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Minutes of #14 Remote E-Beam Meeting (draft)

The meeting was held on Zoom on 24/11/2021 - See indico

Participants

Nicolo Biancacci, Jean Cenede, Roberto Corsini, Davide Gamba, Mikko Karppinen, Andrea Latina, Oliver Meusel (Frankfurt University), Laurette Ponce, Adriana Rossi, Kathrin Schulte-Urlichs (GSI), James Storey, Luke Von Freeden, Fredrik Wenander, Michail Zampetakis,

New AD e-cooler particle tracking in Opera (Luke Von Freeden)

Luke started with an introduction about e-cooling.

The simulations done with Opera take only magneto-static effects into account. No self field of the particles (e.g. space charge) nor interactions with walls (e.g. indirect-space charge) effects are taken into account by the solver presently used by Luke within Opera. By default, particle emitters can be defined only on a square grid with uniform spacing between emitter. Luke overcome this limitation by profiting of in-opera python console which allows to programmatically define a local coordinate system as as placing emitters at any desired location.

The magneto-static system is solved on a 3D triangular mesh. Grading of the mesh can be adjusted, and made more fine at location of interest, e.g. where the beam is supposed to pass. Typical mesh steps are of the order of 15 mm. (Note: in slide "Numerical sensitivity" says "0.30mm largest delta between meshes": what is this?)

Typical time to solve magneto-static system is of the order of 4 hours for the new AD e-cooler. This time can vary considerably depending on the level of complexity of the object (e.g. number of coils, ferromagnetic material) as well as mesh size at the location of interest.

After magneto-static system is solved, tracking of particles is fast (minutes time scale) and one can track particles with a step size smaller than the magnetic mesh size: in this case, fields are interpolated using non-linear interpolation at the particle location. A first study varying the magneto-static mesh size and track discretization (0.5 mm or 1 mm) did not show major variation of the tracked particles from the gun till the end of the e-cooler drift, suggesting that mesh size is probably good enough.

Another study snows that the impact of the orbit correct for the ions (i.e. external to the path of the electron) has a strong effect on the orbit of the e^- (20 mm transverse drift!). A small effect (a few mm) is also visible just for the presence of the ferromagnetic material of the orbit corrector itself.

Luke commented that it is not possible to "solve" the trajectory of the e^- particles just using harmonic analysis of the system, i.e. by decomposing the field in transverse magnetic field of different orders (dipole, quadrupole, sextupole and so on) because of the presence of the solenoidal field that couples transverse and longitudinal motion.

Next step of the simulation work will be to optimize for e^- corrector coils powering such to optimize the e^- trajectory toward a more ideal trajectory.

Discussion

Adriana stressed that those simulation will not be able to show all the dynamics of the e^- beam transport due to missing space-charge considerations. On the other hand, the approach for the HEL design is similar:

  • first the single particle trajectory is optimized in Opera
  • then the magneto-static solution from Opera is input to CST for detailed tracking simulation of the e^- beam.

The choice of this workflow instead of using CST directly is due to poor magnetic modeling capabilities of CST. Mikko commented that other solver within Opera could take into account electrostatic and self-field effects. Luke had a preliminary look at the "scala" solver, which in principle could be tried. On the other hand, he also believe the present strategy (Opera + CST) is probably the most convenient.

Adriana asked how easy would be to setup a similar simulation framework for the HEL. Luke commented that about 2 weeks time are need to model an e-cooler like object in Opera, then running time depends on the desired accuracy and number of configurations to be studied. Mikko stressed that adding small aspect ratio components to a big object can quickly result in meshing problems (and so long running time). More in general, for "conductor-dominated" systems the modeling is much faster/easier than when one have to take into account ferromagnetic material around the object, as those then require additional meshes to include them.

Adriana pointed out that in HEL the girder material choice had a strong impact on magnetic field, and asked if similar studies could be possible here. Luke replied that indeed the presence of the ion orbit corrector iron shows an effect on the e^- trajectory. Including the e-cooler girder in the simulation might be needed at a second stage to refine the e^- trajectory correction scheme.

Adriana asked if one can do programatic scans of the geometry in Opera (e.g. moving the position of a steering coil) Luke replied this is in principle possible using the python terminal inside Opera, but might require some thinking. Typically they manually create a few configurations and run the magneto-static solver over night go get all solutions ready for tracking the day after.

Luke asked if the electron beam has indeed to be centered. Davide replied in the ideal world e^- should be well centered in the vacuum chamber and should occupy a most of the chamber such to well intercept the ion beam as well as to minimise the e^- transverse temperature (reason for implementing the beam expansion). In practice, ions are steered such that the are parallel to e^- beam but not necessarily centered to each other: a small offset can be beneficial on cooling efficiency profiting of e^- space charge and dispersion effects in the circulating ion beam. Davide suggest to include the pbar trajectory in the next studies, as this seems to be computationally inexpensive and could give some hint on the effect one should expect on the pbar beam and therefore compute the needed strength of nearby orbit correctors to ensure a good overlap of e^- and pbar beams.