UCLA Local Group: The LAPD Program Overview
Objective: To explore in the laboratory under controlled conditions fundamental plasma processes that play a major role in the behavior of naturally occurring plasmas, e.g., the auroral ionosphere, the magnetosphere, the solar wind, the solar corona, and the interstellar medium.
Motivation: Fundamental plasma processes are responsible for large releases of energy, generation of powerful electromagnetic radiation and acceleration of energetic particles in naturally occurring plasmas. These by-products of plasma interactions can have consequences to earth bound activities or to man-made systems that are exposed to such environments. At the basic level, the phenomena investigated plays a key role in determining the large scale structure of the observable universe.
Need for Laboratory Studies: Knowledge of the behavior of naturally occurring plasmas is derived primarily from remote point measurements at a given time, e.g., magnetic fluctuations measured by a ground-based magnetometer when a magnetic substorm develops, or measurements of scintillations in radio noise coming from distant stars. Complementary information is also derived from spacecraft measurements, but its value is limited in forming a complete picture because the phenomena exhibits temporal and spatial structures that are not resolvable. Furthermore, since in the natural environment phenomena are highly variable, spacecraft may not sample enough events to provide sufficient data to clearly identify and understand some processes.
Requirements of a Laboratory Device: A strict set of requirements must be met by a laboratory device that aims to explore fundamental processes that regulate flows of particles and energy in a naturally occurring plasma. A device suitable for studying processes involving waves must be long enough to accommodate several axial wavelengths, and must also have large cross section in order for the effect of walls to be minimal. To study low frequency processes (essential for understanding energy and momentum tranfer) the plasma must be long lived and the neutral density very low in order to minimize collisional effects.
Capabilities of the Large Plasma Device (LAPD): The LAPD is the finest basic plasma research device in the world. It is the culmination of many years of research into plasma sources and plasma confinement schemes. The machine produces a quiescent, highly ionized, ten meter long ( corresponding to a million Debye lengths) plasma in which the ions can be strongly magnetized; the plasma diameter is fifty centimeters (four hundred ion Larmor radii). The plasma source is reliable and durable; it permits continuous experimentation for several months. Highly reproducible plasmas are created whose density profiles can be taylored to provide a variety of conditions encountered in naturally occurring plasmas. An important element of the LAPD facility is its flexibility of operation. The broad range of operational conditions permit the investigation of a large class of different phenomena with relative ease. For instance, it is possible that during the morning the device could be used to investigate whistler waves, while, in the afternoon, a detailed experiment on striations could take place, and, in the evening, a beam injection problem could be pursued. All of these studies require different plasma parameters and settings that are reproducible and achievable in a short time, but such flexibility is provided by the LAPD. The operational flexibility has been achieved by features built into the machine design. For example, the axial magnetic field can be shaped by independent magnet power supplies. Cross-field density gradients can be produced in the plasma using differential cathode coating, and field-aligned density gradients can be established through external control of the neutral pressure. Counter-streaming plasmas and beams can be produced by biasing three electrically-isolated segments of the ten-meter long chamber. Finally, the plasma can be accessed through 128 radial ports with dozens of pumpdown stations that allow an array of probes and antennas to be switched without disturbing the plasma.
Diagnostics: The LAPD laboratory has a variety of plasma diagnostics that allow volumetric measurements with computer controlled probes. These include Langmuir and emissive probes, magnetic induction loops and electric dipoles. Density measurements are obtained with a 70 GHz microwave interferometer, and a tunable dye laser for LIF (Laser Induced Fluorescence) samples microscopic properties of ions in the plasma. The laboratory is also developing new plasma probing techniques, e.g., nanotechnology and micro-machining techniques are being used to construct microscopic plasma sensors. The LAPD laboratory is equipped with the latest in computer technology for taking and storing data, analyzing data, and displaying the results of data analysis. The facility has over fifty gigabytes of disk storage, CD storage, five 3D graphics work stations, and the capacity to produce Hi-8 and VHS videos from digital data. This computational/graphics capability is essential for the successful exploration in the laboratory of phenomena having relevance to naturally occurring plasmas, the reason being that the phenomena is intrinsically 3-dimensional and not time-stationary.
Scientific Methodology: Because the LAPD program aims to solve fundamental plasma problems that have relevance to complex events encountered in naturally occurring plasmas, a broad-based interdisciplinary team of scientists is associated with this activity. The principal researchers include W. Gekelman (basic plasma experimentalist, expert on computer visualization, analysis of large data sets, and novel diagnostics), J. Maggs (space theoretician, basic experimentalist, expert on spacecraft measurements), and G. Morales (basic plasma theoretician, expert on basic experiments and ionospheric heating). The studies pursued are selected carefully on the basis of their relevance to major questions related to naturally occurring plasmas. A systematic approach is followed: 1) controlled conditions are generated in which a particular scenario is emphasized, 2) exploratory measurements are taken to delineate the phenomena, 3) massive data collection of all relevant variables in three dimensions is undertaken, 4) data analysis is pursued and results are rendered in visual form, 5) a parallel theory effort is pursued to predict the outcome of the experiment based on first principles, 6) a quantitative comparison is made of the theoretical predictions to the various projections of the data, 7) an integrated perspective is formed of the role of the process in the behavior of naturally occurring plasmas.
