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Self-consistent spectrum of electrostatic drift wave fluctuations due to electron phase space correlations in a sheared magnetic field

Description: The spectrum of electrostatic universal mode fluctuations due to electron phase space correlations (clumps) in a sheared magnetic field is self-consistently calculated. The pair correlation equation for electrons in a sheared field is derived and renormalized, and an approximate solution for the correlation function is obtained. Using the density correlation function as a source term in Poisson's equation, the renormalized dielectric operator is inverted to obtain the turbulent spectrum. Self-consistency is imposed by requiring that the diffusion coefficient derived from the calculated spectrum equal that used to obtain the spectrum originally. (MOW)
Date: December 1, 1979
Creator: Hirshman, S.P. & Diamond, P.H.
Partner: UNT Libraries Government Documents Department

On the dynamics of turbulent transport near marginal stability

Description: A general methodology for describing the dynamics of transport near marginal stability is formulated. Marginal stability is a special case of the more general phenomenon of self-organized criticality. Simple, one field models of the dynamics of tokamak plasma self-organized criticality have been constructed, and include relevant features such as sheared mean flow and transport bifurcations. In such models, slow mode (i.e. large scale, low frequency transport events) correlation times determine the behavior of transport dynamics near marginal stability. To illustrate this, impulse response scaling exponents (z) and turbulent diffusivities (D) have been calculated for the minimal (Burgers) and sheared flow models. For the minimal model, z = 1 (indicating ballastic propagation) and D {approximately}(S{sub 0}{sup 2}){sup 1/3}, where S{sub 0}{sup 2} is the noise strength. With an identically structured noise spectrum and flow with shearing rate exceeding the ambient decorrelation rate for the largest scale transport events, diffusion is recovered with z = 2 and D {approximately} (S{sub 0}{sup 2}){sup 3/5}. This indicates a qualitative change in the dynamics, as well as a reduction in losses. These results are consistent with recent findings from {rho} scaling scans. Several tokamak transport experiments are suggested.
Date: March 1, 1995
Creator: Diamond, P.H. & Hahm, T.S.
Partner: UNT Libraries Government Documents Department

Investigations in anomalous transport and ignition physics. Annual summary of progress, November 1995--November 1996

Description: During FY 95, research supported by DOE Grant No. DE-FG03-88ER53275 has focused on confinement enhancement by reversed shear, core transport physics, L {yields} H transition theory, and the general theory of plasma dynamics. This report discusses specific accomplishments and lists future plans.
Date: August 1, 1996
Creator: Diamond, P.H. & Rosenbluth, M.N.
Partner: UNT Libraries Government Documents Department

Nonlinear evolution of current driven instabilities in a reversed field pinch configuration

Description: Plasmas can be confined in a toroidal configuration using magnetic fields generated by external coils and internal plasma currents. In a toroidal configuration, the magnetic field line equations constitute a Hamiltonian system with one and one-half degrees of freedom. This fact establishes a simple correspondence between a toroidal magnetic field and a nonlinear Hamiltonian system. This correspondence equates magnetic field lines in real space to orbits of the Hamiltonian system in phase space, and the magnetic field line arc length to time. For the plasma to be well confined the magnetic field lines should form nested flux surfaces, corresponding to constant energy surfaces for the Hamiltonian system. For a magnetic system invariant under a uniparametric group of transformations, such as rotations around the axis of the torus, nested flux surfaces exist. That is, the Hamiltonian is integrable. However, in a more general case without a symmetry, the existence of flux surfaces is not assured. In practice, the symmetry of a magnetic configuration may be broken by the intrinsic character of the configuration (no symmetry in the coil system) or by the dynamical behavior of the system manifested in the appearance of a symmetry-breaking instability. The purpose of this paper is to investigate the latter situation.
Date: January 1, 1986
Creator: Carreras, B.A.; Holmes, J.A. & Diamond, P.H.
Partner: UNT Libraries Government Documents Department

A self-consistent theory of collective alpha particle losses induced by Alfvenic turbulence

