Angular Momentum Transport and Dynamo Studies in the Flowing Magnetized Plasma Experiment

Z. Wang, S.C. Hsu, C.W. Barnes (P-24), K. Noguchi (T-CNLS), now Rice University), D.C. Barnes, H.Li (X-1), X.Z. Tang (T-15), S.A. Colgate (T-6)
Excerpted from LA-14202-PR

Introduction

Angular momentum transport and dynamo effects are two plasma physics problems that bear on fundamental unsolved astrophysical mysteries. Understanding angular momentum transport in weakly magnetized plasmas is important in determining, for example, the details of how galaxies form and evolve starting from nebulous clouds of matter. A solution to the dynamo problem will tell us how magnetic fields in the galaxy are created and amplified. Researchers in Physics Division’s P-24 Plasma Physics Group have developed a laboratory plasma experiment to investigate these two plasma physics problems.

The problem of angular momentum transport is critical for understanding the rate at which matter collapses gravitationally to form compact objects, such as black holes, in the universe. If this happens too quickly, there will be far too many black holes in the universe; however, if this happens too slowly, then galaxies and stars and planets would never form. Because angular momentum conservation is a robust principle in physics, gravitationally collapsing globs of matter tend to become spinning disks in order to conserve their initial angular momentum. These “accretion disks” are ubiquitous in the universe and exist in the centers of galaxies (Figure 1), around binary star systems, and are an evolutionary step in the formation of solar systems. For matter to “accrete” or fall inward toward the gravitational center of a system, the angular momentum must be transported away via friction within the accretion disk plasma. However, for decades the rate at which matter spiraled inward for many accretion disk systems was a mystery because of the inconsistency between the amount of friction thought to exist and the observationally inferred rates of accretion. The rate of interparticle collisions in disks generally cannot account for the necessary frictional forces. However, in the early 1990s, Balbus and Hawley¹ realized that a weak magnetic field in an accretion disk can lead to a plasma instability, the magnetorotational instability (MRI), which was first discovered in the 1950s. This instability could create magnetic turbulence, which can lead to the necessary frictional forces needed for the mass in an accretion disk to fall towards the central object. Over the past decade, much theoretical and computational research has advanced our understanding of the MRI and its ability to lead to increased friction in rotating disks of plasma. However, this instability has not been observed yet in plasma experiments, providing a strong motivation for this work.

The dynamo problem is another astrophysical holy grail: how are magnetic fields created and amplified on galactic scales to their observed values? Many kinds of dynamos exist, but the kind we are concerned with here involves conversion of kinetic energy in plasma flows to magnetic field energy via electrical currents created by the flowing plasma.² This process is thought to occur in galactic accretion disks, which can be thought of as magnetic engines that fill intergalactic space with magnetic energy. Again, much theoretical and numerical simulation research has been done on the dynamo problem in recent decades, with very little experimental verification and tests. Recently, however, dynamo action was observed in a liquid-metal experiment.³ The experiments in P-24 will try to create dynamo action in a plasma experiment for the first time.

Experimental Setup

In P-24’s Flowing Magnetized Plasma (FMP) experiment, researchers generate a plasma using a “coaxial gun” source, which was recycled from magnetic fusion energy research at LANL in the 1980s. The coaxial gun, which consists of two concentric cylinders, is mounted on one end of a large, cylindrical metal vacuum chamber, as seen in Figure 2. Gas is injected into the unit, and a voltage of up to 1 kV is applied across the gap between the cylinders. The gas then breaks down into a plasma as electric current up to 200 kA flows from one gun electrode through the plasma to the other electrode. The plasma flows down the long axis of the vacuum chamber and rotates azimuthally due to electromagnetic forces. By controlling the plasma density and temperature, the plasma rotation profile, and the magnetic field strength, the experiment is expected, based on numerical calculations, to operate in regimes in which both the MRI and dynamo can be observed and studied.

