GPU tracing¶

SIMPLE traces collisionless guiding-centre losses in Boozer coordinates on an NVIDIA GPU with native CUDA kernels. The GPU run computes the segmented loss objective of arXiv:2606.19523, not the full set of CPU options.

Build and run¶

Needs the CUDA toolkit, a C++17 compiler and GNU or NVIDIA Fortran.

cmake -S . -B build-cuda -G Ninja -DSIMPLE_ENABLE_CUDA=ON
cmake --build build-cuda -j
(cd run && ../build-cuda/simple.x simple.in)

&config
isw_field_type = 2
gpu_mode = 'production'
gpu_backend = 'cuda-native'
gpu_method = 'dopri'
/

Requirements: integ_coords = 2, field_input a Boozer chartmap carrying the compact RK tables (rk_Bmod, … as written by libneo’s booz_xform_to_boozer_chartmap), no collisions, no wall, no orbit output, no classification, guiding-centre orbits. VMEC, coil and SPECTRE fields are not supported (#569). Other combinations stop at input validation, before the field setup.

Keys¶

Key	Default	Meaning
`gpu_mode`	`'off'`	`'production'` runs on the GPU
`gpu_backend`	`'cuda-native'`	only `'cuda-native'` (= `'auto'`) runs
`gpu_method`	`'dopri'`	`'dopri'` (Dormand-Prince) or `'cash-karp'`; tolerance `relerr`
`gpu_landreman`	`.true.`	segmented loss objective; must stay on
`gpu_t_block`	`1d-3`	segment length in s, at most `trace_time`
`gpu_loss_tau`	`1d-1`	decay time \(\tau_L\) (s) of the loss weight \(e^{-t/\tau_L}\)
`gpu_maxloss`	`2d-2`	stop threshold, see below
`gpu_min_timestep`	`1d-10`	smallest Runge-Kutta step in s
`gpu_start_coordinates`	`'reference'`	`'boozer'`: `start.dat` is in Boozer coordinates
`gpu_compact_init`	`.false.`	compact initializer; needs a Boozer chartmap and `gpu_start_coordinates = 'boozer'`
`gpu_particle_profile`	`''`	CSV with start/end state, field evaluations, loss step and loss time (s) per marker

Early stop¶

After each segment SIMPLE evaluates \(L = N^{-1}\sum_{\text{lost } i} e^{-t_i/\tau_L}\). When \(L >\) gpu_maxloss, tracing stops and the markers still in the plasma are written as confined to trace_time (exit code 0). The log line observed until gives the time actually traced. For loss fractions over the full trace_time, set gpu_maxloss = 1, which \(L \le 1\) never exceeds. The stop rule is in src/simple_cuda_native_rk54.cu.

The run writes the usual Outputs and uses one GPU per process. Tests (with CUDA): test_cuda_native_rk54, test_cuda_native_bridge.