Wide Radial Region Research Articles

Delivery of Pebbles from the Protoplanetary Disk into Circumplanetary Disks

Abstract Small bodies likely existed in the late stage of planet formation either as remnants of the planetesimal formation stage or as fragments of larger planetesimals. Recent studies suggest that they may have played an important role in the formation of regular satellites of giant planets, but their delivery process into the circumplanetary disk has been poorly understood. Using orbital integration that incorporates the gas flow around the planet obtained by hydrodynamic simulation, we examine delivery of small bodies in the protoplanetary disk into circumplanetary disks. We find that large bodies can be captured when they experience strong gas drag near the midplane of the circumplanetary disk, while particles with Stokes number near unity tend to settle toward the midplane of the protoplanetary disk and can be captured near the outer edge of the circumplanetary disk. On the other hand, small particles coupled to the gas can be delivered into the circumplanetary disk with the vertically accreting gas and are captured near the surface of the circumplanetary disk over a wide radial region, if they are sufficiently stirred off the midplane of the protoplanetary disk. However, if the turbulence in the protoplanetary disk is not sufficiently strong, delivery of small particles by such a mechanism would not be efficient. Also, gas depletion in the vicinity of the planet’s orbit reduces the efficiency of the delivery. In these cases, larger bodies directly captured by gas drag from the circumplanetary disk would be the major building blocks of regular satellites.

Nonlinear coupling induced toroidal structure of edge localized modes

Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that cause fast periodic relaxations of the strong edge pressure gradient in tokamak fusion plasmas. A novel diagnostic method allows the extraction of toroidal mode numbers, rotation velocities and spatial information during the ELM cycle including the crash. While mode number branches –6 and –10 are dominant just before the ELM crash, during the ELM crash –5 are observed in typical discharges with type-I ELMs in the tokamak experiment. These findings are compared to results from nonlinear MHD simulations. Although is linearly dominant, nonlinear coupling in which is particularly important leads to the dominance of –5 during the ELM crash, in excellent agreement with experimental observations. The simultaneous occurrence of these modes over a wide radial region leads to high stochasticity and thus increased transport.

Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma

A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves.

EXCLUSION OF COSMIC RAYS IN PROTOPLANETARY DISKS. II. CHEMICAL GRADIENTS AND OBSERVATIONAL SIGNATURES

The chemical properties of protoplanetary disks are especially sensitive to their ionization environment. Sources of molecular gas ionization include cosmic rays, stellar X-rays and short-lived radionuclides, each of which varies with location in the disk. This behavior leads to a significant amount of chemical structure, especially in molecular ion abundances, which is imprinted in their submillimeter rotational line emission. Using an observationally motivated disk model, we make predictions for the dependence of chemical abundances on the assumed properties of the ionizing field. We calculate the emergent line intensity for abundant molecular ions and simulate sensitive observations with the Atacama Large Millimeter/Sub-millimeter Array (ALMA) for a disk at D=100 pc. The models readily distinguish between high ionization rates ($\zeta\gtrsim10^{-17}$ s$^{-1}$ per H$_2$) and below, but it becomes difficult to distinguish between low ionization models when $\zeta\lesssim10^{-19}$ s$^{-1}$. We find that \htdp\ emission is not detectable for sub-interstellar CR rates with ALMA (6h integration), and that \ntdp\ emission may be a more sensitive tracer of midplane ionization. HCO$^+$ traces X-rays and high CR rates ($\zeta_{\rm{CR}}\gtrsim10^{-17}$ s$^{-1}$), and provides a handle on the warm molecular ionization properties where CO is present in the gas. Furthermore, species like HCO$^+$, which emits from a wide radial region and samples a large gradient in temperature, can exhibit ring-like emission as a consequence of low-lying rotational level de-excitation near the star. This finding highlights a scenario where rings are not necessarily structural or chemical in nature, but simply a result of the underlying line excitation properties.

Plasma rotation effects on magnetic island formation and the trigger of disruptions in reversed shear plasma

Plasma rotation effects on the trigger and the evolution of MHD activities in reversed shear profiles are studied by nonlinear MHD simulations. It is found that, in a rotating plasma, magnetic islands formed around the inner and outer magnetic resonant surfaces, which are stable for the tearing mode, by an external perturbation (driven magnetic island), evolve with different growth rates during an initial growth phase. After the initial growth phase, an outer magnetic island grows rapidly prior to the inner one and triggers a rapid growth of the inner magnetic island. At the final phase, enlarged magnetic islands flatten a q-profile in a wide radial region including the plasma centre. Though this final phase closely resembles a nonlinear destabilization of a spontaneous double tearing mode (Ishii et al 2002 Phys. Rev. Lett. 89 205002), this process can explain the time delay of a plasma edge oscillation in triggering an internal MHD event and disruption.

Comparative analysis of intermittent burst temporal characteristics at the edge of the CASTOR and Tore Supra tokamaks

Temporal characteristics of intermittent positive bursts of plasma density in the scrape-off layer of the CASTOR and Tore Supra tokamaks are investigated. A monotonic radial decrease of average burst rate together with an increase of average burst duration is observed in the Tore Supra tokamak [I. Nanobashvili et al., Czech. J. Phys. 56, 1339 (2006); I. Nanobashvili et al., J. Nucl. Mater. 363–365, 622 (2007)]. At the edge of the CASTOR tokamak the temporal characteristics stay nearly constant within a quite wide radial region. The reason is that radially elongated turbulent structures (streamers) govern the radial transport in CASTOR [P. Devynck et al., Phys. Plasmas 13, 102505 (2006)]. A comparative analysis allows to conclude that radial transport at the edge of the Tore Supra tokamak is governed by blob-like structures. Plasma biasing at the edge of CASTOR significantly modifies the burst characteristics, reduces plasma transport, and considerably improves the confinement.

