6 Search Results

Scaling of intrinsic rotation in DIII-D H-mode plasmas demonstrates a strong correlation with the ion temperature (T _i) and stored plasma thermal energy, indicating a coupling between the turbulent intrinsic momentum flux and the turbulent energy flux. We consider intrinsic rotation to be the toroidal rotation in axisymmetric conditions with no external momentum injection. The DIII-D dimensionless empirical scaling of intrinsic rotation with plasma stored energy has been recently tested by novel experiments on DIII-D that utilize relatively small variations in the plasma shape, namely the triangularity, to modify the intrinsic rotation. Shape variation affects the intrinsic rotation by modifyingmore » the turbulent transport, rather than via changes in the auxiliary heating power, or applied torque. These H-modes are heated by ECH with no external torque input. Balanced torque blips from neutral beams measure the ion flow velocity and T _i. Higher thermal energy and intrinsic angular momentum are correlated with higher triangularity. Turbulent density fluctuations in the pedestal region show a significantly higher level at lower triangularity, with lower energy confinement, possibly the source of greater transport. In DIII-D, the E x B shear, which is mainly driven by the edge pressure gradient term in intrinsic rotation conditions, could provide the dominant symmetry breaking necessary for generating a net turbulent momentum stress and qualitatively agree with the scaling.« less

The dependence of intrinsic torque and momentum confinement time on normalized gyroradius ( ) and collisionality ( ) have been measured in the DIII-D tokamak. Intrinsic torque normalized to temperature is found to have and dependencies of 1:5 0:8 and 0:26 0:04 . This dependence on is unexpectedly favorable (increasing as decreases). The choice of normalization is important and the implications are discussed. The unexpected dependence on is found to be robust, despite some uncertainty in the choice of normalization. The dependence of momentum confinement on does not clearly demonstrate Bohm or gyro-Bohm like scaling, and a weaker dependence onmore » is found. The calculations required to use these dependencies to determine the intrinsic torqe in a future tokamak like ITER are presentend, and the importance of the normalization explained. Based on currently available information, the intrinsic torque predicted for ITER is 33 N m, comprable to the expected torque available from neutral beam injection. The expected average intrinsic rotation associated with this intrinsic torque is small compared to current tokamaks, but it may still aid stability and performance in ITER.« less

A dimensionless empirical scaling for intrinsic toroidal rotation is given; M_A ~β_Nρ*, where M_A is the toroidal velocity divided by the Alfvén velocity, β_N the usual normalized β value, and ρ* is the ion gyroradius divided by the minor radius. This scaling describes well experimental data from DIII-D, and also some published data from C-Mod and JET. The velocity used in this scaling is in an outer location in minor radius, outside of the interior core and inside of the large gradient edge region in H-mode conditions. Furthermore, this scaling establishes the basic magnitude of the intrinsic toroidal rotation andmore » its relation to the rich variety of rotation profiles that can be realized for intrinsic conditions is discussed.« less

Experiments at the DIII-D tokamak have used dimensionless parameter scans to investigate the dependencies of intrinsic torque and momentum transport in order to inform a prediction of the rotation profile in ITER. Measurements of intrinsic torque profiles and momentum confinement time in dimensionless parameter scans of normalized gyroradius and collisionality are used to predict the amount of intrinsic rotation in the pedestal of ITER. Additional scans of Te=Ti and safety factor are used to determine the accuracy of momentum flux predictions of the quasi-linear gyrokinetic code TGLF. In these scans, applications of modulated torque are used to measure the incrementalmore » momentum diffusivity, and results show phenomenology consistent with the E B shear suppression of turbulent transport. These incremental transport measurements are also compared to TGLF results. In order to form a prediction of the rotation profile for ITER, the pedestal prediction is used as a boundary condition to a simulation that uses TGLF to determine the transport in the core of the plasma. The predicted rotation is 20 krad/s in the core, lower than in many current tokamak operating scenarios. TGLF predictions show this rotation is still significant enough to have a strong effect on con nement via E B shear.« less

A relatively simple model for the generation of the radial electric field, Er, near the outboard boundary in a tokamak is presented. The model posits that Er is established to supply the return current necessary to balance the thermal ion orbit loss current. Comparison with DIII-D data is promising. Features of the model that promote a more negative edge Er are higher ion temperature, lower density, lower impurity ion content, and a shorter pathlength for orbit loss. Lastly, these scalings are consistent with experimentally established access to the high-confinement mode edge transport barrier.

Here, bulk ion toroidal velocity profiles, V^D+_||, peaking at 40–60 km/s are observed with Mach probes in a narrow edge region of DIII-D discharges without external momentum input. This intrinsic rotation can be well reproduced by a first principle, collisionless kinetic loss model of thermal ion loss that predicts the existence of a loss-cone distribution in velocity space resulting in a co-Ip directed velocity. We consider two kinetic models, one of which includes turbulence-enhanced momentum transport, as well as the Pfirsch-Schluter (P-S) fluid mechanism. We measure a fine structure of the boundary radial electric field, Er, insofar ignored, featuring largemore » (10–20 kV/m) positive peaks in the scrape off layer (SOL) at, or slightly inside, the last closed flux surface of these low power L- and H-mode discharges in DIII-D. The Er structure significantly affects the ion-loss model, extended to account for a non-uniform electric field. We also find that V^D+_|| is reduced when the magnetic topology is changed from lower single null to upper single null. The kinetic ion loss model containing turbulence-enhanced momentum transport can explain the reduction, as we find that the potential fluctuations decay with radius, while we need to invoke a topology-enhanced collisionality on the simpler kinetic model. The P-S mechanism fails to reproduce the damping. We show a clear correlation between the near core V^C6+_|| velocity and the peak edge V^D+_|| in discharges with no external torque, further supporting the hypothesis that ion loss is the source for intrinsic torque in the present tokamaks. However, we also show that when external torque is injected in the core, it can complete with, and eventually overwhelm, the edge source, thus determining the near SOL flows. Finally, we show some additional evidence that the ion/electron distribution in the SOL is non-Maxwellian.« less