Hantao Ji

Electromagnetic Fluctuations during Fast Reconnection in a Laboratory Plasma

Experimental evidence for a positive correlation is established between the magnitude of electromagnetic fluctuations up to the lower-hybrid frequency range and enhancement of reconnection rates in a well-controlled laboratory plasma. The fluctuations belong to the right-hand polarized whistler wave branch, propagating obliquely to the reconnecting magnetic field, with a phase velocity comparable to the relative drift velocity between electrons and ions. The measured short coherence lengths indicate their strongly nonlinear nature.

Hydrodynamic turbulence cannot transport angular momentum effectively in astrophysical disks

The most efficient energy sources known in the Universe are accretion disks. Those around black holes convert 5–40 per cent of rest-mass energy to radiation. Like water circling a drain, inflowing mass must lose angular momentum, presumably by vigorous turbulence in disks, which are essentially inviscid. The origin of the turbulence is unclear. Hot disks of electrically conducting plasma can become turbulent by way of the linear magnetorotational instability. Cool disks, such as the planet-forming disks of protostars, may be too poorly ionized for the magnetorotational instability to occur, and therefore essentially unmagnetized and linearly stable. Nonlinear hydrodynamic instability often occurs in linearly stable flows (for example, pipe flows) at sufficiently large Reynolds numbers. Although planet-forming disks have extreme Reynolds numbers, keplerian rotation enhances their linear hydrodynamic stability, so the question of whether they can be turbulent and thereby transport angular momentum effectively is controversial. Here we report a laboratory experiment, demonstrating that non-magnetic quasi-keplerian flows at Reynolds numbers up to millions are essentially steady. Scaled to accretion disks, rates of angular momentum transport lie far below astrophysical requirements. By ruling out purely hydrodynamic turbulence, our results indirectly support the magnetorotational instability as the likely cause of turbulence, even in cool disks.

Experimental study of two-fluid effects on magnetic reconnection in a laboratory plasma with variable collisionality

This article describes the recent findings on two-fluid effects on magnetic reconnection in plasmas with variable collisionality in the magnetic reconnection experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. The MRX device has been upgraded to accommodate a variety of reconnection operation modes and high energy density experiments by increasing its capacitor bank energy and extending the discharge duration. As our experimental operation regime has moved from the collisional to the collision-free, two-fluid effects have become more evident. It is observed that the two-dimensional profile of the neutral sheet is changed significantly from the rectangular shape of the familiar Sweet-Parker type to a double wedge shape as the collisionality is reduced and the reconnection rate increases. The recent evolution of our experimental research from the magnetohydrodynamics (MHD) to the two-fluid analysis is presented to illuminate the physics of Hall MHD in a collision-free reconnection layer. In p...

Magnetorotational instability in a rotating liquid metal annulus

Although the magnetorotational instability (MRI) has been widely accepted as a powerful accretion mechanism in magnetized accretion discs, it has not been realized in the laboratory. The possibility of studying MRI in a rotating liquid metal annulus (Couette flow) is explored by local and global stability analysis. Stability diagrams are drawn in dimensionless parameters, and also in terms of the angular velocities at the inner and outer cylinders. It is shown that MRI can be triggered in a moderately rapidly rotating table-top apparatus, using easy-to-handle metals such as gallium. Practical issues of this proposed experiment are discussed.

Conversion of magnetic energy in the magnetic reconnection layer of a laboratory plasma

Magnetic reconnection, in which magnetic field lines break and reconnect to change their topology, occurs throughout the universe. The essential feature of reconnection is that it energizes plasma particles by converting magnetic energy. Despite the long history of reconnection research, how this energy conversion occurs remains a major unresolved problem in plasma physics. Here we report that the energy conversion in a laboratory reconnection layer occurs in a much larger region than previously considered. The mechanisms for energizing plasma particles in the reconnection layer are identified, and a quantitative inventory of the converted energy is presented for the first time in a well-defined reconnection layer; 50% of the magnetic energy is converted to particle energy, 2/3 of which transferred to ions and 1/3 to electrons. Our results are compared with simulations and space measurements, for a key step towards resolving one of the most important problems in plasma physics.

