Download or read book Dependence of the L to H mode Power Threshold on Toroidal Rotation and the Link to Edge Turbulence Dynamics written by and published by . This book was released on 2008 with total page 10 pages. Available in PDF, EPUB and Kindle. Book excerpt: The injected power required to induce a transition from L-mode to H-mode plasmas is found to depend strongly on the injected neutral beam torque and consequent plasma toroidal rotation. Edge turbulence and flows, measured near the outboard midplane of the plasma (0.85
Download or read book Transport driven Scrape off Layer Flows and the X point Dependence of the L H Power Threshold in Alcator C Mod written by Brian LaBombard and published by . This book was released on 2004 with total page 62 pages. Available in PDF, EPUB and Kindle. Book excerpt: Factor of ~2 higher power thresholds for low- to high-confinement mode transitions (LH) with unfavorable x-point topologies in Alcator C-Mod [Phys. Plasmas 1, 1511 (1994)] are linked to flow boundary conditions imposed by the scrape-off layer (SOL). Ballooning-like transport drives flow along magnetic field lines from low- to high-field regions with toroidal direction dependent on upper/lower x-point balance; the toroidal rotation of the confined plasma responds, exhibiting a strong counter-current rotation when Bx B points away from the x-point. Increased auxiliary heating power (rf, no momentum input) leads to an L-H transition at approximately twice the edge electron pressure gradient when Bx B points away. As gradients rise prior to the transition, toroidal rotation ramps toward the co-current direction; the H-mode is seen when the counter-current rotation imposed by the SOL flow becomes compensated. Remarkably, L-H thresholds in lower-limited discharges are identical to lower x-point discharges; SOL flows are also found similar, suggesting a connection.
Download or read book Dependence of H mode Power Threshold on Global and Local Edge Parameters written by and published by . This book was released on 1995 with total page 4 pages. Available in PDF, EPUB and Kindle. Book excerpt: Measurements of local electron density n{sub e}, electron temperature T{sub e}, and ion temperature T{sub i} have been made at the very edge of the plasma just prior to the transition into H-mode for four different single parameter scans in the DIII-D tokamak. The means and standard derivations of n{sub e}, T{sub e}, and T{sub i} under these conditions for a value of the normalized toroidal flux of 0.98 are respectively, 1.5 " 0.7 x 1019 m−3, 0.051 " 0.016 keV, and 0.14 " 0.03 keV. The threshold condition for the transition is more sensitive to temperature than to density. The data indicate that the dependence is not as simple as a requirement for a fixed value of the ion collisionality.
Download or read book Configuration and Heating Power Dependence of Edge Parameters and H mode Dynamics in National Spherical Torus Experiment NSTX written by C. E. Bush and published by . This book was released on 2003 with total page 34 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Toroidal Rotation and Momentum Transport in Alcator C Mod Plasmas with No Momentum Input written by John E. Rice and published by . This book was released on 2003 with total page 30 pages. Available in PDF, EPUB and Kindle. Book excerpt: The time evolution of toroidal rotation velocity profiles has been measured in Alcator C-Mod [I.H. Hutchinson et al., Phys. Plasmas (1), 1511 (1994)] plasmas using a tangentially viewing x-ray spectrometer array. The strong co-current toroidal rotation in enhanced D[alpha] (EDA) high confinement mode (H-mode) plasmas is observed to propagate in from the edge on a time scale similar to the energy confinement time. The ensuing steady state rotation velocity profiles in both Ohmic and ion cyclotron range of frequencies (ICRF) heated EDA H-modes, which are generated in the absence of any external momentum input, are found to be relatively flat. These profiles may be simulated by a simple diffusion model with the boundary condition of an edge rotation, which appears during the H-mode period. The observed profiles are well matched by the simulations using a momentum diffusivity of [approx.]0.1 m2/s, which is much larger than the calculated neo-classical value, and the momentum transport may be regarded as anomalous. The Alcator C-Mod rotation observations have been compared in detail with the calculations of neo-classical and sub-neo-classical theory, to the predictions from modeling of ICRF wave induced energetic ion orbit shifts, and to estimates from turbulence driven mechanisms. The magnitude and scalings of the observed rotation results are in accord with neo-classical and sub-neo-classical calculations, but the measured momentum diffusivity is higher than the predictions by a large fraction. The prediction of rotation reversal with a high magnetic field side resonance location for ICRF wave induced ion orbit shifts has not been observed in the experiments. While the turbulence driven rotation calculations are mostly qualitative, they represent some of the observed features.
