Neutral Beam Plasma Attenuation Calculator
Auxiliary Plasma Heating: Calculate neutral beam ionization cross-section ($\sigma_{eff}$), plasma optical depth ($\tau$), shine-through first-wall heat load, and deposition profile.
Neutral Beam Injection (NBI) Parameters
Attenuation, Trapping & Shine-Through
Neutral Beam Penetration & Optical Depth Physics
Neutral beam heating efficiency depends directly on the optical depth $\tau = \int n_e \sigma_{eff} dl$. If $\tau$ is too small, excessive shine-through damages the vessel wall; if $\tau$ is too large, the beam is stopped prematurely at the edge, failing to heat the core.
1. Governing Equations
τ = ∫ n_e · σ_eff · dl ≈ n̄_e · σ_eff · L f_shine = exp(-τ) P_shine = P_inj · exp(-τ) P_abs = P_inj · (1 - exp(-τ))
2. Three Atomic Ionization Channels
- Charge Exchange: Dominates at lower beam energies.
- Proton/Ion Impact Ionization: Dominates at intermediate and high energies.
- Electron Impact Ionization: Contributes a steady ionization baseline across all beam energies.
Frequently Asked Questions
What is Neutral Beam Injection (NBI) in fusion devices?
NBI is a primary auxiliary heating and current drive system in tokamaks and stellarators. Energetic neutral atoms (deuterium or hydrogen accelerated to $50\sim 1000\,\text{keV}$) are injected into the torus. Because they carry no electrical charge, they freely cross the confining magnetic field lines until they collide with background plasma ions and electrons, undergo ionization, and become magnetically trapped energetic fast ions.
Why is beam shine-through dangerous to the first wall?
Shine-through refers to the fraction of high-energy neutral atoms that travel completely through the plasma chord without being ionized $(P_{shine} = P_0 e^{-\tau})$. These neutrals strike the opposite first wall with their full kinetic energy, generating concentrated localized surface heat fluxes. If heat flux exceeds the cooling limit of the tungsten or beryllium armor (typically $> 5\,\text{MW/m}^2$), armor ablation or water cooling tube melting occurs.
Why do reactor-scale tokamaks like ITER require negative-ion NBI (N-NBI)?
Positive ion neutral beam systems suffer an exponential drop in neutralization efficiency above $100\,\text{keV}$ (falling below $10\%$ at $200\,\text{keV}$). Because large fusion devices like ITER require beam energies of $1.0\,\text{MeV}$ ($1000\,\text{keV}$) to penetrate the dense core, they must use Negative Ion Neutral Beams ($D^-$), which maintain a high neutralization efficiency of $\sim 60\%$ in gas cells.