Engineering topics

Intrator, T.

Publications and source records attributed to Intrator, T..

14 records

NASA NTRS1991

Space charge enhanced plasma gradient effects on satellite electric field measurements

It has been recognized that plasma gradients can cause error in magnetospheric electric field measurements made by double probes. Space charge enhanced Plasma Gradient Induced Error (PGIE) is discussed in general terms, presenting the results of a laboratory experiment designed to demonstrate this error, and deriving a simple expression that quantifies this error. Experimental conditions were not identical to magnetospheric conditions, although efforts were made to insure the relevant physics applied to both cases. The experimental data demonstrate some of the possible errors in electric field measurements made by strongly emitting probes due to space charge effects in the presence of plasma gradients. Probe errors in space and laboratory conditions are discussed, as well as experimental error. In the final section, theoretical aspects are examined and an expression is derived for the maximum steady state space charge enhanced PGIE taken by two identical current biased probes.

Diebold, Dan↗

NASA NTRS1988

Temporal evolution of collisionless sheaths

Experimental measurements have been performed to determine the temporal evolution of Langmuir sheaths near an electrode to which a negative step bias is applied in a collisionless argon plasma. The plasma was produced by a hot-filament discharge in a multidipole device. Plasma potential data were obtained using emissive probes with two different techniques: time-resolved sampling and time-averaged techniques. The sheath is found to initially form close to the electrode, to extend to a maximum separation, and to contract to a steady-state value. The time scale required to reach a steady state is close to the time scale of the presheath relaxation. Characteristics of sheaths in RF plasmas are also measured using a parallel-plate plasma capacitor. It is observed that the plasma potential profile has significant variation with frequency, even for frequencies as low as 1 kHz which are far below the ion plasma frequency (about 1 MHz).

Cho, M. H.↗

NASA NTRS1988

The virtual cathode as a transient double sheath

The two-dimensional plasma potential measurements are given of a space-charge dominated double sheath near a hot cathode. Laboratory data show that a virtual cathode is a self-consistent solution only for a transient cathode-plasma system. Slow charge exchange ions get trapped in the potential dip that forms the virtual cathode and eventually destroy it.

Intrator, T.↗

NASA NTRS1988

Multiple valued floating potentials of Langmuir probes

It is shown that Langmuir probes can have three different floating potentials in plasmas produced by a hot filament discharge in a multi-dipole device when the primary and secondary electron currents are comparable. The measured floating potential depends on the probe's initial condition - the most negative and the least negative potentials are found to be stable and the in-between value is found to be unstable. Results are compared to a simple theoretical model.

Nam, Cheol-Hee↗

NASA NTRS1987

Pumping potential wells

Nonmonotonic plasma potential structures are a common feature of many double layers and sheaths. Steady state plasma potential wells separating regions having different plasma potentials are often found in laboratory experiments. In order to exist, all such structures must find a solution to a common problem. Ions created by charge exchange or ionization in the region of the potential well are electrostatically confined and tend to accumulate and fill up the potential well. The increase in positive charge should eliminate the well. Nevertheless, steady state structures are found in which the wells do not fill up. This means that it is important to take into account processes which pump ions from the well. As examples of ion pumping of plasma wells, potential dips in front of a positively biased electron collecting anode in a relatively cold, low density, multidipole plasma are considered. Pumping is provided by ion leaks from the edges of the potential dip or by oscillating the applied potential. In the former case the two-dimensional character of the problem is shown to be important.

Hershkowitz, N.↗

NASA NTRS1987

Pumping potential wells

Nonmonotonic plasma potential structures are a common feature of many double layers and sheaths. Steady state plasma potential wells separating regions having different plasma potentials are often found in laboratory experiments. In order to exist, such structures all must find a solution to a common problem. Ions created by charge exchange or ionization in the region of the potential well are electrostatically confined and tend to accumulate and fill up the potential well. The increase in positive charge should eliminate the well. Nevertheless, steady state structures are found in which the wells do not fill up. This means that it is important to take into account processes which 'pump' ions from the well. As examples of ion pumping of plasma wells, potential dips in front of a positively biased electro collecting anode in a relatively cold, low density multidipole plasma is considered. Pumping is provided by ion leaks from the edges of the potential dip or by oscillating the applied potential. In the former case the two dimensional character of the problem is shown to be important.

