Role of PVA Flakes in Promoting Self-Degradation of Sodium Metasilicate-Activated Cement under a Hydrothermal Environment at ≥150°C

We investigated the effect of flakes of polyvinyl alcohol (PVA) on the self-degradation of sodium metasilicate (SMS)-activated slag/Class C fly ash cement, which is used as temporary sealer for fractures in Enhanced Geothermal System (EGS) wells under a hydrothermal environment at ≥150°C and at pressure of 1000 psi. The reactions between PVA and SMS dissolved in an aqueous medium at 85°C led to the formation of a colloidal hydroxylated silicate-cross-linked PVA gel in the cement body. This gel-incorporated cement had a compressive strength >2000 psi, so ensuring that it adequately plugs the fractures. Increasing the hydrothermal temperature to ≥150°C triggered the transformation of the gel into a sol. This in-situ gel-->sol phase transition played a pivotal role in promoting the cement's self-degradation because of the extensive spreading of sol in the cement body, and its leaching from the cement. In contrast, when gel was dry heated at ≥150°C, the gel-->xerogel phase transformation engendered the molecular fragmentation of PVA. This fragmentation yielded polysilicate cross-linked PVA derived from the combination of extended-chain scission, carboxylation and condensation, so that it no longer served as a self-degrading promoter of cement.

Dynamics of Zonal Flows in Helical Systems

A theory for describing collisionless long-time behavior of zonal flows in helical systems is presented and its validity is verified by gyrokinetic-Vlasov simulation. It is shown that, under the influence of particles trapped in helical ripples, the response of zonal flows to a given source becomes weaker for lower radial wave numbers and deeper helical ripples while a high-level zonal-flow response, which is not affected by helical-ripple-trapped particles, can be maintained for a longer time by reducing their bounce-averaged radial drift velocity. This implies a possibility that helical configurations optimized for reducing neoclassical ripple transport can simultaneously enhance zonal flows which lower anomalous transport

Collisionless damping of geodesic acoustic modes

Collisionless time evolutions of geodesic acoustic modes (GAMs) in tokamaks are investigated by the gyrokinetic theory and simulation. It is shown that the collisionless damping of the GAM oscillations is enhanced when the ratio of the typical drift orbit width of passing ions to the radial wavelength of the zonal flow increases

Gyrokinectic Theory and Simulation of Zonal Flows and Turbulence in Helical Systems

Gyrokinetic theory and simulations on ion heat transport physics in helical systems have recently been developed. Damping processes of zonal flows driven by ion temperature gradient (ITG) turbulence in helical systems have been analytically investigated based on the gyrokinetic theory as a generalization of the previous work by Rosenbluth and Hinton for tokamaks. A collisionless response function of the zonal flow to given source terms is derived by taking account of the helical geometry and finite-orbit-width effects. Validity of the analytical predictions are verified by the Eulerian gyrokinetic code (GKV code) with very-high resolution of the phase space. The GKV simulation extended to take account of helical-ripple-trapped particles is also applied to the ITG turbulence in helical systems. The ITG turbulent transport level in a model case for the inward-shifted magnetic-axis configuration with a stronger instability drive is effectively suppressed by the zonal flow, and is reduced to a level comparable to that in the less unstable case for the standard configuration with smaller side-band helical field components

Study of electromagnetic microinstabilities in helical systems with the stellarator expansion method

Electromagnetic microinstabilities in helical systems are studied by numerically solving integral eigenmode equations, which are derived from the ion gyrokinetic equation, the quasineutrality equation, the Amp?re\u27s law, and the massless electron approximation. The stellarator expansion technique is used to evaluate finite-beta effects on the guiding-center drift in the helical configuration, where the toroidal plasma shift and the magnetic shear strongly influence the magnetic curvature and accordingly the stability of both magnetohydrodynamics (MHD) and kinetic modes. The kinetic integral equations are shown to reduce to the ideal MHD ballooning mode equation in the fluid limit, from which the Mercier criterion is obtained. For helical geometry like the Large Helical Device (LHD) [Motojima, et al., Nucl. Fusion 43, 1674 (2003)], it is confirmed that, when increasing the beta value, the ion temperature gradient mode is stabilized while the kinetic ballooning mode (KBM) is destabilized due to the unfavorable geodesic curvature resulting from the negative magnetic shear combined with the toroidal plasma shift. Also, dependencies of these kinetic-mode properties on the poloidal wave number and the magnetic shear are investigated. It is found that the KBM-unstable parameter region is narrower than the Mercier-unstable region in the LHD-like configuration

Nondissipative kinetic simulation and analytical solution of three-mode equations of the ion temperature gradient instability

A nondissipative drift kinetic simulation scheme, which rigorously satisfies the time-reversibility, is applied to the three-mode coupling problem of the ion temperature gradient (ITG) instability. It is found from the simulation that the three-mode ITG system repeats growth and decay with a period which shows a logarithmic divergence for infinitesimal initial perturbations. Accordingly, time average of the mode amplitude vanishes, as the initial amplitude approaches zero. An exact solution is analytically given for a class of initial conditions. An excellent agreement is confirmed between the analytical solution and numerical results. The results obtained here provide a useful reference for basic benchmarking of theories and simulations of the ITG modes