Dynamics of turbulent transport dominated by the geodesic acoustic mode near the critical gradient regime

The effects of geodesic acoustic modes (GAMs) on the toroidal ion temperature gradient turbulence and associated transport near the critical gradient regime in tokamak plasma are investigated based on global Landau-fluid simulations and extended predator-prey modeling analyses. A new type of intermittent dynamics of transport accompanied with the emission and propagation of the GAMs, i.e., GAM intermittency [K. Miki et al., Phys. Rev. Lett. 99, 145003 (2007)], has been found. The intermittent bursts are triggered by the onset of spatially propagating GAMs when the turbulent energy exceeds a critical value. The GAMs suffer collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region. The stationary zonal flows gradually increase due to the accumulation of non-damped residual part over many periods of quasi-periodic intermittent bursts and eventually quench the turbulence, leading to a nonlinear upshift of the linear critical gradient; namely, the Dimits shift. This process is categorized as a new class of transient dynamics, referred to as growing intermittency. The Dimits shift is found to be established through this dynamical process. An extended minimal predator-prey model with collisionless damping of the GAMs is proposed, which qualitatively reproduce the main features of the growing intermittency and approximately predict its various time scales observed in the simulations.

Observation of the Internal Structure of Energetic Particle-Driven MHD Modes in a Large Helical Device

For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfven eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation.

Transport and Stability Studies in Negative Central Shear Advanced Tokamak Plasmas

Achieving high performance for long duration is a key goal of advanced tokamak research around the world. To this end, tokamak experiments are focusing on obtaining (a) a high fraction of well-aligned noninductive plasma current, (b) internal transport barriers (ITBs) in the ion and electron transport channels over a wide radial region with transport approaching neoclassical values, and (c) control of resistive wall modes and neoclassical tearing modes that limit the achievable beta. A current profile that yields a negative central magnetic shear (NCS) in the core is consistent with this focus; NCS is conducive for obtaining ITBs, a high degree of bootstrap current alignment, and reaching the second stability region for ideal ballooning modes, while being stable to ideal kink modes at high beta with wall stabilization and neoclassical tearing modes in the core NCS region. Much progress has been made in obtaining advanced performance in several tokamaks through an increasing understanding of the stability and transport properties of tokamak plasmas. Radio-frequency and neutral beam current drive scenarios are routinely developed and implemented in experiments to access new advanced regimes and control plasma profiles. Short-duration and sustained ITBs have been obtained in the ion and electron channels. The formation of an ITB is attributable to the stabilization of ion and electron temperature gradient and trapped electron modes by the negative shear and by the enhanced E × B flow shear rate and rarefaction of resonant surfaces near the rational qmin values. The progress in understanding the underlying physics in such plasmas and the development of techniques and technology would be of interest in stellarator efforts.

Monte Carlo simulations of central ion temperature measurements using neutral particle analysers

A detailed Monte Carlo-based numerical analysis of ASDEX Upgrade discharges is presented, examining the applicability of neutral particle analysers (NPAs) for measuring the central ion temperature from the neutral particle spectra above the neutral beam injection energy. The analysis is motivated by an earlier work carried out for TFTR (Fiore C L et al 1988 Nucl. Fusion 28 1315), where the beams were quite tangential. This work is now extended to investigate the applicability of the method when the beams are perpendicular. The Monte Carlo simulations take fully into account the effects due to finite orbits and realistic magnetic geometry, and reveal that the fast ion distribution above the beam energy, generated by collisions, reflects the background temperature with good accuracy. However, it is found that the use of NPA tends to give temperature values that are up to 30% lower than the on-axis temperature Ti(0) assumed for the background. This is because the neutral fluxes measured by NPA originate from a wide radial region, and thus the measured spectrum is a chord-averaged one. A fairly tangential viewing is found to be optimal because in that case the signal originating from the centre of the plasma is enhanced relative to the signal from outer radii. It is concluded that, in discharges where the slowing down from electrons does not dominate the beam ion dynamics, NPA can give a reliable estimate of an average central temperature, Ti(ρ = 0,...,0.3).

Global ship waves in poloidally rotating tokamak plasma

Plasma rotation can lead to excitation of ship waves that have zero frequency in the laboratory frame. Ship waves of a drift type that have a global structure of a quasimode in a poloidally rotating toroidal plasma are considered for an axisymmetric, large aspect-ratio tokamak with concentric, circular magnetic surfaces. The analysis performed is based on rigorously derived eigenmode equations, coupled in poloidal mode numbers through the toroidal effects. Analytical dispersion relations for global drift ship waves are obtained and their simple solutions are found for some limiting cases. These global ship waves, resonantly excited due to a plasma rotation near the plasma edge, may give a simple explanation for the observed phenomenon of the fast response of the electron transport over a wide radial region in transitions from the low (L) to high (H) confinement regime (L–H transitions).

Evolution of transport through the L-H transition in JET

The evolution of the energy, momentum and particle transport through the L-H transition are determined in JET NBI heated discharges. Both normal and periodic L-H transitions are studied. It is found that all of the transport coefficients drop at the transition over a wide radial region and not just in the edge region as was previously thought to be the case. Indeed it is shown by two different modelling techniques that the conventional model in which the transport changes in a narrow region at the edge cannot explain the time behaviour of the electron temperature. Measurements of the fluctuation level by reflectometry also show a very fast drop over a wide radial region