Description: The nonlinear dynamics of kinetic Alfven waves, resonantly excited by energetic ions/alpha particles, is investigated. It is shown that {alpha}-particles govern both linear instability and nonlinear saturation dynamics, while the background MHD turbulence results only in a nonlinear real frequency shift. The most efficient saturation mechanism is found to be self-induced profile modification. Expressions for the fluctuation amplitudes and the {alpha}-particle radial flux are self-consistently derived. The work represents the first self-consistent, turbulent treatment of collective {alpha}-particle losses by Alfvenic fluctuations.
Date: January 1, 1992
Creator: Biglari, H. & Diamond, P. H.
Partner: UNT Libraries Government Documents Department

U.S. Transport Task Force 2010

Description: The Transport Task Force (TTF) Meeting is a venue for vigorous scientific discourse and discussion on topics in transport and turbulence in fusion plasmas. Its participation is international. The 2010 meeting was highly effective, with 139 registered participants and 131 presentations. This is remarkable for an even year (IAEA year) meeting. The meeting clearly fostered progress in understanding and control of turbulent transport.
Date: September 21, 2011
Creator: Diamond, P.H.
Partner: UNT Libraries Government Documents Department

Study of resistive pressure-gradient-driven turbulence

Description: Previous studies have shown the resistive pressure-gradient-driven turbulence (RPGDT) is a likely cause of observed turbulent fluctuations and anomalous transport in magnetically confined plasmas. More recent study of RPGDT found a true saturation criterion and predicted significantly larger pressure diffusivity over simple mixing-length estimate. In this study, we investigate wavenumber spectrum for more detailed characteristics of this driven turbulence and consider an electromagnetic model with electron temperature evolution to study the effect of magnetic fluctuations on thermal transport.
Date: January 1, 1987
Creator: Lee, G.S.; Garcia, L.; Carreras, B.A. & Diamond, P.H.
Partner: UNT Libraries Government Documents Department

The dynamics of marginality and self-organized criticality as a paradigm for turbulent transport

Description: A general paradigm, based on the concept of self-organized criticality (SOC), for turbulent transport in magnetically confined plasmas has been recently suggested as an explanation for some of the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas. This model describes the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata realization of such a model have found that noise driven SOC systems can maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. The addition of sheared flow into the dynamics leads to a large reduction of the system-scale transport events and a commensurate increase in the fluctuation-scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed.
Date: December 31, 1995
Creator: Newman, D.E.; Carreras, B.A.; Diamond, P.H. & Hahm, T.S.
Partner: UNT Libraries Government Documents Department

Sandpile dynamics as a paradigm for turbulent transport

Description: To shed some light on the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas, a model for transport has been developed based on the concept of self-organized criticality (SOC). This model seeks to describe the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata model have found that SOC systems maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics in contrast to naive marginal stability arguments. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. However, the addition of sheared flow into the dynamics leads to a large reduction of the system-scale transport events and a commensurate increase in the fluctuation-scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed.
Date: December 31, 1995
Creator: Newman, D.E.; Carreras, B.A. & Diamond, P.H.
Partner: UNT Libraries Government Documents Department

Trapped Electron Precession Shear Induced Fluctuation Decorrelation

Description: We consider the effects of trapped electron precession shear on the microturbulence. In a similar way the strong E x B shear reduces the radial correlation length of ambient fluctuations, the radial variation of the trapped electron precession frequency can reduce the radial correlation length of fluctuations associated with trapped electrons. In reversed shear plasmas, with the explicit dependence of the trapped electron precession shearing rate on B(subscript)theta, the sharp radial gradient of T(subscript)e due to local electron heating inside qmin can make the precession shearing mechanism more effective, and reduce the electron thermal transport constructing a positive feedback loop for the T(subscript)e barrier formation.
Date: July 29, 2002
Creator: Hahm, T.S.; Diamond, P.H. & Kim, E.-J.
Partner: UNT Libraries Government Documents Department

