Bibliothek

Notizen: We describe in detail the design and performance of a multikilowatt Pockels cell for use in high average power laser systems. The Pockels cell is a gas-cooled, transversely excited electro-optic switch based on KD*P as the electro-optic material. Matched pairs of crystals with different crystallographic orientations are used to make the switch performance insensitive to the operating point and ambient temperature. Excellent switching performance and low-wave-front distortion have been achieved at average power levels in excess of 1 kW.

Notizen: The anisotropic distribution of turbulent kinetic energy in fully developed channel flows is examined by using an algebraic preclosure which relates the Reynolds stress to the mean field gradient and to a prestress correlation, (I[underaccent underbar-underbar [below]+τR∇〈u(underbar)〉)T⋅〈u(underbar)′u(underbar)′〉⋅(I[underaccent underbar-underbar [below]+τR∇〈u(underbar)〉)=τR2〈f(underbar)′f(underbar)′〉. Local fluctuations in the pressure field and in the instantaneous Reynolds stress are responsible for the prestress correlation τR2〈f(underbar)′f(underbar)′〉. Closure requires a phenomenological model for the anisotropic prestress 2kH[underaccent underbar-underbar [below], defined by 2kH[underaccent underbar-underbar [below]≡τR2〈f(underbar)′f(underbar)′〉−2αI[underaccent underbar-underbar [below]/3. The prestress coefficient α(=τR2〈f(underbar)′⋅f(underbar)′〉/2) depends algebraically on the components of the Reynolds stress, the mean velocity gradient, the relaxation time τR, and the turbulent kinetic energy k. Previously reported direct numerical simulations (DNS) results for fully developed channel flows (δ+=395) are used to evaluate the behavior of the Reynolds stress for an isotropic prestress (IPS) correlation (i.e., H[underaccent underbar-underbar [below]=O[underaccent underbar-underbar [below]). The IPS theory predicts the existence of a nonzero primary normal stress difference and shows that a significant transfer of kinetic energy occurs from the transverse and normal components of the Reynolds stress to the longitudinal component for τR||∇〈u(underbar)〉||(very-much-greater-than)1. The spatial distributions of the two nontrivial invariants of the anisotropic stress predicted by the IPS theory are consistent with DNS results for 10≤y+≤395. The practical utility of the isotropic prestress theory is further demonstrated by predicting the low-order statistical properties of the turbulence in the outer region of fully developed channel flows. Transport equations for the turbulent kinetic energy and the turbulent dissipation are used to estimate the spatial distributions of the turbulent time scales k/cursive-epsilon and τR. © 1999 American Institute of Physics.

Notizen: In recent experiments with combined fast wave current drive (FWCD) and deuterium neutral beam injection on the DIII-D tokamak [Luxon et al., Fusion Technol. 8, 441 (1985)], an enhanced fusion reactivity and fast ion energy content have been observed in the presence of FWCD, with a concomitant low FWCD efficiency [Petty et al., Radio Frequency Power in Plasmas (AIP, New York, 1997), p. 225]. In this paper, we investigate whether high-harmonic ion cyclotron damping could be responsible for the low FWCD efficiency in these experiments, since a number of high-harmonic hydrogen and deuterium cyclotron resonance layers existed in the plasma. The main analysis tool is the ion cyclotron range of frequencies code PION [Eriksson et al., Nucl. Fusion 33, 1037 (1993)], modified to allow multiple frequencies simultaneously as was done in the DIII-D experiments. According to the PION modeling, high harmonic damping of fast wave power can give rise to enhanced fusion reactivity and fast ion energy content, which is consistent with the experimental observations. © 2002 American Institute of Physics.

Notizen: The scalings of heat transport with safety factor (q), normalized collisionality (ν), plasma beta (β), and relative gyroradius (ρ*) have been measured on the DIII-D tokamak [Fusion Technol. 8, 441 (1985)]. The measured ρ*, β and ν scalings of heat transport indicate that E×B transport from drift wave turbulence is a plausible basis for anomalous transport. For high confinement (H) mode plasmas where the safety factor was varied at fixed magnetic shear, the effective (or one-fluid) thermal diffusivity was found to scale like χeff∝q2.3±0.64 , with the ion and electron fluids having the same q scaling to within the experimental errors except near the plasma edge. The scaling of the thermal confinement time with safety factor was in good agreement with this local transport dependence, τth∝q−2.42±0.31 ; however, when the magnetic shear was allowed to vary to keep q0 fixed during the (edge) safety factor scan, a weaker global dependence was observed, τth∝q95−1.43±0.23. This weaker dependence was mainly due to the change in the local value of q between the two types of scans. The combined ρ*, β , ν and q scalings of heat transport for H-mode plasmas on DIII-D reproduce the empirical confinement scaling using physical (dimensional) parameters with the exception of weaker power degradation. © 1998 American Institute of Physics.