Download or read book H mode Access and Characteristics at Near unity Aspect Ratio on the Pegasus Toroidal Experiment written by and published by . This book was released on 2015 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The high-confinement (H-mode) operating regime is achieved at near-unity aspect ratio (A ~1.2) in the Pegasus Toroidal Experiment. Ohmic H-mode is attained via high-field-side fueling and low edge recycling in both limited and diverted magnetic topologies. This features of this regime are: formation of a quiescent edge; reduced Da emissions; increased core rotation; formation of edge current and pressure pedestals; measured energy confinement consistent with the IPB98(y,2) scaling [1]; and increased heating in the electron and ion channels. The H-mode power threshold, PLH, behaves quite differently at low-A when compared with high-A operations. The threshold has been studied in both limited and favorable single null diverted plasmas in Pegasus, with PLH =10-20x higher than projected by the conventional ITPA08 scaling [2]. This continues and emphasizes the trend indicated from other low-A tokamaks, MAST and NSTX, that increasingly more power than predicted by the scaling is required as A decreases. Unlike at higher-A, no minimum PLH with density is observed. Also in contrast to higher-A tokamaks, where PLH is>2x higher in limited plasmas than diverted plasmas, the threshold in approximately the same in both limited and favorable single null diverted Pegasus plasmas. Some of these PLH results are consistent with the FM3 model for the L-H transition [3]. Two classes of edge localized modes (ELMs) have been observed. Small, Type III-like ELMs are present at low Ohmic input power POH and have toroidal mode number n
Download or read book H Mode Turbulence Power Threshold ELM and Pedestal Studies in NSTX written by and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: High-confinement mode (H-mode) operation plays a crucial role in NSTX [National Spherical Torus Experiment] research, allowing higher beta limits due to reduced plasma pressure peaking, and long-pulse operation due to high bootstrap current fraction. Here, new results are presented in the areas of edge localized modes (ELMs), H-mode pedestal physics, L-H turbulence, and power threshold studies. ELMs of several other types (as observed in conventional aspect ratio tokamaks) are often observed: (1) large, Type I ELMs, (2) ''medium'' Type II/III ELMs, and (3) giant ELMs which can reduce stored energy by up to 30% in certain conditions. In addition, many high-performance discharges in NSTX have tiny ELMs (newly termed Type V), which have some differences as compared with ELM types in the published literature. The H-mode pedestal typically contains between 25-33% of the total stored energy, and the NSTX pedestal energy agrees reasonably well with a recent international multi-machine scaling. We find that the L-H transition occurs on a [approx]100 [micro]sec timescale as viewed by a gas puff imaging diagnostic, and that intermittent quiescent periods precede the final transition. A power threshold identity experiment between NSTX and MAST shows comparable loss power at the L-H transition in balanced double-null discharges. Both machines require more power for the L-H transition as the balance is shifted toward lower single null. High field side gas fueling enables more reliable H-mode access, but does not always lead to a lower power threshold e.g., with a reduction of the duration of early heating. Finally the edge plasma parameters just before the L-H transition were compared with theories of the transition. It was found that while some theories can separate well-developed L- and H-mode data, they have little predictive value.
Download or read book H mode Threshold Power Scaling and The gradient B Drift Effect written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: One of the largest influences on the H-mode power threshold (P[sub TH]) is the direction of the ion[gradient]B drift relative to the X-point location, where factors of 2--3 increase in P[sub TH] are observed for the ion[gradient]B drift away from the X-point. It is proposed that the threshold power scaling observed in single-null configurations with the ion[gradient]B drift toward the X-point location (P[sub TH][approximately] nB, where n is the plasma density, and B is the toroidal field) is due to the scaling of the magnitude of the[gradient]B drift effect. Hinton and later Hinton and Stebler have modeled this effect as neoclassical cross field fluxes of both heat and particles driven by poloidal temperature gradients on the open field lines in the scrape-off layer (SOL). The[gradient]B drift effect influences the power threshold by affecting the edge conditions needed for the L-H transition. It is not essential for the L-H transition itself since transitions are observed with either direction of B. Predictions of this model include saturation of the B scaling of P[sub TH] at high field, 1/B scaling of P[sub TH] with reverse B, and no B scaling of P[sub TH] in balanced double-null configurations. This last prediction is consistent with the observed scaling of p[sub TH] in double-null plasma sin DIII-D.
