Confinement and Transport in Spherical Tokamaks: A Review

Confinement and Transport in Spherical Tokamaks: A Review

Confinement and Transport in Spherical Tokamaks: A Review 150 150 Mathew
UKAEA-CCFE-PR(22)40

Confinement and Transport in Spherical Tokamaks: A Review

In this paper, we review the confinement and transport properties observed and predicted in low aspect ratio tokamaks, or spherical tokamaks (STs), which can depart significantly from those observed at higher aspect ratio. In particular, thermal energy confinement scalings show a strong, near linear dependence of energy confinement time on toroidal magnetic field, while the dependence on plasma current is more modest, the opposite of what is seen at higher aspect ratio. Furthermore, STs show a strong increase of normalized confienemnt with decreasing collisionality, a dependence that is much stronger than that at higher aspect ratio. These differences reflect the fundamental differences in transport in STs due to the more extreme toroidicity than plasmas at higher aspect ratio, and to the relatively larger E × B shearing rates, both of which can suppress electrostatic drift wave instabilities at both ion and electron gyroradius scales. In addition, the importance of much stronger electro-magnetic effects due to the ST operating at high β T are clearly observed and inferred. These latter effects bring into light importance both Microtearing modes and Kinetic Ballooning modes have a much stronger impact in the core of STs plasmas than at higher aspect ratio. These differences have led to inferring a very strong improvement in normalized confinement with decreasing collisionality, Ωτ E ∝ ν e,∗ −1 , much stronger than at higher aspect ratio, which bodes well for an ST-based fusion pilot plant should this trend continue. Gyrokinetic studies, coupled with low- and high-k turbulence measuements, have shed light on the underlying physics controlling transport. At lower β, both ion- and electron-scale electrostatic drift turbulence may be responsible for transport, while at higher β, MTMs, KBMs, and hybrid TEM-KBMs play a role. Flow shear will, of course affect the balance between ion- and electron-scale modes. Non-linear gyrokinetic simulations find regimes where the electron heat flux decreases with decreasing collisionality, consistent with the experimental global normalized confinement scaling. The ST is unique in that the relatively low toroidal magnetic field allows for localized measurements of electon-scale turbulence, and this coupled with turbulence measurements at ion-scales has facilitated detailed comparisons with gyrokinetic simulations. These data have provided compelling evidence for the presence of ITG and ETG turbulence in some plasmas, and direct experimental support for the impact of experimental actuators like γ E , R/L n and magnetic shear on turbulence and transport.

Collection:
Journals
Journal:
Plasma Physics and Controlled Fusion
Publisher:
IOP (Institute of Physics)
Published date:
17/11/2021