Experimental measurements of magnetic field, electron density and temperature, and ion-flow speeds are required to characterize the plasma configurations created by the coaxial gun and to detect the MRI and dynamo processes. Various diagnostics are installed on the vacuum chamber to make these measurements, including

1. edge and internal magnetic probe coils to measure all three components of the magnetic field,
2. a triple Langmuir probe to measure electron temperature and density and floating electric potential,
3. a Mach probe to measure ion-flow speed,
4. a charge-coupled device camera to capture movie sequences of plasma evolution as seen in visible light emission, and
5. a spectroscopy system to measure ion temperature and flows via Doppler broadened emission lines.

More sophisticated probe arrays are being built by summer students to provide better diagnosis capability for the experiment.

Initial Experimental Data on FMP Plasmas

The first FMP plasma was achieved in September 2003. Coaxial-gun plasmas were created using a new low-voltage technique⁴, allowing longer plasma pulse durations for a given stored energy. Our first goal—to create and characterize the necessary plasma conditions to observe the MRI—requires the establishment of a rotating plasma within a certain range of density, temperature, magnetic field, and lifetime values. These ranges of values were determined from preliminary numerical calculations of the MRI, specifically for the geometry and conditions of the FMP experiment.⁵ These calculations also predict the initial growth rate of the instability and expected modifications to the profiles of the magnetic field and plasma pressure. The FMP diagnostics should be able to detect these signatures as the plasma transitions from a stable to unstable regime with respect to the MRI.

Camera images of the plasma evolution are shown in Figure 3. The images show that a plasma is generated in the gap between the coaxial gun electrodes. Then it flows out of the coaxial gun and quickly fills up the plasma chamber. An azimuthal rotation can also be detected from the images. Various probes, which are scanned in the radial direction, provide profile information on magnetic field strength, ion-flow speed, and electron density and temperature.

The initial experimental data suggest that we may have a quasi-steady plasma equilibrium in which high magnetic pressure at larger radii balances the thermal plasma pressure and at smaller radii balances the rotational-kinetic pressure. The large magnetic pressure at large radii is related to a plasma “diamagnetic” effect in which the axial magnetic flux in the interior is reduced from its initial value by being expelled to the edge. The net reduction is known as the excluded flux. Diamagnetic effect is measured using two large single-turn pickup coils that encircle the plasma. The coils are at different axial positions.The result is summarized in Figure 4. We observed that the total excluded flux is maximized when the initial axial magnetic field is around 100 G. From the measured excluded flux and a magnetohydrodynamic equilibrium model, we estimate the plasma β (ratio of the total thermal energy to magnetic field energy) is between 30%–60%. We have also observed growth of the local magnetic field (Figure 5), which will be investigated further in terms of both the MRI and dynamo.

Present experimental runs are now dedicated to characterizing the radial profiles of magnetic field, ion flow, and electron density and temperature more completely. This characterization will also be done for a variety of bias schemes on an internal bias plate in hopes of achieving the most desirable profiles for observing the MRI.

Future Work

We plan to deduce the viscous damping term in the plasma force equation by measuring all of the other terms in the equation. This deduced viscosity will be compared to the theoretical value of viscosity based simply on Coulomb collisions between plasma particles. If the MRI is active, then we should expect that the experimentally deduced value for viscosity is higher than the classical value. Both MRI and dynamo will be studied by controlling flow profiles.

References

1. S.A. Balbus and J.F. Hawley, “Instability, turbulence, and enhanced transport in accretion disks,” Reviews of Modern Physics 70, 1–53 (1998).
2. Z. Wang et al., “Laminar plasma dynamos,” Physics of Plasmas 9, 1491–1494 (2002).
3. A. Gailitis et al., “Detection of a flow induced magnetic field eigenmode in the Riga Dynamo Facility,” Physical Review Letters 84, 4365–4368 (2000).
4. Z. Wang et al., “A Penning-assisted sub-kilovolt coaxial plasma source,” submitted to Journal of Applied Physics (2004).
5. K. Noguchi and V.I. Pariev, “Magnetorotational instability in a Couette flow of plasma,” in American Institute of Physics Conference Proceedings 692, 285–292 (2003).

Acknowledgment

This work is supported by Laboratory-Directed Research and Development/ Exploratory Research funding.

For further information, contact Zhehui Wang, 505-665-5353, zwang@lanl.gov.