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<p><b>New page</b></p><div>{{Short description|Magnetic field component encircling the plasma cross-section in toroidal systems}}<br />
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[[Book:Quantum_Collection/Matter_(by_scale)#Fields|← Back to Matter (by scale) Fields]]<br />
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The '''poloidal field''' is the component of the magnetic field that runs around the short (poloidal) direction of a toroidal plasma, such as in a [[Physics:Quantum Tokamak|tokamak]]. Together with the toroidal field, it forms helical magnetic field lines that confine charged particles within the plasma.<br />
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In magnetic confinement systems, the combination of toroidal and poloidal fields is essential for achieving stable confinement. Without a poloidal component, particles would drift across field lines and escape the plasma.<br />
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<div style="float:right; border:1px solid #e0d890; background:#fff8cc; padding:6px; margin:0 0 1em 1em; width:400px;"><br />
[[File:Tokamak poloidal toroidal fields.svg|400px]]<br />
<div style="font-size:90%;">Magnetic field geometry in a tokamak: the toroidal field wraps around the torus, while the poloidal field encircles the plasma cross-section, producing helical field lines.</div><br />
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== Physical meaning ==<br />
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In a toroidal geometry, two directions are defined:<br />
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* '''Toroidal direction''' — around the major axis of the torus <br />
* '''Poloidal direction''' — around the minor cross-section <br />
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The poloidal field follows the second direction and is typically weaker than the toroidal field. However, it plays a crucial role in shaping the magnetic topology.<br />
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The resulting helical field lines guide charged particles, preventing them from drifting freely across the plasma.<br />
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== Generation of the poloidal field ==<br />
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In a tokamak, the poloidal field is primarily generated by an electric current flowing through the plasma itself. This current is induced by transformer action and produces a magnetic field that encircles the plasma column.<br />
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Additional external coils can also contribute to the poloidal field, allowing control of plasma position and shape.<br />
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== Role in confinement ==<br />
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The poloidal field is essential for magnetic confinement:<br />
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* It twists magnetic field lines into helices <br />
* It reduces particle drift losses <br />
* It contributes to closed magnetic surfaces <br />
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Without the poloidal component, the magnetic field would be purely toroidal, and charged particles would gradually escape due to curvature and gradient drifts.<br />
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== Relation to safety factor ==<br />
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The ratio between toroidal and poloidal field components determines the '''safety factor''':<br />
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<math><br />
q = \frac{\text{toroidal turns}}{\text{poloidal turns}}<br />
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This parameter describes how magnetic field lines wind around the torus and is a key quantity in plasma stability analysis.<br />
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== Connection to plasma stability ==<br />
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The strength and structure of the poloidal field influence many plasma instabilities:<br />
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* [[Physics:Kink instability|Kink instability]] depends on field line twist <br />
* [[Physics:Tearing mode|Tearing mode]] occurs at rational surfaces <br />
* [[Physics:Ballooning instability|Ballooning instability]] is affected by field curvature <br />
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Careful control of the poloidal field is therefore required to maintain stable confinement.<br />
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== Applications ==<br />
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Poloidal fields are central to:<br />
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* [[Physics:Quantum Tokamak|Tokamak]] operation <br />
* [[Physics:Quantum Magnetically confined plasmas|Magnetic confinement systems]] <br />
* Plasma shaping and equilibrium control <br />
* Fusion reactor design <br />
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They are also relevant in astrophysical plasmas where toroidal and poloidal field components coexist.<br />
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== Physical interpretation ==<br />
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The poloidal field represents the coupling between plasma current and magnetic confinement. It transforms a simple toroidal field into a structured, self-consistent system capable of confining high-temperature plasma.<br />
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Together with the toroidal field, it defines the geometry of magnetically confined plasmas.<br />
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=See also=<br />
{{#invoke:PhysicsQC|tocHeadingAndList|Physics:Quantum basics/See also}}<br />
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=References=<br />
{{reflist|3}}<br />
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{{Author|Harold Foppele}}<br />
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{{Sourceattribution|Poloidal field|1}}</div>imported>WikiHarold