Bootstrap current density

The bootstrap current density is tracked with the unknown j_bs, and is disabled by default as it typically vanishes during disruptions as the plasma profiles flattens. It is determined mainly by gradients in the the density and temperature profiles, following the formula

\[\frac{j_\mathrm{bs}}{B} = -\frac{G}{ \langle B^2\rangle \psi'_\mathrm{ref} } \bigg[ \mathcal{L}_{31} \frac{p}{n} \frac{ \partial \ln n }{ \partial r } +( \mathcal{L}_{31} + \mathcal{L}_{32} ) p_e \frac{ \partial \ln T_e }{ \partial r } +\mathcal{L}_{31} (1+\alpha) \sum_i p_i \frac{ \partial \ln T_i }{ \partial r } \bigg],\]

where \(p\) and \(n\) are the total pressure and density, respectively, summed over the thermal electrons and all ion species. In case ion temperatures are not evolved in time, then ions are assumed to be in thermal equilibrium with the electrons, and \(T_i=T_e\). The coefficients \(\mathcal{L}_{31}\), \(\mathcal{L}_{32}\), and \(\alpha\) are functions of the local fraction of trapped particles, the effective charge, and the electron collisionality (or ion collisionality for \(\alpha\)).

Bootstrap models

Various models exist, and these mainly vary in how the coefficients \(\mathcal{L}_{31}\), \(\mathcal{L}_{32}\), and \(\alpha\) are defined. Currently, only the model given in Redl et al. (2021) is implemented in DREAM.

The following table summarises the available options:

Name	Description
BOOTSTRAP_MODE_DISABLED	Neglects the bootstrap current, setting \(j_\mathrm{bs}=0\).
BOOTSTRAP_MODE_REDL	Uses the model given in the Redl paper.

The bootstrap current model can be set following the following example:

import DREAM.Settings.Equations.BootstrapCurrent as BootstrapCurrent

ds = DREAMSettings()

ds.eqsys.j_bs.setMode(BootstrapCurrent.BOOTSTRAP_MODE_REDL)

Initialisation mode

When providing the initial current density in a simulation, it can either be the total current density \(j_\mathrm{tot}\) or the Ohmic current density \(j_\Omega\) that is known. By default, it is assumed that it is the total that is provided, from which the Ohmic component is then calculated by subtracting the bootstrap current density (and other components e.g. the runaway current).

However, if it is the Ohmic component that is known, simply include the following in the simulation setup script:

ds.eqsys.j_bs.setInitMode(BootstrapCurrent.BOOTSTRAP_INIT_MODE_OHMIC)

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