An innovative new approach has been developed for modeling the expansion of laser-generated plumes into low-pressure gases where initially the mean free path may be long enough for interpenetration of the plume and background. The model is based on a combination of multiple elastic scattering and hydrodynamic formulations. Although relatively simple in structure, it gives excellent fits to new experimental data for Si in He and Ar, and provides for the first time a detailed, coherent explanation of the observed splitting of the plume into a fast and slow component. [S0031-9007(97)03820-9] PACS numbers: 79.20.Ds, 52.50.JmPulsed laser deposition (PLD) has become an important technique for depositing a variety of materials [1,2], most notably thin films [3] and superlattices [4] of high-T c superconductors. As a consequence, a very active field of research into laser ablation phenomena underlying PLD has developed [5]. But this is not a new field since it dates back to the earliest days of the laser era when many materials were irradiated with high-powered laser pulses [6][7][8]. Thus, the work reported here has applications far beyond the PLD process itself. More recent work has provided a wealth of new diagnostics with which to study the laser ablation process [9]. It is of crucial importance to know the constitution and dynamical behavior of the plume of ablated material in order to understand how film growth can be optimized by varying the laser parameters, the targetsubstrate distance, and ambient gases introduced into the deposition chamber. In particular, the often observed but little understood phenomenon of "plume splitting" [10] into fast (ϳ vacuum speed) and background-slowed components is of great interest because the fast component may damage the growing film or otherwise affect its microstructure. Also, clustering of film constituents in the gas phase or on the surface may cause problems, but may also provide a technologically important method of producing nanostructures [5,11,12].In this paper, a new modeling approach, combining multiple scattering and hydrodynamical elements, is described and applied to recently obtained experimental data on Si ablated into He and Ar gases. The resulting model is remarkably successful in describing quantitatively the data and resolves long-standing uncertainties about the interpretation of many previous experimental observations.Silicon was selected for the target in the experiments because it is well characterized, is readily obtained as single crystals, and has been thoroughly studied in the laser-annealing regime [13]. Background gases of He and Ar were chosen because their ionization energies are high (25 and 16 eV, respectively), hence avoiding ionization, and because one is lighter and one heavier than Si. KrF laser pulses of 3.0 J͞cm 2 provided a good supply of singly ionized Si for the ion-probe detector while avoiding higher ionization states. Measurements revealed only neutral and singly ionized Si in the plume and only neutral atoms in the background. A de...

Measured turbulence characteristics (correlation lengths, spectra, etc.) in low-confinement (L-mode) and high-performance plasmas in the DIII-D tokamak [Luxon et al., Proceedings Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] show many similarities with the characteristics determined from turbulence simulations. Radial correlation lengths Δr of density fluctuations from L-mode discharges are found to be numerically similar to the ion poloidal gyroradius ρθ,s, or 5–10 times the ion gyroradius ρs over the radial region 0.2<r/a<1.0. Comparison of these correlation lengths to ion temperature gradient gyrokinetic simulations (the UCLA-University of Alberta, Canada UCAN code [Sydora et al., Plasma Phys. Controlled Fusion 38, A281 (1996)]) shows that without zonal flows simulation values of Δr are very long, spanning much of the 65 cm minor radius. With zonal flows included, these decrease to near the measured values in both magnitude and radial behavior. In order to determine if Δr scaled as ρθ,s or 5–10 times ρs, an experiment was performed which modified ρθs while keeping other plasma parameters approximately fixed. It was found that the experimental Δr did not scale as ρθ,s, which was similar to low-resolution UCAN simulations. Finally, both experimental measurements and gyrokinetic simulations indicate a significant reduction in the radial correlation length from high-performance quiescent double barrier discharges, as compared to normal L-mode, consistent with reduced transport in these high-performance plasmas.