Dissipative trapped electron modes in l=2 torsatrons

Description: Trapped electron modes can play an important role in enhancing losses in a toroidal confinement device. They could be one cause of the deterioration of confinement with beta in tokamaks. For straight stellarators and for a model field, it has been shown that the helical ripple and short connection lengths allow for strongly localized solutions to the drift wave equation. Therefore, it is suggested that trapped electron modes in stellarators can be more unstable than in tokamaks. This is particularly the case for low-shear configurations. In this paper, we consider this problem for a realistic 3-D stellarator. We use as input a 3-D equilibrium and include the finite-beta effects self-consistently. 8 refs., 2 figs.
Date: January 1, 1989
Creator: Carreras, B.A.; Dominguez, N.; Leboeuf, J.N.; Lynch, V.E. & Diamond, P.H.
Partner: UNT Libraries Government Documents Department

Transport suppression by diamagnetic phase shift as a possible mechanism to the L to H transition

Description: In the presence of a pressure gradient, the phase velocity of drift-wave or ITG-mode turbulence is different from the poloidal E{times}B rotation velocity of guiding centers. This results in an E{times}B turbulent particle diffusion being suppressed by the phase shift due to the diamagnetic rotation velocity {upsilon}{sub {theta}} = (c/enB)dP/dr. This shift cannot be eliminated by a frame change and affects the plasma transport. For {upsilon}{sub {theta}} well above the turbulent velocity v = {minus}c{gradient}{phi}{times}B/B{sup 2}, the radial diffusivity is suppressed as D {proportional_to} {upsilon}{sub {theta}}{sup {minus}3}. This results in a non-monotonous particle flux {Gamma}({gradient}n) {approx_equal} {minus}D{sub 0}{gradient}n[1+{alpha}({gradient}n){sup 2}]{sup 3/2} such as the transport can develop a bifurcation at a realistic density gradient.
Date: September 28, 1994
Creator: Isichenko, M. B. & Diamond, P. H.
Partner: UNT Libraries Government Documents Department

Flow shear suppression of turbulence using externally driven ion Bernstein and Alfven waves

Description: The utilization of externally-launched radio-frequency waves as a means of active confinement control through the generation of sheared poloidal flows is explored. For low-frequency waves, kinetic Alfven waves are proposed, and are shown to drive sheared E {times} B flows as a result of the radial variation in the electromagnetic Reynolds stress. In the high frequency regime, ion Bernstein waves are considered, and shown to generate sheared poloidal rotation through the pondermotive force. In either case, it is shown that modest amounts of absorbed power ({approximately} few 100 kW) are required to suppress turbulence in a region of several cm radial width.
Date: January 1, 1992
Creator: Biglari, H.; Ono, M.; Diamond, P. H. & Craddock, G. G.
Partner: UNT Libraries Government Documents Department

Dithering transitions in resistive pressure-gradient-driven turbulence

Description: A self-consistent model of the L to H transition has been derived from coupled nonlinear envelope equations for the fluctuation level, and radial electric field shear, E{prime}{sub r}. This model is based on general properties of the coupling between turbulence and averaged sheared flows. To extract these generic properties several turbulence models have been investigated. Here, some of the results obtained with the resistive pressure-gradient-driven turbulence are presented with special emphasis on the structure of the Reynolds stress and the existence of limit cycle solutions.
Date: December 31, 1994
Creator: Garcia, L.; Carreras, B. A.; Lynch, V. E. & Diamond, P. H.
Partner: UNT Libraries Government Documents Department

On the Dynamics of Edge-core Coupling

Description: One of the nagging, unresolved questions in fusion theory is concerned with the extent of the edge. Gyrokinetic particle simulations of toroidal ion temperature gradient (ITG) turbulence spreading using the Gyrokinetic Toroidal Code (GTC) [Z. Lin et al., Science 281, 1835 (1998)] and its related dynamical model have been extended to a system with radially varying ion temperature gradient, in order to study the inward spreading of edge turbulence toward the core plasma. Due to such spreading, the turbulence intensity in the core region is significantly enhanced over the value obtained from simulations of the core region only, and the precise boundary of the edge region is blurred. Even when the core gradient is within the Dimits shift regime (i.e., dominated by self-generated zonal flows which reduce the transport to a negligible value), a significant level of turbulence can penetrate to the core due to spreading from the edge. The scaling of the turbulent front propagation speed is closer to the prediction from a nonlinear diffusion model than from one based on linear toroidal coupling.
Date: August 26, 2005
Creator: Hahm,T.S.; Diamond, P.H.; Lin, Z.; Rewoldt, G.; Gurcan, O. & Ethier, S.
Partner: UNT Libraries Government Documents Department

