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Internal Transport Barrier: Difference between revisions
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No generally accepted definition for Internal Transport Barriers (ITBs) exists. Vaguely speaking, a radially localized reduction of transport | No generally accepted definition for Internal Transport Barriers (ITBs) exists. Vaguely speaking, the term refers to a radially localized reduction of transport for ions or electrons. | ||
ITBs can be actively produced by modifying the current profile using external means. | |||
<ref>R.C. Wolf, ''Internal transport barriers in tokamak plasmas'', [[doi:10.1088/0741-3335/45/1/201|Plasma Phys. Control. Fusion '''45''' (2003) R1-R91]]</ref> | |||
They are used to improve plasma confinement and stability properties, and to drive additional [[Bootstrap current|bootstrap current]]. Therefore, they are included in some alternative operational scenarios for [[ITER]]. | |||
== Physical mechanism == | == Physical mechanism == | ||
The mechanism for the formation of Internal Transport Barriers in magnetically confined plasmas is complex and not fully understood. | The mechanism for the formation of Internal Transport Barriers in magnetically confined plasmas is complex and not fully understood. | ||
The general consensus is that ITBs are the consequence of turbulence suppression due to sheared (''E'' × ''B'') flows. | |||
<ref>K.H. Burrell, ''Effects of E × B velocity shear and magnetic shear on turbulence and transport in magnetic confinement devices'', [[doi:10.1063/1.872367|Phys. Plasmas '''4''' (1997) 1499]]</ref> | |||
Such sheared flows can be generated by the turbulence itself via the [[Reynolds stress]] mechanism. | |||
This process is related to the formation of the [[H-mode]] barrier. | |||
Factors contributing to the | ITBs are often found to be associated with rational magnetic surfaces. | ||
<ref> | |||
* | Factors contributing to the formation of ITBs include: | ||
* [[Magnetic shear]] and the shape of the | <ref>J.W. Connor et al, ''A review of internal transport barrier physics for steady-state operation of tokamaks'', [[doi:10.1088/0029-5515/44/4/R01|Nucl. Fusion '''44''' (2004) R1-R49]]</ref> | ||
* The power deposited inside the [[Flux surface|magnetic surface]], and/or local pressure gradients | |||
* [[Magnetic shear]] and the shape of the [[Rotational transform|rotational transform]] profile (e.g., reversed shear) | |||
* MHD activity | * MHD activity | ||
* Momentum torques | * Momentum torques (poloidal or toroidal) | ||
* Enhanced collisionless losses of trapped particles, generating a radial electric field <ref>U. Stroth et al, ''Internal Transport Barrier Triggered by Neoclassical Transport in W7-AS'', [[doi:10.1103/PhysRevLett.86.5910|Phys. Rev. Lett. '''86''' (2001) 5910 - 5913]]</ref> | |||
* Reduced collisional damping, allowing the growth of zonal flows <ref>K. Itoh et al, ''Physics of internal transport barrier of toroidal helical plasmas'', [[doi:10.1063/1.2435310|Phys. Plasmas '''14''' (2007) 020702]]</ref> | |||
The relation between some of these factors can be understood from the steady state ion force balance for the radial electric field: | |||
:<math>E_r = \frac{1}{Zen_e}\nabla p_i -v_\theta B_\phi + v_\phi B_\theta</math> | |||
Thus, gradients in any of the quantities appearing in this equation may lead to sheared ''E'' × ''B'' flows. | |||
== References == | == References == | ||
<references /> | <references /> | ||