[en] This paper reviews the application to magnetic-confinement fusion experiments of dimensional analysis, which holds that the behavior of physical systems can be determined from the scaling of phenomena with the set of dimensionless parameters that embody the governing physics. This paper begins by explaining the two most well-known approaches to dimensional analysis, and shows that the principle of similarity has been demonstrated in high-temperature plasmas of different physical size. Next, the measured dependences of cross-magnetic-field transport and edge plasma characteristics on dimensionless parameters are examined. These dimensionless parameter scans are generally in good agreement with drift wave models of turbulent transport (i.e., microturbulence), although some discrepancies remain. Finally, the benefits of incorporating dimensional analysis into the extrapolation of plasma behavior from present-day experiments to future burning plasma devices are discussed. The experiments reviewed in this paper have greatly improved our understanding of the underlying physics of many plasma phenomena

[en] Recent experiments on tokamaks around the world [1-5] have demonstrated discharges with moderately high performance in which the q-profile remains stationary, as measured by the motional Stark effect diagnostic, for periods up to several τ_R. Hybrid discharges are characterize by q_min ∼ 1, high β_N, and good confinement. These discharges have been termed hybrid because of their intermediate nature between that of an ordinary H-mode and advanced tokamak discharges. They form an attractive scenario for ITER as the normalized fusion performance (β_NH_89P/q₉₅²) is at or above that for the ITER baseline Q_fus = 10 scenario, even for q₉₅ as high as 4.6. The startup phase is thought to be crucial to the ultimate evolution of the hybrid discharge. An open question is how hybrid discharges achieve and maintain their stationary state during the initial startup phase. To investigate this aspect of hybrid discharges, we have used the CORSICA code to model the early stages of a discharge. Results clearly indicate that neoclassical current evolution alone is insufficient to account for the time evolution of the q-profile and that an addition of non-inductive current source must be incorporated into the model to reproduce the experimental time history. We include non-inductive neutral beam and bootstrap current sources in the model, and investigate the difference between simulations with these sources and the experimentally inferred q-profile. Further, we have made preliminary estimates of the spatial structure of the current needed to bring the simulation and experiment into agreement. This additional non-inductive source has not been tied to any physical mechanism as yet. We present these results and discuss the implications for hybrid startup on ITER

[en] Previous work has summarized the physics and first results of benchmarking the trapped gyro-Landau-fluid (TGLF) model for turbulent transport driven by trapped ion and electron modes, ion and electron temperature gradient (ETG) modes, and electromagnetic kinetic ballooning modes including the effects of shaped geometry. Recently, an improved collision model was implemented which provides a more accurate fit to a transport database of nonlinear collisional GYRO[J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] simulations of long wavelength driftwave turbulence. The impact of the new collision model on TGLF modeling results was unknown. Using the improved TGLF model we obtain excellent agreement with the ion and electron temperature profiles from 30 DIII-D [A. Mahdavi and J. L. Luxon, Fusion Sci. Technol. 48, 2 (2005)] hybrid discharges. The transport results show that the electron energy transport tends to be dominated by short wavelength ETG modes in cases where the ion energy transport approaches neoclassical levels. The hybrid regime has significant energy confinement improvement from ExB velocity shear which is well predicted by TGLF. Weak magnetic shear and low safety factor are also shown to enhance the hybrid regime energy confinement. In high normalized β hybrids, we find that finite β effects noticably reduce the predicted electron energy transport and improve agreement with the measured electron temperature profiles.

[en] Global gyrokinetic simulations of DIII-D [M. A. Mahdavi and J. L. Luxon, in 'DIII-D Tokamak Special Issue', Fusion Sci. Technol. 48, 2 (2005)] L- and H-mode dimensionally similar discharge pairs are treated in detail. The simulations confirm the Bohm scaling of the well-matched L-mode pair. The paradoxical but experimentally apparent gyro-Bohm scaling of the H-mode pair at larger relative gyroradius (rho-star) and lower transport levels is due to poor profile similarity. Simulations of projected experimental plasma profiles with perfect similarity show both the L- and H-mode pairs to have Bohm scaling. A ρ_* stabilization rule for predicting the breakdown of gyro-Bohm scaling from simulations of a single discharge is presented

[en] This paper evaluates a new type of ambipolar plasma thruster that uses multiply-charged ions as propellant. Beginning with an electron cyclotron heated (ECH) mirror plasma with energetic electrons, the ion charge state distribution and confining electrostatic potentials are self-consistently modeled for different fill gases using the particle, charge, and energy conservation equations and Pastukhov-flow confinement. The specific impulse is found to be high (∼6000 s) and easily varied. Although the thrust efficiency is low, 24% for double-ended operation and 45% for single-ended operation, this ambipolar thruster is capable of producing high thrust in a compact source because ECH mirror plasmas can operate at high density

