Local reversed magnetic shear and the formation of a transport barrier

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:489366

Drake, J F; Lau, Y T; Guzdar, P N ^[1]

---

1. Univ. of Maryland, College Park, MD (United States); and others

We derive a set of 3-D nonlinear equations describing drift-resistive ballooning modes in a torus including the self-consistent modification of the local magnetic shear due to the shift of the flux surfaces in a plasma torus. 3-D simulations using these equations reveal that a bifurcation of the transport occurs when the local magnetic shear on the outside midplane reverses sign. Resistive ballooning modes in the edge of tokamak plasmas axe unstable at low values of the poloidal {beta}. As the local edge gradients increase, compression of the poloidal magnetic flux on the outside of the torus causes the local magnetic shear to reverse, which stabilizes resistive ballooning modes. The same mechanism stabilizes ideal modes at higher values of poloidal {beta}. The bifucation results when a local increase in the pressure gradient causes the negative shear stabilization of the edge turbulence to beat out the increase in transport resulting from the larger gradient. The complete structure of the bifurcation diagram, anomalous transport rate D versus the control parameter {beta}{sub p}, has been mapped out by completing a series of 3-D simulations. This fully self-consistent bifurcation diagram reveals significant hysteresis, i.e., the transport barrier is maintained at lower values of {beta}{sub p} than is required for the formation of the barriers as expected from the experimental observations. The approach taken in these calculations to map the structure of the transport bifurcation can be used as a template for transport calculations being carried out on internal transport barriers. The formation of local regions of negative shear have been measured in both H and VH mode on the outside midplane of the edge of DIII-D, consistent with the present theory.

• OSTI ID: 489366
• Report Number(s): CONF-960354--
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
BALLOONING INSTABILITY
BIFURCATION
CHARGED-PARTICLE TRANSPORT THEORY
MAGNETIC CONFINEMENT
PLASMA SIMULATION
SHEAR
TOKAMAK DEVICES