Ring mode

Ring mode tracks a beam turn after turn around a synchrotron. It is opt-in: a linac session never sees it, and switching it on changes nothing in the linac tabs or their results. It stores a beam at the ring's fixed energy or accelerates it through a ramp — the reference energy, the RF voltage and the RF phase programmed against time, transition crossed with its phase jump — with the RF bucket, chromaticity, nonlinear magnets, apertures and space charge (the 2.5-D model), and fills it turn by turn through a stripping foil with bump and painting magnets (injection).

Switching it on

Simulate → Ring Mode adds the Ring tab after the linac tabs and shows it; unticking the entry hides the tab again (a run in progress stops). A project remembers ring mode in a "ring" section, written only once ring mode has been used — a linac project is saved exactly as before.

When the lattice you open closes on itself — its horizontal bends add up to 360° and its survey returns to the start, or the deck declares ;@LG ring — the Lattice tab shows a note offering ring mode: Use ring mode switches it on, Keep as line hides the note for this lattice (the choice is saved with the project), Why? shows the evidence. Ring mode is never switched on by itself, and a lattice that does not bend through 360° never loads any of it.

Outside the GUI, python -m linac_gen ring runs the same computations (Command line), and the AI assistant has ring tools of its own — the verdict, the optics, the ramp and its preview, ring runs (AI Assistant).

The Ring tab

The tab opens with the verdict for the loaded lattice — whether it closes on itself, its length, the RF harmonic number when the deck gives one, and the reference beam — and a line of the latest optics (tunes, chromaticities, transition gamma). Below it are five pages.

Ring & Optics computes the ring's periodic optics from the ring kernels: tunes with their integer part, chromaticities, momentum compaction and transition, the Twiss functions and dispersion along the ring, and with RF the synchrotron tune and the bucket. Use the matched beam in the Beam tab gives the Beam tab the beam matched to the ring at the start of the run: the Twiss functions in x, y and, with the RF on, z, the dispersion and the closed orbit, from the optics of the run's first turn. The element model (Cartesian or curvilinear multipoles), the pole-face model and the physical constants are chosen here and saved with the project. See Ring optics and Element models.

Programs sets the ramp: the reference energy (fixed, flat, a resonant cycle or a table), the RF voltage and phase programs, transition gamma, the clock at turn 0, paraphasing groups; tables are read from and written to CSV files. Preview runs the reference particle alone through the ramp — the turns to the end of the program, transition, the RF frequency, the gain against the voltage — and Run to the end of the program gives the Run page that number of turns. See The ramp.

Space charge sets the 2.5-D model: off, particle-in-cell or a frozen Gaussian, the particles in the bunch, the kicks per turn, the grid, the pipe (open, round, rectangle, ellipse) and the longitudinal kick, with the analytic tune shift for the Beam tab's emittances once the optics are computed. See Space charge.

Injection sets the multi-turn injection through a stripping foil: the turns that inject, the macroparticles each brings and the current; the foil (where it is, its material and thickness, its rectangle); the injected beam (the Beam tab's, a particle file or a Gaussian at the foil; an H⁻ is stripped to a proton of the same velocity); the chopper; and the bump and painting magnets with their waveforms. See Injection.

Run tracks a bunch for a number of turns through the same ring kernels, at the ring's fixed energy or through the ramp of the Programs page, with the space charge of the Space charge page and the injection of the Injection page:

• Particles. The Beam tab's beam, as for a linac run (its distribution, number of particles, emittances and Twiss functions, generated at the ring's species and energy), or none (the injection fills the ring). A run that starts with a beam gives its BMAG against the ring's matched beam. A MAD-X or MAD8 deck whose BEAM gives the energy states the beam its magnets are scaled for: the ring runs at that reference. A lattice that states no beam — a TraceWin or elegant deck, a MAD-X or MAD8 BEAM with only the particle — runs at the Beam tab's species and energy (set them to the ring's); a deck of normalized strengths keeps them at that energy. The Beam tab's longitudinal emittance is in deg·MeV at the Beam tab's frequency.
• Tune probes (one small-amplitude probe gives the tunes; n gives an n × n footprint), snapshot turns (the whole distribution at those turns; last is the last turn), the device (every CPU core, or a CUDA GPU).
• Run and Stop. Stop takes effect at the end of the turn being tracked — a turn of a large bunch can take seconds — and keeps the turns done. The survival and emittances update as it runs. A run that would need more than 70 % of the machine's memory is refused before it starts.
• Results. While it runs, a strip of three plots: the survival, the normalized emittances x and y, and the longitudinal emittance turn by turn (statistics every turn, or every k turns for more than 100 000 turns). When it ends, its results are in the Results tab (Show in the Results tab, or the Results tab's Run shown: Ring run): the beam turn by turn in its tiles — sizes, emittances, Twiss, energy, survival, centroid, dispersion, the Σ matrix — and, under RING RUN, a ramp's reference energy, synchronous phase, voltage and frequency with the transition turn marked, the bunch in eV·s, phase space at the snapshot turns with the separatrix of the turn's RF bucket, losses along the ring and over the turns, the NAFF tunes and the footprint on a tune diagram, the run's periodic optics, and with a foil the injection turn by turn, the closed orbit at the foil and the foil's hit map (Ring runs in the Results tab). Every run, a stopped one too, is written to the calc directory as <time>_ring.h5 — one stopped by quitting HELIX too, if it ends its turn and writes its results within the five seconds HELIX waits.

See Ring runs. Every computation works on a copy of the lattice: the lattice in the window is never modified.

A ring that is unstable at the reference energy — the magnets scaled for another energy, an RF phase on the wrong side of transition — is refused with a message, rather than tracked until the numbers overflow. So are cavities that would give the reference particle energy in a run without a ramp: the reference energy changes only through a ramp.