and the phase-space plot are different, as following:
Best regards,
Emily

[Emily]

Dear Didier,
However, at the end of the lattice, the rms beam sizes from the envelope plot are exactly same with those from the phase-space plot, as following:
Best regards,
Emily

[Didier]

Dear Emily,

Ok, I understand now.
The difference comes from the definition of the betX-Y function plotted in the envelope graph which is different from the one used in the twiss parameter of the beam distribution.
The typical difference is that the emittance used to calculate the twiss parameter (env) is the starting emittance of the simulation. This is the definition we usually use here, but I confess I don't remember the exact reason (see "Beta X&Y function" chapter in manual (page 164))

Regards,

Didier

[Emily]

Dear Didier,
I checked the beta function, but the calculated beta are different with both beta(env) and beta(par),please see the attached files. Of course, I used the non normalized emittance at the beginning of the structure. Thank you!
Best regards,
Emily

[Didier]

Dear Emily,

I'm sorry for that, but the documentation was not exact about the calculation of the bet, which explains your discrepancies in the results. I have corrected it as follows.

betxy.png (27.44 KiB) Viewed 3323 times

Regards,

Didier