Hershkowitz, N.↗

NASA NTRS1987

Self-similar potential in the near wake

The plasma potential is measured near the edge of an electrically floating obstacle placed in a steady-state, supersonic, unmagnetized, neutral plasma flow. Equipotential contours show the sheath of the upstream side of the obstacle wrapping around the edge of the obstacle and fanning out into the near wake. Both fluid theory and the data find the near-wake plasma potential to be self-similar when ionization, charge exchange, and magnetic field can be neglected. The theory also finds that fluid velocity is self-similar, the near wake is nonneutral, and plasma density is not self-similar. Strong electric fields are found near the obstacle and equipotential contours are found to conform to all boundaries.

Diebold, D.↗

NASA NTRS1986

Techniques for using emitting probes for potential measurement in RF plasmas

Investigations of the effects of RF on plasma potential measurements with electron emitting probes and methods for interpreting data are presented. Techniques correspond to the floating and inflection point methods of single-emitting and differential emitting probes, respectively. A simple method of measurement of plasma potential fluctuations is given which makes use of time-averaged emitting probe I-V characteristics.

Wang, E. Y.↗

NASA NTRS1984

Nonlinear self-contraction of electron waves

Laboratory evidence is presented of modulationally unstable electron wave packets which can be described by a nonlinear geometrical optics theory. Growth times for self-contraction are found to be much faster than ion response times and the bursts do not appear to be related to Zakharov Langmuir-wave collapse.

Intrator, T.↗

NASA NTRS1984

Laboratory evidence for ion-acoustic-type double layers

The formation of an ion-acoustic-type double layer was observed in the laboratory for the first time. The rarefactive part of a long-wavelength ion-acoustic wave grew in amplitude because of the presence of drifting electrons. The corresponding current limitation led to the formation of the double layer.

Chan, C.↗

NASA NTRS1984

Measurement of vacuum space potential by an emissive probe

The use of an electron-emitting probe (Langmuir and Compton, 1931) to measure the space potential in the vacuum between parallel metal plates is demonstrated, extending the inflection-point interpretation technique of Smith et al. (1979) to the vacuum case. Measurements are made at the off half cycle of during half-wave-rectified sine-wave heating of 3-mm-long 30-micron-diameter W probes positioned with 1-mm precision between 18 x 23-cm Al or Cu plates 7.6 cm apart. The results are presented graphically: potentials of up to + or 30 V are measured with accuracy + or - 0.4 V.

Cho, M. H.↗

NASA NTRS1984

Experimental observations of nonlinearly enhanced 2omega-UH electromagnetic radiation excited by steady-state colliding electron beams

Counterstreaming large-diameter electron beams in a steady-state laboratory experiment are observed to generate transverse radiation at twice the upper-hybrid frequency (2omega-UH) with a quadrupole radiation pattern. The electromagnetic wave power density is nonlinearly enhanced over the power density obtained from a single beam-plasma system. Electromagnetic power density scales exponentially with beam energy and increases with ion mass. Weak turbulence theory can predict similar (but weaker) beam energy scaling but not the high power density, or the predominance of the 2omega-UH radiation peak over the omega-UH peak. Significant noise near the upper-hybrid and ion plasma frequencies is also measured, with normalized electrostatic wave energy density W(ES)/n(e)T(e) approximately 0.01.

Intrator, T.↗

NASA NTRS1984

Experimental observations of self-similar plasma expansion

The present investigation is concerned with measurements of the plasma potential profile of an expanding plasma, taking into account the demonstration of the self-similar behavior of such a plasma. The obtained experimental results are consistent with self-similar solutions reported by Crow et al. (1975). It is found that the quasi-neutrality condition breaks down early during the experiment. A consideration of the effect of charge separation is, therefore, required. Attention is given to the evolution of the potential profiles of the expanding plasma as a function of time, the accelerated ion fronts, and the sheath formation of the expanding plasma into a floating boundary.

Chan, C.↗

NASA NTRS1983

Beam-plasma interactions in a positive ion-negative ion plasma

An electron-free plasma consisting of negative ions /SF6(-)/ and positive ions /Ar(+)/, and negligible neutral-ion collision frequencies has been created in the laboratory. This plasma has a mass ratio of approximately 3.5-similar to many computer particle-in-cell simulated systems. A fluid description of this positive and negative ion confinement (PANIC) plasma is given and compared to experimental measurements of a beam-plasma instability for both beam species and a wide range of beam energies. The fluid dispersion relation and most growing modes are predicted to be insensitive to many parameters of the PANIC beam-plasma system, and found to the consistent with the data.

Intrator, T.↗