Some undocumented parameters? [SOLVED]

[hendrik.haehnel]

Dear Romuald,
I was looking through the ESS RFQ input file and noticed some input parameters that are not documented. Could you tell me, what they do?

theSpecialLostFlag 0
theEquivalentBeamFlag 0
theNewPotFlag 0
theDirectionFlag 1
LBECompensation 1.0
theRhoR0RatioInRMS 1

Best regards,
Hendrik

[RomualdDuperrier]

Hi Hendrik,


there are indeed a few experimental commands with Toutatis that are not documented due to their experimental nature. I do not know so much how some of them came into the ESS RFQ input file.

Anyhow, here are some explanations:

theSpecialLostFlag 0 => a square with side size equal to 2 times the minimum aperture is used for estimating beam loss instead of vane shapes. It supported comparisons with Parmetq that estimates losses this way. This feature does not work anymore. I am thinking about restoring it. This could be a problem when vane positioning errors are considered.

theEquivalentBeamFlag 0 => For Toutatis, particles that are "lost in energy" (not in bucket anymore) but did not hit electrodes are still transported. These low speed particles represent actually more and more real particles as the difference in term of speed between the reference particle and these ones increases. This is nothing else than keeping the current constant at the moment you do not loose the particles or considering that the window capturing the motion of the particles of interest sees more and more low speed particles coming from the previous bunches. To take into account this effect, Toutatis adjusts the weight of the particles considering their beta. This is the default behaviour. Setting theEquivalentBeamFlag to 0, disables this feature. Although, the simulation is less realistic, it helps you testing the space charge effect of these low speed particles on the particles of interest.

theNewPotFlag 0 => it was a way to test alternative analytical formulation for the potential. This is obsolete.

theDirectionFlag 1 => it multiplies the dt in the motion integrator by -1 such that the particles go backward. Useless if the emittance grows as it presents not physical simulation. I would avoid.

LBECompensation 1.0 => this one works and helps at taking into the image effects at RFQ entrance plate inducing an energy spread. The factor can vary from 0 to 1 (1 means full compensation if I remember). The model assumes an uniform beam. I suggest you change the factor and look at the z zp plot to see the effect.

theRhoR0RatioInRMS 1 => this one also works and induces that the rho / R0 ratio is also applied in the Radial Matching Section. Thus, the rho starts with a large value as R0 is large, and the rho decreases as R0 decreases. The idea was to test the effect of the multipoles content if the ratio is not kept constant. As far as I remember, this feature is useless because if you do not maintain the rho/R0 constant, yes the non linearity is increased but as the electrode is farer, the beam does not see it so much.

I hope it helps,

Romuald

[hendrik.haehnel]

Dear Romuald,
thank you very much for answering my curiosity.

Best regards,
Hendrik