CEA Codes forum

Dear Didier,

There is a "Ki" parameter in the FIELD_MAP statement for space charge compensation. Are the two models below equivalent?

(1)
FIELD_MAP x x x x x x 0.5

(2)
SPACE_CHARGE_COMP 0.5
FIELD_MAP x x x x x x 0

Best regards,
Wai-Ming

Dear Wai-Ming,

If you use the command "FIELD_MAP x x x x x 0.5", you have to generate and add to your project a field map containing the space charge compensation according to z (and of course, you can make it vary as a function of z). In that case, the SCC field map will be scaled down by a factor of 0.5.

If you use
SPACE_CHARGE_COMP 0.5
FIELD_MAP x x x x x 0

then, the SCC will be set uniformly to 50% in the following field map and in the rest of the beam line as long as you don't change the SCC.

I hope it helped.

Best regards,

Nicolas

Dear Nicolas,

Thank you for your answer. Do you mean that Ki is a scaling factor for the strength of the field map?
My understanding to SPACE_CHARGE_COMP is scaling factor for the beam current in space charge calculation during transport.

Best regards,
Wai-Ming

>Do you mean that Ki is a scaling factor for the strength of the field map?

Yes, Ki is the scaling factor for the strength of the space charge compensation field map (that you have to generate).

> My understanding to SPACE_CHARGE_COMP is scaling factor for the beam current in space charge calculation during transport.
 
That's true.

By the way, in the case of a space charge compensation field map, the effect is the same: it applies a scaling factor on the beam current for the space charge calculation, but it can vary as a function of z, as it is a field map.

Best regards,

Nicolas.

Dear Nicholas,

I have seen you used the term "space charge compensation field map" or "field map containing the space charge compensation according to z". I do not understand what that is. Using a 1D electric field map as an example (edz), it is a list of numbers for the electric field strength with a few prefixes. Space charge compensation is about reducing the beam current during transport. How can I incorporate space charge compensation into a list of electric field strength and make it a space charge compensation field map?

Best regards,
Wai-Ming

Dear Wai-Ming,

In fact, you have to create a file with the extension ".scc" that contains the space charge compensation map. This file format is described in the "Current or space charge compensation map" section of the TraceWin documentation. Basically, it's a two-column file (with a header "0 N", N being the number of steps in the file) where you can define, for each step i, the space charge compensation factor SCCi at the position zi. Then Tracewin interpolates between steps. That way, it is possible to have a space charge compensation that depends on z.

Then, you can call this file (the scc map) with the command "FIELD_MAP" with the parameter Ki (7th parameter) different from 0, ki being the scaling factor of the scc map.

I hope this helps.

Nicolas.

Thank you, Nicholas.

Wai-Ming