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A Faraday disk dynamo is a DC generator, producing an emf that is constant in time. It consists of a conducting disk that rotates with a constant angular velocity in the magnetic field, perpendicular to the disk's plane. The rotation of the disk causes a change in magnetic flux, which induces an emf, causing opposite charges to develop on the rim and in the center of the disk. The polarity of the induced emf can be determined by the direction of the magnetic field and the direction of the...

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Related Experiment Video

Updated: May 18, 2026

A 100 KW Class Applied-field Magnetoplasmadynamic Thruster

A 100 KW Class Applied-field Magnetoplasmadynamic Thruster

Published on: December 22, 2018

Not much helicity is needed to drive large-scale dynamos.

Jonathan Pietarila Graham¹, Eric G Blackman, Pablo D Mininni

¹Solid Mechanics and Fluid Dynamics (T-3) and Center for Nonlinear Studies, Los Alamos National Laboratory MS-B258, Los Alamos, New Mexico 87545, USA.

Physical Review. E, Statistical, Nonlinear, and Soft Matter Physics

|September 26, 2012

Summary

Large-scale magnetic fields in rotating astrophysical objects are amplified by helical turbulence. This study quantifies the minimum helicity fraction needed, finding it decreases with increasing scale separation, even for small fractions.

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Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

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Last Updated: May 18, 2026

A 100 KW Class Applied-field Magnetoplasmadynamic Thruster

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Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

Optimization, Test and Diagnostics of Miniaturized Hall Thrusters

Published on: February 16, 2019

Area of Science:

Astrophysics
Dynamo Theory
Magnetohydrodynamics

Background:

Large-scale magnetic fields are crucial in astrophysical rotators.
Turbulent helical velocities are known to amplify these fields.
The minimum helicity fraction required for this amplification was not well-quantified.

Purpose of the Study:

To quantify the minimum fractional kinetic helicity (f(h,C)) required for large-scale dynamo action.
To investigate the relationship between helicity fraction and the ratio of forcing to large-scale wavenumbers (k(F)/k(min)).

Main Methods:

Direct numerical simulations of a simple helical dynamo.
Theoretical modeling based on the back-reaction of nonhelical fields.

Main Results:

The minimum helicity fraction (f(h,C)) decreases as the ratio k(F)/k(min) increases.
For k(F)/k(min) ≥ 8, f(h,C) was found to be less than or equal to 3%.
Small helicity fractions significantly impact magnetic spectra even with moderate scale separation.

Conclusions:

A theoretical framework was developed to explain simulation results.
Very low helicity fractions are sufficient for strong magnetic field amplification in astrophysical dynamos.
This has implications for understanding magnetic field generation in turbulent systems.