Time-step check of the case (b) pedestal growth rates
Entry 73 found three toroidal mode numbers of the TORAX ITER hybrid state growing at the pedestal: n = 12, 16 and 20. Decision 7 asks that a growth rate γ change by no more than the agreed tolerance when the time step is halved and when the grid is doubled. This entry is the time-step half of that check.
Method
Repeating the slow rise from round-off at half the time step would have cost several jobs. We therefore restarted each mode twice from the same saved state, once at Δt = 1 and once at Δt = 0.5, and fitted both runs over the same window. All three rules were fixed before the job ran, with every input recorded by checksum: - which modes to restart: those growing in entry 73; - which saved state: the first one inside each mode’s fit window; - how to compare the runs.
What happened
All six runs reached the time limit with no solver abort. Halving the time step changed γ by at most about one part in ten thousand, and every mode stayed at the pedestal (restarting n = 12, 16 and 20 from the same saved state at Δt = 1 and Δt = 0.5 changes γ by 1.3e-5, 9.6e-5 and 7.9e-5, against the 5% tolerance of decision 7; the location stays at the pedestal in both runsC-248). As a control, the Δt = 1 restarts reproduce the original job’s growth rates almost exactly (the Δt = 1 restarts reproduce the original job’s growth rate over the shared window to 9e-10–3e-8C-249). The case (b) pedestal growth rates are therefore converged in time step (γ√(μ₀ρ₀) = 0.00218 (n = 12), 0.00192 (n = 16) and 0.00288 (n = 20), flow perturbation peaked at ψ_N 0.925–0.975; converged in time step (C-248)C-251).
The same job used its spare cores to run the axisymmetric start of case (b) on the doubled grid. It passed its health check and is kept as the starting state for a grid check (50 axisymmetric steps on the 160 × 256 grid in 40 min on 64 cores; the state passed its health check and is kept as the start for a grid checkC-250).
What is still open
The grid check. On the doubled grid these modes would again have to rise from round-off, which takes many jobs, and JOREK in this version cannot start the fine grid from the coarse mode. The owner’s decision on how to proceed is pending (OQ-43). Until then the case (b) growth rates are converged in time step only, on a fixed boundary at the separatrix that stabilises peeling modes artificially.
Technical details → JOREK. Decision → OQ-43.
JOREK acknowledgement. JOREK is developed by the JOREK team and is open source under the GNU LGPL v3. Results here use JOREK at commit 01a52be3. Citations: M. Hoelzl et al., Nuclear Fusion 61, 065001 (2021); M. Hoelzl et al., Nuclear Fusion 66, 116006 (2026). For a list of JOREK team members, refer to the author list of M. Hoelzl, G.T.A. Huijsmans, F.J. Artola, E. Nardon, M. Becoulet, N. Schwarz, A. Cathey, S.J.P. Pamela et al., Nuclear Fusion 64, 112016 (2024).
Technical details → JOREK › case b dt check
Decisions → OQ-39 · OQ-43