Magnetorotational instability of dissipative couette flow

Axisymmetric stability of viscous resistive magnetized Couette flow is re-examined, with the emphasis on flows that would be hydrodynamically stable according to Rayleighs criterion: opposing gradients of angular velocity and specific angular momentum. In this regime, magnetorotational instabilities (MRI) may occur. Previous work has focused on the Rayleigh-unstable regime. To prepare for an experimental study of MRI, which is of intense astrophysical interest, we solve for global linear modes in a wide gap with realistic dissipation coefficients. Exchange of stability appears to occur through marginal modes. Velocity eigenfunctions of marginal modes are nearly singular at conducting boundaries, but magnetic eigenfunctions are smooth and obey a fourth-order differential equation in the inviscid limit. The viscous marginal system is of tenth order; an eighth-order approximation previously used for Rayleigh-unstable modes does not permit MRI. Peak growth rates are insensitive to boundary conditions. They are predicted with surprising accuracy by WKB methods even for the largest-scale mode. We conclude that MRI is achievable under plausible experimental conditions using easy-to-handle liquid metals such as gallium.

Experimental Test of the Sweet-Parker Model of Magnetic Reconnection

We report a quantitative experimental test of the Sweet-Parker model of magnetic reconnection in a controlled laboratory plasma. It is found that the observed reconnection rate cannot be explained by the Sweet-Parker model unless the model is generalized to incorporate compressibility, downstream pressure, and the effective resistivity. The latter is significantly enhanced over its classical values in the collisionless limit.

Experimental investigation of the neutral sheet profile during magnetic reconnection

During magnetic reconnection, a “neutral sheet” current is induced, heating the plasma. The resultant plasma thermal pressure forms a stationary equilibrium with the opposing magnetic fields. The reconnection layer profile holds significant clues about the physical mechanisms which control reconnection. In the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)], a quasi steady-state and axisymmetric neutral sheet profile has been measured precisely using a magnetic probe array with spatial resolution equal to one quarter of the ion gyro-radius. It was found that the reconnecting field profile fits well with a Harris-type profile [E. G. Harris, Il Nuovo Cimento 23, 115 (1962)], B(x)∼tanh(x/δ). This agreement is remarkable since the Harris theory does not take into account reconnection and associated electric fields and dissipation. An explanation for this agreement is presented. The sheet thickness δ is found to be ∼0.4 times the ion skin depth, which agrees with a generalized...

Numerical study of tilt stability of prolate field-reversed configurations

Global stability of the field-reversed configuration (FRC) has been investigated numerically using both three-dimensional magnetohydrodynamic and hybrid (fluid electron and δf particle ion) simulations. The stabilizing effects of velocity shear and finite ion Larmor radius (FLR) on the n=1 internal tilt mode in the prolate FRCs have been studied. Sheared rotation is found to reduce the growth rate, however a large rotation rate with Mach number of M≳1 is required in order for significant reduction in the instability growth rate to occur. Kinetic effects associated with large thermal ion orbits have been studied for different kinetic equilibria. The simulations show that there is a reduction in the tilt mode growth rate due to FLR effects, but complete linear stability has not been found, even when the thermal ion gyroradius is comparable to the distance between the field null and the separatrix. The instability existing beyond the FLR theory threshold could be due to the resonant interaction of the wave with ions whose Doppler shifted frequency matches the betatron frequency.

Initial results from coaxial helicity injection experiments in NSTX

Coaxial helicity injection has been investigated on the National Spherical Torus Experiment (NSTX). Initial experiments produced 130 kA of toroidal current without the use of the central solenoid. The corresponding injector current was 20 kA. Discharges with pulse lengths up to 130 ms have been produced.