Chopper definition

[Bruce Yee]

Dear Didier,

Thanks for your help and time,

I am doing chopper simulation, and I would like to confirm some details about the definition in TraceWin.

According to the manual, the chopper is defined as

Chopper_TraceWin.png (21.88 KiB) Viewed 1058 times

I compared it with other definitions in the literature:

Chopper definition.png (59.43 KiB) Viewed 1058 times

in this case E= V/d

Chopper definition_2.png (94.12 KiB) Viewed 1058 times

in this case Va= 1/2 *mo gama * vo^2

The definitions are similar, but the other cases use the distance between plates as "d" or "D". However, TraceWin uses "D" as the distance between the plates and the axis.
In that case, there is a factor of 2 in the denominator that appears in the other cases, but not in TraceWin. Could you please tell me the difference?

Last, about the parameter C (the chopper transverse position),

I thought that it was the location of the chopper regarding the beam axis ("D" parameters), however if I use C= 60 the chopper location is 0 (left) and if I use C=0 the location is +/- 60mm (right size).

Chopper_c_param.png (215.85 KiB) Viewed 1058 times

Could you please explain me its meaning?

Best regards,

Bruce

[Didier]

Dear Bruce,

I've already noticed this factor of 2 depending on the publications and I think that sometimes it comes from confusion between the distance of the plates from the axis or between the plates, it's not always very clear.
However, for me, voltage and distance are from the 0 axis, so E=U/D and to convince myself, I'll compare it with an equivalent electrostatic field map, like follwing exemple:

Code: Select all

; here we have E=0.1MV/m
CHOPPER 1 1000 10.0 0 0
DRIFT 100 75 0

&

Code: Select all

; here we create a 100mm electric field map with E=0.1MV/m
MULTIPOLE 1 100 100 -0.1 10.0 0 0 1

Both give the same deviation, so I have no doubts about the CHOPPER element.

Concerning the 'C' parameter, it's just a deviation in the transverse direction of the position of the plates, so it won't have any effect in envelope mode because the field remains the same, but it will generate losses in tracking mode in your case, since one of the plates is 120 mm from the axis and the other is on the beam axis. In general, the two plates are slightly offset in the direction of deflection (few mm) to prevent the deflected beam from impacting one of the plates towards the exit.

Regards,

Didier

[Bruce Yee]

Dear Didier,

Thank you very much for your explanation.
Please let me confirm the following:
In your chopper definition, both plates are charged with the same voltage amplitude but opposite polarity, right?

My chopper, one of the plates is grounded. So, to simulate in TraceWin, I must use half of the voltage amplitude, right?

Best regards,

Bruce

[Didier]

Dear Bruce,

I can confirm everything you wrote.

Regards,

Didier

[Bruce Yee]

Dear Didier,

Thank you very much for the confirmation.

Best regards,

Bruce