Collision Lengths Research Articles

Plasma flows generated by an annular thermionic cathode in a large magnetized plasma

A LaB6 thermionic emitter of annular shape is used in the Large Plasma Device at the University of California, Los Angeles to create off-axis heating conditions for various transport studies. Since the emitter is biased relative to a distant anode, which is many collision lengths away, the entire magnetized plasma develops a self-consistent, potential structure that simultaneously generates transverse and axial flows with shear. This study uses swept Langmuir probe techniques and Mach probes to map the flow patterns and their dependence on bias and plasma parameters. By implementing additional biasing configurations, it is possible to control the magnitude of the flows and their shear strength. The experimental measurements, including the self-consistent currents, are compared to predictions of a model that incorporates the boundary conditions associated with thermionic injection, combined with a Braginskii transport code for the electron temperature.

Adiabatic Expansion of Electron Gas in a Magnetic Nozzle

A specially constructed experiment shows the near perfect adiabatic expansion of an ideal electron gas resulting in a polytropic index greater than 1.4, approaching the adiabatic value of 5/3, when removing electric fields from the system, while the polytropic index close to unity is observed when the electrons are trapped by the electric fields. The measurements were made on collisionless electrons in an argon plasma expanding in a magnetic nozzle. The collision lengths of all electron collision processes are greater than the scale length of the expansion, meaning the system cannot be in thermodynamic equilibrium, yet thermodynamic concepts can be used, with caution, in explaining the results. In particular, a Lorentz force, created by inhomogeneities in the radial plasma density, does work on the expanding magnetic field, reducing the internal energy of the electron gas that behaves as an adiabatically expanding ideal gas.

Experimental characterization of a transition from collisionless to collisional interaction between head-on-merging supersonic plasma jetsa)

We present results from experiments on the head-on merging of two supersonic plasma jets in an initially collisionless regime for the counter-streaming ions. The plasma jets are of either an argon/impurity or hydrogen/impurity mixture and are produced by pulsed-power-driven railguns. Based on time- and space-resolved fast-imaging, multi-chord interferometry, and survey-spectroscopy measurements of the overlapping region between the merging jets, we observe that the jets initially interpenetrate, consistent with calculated inter-jet ion collision lengths, which are long. As the jets interpenetrate, a rising mean-charge state causes a rapid decrease in the inter-jet ion collision length. Finally, the interaction becomes collisional and the jets stagnate, eventually producing structures consistent with collisional shocks. These experimental observations can aid in the validation of plasma collisionality and ionization models for plasmas with complex equations of state.

On the thermophoretic force close to walls in dusty plasma experiments

One observes in radiofrequency-heated vacuum chambers that dust, if present or being produced within the chamber, may float in layers close to both the upper and lower electrodes. Important forces on the dust are the electric force, gravity, plasma drag and the thermophoretic force, which is caused by temperature gradients in the background neutral gas in the vacuum chamber. We here discuss the thermophoretic force and show that the normally adopted formula for this force, which is computed on the assumption of an infinite plasma in all directions from the dust, must be modified when close to the plasma chamber walls. Taking into account the closeness of the plasma walls, we find that the thermophoretic force will be reduced out to many neutral gas molecular collision lengths from the wall, compared with the results from the standard formula. This modification of the thermophoretic force should be of importance for the force equilibrium and stability of dust in the dust layers observed in so many dust levitation experiments.

Contrast in multicomponent systems

The forward scattering of radiation by multicomponent systems is expressed in terms of the chemical potentials and of the collision lengths associated with the components. Contrast lengths can be defined in various unequivalent ways. The formula giving the scattering cross section in terms of such contrasts is derived geometrically. This approach allows a discussion concerning the best definition of the contrast lengths.

Muon identification using multiwire proportional chambers

Test data are summarized for a muon identifier consisting of iron absorber plates followed by multiwire proportional chambers. Because of the penetrating secondaries that result from hadronic interactions, conventional muon identifiers lacking spatial resolution require many collision lengths to attenuate, and thus to separate incident hadrons from muons. By using multiwire proportional chambers, hadronic interactions can be detected well before the subsequent hadron cascades have been fully absorbed. This permits a substantial reduction in absorber thickness and is important in applications in which the muons originate in a large volume or in which they are to be identified over a large area or large solid angle. A typical result from these tests is that at 3 GeV/ c a single multiwire proportional chamber, following only 50 cm of iron, can reject pions with 96±1% efficiency while accepting 96% of the incident muons. An important advantage of this technique is that the performance is expected to be rather independent of energy above a few GeV.

Collision Lengths of Neutral, Penetrating-Shower-Producing Cosmic Radiation in Light and Heavy Water

Collision Lengths of Neutral, Penetrating-Shower-Producing Cosmic Radiation in Light and Heavy Water

Observations on Cosmic-Ray Penetrating Showers at High Altitude, Sea Level and Below Ground

Penetrating showers have been investigated, using Geiger counters, at a depth of 30 m. below ground, at sea level and at an altitude of 3,457 m. The influence of the geometry of the absorbers was studied. The transition curves for local showers, corrected for this geometric effect, give the following collision lengths of the generating particles: Pb, 180±40; Al, 85±15; Paraffin similar 80 gm/cm2. These values correspond to a collision cross section close to the geometric cross section of the nuclei. From the results obtained for extensive penetrating showers, it is concluded that most of the penetrating particles in these showers appear to be generated in the atmosphere. In 40 days, no penetrating showers were recorded below ground. Using photographic plates, the density distribution of shower tracks associated with nuclear disintegrations was found to be similar at sea level and 3,457 m.