Overlapping bunch

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Max
Posts: 50
Joined: 31 Jul 2008, 12:16

Overlapping bunch

Post by Max » 24 Aug 2010, 17:20

Hi Michael and Co.,
I have an unusual situation. I am trying to apply the transverse and longitudinal (&insert_sceffects
and LSCDRIFT) space charge forces in the UMER ring (11.52 m circumference, 10 keV electron energy).
The longitudinal debunching (no rf) due to the space charge is so strong that at a certain turn some tail particles lag the head
by a length longer than the circumference. What happens at this point ? Does the space charge algorithm recognizes
the one turn overlap ? The program continues to run for a turn or so even after the
separation between the head and tail is somewhat longer than the circumference, after which all particles are lost. I wonder whether the loss is due to physics or to a problem in the algorithm when particles overlap.
Thanks, Max

xiaoam
Posts: 17
Joined: 12 Jun 2008, 09:27

Re: Overlapping bunch

Post by xiaoam » 25 Aug 2010, 16:39

Hi Max,

I don't think that elegant can handle such a case. Part of the reason is: the bunch length is much longer than individual element's dimension and it has curvature. This makes the longitudinal space charge calculation invalid. Also, I am not sure if the space charge can handle such low energy beam properly.

If you just intent to estimate if beam can survive after certain number of turns, I suggest you may do following:
1. Treat the ring as a cell of beamline, multiple turns equivalent to multiple cells. Then tracking with LSCDRIFT and space charge. End when the bunch length reaches the circumference.
2. The maximum energy deviation will be smaller than what you get from step 1 (Curved bunch), and the number of turns to reach such a condition will be larger. The transverse emittance will be larger than what you get from step 1 (the bunch dilute slowly). After that, I think you can treat the beam as a longitudinal uniformly distributed coasting beam.
3. Calculate dynamic aperture for on-momentum and off-momentum (maximum energy deviation) case for estimating how stable the ring is.

For detail, you have to find another code.

Best regards,

aimin

Max
Posts: 50
Joined: 31 Jul 2008, 12:16

Re: Overlapping bunch

Post by Max » 25 Aug 2010, 17:37

Dear Aimin,
I thank you for your clear reply.
Cheers, Max

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