Process reference: nbi

Generated from the docstrings of vaft.process.nbi: 1 public functions. Each entry is what vaft.process.describe("nbi.<name>") prints. All process categories

Overview

A transparent reference for NBI deposition: given a path coordinate and the attenuation coefficient along it, the neutral survival, the fast-ion birth per path cell and the shine-through, and – with the beam power and energy – the particle and power bookkeeping. It composes the relations of vaft.formula.nbi and adds nothing to their physics.

It is deliberately not NUBEAM, ASCOT5 or BEAMS3D. What each layer answers:

============================== =========== ============================== quantity this layer full solver (vaft.code.*) ============================== =========== ============================== beam particle rate yes yes neutral attenuation on a path yes yes, 3-D beamlets and footprint birth profile along the path yes yes, in (R, Z) and velocity shine-through yes yes, with divergence beam-stopping atomic data input built in prompt / delayed losses no yes slowing down, heating, drive no yes 3-D orbit effects no yes ============================== =========== ==============================

The “birth” power profile is where neutrals become fast ions, not where the plasma is heated. Related work: the VEST NBI description (#265) feeds the path and components; NUBEAM results and their IMAS mapping (#592) are what this is compared against, never re-derived; generic scales and orbits belong to #1064 and #1092; the diagrams follow #890/#1090.

Functions

neutral_beam_attenuation_along_pathNeutral survival, fast-ion birth and shine-through of one beam component on a 1-D path.

neutral_beam_attenuation_along_path

Machine-independent source 
neutral_beam_attenuation_along_path(s, alpha, *, beam_power=None, beam_energy_eV=None)

Neutral survival, fast-ion birth and shine-through of one beam component on a 1-D path.

NameTypeUnitDescription
snumpy.ndarraym

Path coordinate from the entry into the attenuating region to the exit, strictly increasing, 1-D.

alphanumpy.ndarray1/m

Attenuation coefficient $\sum_j n_j\sigma_j$ at each s, for the beam’s species and component energy, non-negative.

beam_powerfloat, optionalW

Power of this component at the entry.

beam_energy_eVfloat, optionaleV

Energy per particle of this component; with beam_power it adds the particle and power bookkeeping.
Returns
—NeutralBeamAttenuationany

Node quantities optical_depth [-] and survival_fraction [-]; per cell birth_fraction_per_cell [-] and birth_fraction_density [1/m]; shine_through_fraction and absorbed_neutral_fraction [-]; with power and energy also particle_rate [1/s], birth_rate_density [1/(m s)], power_birth_profile [W/m], shine_through_power and absorbed_beam_power [W].

Raises

  • ValueError s not strictly increasing or not 1-D, alpha of another length, negative or non-finite; only one of beam_power and beam_energy_eV given; a negative or non-finite power or a non-positive energy.
Processing steps · Applicability · Limitations · Provenance

Processing steps

  1. Optical depth at the nodes by trapezoidal quadrature, vaft.formula.nbi.neutral_beam_optical_depth.
  2. Survival $S = e^{-\tau}$ at the nodes.
  3. Births per cell $S_i - S_{i+1}$ – the neutrals lost between two nodes – and their density over the cell width; they telescope, so $\sum_i (S_i - S_{i+1}) + S_\mathrm{exit} = 1$ exactly on any grid.
  4. Shine-through $S_\mathrm{exit}$; absorbed fraction $1 - S_\mathrm{exit}$.
  5. With power and energy: $\dot N_b = P_b/E_b$, then rate and power per unit path in each cell, shine-through and absorbed power, which add up to $P_b$ exactly.

Applicability

Machine-independent. One energy component, a straight prescribed path, a supplied attenuation coefficient: no beam-stopping atomic data, no divergence, footprint, orbits, slowing down or losses after ionisation. Compare with, never substitute for, a full NBI code (vaft.code.nubeam).

Limitations

On a coarse grid ($\alpha\,\Delta s$ not small) the cell births remain exactly conserved, but the optical depth inherits the trapezoidal error of step 1, so the split between cells and the shine-through converge only as the grid is refined; the point density vaft.formula.nbi.beam_birth_probability_density does not integrate to the absorbed fraction there.

Provenance

  1. Issue #1136, the reduced NBI reference layer.
  2. J. Wesson, Tokamaks, 4th ed., Oxford University Press (2011), Sec. 5.4.

API

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