pepper-pot emittance measurement

[QJohn]

Hi,
I'm a newer to the elegant software. Now I'm using the elegantexamples(download from the website) named pepper-pot to measure the emittance. I just add a drift length in the beamline, then output the file %s.sig. I know that this output file can record the emittance. I wonder if this recorded emittance is the emittance which I want to get by using the pepper-pot method. thank you very much.

[michael_borland]

If you use the pepper-pot element in elegant, the emittance downstream of the pepper-pot plot is the emittance all the beamlets. This isn't what you want. You need to analyze the beamlets to get the emittane of the original beam. The best way is to scan an aperture (to simulate one beamlet at a time) and "measure" the beam properties. See attached.

--Michael

[QJohn]

Thank you very much, Mr. Michael. I'm very glad that you can give me so much help. When I run your attached .zip file, I found two questions,
1, I get the output results: exEst (m) = 4.202312e-005 eyEst (m)= 2.935307e-005 betaxEst(m)=2.158480e+001 betayEst(m)=1.510622e+001
I don't know what are these parameters mean, are they the emittance that I want to measure by pepper-pot method?
2, I don't know what is the l mean in the first row of your lattice.lte file
thank you very much !

[michael_borland]

1, I get the output results: exEst (m) = 4.202312e-005 eyEst (m)= 2.935307e-005 betaxEst(m)=2.158480e+001 betayEst(m)=1.510622e+001
I don't know what are these parameters mean, are they the emittance that I want to measure by pepper-pot method?

Yes, that's right. They approximately agree with the values set in the &bunched_beam command in run.ele. The fact that we only get approximate agreement is presumably a limitation of the pepper-pot method.

2, I don't know what is the l mean in the first row of your lattice.lte file
thank you very much !

I think you mean the value of 0.005 that is assigned to the "l" parameter in the definition of the RC element:

Code: Select all

rc: rcol,x_max=0.001,y_max=0.001,l=.005

If so, that's the thickness of the aperture in the beam direction. A real aperture has to have some non-zero thickness or it won't actually stop the beam.

--Michael

[QJohn]

1, I get the output results: exEst (m) = 4.202312e-005 eyEst (m)= 2.935307e-005 betaxEst(m)=2.158480e+001 betayEst(m)=1.510622e+001
I don't know what are these parameters mean, are they the emittance that I want to measure by pepper-pot method?

Yes, that's right. They approximately agree with the values set in the &bunched_beam command in run.ele. The fact that we only get approximate agreement is presumably a limitation of the pepper-pot method.

2, I don't know what is the l mean in the first row of your lattice.lte file
thank you very much !

I think you mean the value of 0.005 that is assigned to the "l" parameter in the definition of the RC element:

Code: Select all

rc: rcol,x_max=0.001,y_max=0.001,l=.005

If so, that's the thickness of the aperture in the beam direction. A real aperture has to have some non-zero thickness or it won't actually stop the beam.

--Michael

Thank you very much, Michael. I understanded your explainaton of the l parameter. I now have another question in the lattice.lte fle.What are the meaning of the parameters x_max and y_max? And how to set the holes' diameters of the pepper-pot mask Thank you very much!

[michael_borland]

Thank you very much, Michael. I understanded your explainaton of the l parameter. I now have another question in the lattice.lte fle.What are the meaning of the parameters x_max and y_max? And how to set the holes' diameters of the pepper-pot mask Thank you very much!

x_max and y_max are the half-apertures of the holes, which are assumed to be rectangular. Check the manual page for the RCOL element for more information. For round holes, look at the ECOL element.

--Michael

[QJohn]

x_max and y_max are the half-apertures of the holes, which are assumed to be rectangular. Check the manual page for the RCOL element for more information. For round holes, look at the ECOL element.

--Michael[/quote]

Thank you Michael. Now I want to let the electron beam go through some quadrpoles first, then through the pepper-pot mask and to measure the emmittance. I just add some quads in the lattce.lte fle and add the quads' line in the beamline.

Code: Select all

q1: quad,l=0.5,k1=10
l1: drift,l=0.25
q2: quad,l=0.5,k1=-10
l2: drift,l=1.5
bll: line=(q1,l1,q2,l2)
rc: rcol,x_max=0.001,y_max=0.001,l=.005
d1: drift,l=1
bl: line=(bll,rc,d1)

When I run the script, at the last of the process, it has some erros as follws,
error: squre root of negative number
*stop*
too few items on stack(drve)
*stop*
Error for G:\SDDS Toolkit \sddsprocess.exe:
unable to compute rpn expression-- rpn error (SDDS_ComputerDefinedColumn)
Because I don't know how to upload the image, so I have to print these error messages here. Thank you very much!

[michael_borland]

Very likely the problem is that you haven't adjusted the quadrupoles to give the condition alphax=alphay=0 at the position of the pepper pot plate. Without this, the analysis is more complex than what my simple example supports. In fact, I think you'd need to observe the beam at two locations downstream of the pepper pot plate to obtain the emittance in that case.

--Michael

[QJohn]

Very likely the problem is that you haven't adjusted the quadrupoles to give the condition alphax=alphay=0 at the position of the pepper pot plate. Without this, the analysis is more complex than what my simple example supports. In fact, I think you'd need to observe the beam at two locations downstream of the pepper pot plate to obtain the emittance in that case.

--Michael

Hi, Michael, I have another question, I found that the elegant has an element named PEPPOT. It is about the pepper-pot plate. Why do we use the element ECOL or RCOL other than the PEPPOT element to achieve the purpose that measuring the emittance by pepper-pot method? Thank you very much!

[michael_borland]

The reason to use the RCOL or ECOL element is that it allows easily separating the beamlets. With the PEPPERPOT element, you have to separate the beamlets yourself in postprocessing. In a real experiment with a pepper-pot plate, that's what you'll have to do, but for developing the basic method, it is easier to do one beamlet at a time.

--Michael