Abstract
Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator CMod tokamakplasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidalmagnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed cocurrent at low density and abruptly changes direction to countercurrent when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified, since these processes occur in a particular range of the collisionality.
The authors thank C. L. Angioni, D. Pace, J. L. Terry, and A. E. White for enlightening discussions, R. S. Granetz for magnetics measurements, and the Alcator CMod operations group for expert running of the tokamak. Work supported at MIT by DoE Contract No. DEFC0299ER54512.
I. INTRODUCTION
II. EXPERIMENTAL SETUP
III. OHMIC CONFINEMENT SATURATION AND ROTATION REVERSALS
IV. ROTATION REVERSAL CHARACTERISTICS
V. DISCUSSION AND CONCLUSIONS
Key Topics
 Toroidal plasma confinement
 17.0
 Magnetic fields
 13.0
 Self organized systems
 13.0
 Ion temperature gradient mode
 8.0
 Magnetic field reversals
 7.0
H05H1/02
Figures
The energy confinement time (from kinetic profiles) as a function of average electron density for a series of 5.2 T, 0.81 MA Ohmic discharges. The shaded vertical bar indicates the boundary between the LOC and SOC regimes. The dashed line is the neoAlcator scaling, the solid line is the best fit to the low density points, and the dashdot line is the ITER89 P Lmode scaling.
The energy confinement time (from kinetic profiles) as a function of average electron density for a series of 5.2 T, 0.81 MA Ohmic discharges. The shaded vertical bar indicates the boundary between the LOC and SOC regimes. The dashed line is the neoAlcator scaling, the solid line is the best fit to the low density points, and the dashdot line is the ITER89 P Lmode scaling.
The energy confinement times from magnetics (top) and the core toroidal rotation velocities (bottom) as a function of line averaged electron density for 5.2 T discharges with plasma currents of 0.62 MA (left) and 1.0 MA (right). The vertical dashed lines indicate the locations of the co to countercurrent rotation boundaries.
The energy confinement times from magnetics (top) and the core toroidal rotation velocities (bottom) as a function of line averaged electron density for 5.2 T discharges with plasma currents of 0.62 MA (left) and 1.0 MA (right). The vertical dashed lines indicate the locations of the co to countercurrent rotation boundaries.
The transition density between the LOC and SOC regimes (top) and the critical density for core toroidal rotation reversals (bottom) as a function of plasma current for fixed magnetic field. The dotted lines have the same slope. The dashed line is an empirical scaling.^{3}
The transition density between the LOC and SOC regimes (top) and the critical density for core toroidal rotation reversals (bottom) as a function of plasma current for fixed magnetic field. The dotted lines have the same slope. The dashed line is an empirical scaling.^{3}
The electron and ion temperatures (top), their ratio (second frame), effective Z (third frame), and inverse density gradient scale length (bottom) at r/a = 0.6 (R = 0.80 m) as a function of electron density for 0.62 MA, 5.2 T discharges. The dotted vertical line indicates the LOC/SOC transition density.
The electron and ion temperatures (top), their ratio (second frame), effective Z (third frame), and inverse density gradient scale length (bottom) at r/a = 0.6 (R = 0.80 m) as a function of electron density for 0.62 MA, 5.2 T discharges. The dotted vertical line indicates the LOC/SOC transition density.
The core toroidal rotation velocities as a function of for plasma currents of 0.62 MA (top) and 1.0 MA (bottom). Vertical lines indicate the co to countercurrent rotation boundary.
The core toroidal rotation velocities as a function of for plasma currents of 0.62 MA (top) and 1.0 MA (bottom). Vertical lines indicate the co to countercurrent rotation boundary.
The core toroidal rotation velocities as a function of for plasma currents of 0.62 MA (top) and 1.0 MA (bottom). Vertical lines indicate the co to countercurrent rotation boundary.
The core toroidal rotation velocities as a function of for plasma currents of 0.62 MA (top) and 1.0 MA (bottom). Vertical lines indicate the co to countercurrent rotation boundary.
The transition density from LOC to SOC as a function of major radius for different devices at fixed values of q. The solid curve represents 1/R.
The transition density from LOC to SOC as a function of major radius for different devices at fixed values of q. The solid curve represents 1/R.
The ratio Z_{eff}/ as a function of density for the 5.2 T, 0.62 MA discharges of Fig. 4, in the vicinity of the LOCSOC transition point.
The ratio Z_{eff}/ as a function of density for the 5.2 T, 0.62 MA discharges of Fig. 4, in the vicinity of the LOCSOC transition point.
The electron density (top), plasma current (middle), and core rotation velocity (bottom) for a 6.3 T discharge with downward current ramps.
The electron density (top), plasma current (middle), and core rotation velocity (bottom) for a 6.3 T discharge with downward current ramps.
The core rotation velocity as a function of plasma current at fixed magnetic field and electron density. The solid line is the best linear fit.
The core rotation velocity as a function of plasma current at fixed magnetic field and electron density. The solid line is the best linear fit.
Time histories of the electron density (top), toroidal magnetic field (middle), and core rotation velocity (bottom) in a 0.8 MA plasma with a downward magnetic field ramp.
Time histories of the electron density (top), toroidal magnetic field (middle), and core rotation velocity (bottom) in a 0.8 MA plasma with a downward magnetic field ramp.
The core toroidal rotation velocity as a function of toroidal magnetic field at fixed plasma current and electron density.
The core toroidal rotation velocity as a function of toroidal magnetic field at fixed plasma current and electron density.
The density fluctuation spectrum S(k,f) of the difference between dispersion plots taken at two times during a 5.2 T, 1.0 MA discharge.
The density fluctuation spectrum S(k,f) of the difference between dispersion plots taken at two times during a 5.2 T, 1.0 MA discharge.
Contour plots of the linear growth rates (in units of c_{ s }/a) of the most unstable modes (with 0.25 < 0.75) in the a/L_{ n }a/L_{ T } plane, for r/a = 0.6. The + signs indicate the operational point of discharges with n_{ e } = 0.3 × 10^{20}/m^{3} (left) and 1.2 × 10^{20}/m^{3} (right).
Contour plots of the linear growth rates (in units of c_{ s }/a) of the most unstable modes (with 0.25 < 0.75) in the a/L_{ n }a/L_{ T } plane, for r/a = 0.6. The + signs indicate the operational point of discharges with n_{ e } = 0.3 × 10^{20}/m^{3} (left) and 1.2 × 10^{20}/m^{3} (right).
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