Turbulent Equipartition Theory of Toroidal Momentum Pinch

Description: The mode-independet part of magnetic curvature driven turbulent convective (TuroCo) pinch of the angular momentum density [Hahm et al., Phys. Plasmas 14,072302 (2007)] which was originally derived from the gyrokinetic equation, can be interpreted in terms of the turbulent equipartition (TEP) theory. It is shown that the previous results can be obtained from the local conservation of "magnetically weighted angular momentum density," nmi U|| R⁄B2, and its homogenization due to turbulent flows. It is also demonstrated that the magnetic curvature modification of the parallel acceleration in the nonlinear gyrokinetic equation in the laboratory frame, which was shown to be responsible for the TEP part of the TurCo pinch of angular momentum density in the previous work, is closely related to the Coriolis drift coupling to the perturbed electric field. In addition, the origin of the diffusive flux in the rotating frame is highlighted. Finally, it is illustratd that there should be a difference in scalings between the momentum pinch originated from inherently toroidal effects and that coming from other mechanisms which exist in a simpler geometry.
Date: January 31, 2008
Creator: Hahm, T. S.; Diamond, P. H.; Gurcan, O. D. & Rewaldt, G.
Partner: UNT Libraries Government Documents Department

Electrostatic toroidal drift mode turbulence in tokamaks

Description: The kinetic theory of turbulent effects due to electron orbit stochasticity was combined with linear response in toroidal geometry. A review of the linear theory is given. The toroidal electron response is given in the ballooning representation. (MOW)
Date: January 1, 1980
Creator: Hirshman, S.P.; Diamond, P.H.; Rosenbluth, M.N.; Chen, L.; Molvig, K.; Whitson, J.C. et al.
Partner: UNT Libraries Government Documents Department

Turbulence Spreading into Linearly Stable Zone and Transport Scaling

Description: We study the simplest problem of turbulence spreading corresponding to the spatio-temporal propagation of a patch of turbulence from a region where it is locally excited to a region of weaker excitation, or even local damping. A single model equation for the local turbulence intensity I(x, t) includes the effects of local linear growth and damping, spatially local nonlinear coupling to dissipation and spatial scattering of turbulence energy induced by nonlinear coupling. In the absence of dissipation, the front propagation into the linearly stable zone occurs with the property of rapid progression at small t, followed by slower subdiffusive progression at late times. The turbulence radial spreading into the linearly stable zone reduces the turbulent intensity in the linearly unstable zone, and introduces an additional dependence on the rho* is always equal to rho i/a to the turbulent intensity and the transport scaling. These are in broad, semi-quantitative agreements with a number of global gyrokinetic simulation results with zonal flows and without zonal flows. The front propagation stops when the radial flux of fluctuation energy from the linearly unstable region is balanced by local dissipation in the linearly stable region.
Date: October 20, 2003
Creator: Hahm, T.S.; Diamond, P.H.; Lin, Z.; Itoh, K. & Itoh, S.-I.
Partner: UNT Libraries Government Documents Department

Center for Momentum Transport and Flow Organization (CMTFO). Final technical report

