Internal Transport Barrier

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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. [1] They are used to improve plasma confinement and stability properties, and to drive additional bootstrap current. Therefore, they are included in some alternative operational scenarios for ITER.

Physical mechanism

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. [2] 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.

ITBs are often found to be associated with rational magnetic surfaces.

Factors contributing to the formation of ITBs include: [3]

  • The power deposited inside the magnetic surface, and/or local pressure gradients
  • Magnetic shear and the shape of the rotational transform profile (e.g., reversed shear)
  • MHD activity
  • Momentum torques (poloidal or toroidal)
  • Enhanced collisionless losses of trapped particles, generating a radial electric field [4]
  • Reduced collisional damping, allowing the growth of zonal flows [5]

The relation between some of these factors can be understood from the steady state ion force balance for the radial electric field:

$ E_r = \frac{1}{Zen_e}\nabla p_i -v_\theta B_\phi + v_\phi B_\theta $

Thus, gradients in any of the quantities appearing in this equation may lead to sheared E × B flows.

References

  1. R.C. Wolf, Internal transport barriers in tokamak plasmas, Plasma Phys. Control. Fusion 45 (2003) R1-R91
  2. K.H. Burrell, Effects of E × B velocity shear and magnetic shear on turbulence and transport in magnetic confinement devices, Phys. Plasmas 4 (1997) 1499
  3. J.W. Connor et al, A review of internal transport barrier physics for steady-state operation of tokamaks, Nucl. Fusion 44 (2004) R1-R49
  4. U. Stroth et al, Internal Transport Barrier Triggered by Neoclassical Transport in W7-AS, Phys. Rev. Lett. 86 (2001) 5910 - 5913
  5. K. Itoh et al, Physics of internal transport barrier of toroidal helical plasmas, Phys. Plasmas 14 (2007) 020702