Download or read book Core Turbulence and Transport Reduction in DIII D Discharges with Weak Or Negative Magnetic Shear written by and published by . This book was released on 1997 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt: Core turbulence fluctuation levels have been suppressed in DIII-D discharges with weak or negative magnetic shear (NCS) near the magnetic axis. In some weak magnetic shear discharges the ion thermal transport has been reduced to neoclassical levels throughout the whole plasma. The cause of the transport reduction is investigated by calculating the stability of toroidal drift waves, i.e., ion temperature gradient modes (ITG) and trapped electron modes (TE), with a comprehensive gyrokinetic linear stability code. It is found that the ITG modes and TE modes are stabilized by ExB velocity shear. The ExB velocity shear is primarily responsible for the spontaneous growth of a region of suppressed ion thermal transport. Surprisingly, the negative magnetic shear and Shafranov shift are only weak stabilizing influences for the ITG and TE modes in the DIII-D cases studied. Negative magnetic shear does eliminate the ideal magnetohydrodynamic ballooning mode instability which is a necessary access criteria for these improved core confinement regimes. Dilution of the thermal ions by fast ions from the heating beams and hot ions compared to electrons are found to be important stabilizing influences in the core.
Download or read book Transport and Performance in DIII D Discharges with Weak Or Negative Central Magnetic Shear written by and published by . This book was released on 1996 with total page 30 pages. Available in PDF, EPUB and Kindle. Book excerpt: Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak have been produced by combining the benefits of a hollow or weakly sheared central current profile with a high confinement (H-mode) edge. In these discharges, low power neutral beam injection heats the electrons during the initial current ramp, and {open_quotes}freezes in{close_quotes} a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L-mode) to high (H-mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high performance phase is terminated nondisruptively at much higher [beta]{sub T} (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang-Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E x B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium-tritium fuel mixture indicates that such plasmas could produce a DT fusion gain Q{sub DT} = 0.32.
Download or read book Local Analysis of Confinement and Transport in Neutral Beam Heated DIII D Discharges with Negative Magnetic Shear written by and published by . This book was released on 1996 with total page 13 pages. Available in PDF, EPUB and Kindle. Book excerpt: High triangularity double-null discharges with weak or negative central magnetic shear and with both an L-mode and an H-mode edge have been produced on DIII-D. The L-mode edge cases are characterized by peaked toroidal rotation, ion temperature, and plasma density profiles with reduced ion transport in the negative shear region. The H-mode edge cases have broader profiles consistent with reduced ion transport, to the neoclassical level, over the entire plasma cross section. The L-mode edge cases have a greater reduction in central ion diffusivity with stronger negative shear while the H-mode edge cases do not exhibit this dependence. Plasma fluctuation measurements show that a dramatic reduction in turbulence accompanies the improved ion confinement. Calculations of sheared E x B flow indicate that this mechanism can overcome the [eta]{sub i} mode growth rate in the region of reduced transport.
Download or read book Rotational and Magnetic Shear Stabilization of Magnetohydrodynamic Modes and Turbulence in DIII D High Performance Discharges written by and published by . This book was released on 1996 with total page 27 pages. Available in PDF, EPUB and Kindle. Book excerpt: The confinement and the stability properties of the DIII-D tokamak high performance discharges are evaluated in terms of rotational and magnetic shear with emphasis on the recent experimental results obtained from the negative central magnetic shear (NCS) experiments. In NCS discharges, a core transport barrier is often observed to form inside the NCS region accompanied by a reduction in core fluctuation amplitudes. Increasing negative magnetic shear contributes to the formation of this core transport barrier, but by itself is not sufficient to fully stabilize the toroidal drift mode (trapped- electron-[eta]{sub i}mode) to explain this formation. Comparison of the Doppler shift shear rate to the growth rate of the [eta]{sub i} mode suggests that the large core {bold E x B} flow shear can stabilize this mode and broaden the region of reduced core transport . Ideal and resistive stability analysis indicates the performance of NCS discharges with strongly peaked pressure profiles is limited by the resistive interchange mode to low [Beta]{sub N}> 2.3. This mode is insensitive to the details of the rotational and the magnetic shear profiles. A new class of discharges which has a broad region of weak or slightly negative magnetic shear (WNS) is described. The WNS discharges have broader pressure profiles and higher values than the NCS discharges together with high confinement and high fusion reactivity.
Download or read book written by and published by . This book was released on 1983 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Turbulent Transport in Magnetized Plasmas written by Wendell Horton and published by World Scientific. This book was released on 2012 with total page 518 pages. Available in PDF, EPUB and Kindle. Book excerpt: The book explains how magnetized plasmas self-organize in states of electromagnetic turbulence that transports particles and energy out of the core plasma faster than anticipated by the fusion scientists designing magnetic confinement systems in the 20th century. It describes theory, experiments and simulations in a unified and up-to-date presentation of the issues of achieving nuclear fusion power.
Download or read book Confinement and Stability of DIII D Negative Central Shear Discharges written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Negative central magnetic shear (NCS) discharges with[Beta][sub N][le] 4, H[le] 3, and up to 80% of the current non-inductively driven are reproducibly produced in the DIII-D tokamak. Strong peaking of T[sub i], plasma rotation, and in some cases n[sub e] are observed inside the NCS region. Transport analysis shows that the core ion thermal diffusivity is substantially reduced and near the neoclassical value after the formation of the internal transport barrier. The negative central shear is necessary but not sufficient for the formation of this transport barrier. The power required for the formation appears to increase with the toroidal magnetic field. The high performance phase of H-mode NCS discharges often ends with an ELM-like collapse initiated from the edge whereas the L-mode discharges which have a more peaked pressure profile tend to end with a more global n= 1 MHD event.
Download or read book Progress Towards Sustainment of Internal Transport Barriers in 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: Neutral beam heated discharges with internal transport barriers (ITB) have been observed in most of the world's major tokamaks including DIII-D, TFTR, JT-60U and JET. Improved core confinement has been observed over a range of q profiles, including negative central magnetic shear (NCS) and weak positive shear. In discharges with an L-mode edge, the duration of ITBs is generally limited to a few energy confinement times ([tau][sub e]) by low-n MHD activity driven by the steep core pressure gradient or by q[sub min] passing through low-order rational values. In order for ITBs to be useful in achieving a more compact advanced tokamak, the radius of the ITB must be expanded and the pressure gradient must be controlled in order to optimize[beta][sub N] and the bootstrap alignment. In this paper, the authors discuss the results of recent experiments on DIII-D to produce and sustain ITBs for longer pulse lengths. Three techniques are evaluated: (1) reduction of neutral beam power (P[sub NBI]) and plasma current to form a weaker ITB which can be sustained; (2) use of an ELMing H-mode edge to help broaden the pressure profile and improve MHD instability; (3) modification of the plasma shape (squareness) to achieve smaller ELMs with a lower density pedestal at the edge.
Download or read book 16th IEEE NPSS Symposium Fusion Engineering written by and published by . This book was released on 1995 with total page 900 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Fusion Technology 1996 written by C. Varandas and published by Elsevier. This book was released on 2012-12-02 with total page 1057 pages. Available in PDF, EPUB and Kindle. Book excerpt: The objective of these proceedings was to provide a platform for the exchange of information on the design, construction and operation of fusion experiments. The technology which is being developed for the next step devices and fusion reactors was also covered.