Download or read book Characterization of Core and Edge Turbulence in L and Enhanced D alpha H mode Alcator C Mod Plasmas written by Nils Plesner Basse and published by . This book was released on 2005 with total page 96 pages. Available in PDF, EPUB and Kindle. Book excerpt: The recently upgraded phase-contrast imaging (PCI) diagnostic is used to characterize the transition from the low (L) to the enhanced D[alpha] (EDA) high (H) confinement mode in Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas, 1511 (1994)] plasmas. PCI yields information on line integrated density fluctuations along vertical chords. The number of channels has been increased from 12 to 32, the sampling rate from 1 MHz to 10 MHz. This expansion of diagnostic capabilities is used to study broadband turbulence in L- and EDA H-mode and to analyze the quasi-coherent (QC) mode associated with EDA H-mode. Changes in broadband turbulence at the transition from L- to EDA H-mode can be interpreted as an effect of the Doppler rotation of the bulk plasma. Additional fluctuation measurements of D[alpha]-light and the poloidal magnetic field show features correlated with PCI in two different frequency ranges at the transition. The back transition from EDA H- to L-mode, the so-called enhanced neutron (EN) mode, is investigated by new high frequency (132 and 140 GHz) reflectometer channels operating in the ordinary (O) mode. This additional hardware has been installed in an effort to study localized turbulence associated with internal transport barriers (ITBs). The EN-mode is a suitable candidate for this study, since an ITB exists transiently as the outer density decreases much faster than the core density in this mode. The fact that the density decays from the outside inward allows us to study fluctuations progressing towards the plasma core. Our results mark the first localized observation of the QC-mode at medium density: 2.2 x 1020 m-3 (132 GHz). Correlating the reflectometry measurements with other fluctuating quantities provides some insight regarding the causality of the EN-mode development.
Download or read book The Influence of Toroidal Rotation on Global Confinement in JET H mode Plasmas with Different Heat Mix written by I︠U︡riĭ Nikolaevich Dnestrovskiĭ and published by . This book was released on 2013* with total page 13 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book H mode Regimes and Observations of Central Toroidal Rotation in Alcator C mod written by Martin Greenwald and published by . This book was released on 1998 with total page 8 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book H Mode Turbulence PowerThreshold ELM and Pedestal Studies in NSTX written by R. Maingi and published by . This book was released on 2004 with total page 12 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Edge Turbulence Measurements During the L to H mode Transition by Phase Contrast Imaging on DIII D written by Miklos Porkolab and published by . This book was released on 1995 with total page 8 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Government Reports Announcements Index written by and published by . This book was released on 1996 with total page 564 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Edge Turbulence Measurements During the L to H mode Transition by Phase Contrast Imaging on DIII D written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A CO[sub 2]-laser imaging diagnostic measures the vertical line integrals of the density fluctuations at 16 radial locations within a 7.6-cm-wide region at the outer edge of the DIII-D tokamak. The phase-contrast technique provides a flat response to wave numbers in the range 1--16 cm[sup[minus]1] and is mostly sensitive to radial wave vectors. This last property ensures that fluctuations are detected essentially in the plasma frame, since the E x B drift associated with the radial electric field is mainly in the poloidal direction. The radial resolution is 0.5 cm, the data acquisition bandwidth is 1 MHz, and the density sensitivity is on the order of 10[sup 9] cm[sup[minus]3]. The DIII-D Phase Contrast Imaging (PCI) diagnostic has been employed in the study of plasma turbulence, particularly at the transition from L- to H-mode. Past work has shown that the average amplitude of the line-integrated fluctuations in the plasma is reduced at the onset of the H-mode, in agreement with measurements by other diagnostics in DIII-D and elsewhere. In this paper the authors report on measurements of turbulence carried out during a study of low-power L- to H-mode transitions. As the input power approaches the threshold for H-mode, the transition occurs on a progressively slower time scale. The new results are consistent with past observations in L-mode and in the fully developed H-mode; novel features emerge, however, in the transient phase immediately following the transition. The phenomenology of these events, which lack a theoretical explanation at present, will be discussed in the next section.
Download or read book Fusion Energy Update written by and published by . This book was released on 1986 with total page 112 pages. Available in PDF, EPUB and Kindle. Book excerpt:
