space charge simulation in rings

[li.chao]

Dear Michael,

I am studying space-charge effects in rings and would appreciate your advice on a surprising result from an element-by-element tracking test.

I attached the input file that shows how I set up the simulation. In the run I apply a space-charge kick at every quadrupole (element-by-element tracking). For the test I used a flat beam with horizontal emittance emitx=20 pm, and vertical emittance emity=2 pm. I therefore expected stronger emittance growth in the vertical plane.

With a bunch charge of 7.6 nC the attached figure shows the result: contrary to my expectation, the horizontal emittance starts to increase first and more rapidly.

sc_res.png (6.01 KiB) Viewed 404 times

Could you please help me check what might cause this behaviour?

Yours Chao

[michael_borland]

Chao,

I see that the emittance initially increases, then goes down. Can you check to see if particles are lost from the simulation?

--Michael

[li.chao]

Dear Michael,

Many thanks for your reply. I checked the data of transmission in this simulation, and there is no particle loss.

Meanwhile, I did a simple scan on beam current from 0.1 mA to 1mA, the results is shown below

There are two groups of curves, each including 10 (0.1 mA to 1 mA with a 0.1 mA scan step).
The group with an initial growth corresponds to x-plane.

Even in 0.1 mA beam current, there exits initial emittance growth in the x plane
It seems like the initial emittance growth comes from somewhere else rather than the space-charge kicks.

Could you please confirm this and help me figure out where it comes from?

Many thanks.

Yours Chao

[michael_borland]

Chao,

The problem is that the beam is not matched to the lattice at the injection point. You can fix this with

Code: Select all

sddsmatchtwiss train.sdds rematched.sdds -xplane=filename=lattice.twi -yplane=filename=lattice.twi
mv train.sdds old-train.sdds
mv rematched.sdds train.sdds

where "lattice.twi" is a file with the twiss parameters computed for the full ring.

When I do this, I don't see the emittance jump at the beginning.

--Michael