[en] In strongly driven ECCD experiments, consideration of radial transport can be crucial for accurate modeling of otherwise localized electron cyclotron current drive. The DIII-D experiment is in an intermediate driven regime with t_transport∼t_slowing for the EC driven electrons. We report computational results from the CQL3D Fokker-Planck simulation code showing radial spreading of driven ECCD in DIII-D. Progress on implementation of a new iterative sparse matrix fully-implicit solve for the full 3D electron distribution, f(u,θ_u,ρ,t) and toroidal electric potential, V_loop(ρ,t) is described. We give a new algorithm for implicit determination of the self-consistent solution of the Ampere-Faraday equation for the time-dependent toroidal electric field

[en] The ion thermal diffusivities (χ_i) in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] discharges exhibit a strong nonlinear dependence on the measured temperature gradients. In low confinement mode (L-mode) discharges with low toroidal rotation, the ion thermal diffusivity, χ_i, has an approximately Heaviside function dependence on the major radius divided by the radial scale length of the ion temperature, R/L_Ti. When R/L_Ti is less than a critical value, the χ_i's are very small. When R/L_Ti is about equal to the critical value, χ_i increases rapidly. Although the gradient profiles for high confinement (H-mode) have a different shape, they still show a critical gradient type of behavior. This type of dependence is consistent with the predictions for transport, which is dominated by ion temperature gradient modes and is a strong indicator that these modes are the main contributors toward L-mode transport in DIII-D and a major contributor to transport in a certain region of DIII-D H-mode discharges. When strong rotational shear is present, the thermal confinement is improved in regions of the plasma. In these regions, the dependence of the diffusivities on the gradients is changed. The type of change is consistent with the physical picture that when the E x B shearing frequency is greater than the maximum linear growth rate of the modes as calculated without shear, then the modes are stabilized and the transport is reduced

[en] A review of the application of dimensionless parameter scaling techniques to magnetic fusion experiments is presented. Because the methods of this type of analysis are not generally known, a detailed discussion of the basis for these techniques is given, including examples. The primary applications and successes of these methods in magnetic fusion research are in the area of transport of energy and particles across surfaces of constant magnetic flux. The experimental justification for the use of these techniques to describe transport is given, and the applications are reviewed. The two key applications of these techniques, the identification of the underlying physical mechanisms that cause transport and the projection of the transport in future devices from present-day experiments, are extensively discussed. Comparison of the results of dimensionless parameter scaling experiments with the regression analysis of multi-machine databases points to limitations in the databases and the analysis of them as the source of the discrepancies. These discrepancies have significant implications for the design optimization of tokamaks, which are discussed here. Finally, the application of dimensionless parameter scaling techniques to plasma stability, to the boundary region between closed and open field lines and to divertor operation in the open field line region are reviewed and discussed. (review article)

[en] The radial profile of the Pfirsch-Schlueter current has been measured directly for the first time in a tokamak using motional Stark effect (MSE) polarimetry. The MSE diagnostic measures the vertical component of the magnetic field (B_z) as a function of the plasma major radius. The analysis technique presented here converts the experimental B_z profile into a flux-surface-average current density as well as a local current density. Taking the appropriate difference between these two quantities yields a direct measurement of the Pfirsch-Schlueter current density, and consequently a MSE-based measurement of the plasma pressure profile. The bootstrap current density can also be directly determined from the B_z profile (with greater approximation) in the collisionless limit. An equilibrium reconstruction is not required for this MSE analysis technique, although a few basic geometric parameters such as the plasma minor radius, elongation and triangularity need to be known. The usefulness of this approach for analysing the current profile is demonstrated using discharges from the DIII-D tokamak

[en] Tokamaks designed for burning plasma operation have significant free energy in the poloidal magnetic field and in the thermal stored energy. Protection of the first wall and the vacuum vessel from the effects of a rapid release of this energy (a disruption) is required. While mitigation of a disruption is feasible, avoidance of the disruption is preferable. Suppression of tearing modes that lead to disruption is a key method of avoidance. Electron cyclotron current drive is a demonstrated technique for suppression of tearing modes. The location of the current drive relative to the tearing mode is the critical parameter for successful suppression. Incorporation of this suppression technique in a machine protection system requires continuous aiming of the electron cyclotron waves (for rapid application to island) and a closed-loop optimization of the effect of the current drive on the mode (for maximum effectiveness). Real-time methods to accomplish both of these tasks have been demonstrated successfully in the DIII-D tokamak