# Reactors

`canterax.ReactorNet` advances constant-pressure adiabatic reactor trajectories using JAX-based ODE solvers.

## Model scope

The current reactor path is:

- zero-dimensional
- constant pressure
- adiabatic

The state vector is arranged as:

```text
[T, Y_0, Y_1, ..., Y_{n-1}]
```

## Basic usage

```python
import cantera as ct
import jax.numpy as jnp

from canterax import ReactorNet, Solution

gas = Solution("gri30.yaml")
gas.TPX = 1200.0, ct.one_atm, "H2:2.0, O2:1.0, N2:3.76"

net = ReactorNet(gas.mech)
sol = net.advance(
    T0=gas.T,
    P=gas.P,
    Y0=jnp.array(gas.Y),
    t_end=1e-3,
)
```

## Solver choices

By default, `advance(...)` uses Diffrax `Kvaerno5`, which is a stiff implicit Runge-Kutta method.

You can also request the experimental custom BDF path:

```python
sol = net.advance(
    T0=gas.T,
    P=gas.P,
    Y0=jnp.array(gas.Y),
    t_end=1e-3,
    solver="bdf",
)
```

## Output shape

Default Diffrax path:

- `sol.ts` contains saved times
- `sol.ys` contains saved states
- `sol.ys[:, 0]` is temperature
- `sol.ys[:, 1:]` are species mass fractions

Experimental BDF path:

- returns a dictionary-like structure with `ts`, `ys`, and `stats`
- `stats` includes step and function-evaluation counters

## Saving trajectories

Use Diffrax `SaveAt` if you want samples at prescribed times.

```python
import diffrax
import jax.numpy as jnp

ts = jnp.linspace(0.0, 1e-3, 200)
saveat = diffrax.SaveAt(ts=ts)
sol = net.advance(
    T0=gas.T,
    P=gas.P,
    Y0=jnp.array(gas.Y),
    t_end=1e-3,
    saveat=saveat,
)
```

## Practical notes

- `Y0` should be a JAX array if you are staying in a JAX-native workflow.
- The reactor implementation is designed for differentiable simulation, but the current docs focus on forward simulation behavior.
- The BDF path is marked experimental and has more limited differentiation support than the default Diffrax solver.

## Validation image

The repository includes parity plots comparing Canterax and Cantera reactor trajectories.

```{image} ../images/gri-30_methane_trajectory.png
:alt: Methane reactor trajectory parity plot
:width: 85%
```