It is shown that the usual picture for the suppression of turbulent transport across a stable sheared flow based on a reduction of diffusive transport coefficients is, by itself, incomplete. By means of toroidal gyrokinetic simulations of electrostatic, collisionless ion-temperature-gradient turbulence, it is found that the nature of the transport is altered fundamentally, changing from diffusive to anticorrelated and subdiffusive. Additionally, whenever the flows are self-consistently driven by turbulence, the transport gains an additional non-Gaussian character. These results suggest that a description of transport across sheared flows using effective diffusivities is oversimplified. It is widely accepted that the rate of the transport (of particles, energy, or any other quantity) carried out by turbulence can be significantly lowered by a perpendicular sheared flow [1]. Since these flows are intrinsically unstable against Kelvin-Helmholz instabilities, their sustainment requires additional stabilizing mechanisms, such as a magnetic field or rotation. These additional mechanisms also introduce waves, which can help drive the flows via the turbulent Reynolds stresses. For instance, drift waves may drive poloidal or toroidal (zonal) sheared flows in tokamak plasmas [2]. These zonal flows are central to the formation of the radial transport barriers characteristic of the enhanced regimes [3] in which the future International Thermonuclear Experimental Reactor tokamak will operate [4]. In atmospheric and oceanic flows, Rossby waves, driven by the change in rotation rate around the local vertical axis with latitude, play an analogous role [5]. The turbulent flux across the flow,À ? ¼ hsṽ ? i (wheres is the advected quantity) can decrease due to a reduction in either the amplitude or the self-coherence of the turbulence [6], or to a shift in the phase between advected and advecting fields [7]. Importantly, the investigation (and modeling) of these various possibilities has traditionally been done by assuming from the start that some effective diffusivity characterizes transport in the absence of the flow, D ? $ l 2 ? = (l ? and being typical transport scales), which is then reduced (via changes in l ? and/or ) by the action of the sheared flow. Note, however, that the nature of transport across the flow must be and remain diffusive for this notion to be applicable. More precisely, this means that the dynamics must be and remain Gaussian and Markovian, so that a finite length l ? and time exist. Otherwise, any description based on these ideas would provide an incomplete, even misleading, explanation. In this Letter we report on the first numerical evidence which suggests that the diffusive assumption is invalid in the presence of sheared zonal flows, driven either self-consistently by turbulence or externally, in magnetically confined toroidal plasmas. In particular, our simulations show that transport across them is subdiffusive for a large range of scales beyond the turbulent decorrelation time. Furthermore, the tr...

In recent experiments in DIII-D, comparison of specific turbulence characteristics with linear and nonlinear modeling has identified common features associated with the ion temperature gradient (ITG) mode. A low-frequency turbulence feature is observed in high-density saturated Ohmic confinement discharges, which is absent in low-density linear Ohmic confinement discharges. The feature is in a range of wavelength (k⊥ρs≈0.2–0.5) and the frequency expected for the ITG mode and onset of the feature is coincident with onset of confinement saturation. The density profile is significantly broader in the high-density discharge, a known destabilizing effect on the ITG mode. Gyrokinetic stability calculations of the growth rate of the most unstable drift ballooning mode show the ITG mode to be more unstable in the high-density discharges.

A new electromagnetic kinetic electron simulation model that uses a generalized split-weight scheme, where the adiabatic part is adjustable, along with a parallel canonical momentum formulation has been developed in threedimensional toroidal flux-tube geometry. This model includes electron-ion collisional effects and has been linearly benchmarked. It is found that for H-mode parameters, the nonadiabatic effects of kinetic electrons increase linear growth rates of the ion-temperature-gradient-driven (ITG) modes, mainly due to trapped-electron drive. The ion heat transport is also increased from that obtained with adiabatic electrons. The linear behaviour of the zonal flow is not significantly affected by kinetic electrons. The ion heat transport decreases to below the adiabatic electron level when finite plasma β is included due to finite-β stabilization of the ITG modes. This work is being carried out using the 'Summit Framework'.