Elegant users forum

Hello,

I am new in Linac simulation using Elegant, and I am facing difficulty in the simulation of TW linac. I am attempting to model first two sections of an electron Linac in Elegant to obtain Twiss parameters and perform beam tracking.

The setup is following:
(1) Sub-harmonic buncher: 499.75 MHz.
(2) Section-1 (TW): 8 buncher cells (tapered length from 18.33 to 29.994 mm) followed by 60 accelerating TW-CI cells.
(3) Section-2 (TW): 114 TW-CG cells.
(4) External solenoid fields are superimposed on the structures.

Modelling approach:
Each cell is modelled as individual TWLA element to accommodate the local solenoid field value.
For the 8 buncher cells, phase is computed as following:
- Cell length are matched to 2.pi/3 mode with an increasing beta profile. Difference in beta in adjacent cells provides energy gain.
- Required phase is derived from the energy gain using sin^-1(Energy-gain/(EzT * gap)).
- Phase continuity is maintained by subtracting the phase slip 2.pi.L/(beta.Lambda) at each cell.

Issues encountered:
(1) Twiss computation failure. NaN for beta, alpha.
(2) Tracking issue: I receive error: missing beam sums pointer (finish_output).
(3) I tried with FOCUSSING=0 or 1, with particles from outside file, and also particles generated by Elegant itself using bunched_beam.

How to perform the optics (beam envelope and Twiss) computation correctly, and then beam tracking? Theses sections are stable and run finely on other codes.

Attached files: linac_2_sections.lte; LINAC_2_SECTIONS_INPUT.ele; beam_initial.sdds

Please help me in this matter, if I am modelling sections incorrectly or my phase computation method requires correction?

Thanks and regards,
Amal

Amal,

The main problem is that the TWLA elements are not set up using phase references. If you break a structure into cells, you should use a phase reference to ensure consistent phasing. See the attached file.

Even after fixing that, there are problems finding the matrices of the TWLAs, which prevents computations with &twiss_output and &moments_output. I'm still looking into that. However, I was able to optimize transmission of the beam using tracking. See the second attached file.

--Michael

Michael

Thank you very much for your reply and clarifying about the phase reference. Tracking is now working with optimized files.

Regards,
Amal.