Richard J. Armstrong

Confinement and turbulent transport in a plasma torus with no rotational transform

In the BLAAMANN device a weakly ionized hydrogen plasma is produced by electrons accelerated from a hot, negatively biased tungsten filament and confined in a toroidal magnetic field of strength up to 0.4 T. The plasma is turbulent, with relative fluctuation levels in ne, phi and Te of 10% or more. The time-averaged state exhibits nested toroidal surfaces of constant potential and pressure, which requires an anomalous cross-field current to remove the space-charge injected by the cathode and the charge accumulated due to the Del B- and curvature drifts. Typical plasma parameters are ne approximately 1016 m-3, Te approximately 1-20 eV, Ti approximately 1 eV. The cross-field diffusion coefficient is typically Dperpendicular to approximately 30 m2 s

Ion-beam diagnostics by means of an electron-saturated plane Langmuir probe

-1 approximately 104*Dperpendicular to classical approximately 101*Dperpendicular to Bohm. Evidence is presented in support of the hypothesis that the plasma goes turbulent because it needs to develop an anomalous current channel, and this turbulence in turn determines the plasma transport and the time-averaged state.

Formation of Ion Phase-Space Vortexes

A new sensitive method for the detection of low‐energy ion beams in low‐pressure nearly collisionless unmagnetized plasma is presented. The method makes use of a swept planar Langmuir probe in the electron saturation regime. An ion beam with direction perpendicular to the plane of the probe produces a double knee (’’beam edge’’) on the characteristic at a positive bias with respect to plasma potential. The magnitude of this bias is a measure of the directed energy of the beam. It seems to be possible to deduce the beam temperature from the form of the beam edge. The amplitude of the beam edge is proportional to the beam ion flux. The size of the effect is much greater than can be inferred from ion collection alone. An explanation is suggested based on the presence of a space‐charge layer in front of the probe produced by the reflected beam and that this layer in turn causes enhanced electron collection by the probe.

Experimental observations of ion phase-space vortices

The formation of ion phase space vortexes in the ion two stream region behind electrostatic ion acoustic shocks are observed in a laboratory experiment. A detailed analysis demonstrates that the evolution of such vortexes is associated with ion-ion beam instabilities and a nonlinear equation for their initial evolution is derived. The results are supported by a numerical particle simulation. Also the possibility of vortex excitation by ion bursts and coalescence of two vortexes are demonstrated. The effects of finite ion temperature, particle trapping and charge exchange collisions are discussed and the results related to ionospheric observations.

Ion beam diagnostics by means of plane Langmuir probes

Abstract Experimental observations of ion phase-space vortices are reported. The ion phase-space vortices form in the region of heated ions behind electrostatic ion acoustic shocks. The results are in qualitative agreement with numerical and analytic studies.

The day to night absorption ratio in auroral zone riometer measurements

An ion beam normally incident on a plane Langmuir probe in a plasma gives rise to an extra knee in the electron saturation part of the current‐voltage characteristics of the probe. Findings from a numerical investigation, together with some experimental results, are presented, confirming that the effect arises from a modulation of the electron collection current because of space‐charge effects from beam ions reflected in front of the probe. It is verified that beam energy, temperature, and, in principle, beam density can be derived from the position, relative shape, and size of the extra ion‐beam‐induced knee of the Langmuir characteristics.

Collisional and Landau damping of ion-acoustic waves in a two electron temperature plasma

Abstract The day to night ratio in ionospheric radio wave absorption has been studied for College, Alaska using a narrow beam riometer array and computer assisted programming of the data. For the period of 6 November 1967 to 17 April 1968 no significant departure from a ratio of unity was found.

Plasma simulation of a Langmuir probe with normal magnetic field

Abstract Propagation of low-amplitude ion-acoustic waves at low frequencies is studied in a DP-machine with a plasma exhibiting two maxwellian electron temperatures. Damping rate and phase velocity are found in agreement with a linear kinetic theory including two electron temperatures, the total damping consisting of exponential Landau decay and a small collisional contribution.

Two-dimensional ion velocity diffusion due to ion-acoustic turbulence

Abstract The response of a probe with varying applied potential is studied by numerical simulation. Each particle is followed in turn in its path through the test volume and leaves a space charge in its track for the next overall iteration. The space charge is inversely proportional to its local speed [L.W. Parker and E.C. Whipple, Ann. Phys. 44 (1967) 126]. Preliminary results indicate that the effects of a magnetic field is strong and consistent with an anomalous diffusion, indicating that we may be simulating some process in real plasmas, in spite of the fact that the simulation method employed in principle gives no information on the temporal development of the plasma. On the other hand, effects of collisions seem to be slight.

Fractal dimension of mesospheric radar backscatter at 2.75 MHz

Abstract The diffusion of an ion beam caused by ion-acoustic turbulence is demonstrated experimentally by measuring the two-dimensional velocity distribution function f b ( v ⊥ , v ⌈ ). The obtained ratio between the perpendicular and the parallel diffusion coefficient D ⊥ / D ⌈ ≈ 21 is in reasonable agreement with three-dimensional quasi-linear theory.