amangkurat: Idealized (1+1)D nonlinear Klein-Gordon solver

Overview

Numerical solver for the $(1+1)$-dimensional nonlinear Klein-Gordon equation using symplectic pseudo-spectral methods. Features adaptive timestepping, Numba acceleration, and scientific data output.

Key Features:

• Pseudo-spectral Fourier discretization (exponential convergence)
• Størmer-Verlet symplectic integration (structure-preserving)
• Adaptive CFL-based timestepping
• Parallel execution with Numba
• NetCDF4 data output + 3D animations

---

Mathematical Foundation

Klein-Gordon Equation

$(1+1)$-dimensional relativistic field equation in natural units ($c = \hbar = 1$):

$$\frac{\partial^2 \phi}{\partial t^2} - \frac{\partial^2 \phi}{\partial x^2} + V'(\phi) = 0$$

Supported Potentials

Potential	$V(\phi)$	$V'(\phi)$	Physics
Linear	$\frac{1}{2}m^2\phi^2$	$m^2\phi$	Free massive scalar field
$\phi^4$	$\frac{\lambda}{4}(\phi^2 - v^2)^2$	$\lambda\phi(\phi^2 - v^2)$	Topological kinks, domain walls
Sine-Gordon	$1 - \cos(\phi)$	$\sin(\phi)$	Breathers, Josephson junctions

Numerical Method

Spatial: Pseudo-spectral Fourier with wavenumbers $k_n = 2\pi n/L$

Temporal: Størmer-Verlet (symplectic leapfrog) $$\phi^{n+1} = 2\phi^n - \phi^{n-1} + \Delta t^2 \left[\nabla^2\phi^n - V'(\phi^n)\right]$$

Stability: CFL condition $\Delta t \leq 0.5/k_{\max}$ enforced via adaptive timestepping

---

Installation

From Source

git clone https://github.com/sandyherho/amangkurat.git
cd amangkurat
pip install -e .

From PyPI

pip install amangkurat

Dependencies: numpy, scipy, matplotlib, netCDF4, tqdm, numba (optional, recommended for 10-100× speedup)

---

Quick Start

Command Line

amangkurat case1           # Linear wave
amangkurat case2           # Kink soliton
amangkurat case3           # Breather
amangkurat case4           # Kink-antikink collision

# Options
amangkurat case1 --cores 8 --output-dir results/ --quiet
amangkurat --all           # Run all cases

Python API

from amangkurat import KGSolver
from amangkurat.core.initial_conditions import KinkIC

# Initialize solver
solver = KGSolver(nx=512, x_min=-30.0, x_max=30.0, adaptive_dt=True, n_cores=4)

# Create initial condition
ic = KinkIC(vacuum=1.0, position=0.0, velocity=0.0)
phi0, phi_dot0 = ic(solver.x)

# Solve
result = solver.solve(
    phi0=phi0, phi_dot0=phi_dot0,
    dt=0.005, t_final=50.0,
    potential='phi4', lambda_=1.0, vacuum=1.0,
    n_snapshots=200
)

# Access results
x = result['x']      # Spatial grid
t = result['t']      # Time snapshots  
phi = result['phi']  # Field evolution (nt × nx)

---

Test Cases

Case	Potential	Initial Condition	Description
1	Linear	Gaussian pulse	Dispersive wave propagation
2	$\phi^4$	Static kink	Topological soliton ($\phi = v\tanh(x/\xi)$)
3	Sine-Gordon	Breather	Time-periodic localized oscillation
4	$\phi^4$	Kink-antikink pair	Collision dynamics ($v = 0.3c$)

---

Configuration

Simple key-value format (configs/case*.txt):

scenario_name = Kink Soliton
nx = 1024
x_min = -50.0
x_max = 50.0
t_final = 50.0
dt = 0.005
adaptive_dt = true

potential = phi4
lambda = 1.0
vacuum = 1.0

ic_type = kink
position = 0.0
velocity = 0.0

n_cores = 0
save_netcdf = true
save_animation = true
n_frames = 200
fps = 30
colormap = inferno

---

Output Files

NetCDF4 data (outputs/*.nc):

• Variables: x(nx), t(nt), phi(nt, nx)
• Metadata: grid parameters, potential type, timestep info

Animated GIFs (outputs/*.gif):

• 3D visualization of $\phi(x,t)$ evolution
• Dynamic camera rotation, color-coded amplitude

Log files (logs/*.log):

• Detailed diagnostics, parameter listings, timing

---

Physical Units

Code uses dimensionless units. Map to physical systems via scale factors:

$\phi^4$ Kinks

Natural scales: Length $\xi = \sqrt{2}/v$, Time $T = \xi/c$, Energy $E_0 = \frac{2\sqrt{2}}{3}\lambda v^3$

Context	Length Scale	Time Scale	Application
Particle physics	$\xi \sim 10^{-18}$ m	$T \sim 10^{-26}$ s	Higgs field (theoretical)
Condensed matter	$\xi \sim 10$ nm	$T \sim 1$ ps	Ferromagnetic domain walls
Josephson junctions	$\lambda_J \sim 1$ μm	$T \sim 10$ ps	Superconducting phase

Conversion:

units = result['units']
x_physical = result['x'] * length_scale_physical / units.length_scale

---

Directory Structure

amangkurat/
├── configs/              # Test configurations
├── src/amangkurat/
│   ├── core/            # Solver + initial conditions
│   ├── io/              # NetCDF + config parser
│   ├── visualization/   # 3D animations
│   └── utils/           # Logger + timer
├── outputs/             # Results (*.nc, *.gif)
├── logs/                # Simulation logs
└── pyproject.toml       # Build config

---

Limitations

• 1D spatial domain only ($(1+1)$D spacetime)
• Periodic boundary conditions (not suitable for scattering)
• Uniform grid (no adaptive mesh refinement)
• CFL-limited timestep (explicit integration)
• Requires smooth solutions (spectral methods)

---

Citation

@software{herho2025_amangkurat,
  author = {Herho, Sandy H. S.},
  title = {amangkurat: Idealized Nonlinear Klein-Gordon Solver},
  year = {2025},
  version = {0.0.1},
  url = {https://github.com/sandyherho/amangkurat}
}