David Leneman

Design, construction, and properties of the large plasma research device−The LAPD at UCLA

The large plasma research device (LAPD), a large, linear plasma research device designed to study space plasma processes, has been constructed at UCLA over the past four years. The LAPD has a 0.5×0.5 m2 oxide‐coated cathode as a source which produces a 10‐m‐long plasma column with densities up to the mid 1012/cm3 range. The linear machine is surrounded by a set of 68 magnet coils which can generate an axial magnetic field of up to 3000 G. The vacuum chamber has 128 radial ports to ensure excellent access for probes and antennas. An internal probe drive capable of moving a set of probes to any position within the plasma column is described in a companion paper. This machine is a scientific instrument in its own right and was designed to be versatile enough to study a large variety of phenomena. The techniques employed in the design and construction are sufficiently useful to be discussed here so that others can benefit from our experience.

Experimental observation of Alfven wave cones

The spatial evolution of the radial profile of the magnetic field of a shear Alfven wave launched by a disk exciter with radius on the order of the electron skin depth has been measured. The waves are launched using wire mesh disk exciters of 4 mm and 8 mm radius into a helium plasma of density about 1.0×1012 cm−3 and magnetic field 1.1 kG. The electron skin depth δ=c/ωpe is about 5 mm. The current channel associated with the shear Alfven wave is observed to spread with distance away from the exciter. The spreading follows a cone‐like pattern whose angle is given by tan θ=kAδ, where kA is the Alfven wave number. The dependence of the magnetic profiles on wave frequency and disk size are presented. The effects of dissipation by electron–neutral collisions and Landau damping are observed. The observations are in excellent agreement with theoretical predictions [Morales et al., Phys. Plasmas 1, 3765 (1994)].

The plasma source of the Large Plasma Device at University of California, Los Angeles

The Large Plasma Device at the University of California, Los Angeles has recently been upgraded. The plasma is now 18m long (the device is 22m long) and is designed to produce a 0.36T axial magnetic field. Its plasma source has also been upgraded, incorporating a 1m square heater, a 72cm diameter cathode and anode, and associated heat shields and reflectors. The barium oxide coated cathode is heated to 750°C and can produce plasmas of diameters up to 0.9m diameter (depending on the magnetic field configuration), and densities up to 7×1012cm−3 with a spatial uniformity of ±10%.

The upgraded Large Plasma Device, a machine for studying frontier basic plasma physics

In 1991 a manuscript describing an instrument for studying magnetized plasmas was published in this journal. The Large Plasma Device (LAPD) was upgraded in 2001 and has become a national user facility for the study of basic plasma physics. The upgrade as well as diagnostics introduced since then has significantly changed the capabilities of the device. All references to the machine still quote the original RSI paper, which at this time is not appropriate. In this work, the properties of the updated LAPD are presented. The strategy of the machine construction, the available diagnostics, the parameters available for experiments, as well as illustrations of several experiments are presented here.

Spectral gap of shear Alfvén waves in a periodic array of magnetic mirrors

A multiple magnetic mirror array is formed at the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum. 62, 2875 (1991)] to study axial periodicity-influenced Alfven spectra. Shear Alfven waves (SAW) are launched by antennas inserted in the LAPD plasma and diagnosed by B-dot probes at many axial locations. Alfven wave spectral gaps and continua are formed similar to wave propagation in other periodic media due to the Bragg effect. The measured width of the propagation gap increases with the modulation amplitude as predicted by the solutions to Mathieu’s equation. A two-dimensional finite-difference code modeling SAW in a mirror array configuration shows similar spectral features. Machine end-reflection conditions and damping mechanisms including electron-ion Coulomb collision and electron Landau damping are important for simulation.

Lithium ion sources for investigations of fast ion transport in magnetized plasmas

In order to study the interaction of ions of intermediate energies with plasma fluctuations, two plasma immersible lithium ion sources, based on solid-state thermionic emitters (Li aluminosilicate) were developed. Compared to discharge based ion sources, they are compact, have zero gas load, small energy dispersion, and can be operated at any angle with respect to an ambient magnetic field of up to 4.0 kG. Beam energies range from 400 eV to 2.0 keV with typical beam current densities in the 1 mAcm(2) range. Because of the low ion mass, beam velocities of 100-300 kms are in the range of Alfven speeds in typical helium plasmas in the large plasma device.