Notizen: Profiles of the noninductive current, driven by direct electron absorption of fast waves in the ion cyclotron range of frequencies, have been determined for DIII-D tokamak discharges [Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. The results clearly indicate the presence of centrally peaked driven current and validate theoretical models of fast wave current drive. © 1996 American Institute of Physics.

Notizen: A noninductive current drive concept, based on internal pressure-driven currents in a low-aspect-ratio toroidal geometry, has been demonstrated on the Current Drive Experiment Upgrade (CDX-U) [Forest et al., Phys. Rev. Lett. 68, 3559 (1992)] and further tested on DIII-D [in Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. For both experiments, electron cyclotron power provided the necessary heating to breakdown and maintain a plasma with high-βp and low collisionality (εβp∼1, ν*≤1). A poloidal vacuum field similar to a simple magnetic mirror is superimposed on a much stronger toroidal field to provide the initial confinement for a hot, trapped electron species. With application of electron cyclotron heating (ECH), toroidal currents spontaneously flow within the plasma and increase with applied ECH power. The direction of the generated current is independent of the toroidal field direction and depends only on the direction of the poloidal field, scaling inversely with magnitude of the later. On both CDX-U and DIII-D, these currents were large enough that stationary closed flux surfaces were observed to form with no additional Ohmic heating. The existence of such equilibria provides further evidence for the existence of some type of bootstrap current. Equilibrium reconstructions show the resulting plasma exhibits properties similar to more conventional tokamaks, including a peaked current density profile which implies some form of current on axis or nonclassical current transport.

Notizen: An algebraic preclosure theory for the Reynolds stress 〈u′u′〉 is developed based on a smoothing approximation which compares the space–time relaxation of a convective-diffusive Green's function with the space–time relaxation of turbulent correlations. The formal preclosure theory relates the Reynolds stress to three distinct statistical properties of the flow: (1) a relaxation time τR associated with the temporal structure of the turbulence; (2) the spatial gradient of the mean field; and, (3) a prestress correlation related to fluctuations in the instantaneous Reynolds stress and the pressure field. Closure occurs by using an isotropic model for the prestress. For simple shear flows, the theory predicts the existence of a nonzero primary normal stress difference and an eddy viscosity coefficient which depends on the temporal relaxation of the turbulent structure and a characteristic time scale associated with the mean field. The asymptotic state of homogeneously sheared turbulence shows that τRS∼1, where S represents the mean shear rate. The Reynolds stress model and a set of recalibrated k−ε transport equations predict that the relaxation of homogeneously sheared turbulence to an asymptotic state requires development distances larger than 20 ×〈uz〉(0)/S, a theoretical result consistent with experimental observations. © 1998 American Institute of Physics.

Notizen: The effect of rotation on the heat and particle transport is measured in the DIII–D tokamak [Fusion Technol. 8, 441 (1985)] for high-confinement mode (H-mode) plasmas with edge localized modes. In a novel experiment, transport is compared for nearly identical scans of the relative gyroradius in co- and counter-rotating plasmas. Since the plasma profiles are the same, the difference in the transport scaling can be attributed to changes in the sheared E×B flow caused by the shift in the toroidal plasma velocity. The ion heat and particle transport are found to be sensitive to the change in the rotation direction and magnitude whereas the electron heat transport is not. Simulations using a gyroLandau-fluid drift wave transport model show that the variation in the ion heat transport for co/counter rotation is due to changes in the E×B shear stabilization, but the electrons appear to be governed by a different transport mechanism. © 2002 American Institute of Physics.

Notizen: The development of techniques for neoclassical tearing mode (NTM) suppression or avoidance is crucial for successful high beta/high confinement tokamaks. Neoclassical tearing modes are islands destabilized and maintained by a helically perturbed bootstrap current and represent a significant limit to performance at higher poloidal beta. The confinement-degrading islands can be reduced or completely suppressed by precisely replacing the "missing" bootstrap current in the island O-point or by interfering with the fundamental helical harmonic of the pressure. Implementation of such techniques is being studied in the DIII-D tokamak [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] in the presence of periodic q=1 sawtooth instabilities, a reactor relevant regime. Radially localized off-axis electron cyclotron current drive (ECCD) must be precisely located on the island. In DIII-D the plasma control system is put into a "search and suppress" mode to make either small rigid radial position shifts of the entire plasma (and thus the island) or small changes in the toroidal field (and, thus, the ECCD location) to find and lock onto the optimum position for complete island suppression by ECCD. This is based on real-time measurements of an m/n=3/2 mode amplitude dBθ/dt. The experiment represents the first use of active feedback control to provide continuous, precise positioning. An alternative to ECCD makes use of the six toroidal section "C-Coil" on DIII-D to provide a large nonresonant static m=1, n=3 helical field to interfere with the fundamental harmonic of an m/n=3/2 NTM. While experiments show success in inhibiting the NTM if a large enough n=3 field is applied before the island onset, there is a considerable plasma rotation decrease due to n=3 "ripple." © 2002 American Institute of Physics.