Description: The Center for Momentum Transport and Flow Organization (CMTFO) is a DOE Plasma Science Center formed in late 2009 to focus on the general principles underlying momentum transport in magnetic fusion and astrophysical systems. It is composed of funded researchers from UCSD, UW Madison, U. Colorado, PPPL. As of 2011, UCSD supported postdocs are collaborating at MIT/Columbia and UC Santa Cruz and beginning in 2012, will also be based at PPPL. In the initial startup period, the Center supported the construction of two basic experiments at PPPL and UW Madison to focus on accretion disk hydrodynamic instabilities and solar physics issues. We now have computational efforts underway focused on understanding recent experimental tests of dynamos, solar tachocline physics, intrinsic rotation in tokamak plasmas and L-H transition physics in tokamak devices. In addition, we have the basic experiments discussed above complemented by work on a basic linear plasma device at UCSD and a collaboration at the LAPD located at UCLA. We are also performing experiments on intrinsic rotation and L-H transition physics in the DIII-D, NSTX, C-Mod, HBT EP, HL-2A, and EAST tokamaks in the US and China, and expect to begin collaborations on K-STAR in the coming year. Center funds provide support to over 10 postdocs and graduate students each year, who work with 8 senior faculty and researchers at their respective institutions. The Center has sponsored a mini-conference at the APS DPP 2010 meeting, and co-sponsored the recent Festival de Theorie (2011) with the CEA in Cadarache, and will co-sponsor a Winter School in January 2012 in collaboration with the CMSO-UW Madison. Center researchers have published over 50 papers in the peer reviewed literature, and given over 10 talks at major international meetings. In addition, the Center co-PI, Professor Patrick Diamond, shared the 2011 Alfven Prize at the EPS ...
Date: July 29, 2013
Creator: Tynan, George R.; Diamond, P. H.; Ji, H.; Forest, C. B.; Terry, P. W.; Munsat, T. et al.
Partner: UNT Libraries Government Documents Department

Nonlinear Flow Generation By Electrostatic Turbulence In Tokamaks

Description: Global gyrokinetic simulations have revealed an important nonlinear flow generation process due to the residual stress produced by electrostatic turbulence of ion temperature gradient (ITG) modes and trapped electron modes (TEM). In collisionless TEM (CTEM) turbulence, nonlinear residual stress generation by both the fluctuation intensity and the intensity gradient in the presence of broken symmetry in the parallel wave number spectrum is identified for the first time. Concerning the origin of the symmetry breaking, turbulence self-generated low frequency zonal flow shear has been identified to be a key, universal mechanism in various turbulence regimes. Simulations reported here also indicate the existence of other mechanisms beyond E × B shear. The ITG turbulence driven “intrinsic” torque associated with residual stress is shown to increase close to linearly with the ion temperature gradient, in qualitative agreement with experimental observations in various devices. In CTEM dominated regimes, a net toroidal rotation is driven in the cocurrent direction by “intrinsic” torque, consistent with the experimental trend of observed intrinsic rotation. The finding of a “flow pinch” in CTEM turbulence may offer an interesting new insight into the underlying dynamics governing the radial penetration of modulated flows in perturbation experiments. Finally, simulations also reveal highly distinct phase space structures between CTEM and ITG turbulence driven momentum, energy and particle fluxes, elucidating the roles of resonant and non-resonant particles.
Date: July 7, 2010
Creator: Wang, W. X.; Diamond, P. H.; Hahm, T. S.; Ethier, S.; Rewoldt, G. & Tang, W. M.
Partner: UNT Libraries Government Documents Department

Effects of Collisional Zonal Flow Damping on Turbulent Transport

Description: Results from 3D global gyrokinetic particle simulations of ion temperature gradient driven microturbulence in a toroidal plasma show that the ion thermal transport level in the interior region exhibits significant dependence on the ion-ion collision frequency even in regimes where the instabilities are collisionless. This is identified as arising from the Coulomb collisional damping of turbulence-generated zonal flows.
Date: October 1, 1999
Creator: Diamond, P.H.; Hahm, T.S.; Tang, W.M.; Lee, W.W. & Lin, Z.
Partner: UNT Libraries Government Documents Department

A self-consistent theory of collective alpha particle losses induced by Alfvenic turbulence

Description: The nonlinear dynamics of kinetic Alfven waves, resonantly excited by energetic ions/alpha particles, is investigated. It is shown that {alpha}-particles govern both linear instability and nonlinear saturation dynamics, while the background MHD turbulence results only in a nonlinear real frequency shift. The most efficient saturation mechanism is found to be self-induced profile modification. Expressions for the fluctuation amplitudes and the {alpha}-particle radial flux are self-consistently derived. The work represents the first self-consistent, turbulent treatment of collective {alpha}-particle losses by Alfvenic fluctuations.
Date: January 1, 1992
Creator: Biglari, H. (Princeton Univ., NJ (United States). Plasma Physics Lab.) & Diamond, P.H. (California Univ., San Diego, La Jolla, CA (United States). Dept. of Physics)
Partner: UNT Libraries Government Documents Department