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1994 with total page 708 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Characteristics of the Scrape off Layer in DIII D High performance Negative Central Magnetic Shear Discharges written by and published by . This book was released on 1996 with total page 21 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this paper the authors present measurements of the global power and particle balance in the high-performance phase of negative central magnetic shear (NCS) discharges and compare with reference VH-mode discharges. The principal differences observed are that NCS has a much lower fraction of the total input power flowing into the boundary, less core radiation, and larger rate of stored energy increase as a fraction of total power. Scrape-off layer (SOL) temperature and divertor heat flux profiles, and radiation profiles at the midplane, are similar to VH-mode. Due to the good core particle confinement and efficient fueling by neutral beam injection (NBI), with little gas puffing, the gas load on the walls and the recycling are very low during the NCS discharges. This results in a rate of density rise relative to beam fueling at the L to H transition time which is 1/3 of the value for VH transitions, which is in turn 1/2 that for L-to-ELMing-H-mode transitions.
Download or read book Production and Stability of High beta DIII D Discharges with Reversed Magnetic Shear written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Plasma configurations with reversed magnetic shear have been proposed for steady-state tokamak operation since the plasma profiles can be made consistent with good confinement, high bootstrap current fraction, and stability at very high beta. The stability of reversed magnetic shear discharges with beta up to 11% has previously been demonstrated in DIII-D. Reversed magnetic shear (RMS) refers to a safety factor profile, q([rho]), which is a non-monotonic function of minor radius, [rho]. The magnetic shear S[equivalent-to] ([rho]/q) dq/d[rho] is negative within the plasma core and positive at the edge. When S
Download or read book Physics of Advanced Tokamaks written by and published by . This book was released on 1997 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Observation and Analysis of Resistive Instabilities in Negative Central Shear in DIII D Discharges with L Mode Edge written by and published by . This book was released on 2002 with total page 7 pages. Available in PDF, EPUB and Kindle. Book excerpt: In DIII-D plasmas with L-mode edge and negative central shear (q{sub axis}-q{sub min} (almost equal to)0.3 to 0.5), an interchange-like instability has been observed [1]. The instability and a subsequent tearing mode cause reduction of the core electron temperature and plasma rotation, and therefore the instability affects discharge evolution and the desired high performance is not achieved. Stability analyses indicate robust ideal stability, while the Resistive Interchange Mode criterion is marginal and the instability appears to be localized initially. Based on this, we believe that the mode is, most likely, a Resistive Interchange Mode. The amplitude of the instability is correlated with the location of the q{sub min} surface and inversely with the fast-ion pressure. There is indication that the interchange-like instability may be ''seeding'' the tearing mode that sometimes follows the interchange-like instability.
Download or read book H mode and VH mode Confinement Improvement in DIII D Investigations of Turbulence Local Transport and Active Control of the Shear in the E times B Flow written by and published by . This book was released on 2001 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: The hypothesis of stabilization of turbulence by shear in the E[times] B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L to H transition. This same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from H-mode to VH-mode. Consequently, the most fundamental question for H-mode studies now is: how is the electric field E[sub r] formed? The radial force balance equation relates E[sub r] to the main ion pressure gradient[triangledown]P[sub i], poloidal rotation[nu][sub[theta]i], and toroidal rotation[nu][sub[phi]i]. In the plasma edge, direct measurements show[triangledown]P[sub i] and[nu][sub[theta]i] are the important terms at the L to H transition, with[triangledown]P[sub i] being the dominant, negative term throughout most of the H-mode. Since E[sub r] is observed to change prior to the change in[triangledown]P[sub i], the authors infer that main ion rotation, probably[nu][sub[theta]i], changes first, triggering the transition. E[sub r] is seen to change prior to the change in fluctuations, consistent with E[times] B shear causing the change in fluctuations and transport. In the plasma core, E[sub r] is primarily related to[nu][sub[phi]i]. There is a clear temporal and spatial correlation between the change in E[times] B shear and the region of local confinement improvement when the plasma goes from H-mode to VH-mode. Direct manipulation of[nu][sub[phi]i] and E[times] B shear using the drag produced by a non-axisymmetric magnetic perturbation has produced clear changes in local transport consistent with the E[times] B shear stabilization hypothesis. The implications of these results for theories of the L to H and H to VH transitions will be discussed. 83 refs., 5 figs.
