3 Matching Results

Search Results

Advanced search parameters have been applied.

Investigations of VH-mode in DIII-D and JET

Description: The VH-mode regime of high confinement has been observed in both DIII-D and JET. VH-mode is characterized by thermal confinement twice that seen in H-mode, with the edge transport barrier penetrating deeper into the plasma. Two mechanisms have been identified as important in achieving this high level of confinement. Expansion of the {rvec E} {times} {rvec B} velocity shear turbulence suppression zone is important in allowing reductions in local transport, while access to the second ballooning stability regime in the edge allows avoidance or elimination of ELMs which impede the confinement improvement. The high performance phase of these discharges is usually terminated by an MHD event which removes energy from a large portion of the plasma cross-section, and is followed by an H-mode phase.
Date: September 1, 1993
Creator: Greenfield, C. M.; Burrell, K. H. & Balet, B.
Partner: UNT Libraries Government Documents Department

Implications from dimensionless parameter scaling experiments

Description: The dimensionless parameter scaling approach is increasingly useful for predicting future tokamak performance and guiding theoretical models of energy transport. Experiments to determine the {rho}* (gyroradius normalized to plasma size) scaling have been carried out in many regimes. The electron {rho}* scaling is always ``gyro-Bohm``, while the ion {rho}* scaling varied with regime. The ion variation is correlated with both density scale length (L mode, H mode) and current profile. The ion {rho}* scaling in the low-q, H-mode regime is gyro-Bohm, which is the most favorable confinement scaling observed. New experiments in {beta} scaling and collisionality scaling have been carried out in low-q discharges in both L mode and H mode. In L mode, global analysis shows that there is a slightly unfavorable {beta} dependence ({beta}{sup {minus}0.1}) and no {nu}* dependence. In H-mode, global analysis finds a weak {beta} dependence ({beta}{sup 0.1}) and an unfavorable dependence on {nu}*. The lack of significant {beta} scaling spans the range of {beta}{sub N} from 0.25 to 2.0. The very small {beta} dependence in L mode and H mode is in contradiction with the standard global scaling relations. This contradiction in H mode may be indicative of the impact on the H-mode database of low-n tearing instabilities which are observed at slightly higher {beta}{sub N} in the {beta} scaling experiments. The measured {beta} and {nu}* scalings explain the weak density dependence observed in engineering parameter scans. It also points to the power of the dimensionless parameter approach, since it is possible to obtain a definitive size scaling from experiments on a single tokamak.
Date: October 1, 1996
Creator: Luce, T.C.; Petty, C.C.; Balet, B. & Cordey, J.G.
Partner: UNT Libraries Government Documents Department

Nondimensional transport experiments on DIII-D and projections to an ignition tokamak

Description: The concept of nondimensional scaling of transport makes it possible to determine the required size for an ignition device based upon data from a single machine and illuminates the underlying physics of anomalous transport. The scaling of cross-field heat transport with the relative gyroradius {rho}*, the gyroradius normalized to the plasma minor radius, is of particular interest since {rho}* is the only nondimensional parameter which will vary significantly between present day machines and an ignition device. These nondimensional scaling experiments are based upon theoretical considerations which indicate that the thermal heat diffusivity can be written in the form {chi} = {chi}{sub B}{rho}*{sup x{sub {rho}}} F({beta}, v*, q, R/a, {kappa}, T{sub e}/T{sub i},...), where {chi}{sub B} = cT/eB. As explained elsewhere, x{sub {rho}} = 1 is called gyro-Bohm scaling, x{sub {rho}} is Bohm scaling, x{sub {rho}} = {minus}1/2 is Goldston scaling, and x{sub {rho}} = {minus}1 is stochastic scaling. The DIII-D results reported in this paper cover three important aspects of nondimensional scaling experiments: the testing of the underlying assumption of the nondimensional scaling approach, the determination of the {rho}* scaling of heat transport for various plasma regimes, and the extrapolation of the energy confinement time to future ignition devices.
Date: July 1, 1996
Creator: Petty, C.C.; Luce, T.C.; Balet, B.; Christiansen, J.P. & Cordey, J.G.
Partner: UNT Libraries Government Documents Department