Microbunching Instability Simulation with LSC

[Aamna_Khan]

Hello,

While tracking with LSC there is an error that the beam is longer than the longitudinal wake function. I've tried to change different parameters but error is same. I have attached the file, please help me to figure out if I'm doing wrong somewhere.
Error: The beam is longer than the longitudinal wake function.
This may produce unphysical results.
The beam length is 8.595542e-11 s, while the wake length is 1.332589e-11 s

Also I want to know that how I can plot current profile and bunch form factor after each component like arc T1, arc T2 etc.. in my lattice file.

Thanks for any help and suggestions,
Aamna

[michael_borland]

Aamna,

The problem is that the beam develops very long tails, which causes it to extend to a length that is greater than the wakefield file. You can extend the wakefield file, but I'm not sure that is entirely practical given the extreme length. I suggest inserting some histogram and watch elements to get a look at the evolution of the beam.
Try adding these statements after &run_setup

Code: Select all

!! Insert histogram elements after every element
&insert_elements
        element_def = "h1: histogram,filename=\"%s.h%05ld\",x_data=0,y_data=0"
        name = *
        type = *,
&end
!! Insert watch elements after every element
&insert_elements
        element_def = "w1: watch,filename=\"%s.w%05ld\",fraction=0.01,mode=\"coord\"",
        name = *
        type = *,
&end

Once the simulation runs, try

Code: Select all

sddsplot -col=dt,dtFrequency mesa.h* -sep -title=@s -mode=x=offset

and

Code: Select all

sddsplot -col=t,p mesa.w* -sep -title=@s -mode=x=meancenter

--Michael

[Aamna_Khan]

Michael,

Thanks. I'll check by increasing the wake length.

One more thing, I've changed the lattice from MAD to elegant by python script. All the elements are same, but in elegant it gives different twiss parameters. As you can see in plots from both MADX and Elegant. How it will affect the tracking part?

Aamna

[michael_borland]

Aamna,

You may need to set END1_FOCUS=1 and END2_FOCUS=1 on the RFCA and RFCW elements. By default, elegant leaves these off. Regardless, tracking and twiss output should agree in elegant.

I am not familiar with what MAD-X does. I know that the standard version of MAD-8 was wrong when there was acceleration (didn't have the end focusing even as an option).

--Michael

[Aamna_Khan]

Hello Michael,

There is still same error . From last three days, every time when I run the elegant file it shows different bunch length with same parameters.
Sometime it shows beam radius is zero in LSCDRIFT.
Error:
The beam length is 9.725086e-11 s, while the wake length is 8.339102e-11 s
(The beam length varies from e+06 to e-11 that is strange.)

Please check the bunched_beam.

Thank you.
Aamna

[michael_borland]

Aamna,

The root of the problem seems to be that your beam is not properly controlled in the transverse plane. This leads to very large slopes, which affects the time of flight, which in turn results in a very long bunch.

To see this, simply disable all the wake and LSC elements (set TRWAKE=0, ZWAKE=0, and LSC=0 on RFCW elements, and set LSC=0 on LSCDRIFT elements).

--Michael