beam current ramping

[li.chao]

Dear Michael，

I am tying to make a test run which ramps the beam current in the first several hundreds turns.
Somehow, not be able to do that.

The attached files shows the script I prepared, could you please take a look and help me to fix it.

PS: I found that the output of the &ramp_elements command, showing the charge ramping as expected. Whereas in the *.wp file, the
Charge column does not change. Since I have longitudinal impedance in the lattice. I did not see extra bunch lengtheing due to the longitudinal
impedance. It seems like that the the charge stays at the initial value during the tracking.

Great thanks

yours Chao

[michael_borland]

Chao,

The problem is that your CHARGE element is downstream of a recirculation element (RC). So the beam only sees the CHARGE element on the first pass through the beamline. I don't think you need the recirculation element.

--Michael

[li.chao]

Chao,

The problem is that your CHARGE element is downstream of a recirculation element (RC). So the beam only sees the CHARGE element on the first pass through the beamline. I don't think you need the recirculation element.

--Michael

Thanks Michael,

Great thanks for your answers. I got it.
For a further question, What is the purpose of of the recirculation element? What this element did to beam?
I noticed that, with IBS tracking, this recirculation element has to be removed also, is it?

yours Chao

[michael_borland]

Chao,

The recirculation element is to allow setup steps to take place prior to multi-turn tracking in a ring. The original inspiration was to allow tracking through a transport line and injection elements, followed by multi-turn tracking.

Although the feature is kept for backward compatibility, it is rarely if ever really needed. It is usually easier to perform to separate runs, e.g., one for the transport line that creates an SDDS output particle file and one for turns in the ring that uses that file.

The other reason not to use recirculation elements is that it can interfere with some calculations, such as IBS tracking. This is due to confusion about where to start the beam size calculations. It could be fixed, but it is easier just to leave the recirculation element out.

--Michael

[li.chao]

Chao,

The recirculation element is to allow setup steps to take place prior to multi-turn tracking in a ring. The original inspiration was to allow tracking through a transport line and injection elements, followed by multi-turn tracking.

Although the feature is kept for backward compatibility, it is rarely if ever really needed. It is usually easier to perform to separate runs, e.g., one for the transport line that creates an SDDS output particle file and one for turns in the ring that uses that file.

The other reason not to use recirculation elements is that it can interfere with some calculations, such as IBS tracking. This is due to confusion about where to start the beam size calculations. It could be fixed, but it is easier just to leave the recirculation element out.

--Michael

I see. Great thanks.

[li.chao]

Hi Michael,

Recently, I am trying to ramp the beam current from a certain value to zero to check the cavity response.
In the simulation, I set 80 bunches, given in train_80b_1KPerBunch_even.sdds and each bunch has 7.6nC charge initially


In the output of ramp_element, ramp.sdds, the total charge is ramped down as expected.
Whereas in the bunch parameter file *.wp* , the bunch charge starts from 15.2 ramp down to 7.6.

Could you please help me to check it out?

Many thanks
Yours Chao

[michael_borland]

Chao,

The problem seems to be that ramp_elements by default has differential=1, so that the ramp is imparted to the element by adding a value to the pre-existing value. Try setting differential=0.

Note that the output of ramp_elements has columns Amplitude, OriginalValue, and NewValue. I'm guessing that OriginalValue is non-zero, and that NewValue ramps from 15.2 down to 7.6.

--Michael