A novel angular motion vacuum feedthrough

A novel angular motion feedthrough to vacuum has been designed and implemented at the Large Plasma Device at UCLA. The mechanism is easy and inexpensive to build. If linear motion capability is added, then one can arbitrarily and precisely position a probe within a specific volume in a vacuum chamber.

Experimental measurements of the propagation of large-amplitude shear Alfvén waves

Experiments on the edge plasma of tokamaks have discovered magnetic fluctuations which are highly correlated along the magnetic field, and are correlated with scale size of the electron inertial length (δ = c/ωpe) across the field. They are, in all probability, shear Alfven waves. The FREJA, FAST, and Interball satellites have frequently encountered density striations in the auroral ionosphere. These can be narrow, also of the order of δ. Intense wave activity has been measured within these structures and tentatively identified as shear inertial (VA>Vthe) Alfven waves. These waves have been studied in great detail in the Large Plasma Device at UCLA. The plasma, which is 10 m in length and 500 ion Larmor radii in diameter (He (λ∥≈2 m), Ar (λ∥≈10 m, 1.5 kG, 40 cm plasma diameter, n = 1.0-4.0×1012 cm-3, fully ionized) supports Alfven waves. Our initial investigations, which will be briefly reviewed, involved low-amplitude (δBwave/B0≈10-4) shear waves launched by modulating a skin depth size current channel, and have examined the wave characteristics in the kinetic (VA<Vthe) and inertial regimes and in magnetic field gradients. Launching higher-power waves (δBwave/B0≥10-3) waves with the use of a helical antenna has extended these studies. Both shear Alfven waves (ω<ωci) and compressional Alfven waves have been investigated. Below fci the wave fields slowly spread across the background magnetic field and the current associated with it forms a rotating spiral. The higher-power wave causes a localized density perturbation when δBwave/B0 exceeds 10-3 even when the wave propagates below the cyclotron frequency. The perturbation is measured using Langmuir probes as well as laser-induced fluorescence (LIF) signal from Ar II ions. We present data of the wave propagation in which the temporal history of the vector magnetic field was acquired at 20 000 spatial locations. The data is used to calculate 3D wave currents, wave phase fronts and energy propagation. In helium the wave pattern is more complex than in argon. We also present the space and time evolution of the density perturbations associated with the wave in an Ar plasma. LIF data was used to directly measure the ion motion in the electric field of the wave, ion polarization currents and the motion of the ions as they form the density non-uniformities.

Experimental observations of shear Alfvén waves generated by narrow current channels

Alfven waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Shear Alfven waves radiated from sources with cross-field scale size of the order of the electron inertial length, δ=c/ωpe, have properties which differ considerably from planar magneto-hydrodynamic waves. Currents of cross-field size δ are common in space plasmas. A series of experiments in the large plasma device (LAPD) at UCLA is presented which illustrates that waves generated by small scale fluctuating currents radiate across magnetic field lines and are associated with complex three-dimensional currents. Waves generated by two sources are observed to constructively interfere to produce large magnetic fields in spatial regions away from source field lines. Volume data sets will be presented and discussed in the light of space plasma applications.

Shear Alfven wave radiation from a source with small transverse scale length

Shear Alfven waves are studied in the kinetic and inertial regimes. The waves are launched from an antenna which is on the order of the electron collisionless skin-depth, δ=c/ωpe, in size. The experiment is performed in the LArge Plasma Device, LAPD [W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles, using a new antenna design that modulates parallel plasma electron current. The plasma is 100 skin-depths in diameter and 3.6 Alfven wavelengths long. The results include the calculation of the wave currents based on measurements of the wave magnetic field. Differences in the perpendicular phase velocity of the wave in the kinetic regime as compared to the inertial regime are also reported. Results generally compare favorably with predictions of a theory which includes collisional damping and kinetic electron dynamics with fluid ions.