Tokamak equilibria with nearly zero central current: the current hole

Abstract

The observation of stable sustainment of the ‘current hole’, namely the nearly zero current density region in the central part of a tokamak plasma, has opened a new class of configurations in tokamak plasmas, and a variety of research from the viewpoints of equilibrium, magnetohydrodynamics (MHD) stability, particle orbits and radial transport has been generated. Some theories and codes have been tested and extended by being applied to extreme conditions in the current hole with very weak poloidal field. The current hole is generated due to a transient negative toroidal electric field established when a large off-axis non-inductive current is rapidly formed. It has been observed in high confinement plasmas with a large fraction of bootstrap current in advanced tokamak operation. The current hole is very stiff against current drive, which suggests that it is a saturated or self-organized system. Appearance of the current hole in ITER and DEMO would be expected in some of the operation scenarios, and its influence and its control methods have been studied. Results of experimental and theoretical studies on the current hole are reviewed.

Analytical solutions for Tokamak equilibria with reversed toroidal current

“…On the other hand, a reversed current density with a negative density in the plasma center has not yet been observed. 3,5,6 However, if the increase in non-inductive current is formed rapidly, the toroidal electric field, transiently negative at the central region, may create a negative central current density. 6 These scenarios open the possibility of a new class of tokamak configurations, stimulating new theoretical and experimental investigations.…”

“…3,5,6 However, if the increase in non-inductive current is formed rapidly, the toroidal electric field, transiently negative at the central region, may create a negative central current density. 6 These scenarios open the possibility of a new class of tokamak configurations, stimulating new theoretical and experimental investigations. In this context, our analytical results may contribute to further investigations on stability and transport in non-nested equilibrium plasma magnetic surfaces.…”

“…5 Whenever a reversed current density is created, the magnetic surfaces are non-nested with magnetic islands since a magnetic field component normal to the average magnetic surface is generated. 5,6 As this new configuration with current hole opens a new regime to be explored in tokamaks, 5 it is useful to know solutions of the Grad-Shafranov equation 1 which describe magnetohydrodynamic (MHD) equilibria with non nested magnetic surfaces.…”

Fully non-inductive plasma start-up with lower-hybrid waves using the outboard-launch and top-launch antennas on the TST-2 spherical tokamak

“…Because of the low density, the same discharge was repeated and the measurements were ensemble averaged to bring the Thomson scattering signal out of the noise floor [19]. In fully non-inductive plasma current rampup, current holes due to back EMF with little particle and energy confinement have been observed elsewhere [20]. The density and temperature profiles were flat inside the current hole (a "box-type" profile) in these discharges.…”

Reduced resistive MHD in Tokamaks with general density

“…The "Current Hole" phenomena has been indirectly observed in JET [23] and JT-60 [21] and is a scenario for the ITER machine. It is of major physical interest to better understand the Current Hole in view of the ITER project, see [20] and references therein. Following [13] we consider that reduced MHD models can be helpful in this direction.…”