Interaction with the Scientific Community: The results of experiments conducted at the LAPD facility are of interest to both the basic plasma physics community and the space science community. Therefore, research results are published in Journals that reach both communities (Phy. Rev. Lett., J. Geophy. Res., Phy. of Plasmas, Geophy. Res. Lett.). Also research results are reported at the Fall meeting of the American Geophysical Union, the Annual meeting of the American Physical Society, Division of Plasma Physics and various international meetings such as the meeting of the International Association of Geomagnetism and Aeronomy. In addition the LAPD laboratory is engaged in organizing the fourth IPELS (Interrelationship between Experiments in the Laboratory and in Space) conference to be held in 1997.
Areas of Research: The research program at LAPD covers a broad variety of topics concerning processes of basic importance as well of topics of current interest in the space plasma community. Two areas in which a great deal of attention is focused are Alfvén waves and field-aligned density striations. Below we discuss these two topics in detail and touch upon some other areas of current and future research.
Alfvén Waves: Alfvén waves are low frequency fluctuations propagating in dense, magnetized plasmas. They exhibit both an electric and magnetic field and have large scale size along the magnetic field but microscopic size across the field. They are radiated by fluctuating magnetic field-aligned currents and, in turn, modulate such currents. Localized structures of Alfvén waves lead to acceleration and heating of both electrons and ions. They are of particular importance because they are the principal carrier of energy in macroscale plasmas.
The study of Alfvén waves is recognized to be crucial for understanding the solar corona, generation of the solar wind, the structure of magnetospheres, generation of the aurora, micropulsations of the geomagnetic field, induced currents in the Earth and man-made power and communication grids, and, on an astrophysical scale, the formation of structure in the intergalactic medium. In the interactions among plasma systems, plasma flows, and macroscopic energy exchanges between different plasma regions the Alfvén waves play an important and leading role. They mediate process on a macroscopic scale size as contrasted with high frequency waves such as Langmuir waves which modify the plasma distribution function locally.
As an illustration of the interest in the study of Alfvén wave research, we note that twenty six papers involving spacecraft measurements, computer simulation, and basic and applied theory were presented at the Fall AGU meeting held in San Francisco (Dec 11/15, 1995). It is noteworthy that only three experimental papers were presented, all from the LAPD laboratory. This is not surprising because the experimental study of Alfvén waves requires a special facility that is large in physical size and capable of producing dense magnetized plasmas. It is a tribute to the uniqueness of the LAPD facility that it is the only basic plasma device capable of providing conditions in which Alfvén wave phenomena can be explored with minimal influence of radial boundaries. The only other plasma devices capable of achieving such conditions are tokamaks used for thermonuclear fusion research. However, tokamak devices are intrinsically toroidal resulting in phenomena that are periodic along the magnetic field direction, a situation not present in naturally occurring plasmas.
In addition, Alfvén wave phenomena are intrinsically three dimensional. A complete picture of their spatial structure requires volume measurements. Such measurements are impossible from a single spacecraft and difficult to obtain using multiple spacecraft. However, volume data measurements are straight forward at the LAPD facility because of its computerized data acquisition and extensive three dimensional graphics capability.
Experiments involving Alfvén waves performed to date at the LAPD facility include the study of propagation of low frequency pulses and wave packets, the interference of waves from two sources, the verification of the existence of Alfvén wave cones, Landau damping of Alfvén waves, the spontaneous growth of drift Alfvén waves in a cross field density gradient, the interaction of Alfvén waves with low frequency density fluctuations , Alfvén wave radiation from modulated currents, and electron heating from Alfvén waves. Many more aspects of the Alfvén wave of interest to space plasma physicists remain to be studied, including field line resonances, reflection from boundaries, and generation of Alfvenic turbulence, a topic of special interest to astrophysicists.
Striations: Another important feature of magnetized plasmas is the formation of magnetic field-aligned plasma structures. Such structures include spicules, granules and coronal arches in the sun, density striations in the auroral and equatorial ionospheres and filamentary magnetic structures in the galactic medium. These types of structures are important in the transport of wave energy through the plasmas and play a role in particle heating. The effect of field-aligned density structures (striations) on various plasma processes can be studied at the LAPD facility because techniques have been developed to produce these structures in a controlled fashion.
Experiments conducted in the LAPD laboratory involving striations include scattering of electromagnetic whistlers and the production of lower hybrid waves through mode conversion at a striation, spontaneous generation of drift Alfvén wave eigenmodes in a striation, and the interaction of Alfven wave cones with density striations.
Other Studies: Other plasma processes important in space plasma physics that can be studied in the LAPD device include interaction of field aligned currents, the effects of bulk flows in plasmas, the generation and evolution of beams and beam generated waves, and the propagation and interaction of whistler and lower hybrid waves in nonuniform plasmas. In addition to these experimental issues the manipulation and analysis of the large data sets gathered during such experiments is also an active area of research at the LAPD facility. A program to develop advanced techniques for visualizing and handling large data sets is underway in collaboration with the computer science department at UCLA.