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Over the past few years, many double-layer-relevant laboratory experiments have been carried out at the University of Wisconsin. Laboratory stair-step double layers, which resemble three or more weak double layers joined in series, have been produced without ionization for the first time. Double-layer floating potential fluctuations have been investigated and progress has been made in developing a novel technique for measuring electron energy distribution functions in low-density double layers (i.e., lambda(sub D) much greater than probe dimensions). A new inductive plasma source has been developed. With this source, magnetized double layers can be routinely produced. These magnetized double layers are often weak stair-step double layers that are oblique to the magnetic field. Laboratory data of emitting probe characteristics taken in tenuous plasmas have helped to quantify space-charge-enhanced plasma gradient induced error in double-layer electric field measurements made by satellite double probes. Also, magnetic sheaths have been experimentally studied and compared with theory.

Presents a study that examined plasma potential measurements of a space-charge dominated double sheath near a hot cathode. Methodology; Background on virtual cathode; Analysis of the current voltage characteristics of the hot cathode.

Focuses on a study which examined the multi-valued floating potential of Langmuir probe in plasmas. Discussion on experimental condition; Details of experimental results; Comparison of findings on a theoretical model.

Presents a study which showed the use of secondary electron-emission-capacitive probes (SECP) to determine the plasma potential measurements in plasmas with hot electrons. Discussion on the floating potential of secondary emissive capacitance probe; Probe construction and circuit; Details of the SECP techniques for measuring plasma potential.

Laboratory experiments and fluid theory find a stationary rarefaction shock in the near wake of an electrically grounded obstacle placed in a steady state, supersonic plasma flow. The shock is only found when two electron temperatures, differing by at least an order of magnitude, are present. These shocks are analogous to rarefaction shocks in plasma free expansions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Plasma flow velocity was determined experimentally from the time delay between fluctuations measured by two spatially separated Langmuir probes in both subsonic and supersonic, steady-state, unmagnetized plasma flows. [ABSTRACT FROM AUTHOR]

High bandwidth capacitive probes are useful tools for potentials measurements in the tokamak scrape off layer. An improved capacitive divider probe design with very high frequency bandwidth is shown. The gain of this system is between 0 and -8 dB for frequencies from 0.1 Hz to more than 100 MHz, with the - 3 dB point at approximately 40 MHz. The probe structure, circuits, performance, and experimental results from the Phaedrus-T tokamak are given. [ABSTRACT FROM AUTHOR]

It is experimentalist demonstrated that, with proper current bias, emissive probes can accurately measure dc electric potential in a vacuum. A comparison is made of the accuracy and time response of this "vacuum current bias" method with two other emissive probe techniques, the inflection point in the limit of zero emission, and the floating potential of a strongly heated probe. [ABSTRACT FROM AUTHOR]

Shows that Alfven waves are indirectly driven by a fast wave antenna array in the Phaedrus-T tokamak. Effect of small fractions of minority ions on the Alfven spectrum as measured at the edge; Identification of an ion-ion hybrid Alfven mode by measuring dispersion properties.

Shows the first experimental evidence of Alfven Wave Current Drive (AWCD) in a tokamak. Estimated efficiency for current driven per unit radio frequency input power; Drop in loop voltage; Changes in plasma resistivity; Effective ionic charge; Plasma inductance and resistance.

Edge equilibrium, turbulence and transport related plasma parameters from the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] torsatron, the ZT-40M [Fusion Technol. 8, 1571 (1985)] reversed-field pinch, the Phaedrus-T [Nucl. Fusion 32, 2040 (1992)] tokamak, the Texas Experimental Tokamak (TEXT) [Nucl. Technol. Fusion 1, 479 (1981)], and the Tokamak Fusion Test Reactor (TFTR) [in Plasma Physics and Controlled Nuclear Fusion Research, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 9] have been obtained using a standardized Langmuir probe array and a consistent set of data analysis packages. Additional data from some other devices have also been furnished via private communications and incorporated from published results. Experimental results over a wide range of parameters are compared and the turbulence contribution to edge transport are assessed. Certain physical properties that are relevant to the modeling of edge turbulence are identified: namely, shear decorrelation of turbulence, the role of resistive dissipation and electron parallel thermal conduction, radial mode structure in sheared magnetic field, and electromagnetic contribution to the parallel Ohm's law. [ABSTRACT FROM AUTHOR]

Heating in the Alfvén resonant regime has been demonstrated in the Phaedrus-T tokamak [Fusion Technol. 19, 1327 (1991)]. Electron heating during injection of radio-frequency (rf) power is indicated by a 30%-40% drop in loop voltage and modifications in sawtooth activity. Heating was observed at a frequency ω[sub rf]≈0.7Ω[sub i] on axis, using a two-strap fast wave antenna operated at 7 and 9.2 MHz with 180° phasing (N[sub ⊥] ∼ 100). Numerical modeling with the fast wave code FASTWA [Plasma Phys. Controlled Fusion 33, 417 (1991)] indicates that for Phaedrus-T parameters the kinetic Alfvén wave is excited via mode conversion from a surface fast wave at the Alfvén resonance and is subsequently damped on electrons. [ABSTRACT FROM AUTHOR]

Triple Langmuir probe data have been obtained in the initial phase of Phaedrus-T operation. The data were obtained during ohmic discharges of roughly 60 kA, 〈n〉 = 5 × 10[sup 18] m[sup -3], T[sub e](O) = 0.5 keV, B[sub T] = 0.7 T, q(a) = 4 and 80 ms duration. Measurements of T[sub e], φ[sub F], and n were made in a radial scan in the scrape-off layer of the plasma. The toroidal and poloidal positions were fixed throughout the scan. The spectra of T[sub e], φ[sub F], and n fluctuations up to 100 kHz were also measured. One of the interesting features of these data is that m = 2 oscillations detected by Mirnov coils were very pronounced in the φ[sub F] and T[sub e] fluctuation spectra, but are not as evident in the n fluctuations. These data are presented and the techniques employed to get these data are discussed. Future plans to extend the maximum frequency resolution of the fluctuation measurements up to and beyond the frequency range of the ICRF power (13-18 MHz) that will be injected into Phaedrus-T are also discussed. [ABSTRACT FROM AUTHOR]

Plasma density turbulence has been measured with the beam emission spectroscopy (BES) diagnostic system, using a low-power neutral beam with He[sup 0] and H[sup 0] as beam species. in general, He[sup 0] (588 nm) provided the best signal-to-noise ratio due to its lower edge plasma background interference. Simultaneous measurements of edge density fluctuations have been made with BES and Langmuir probes; the spectra are seen to be essentially identical, and the fluctuation amplitudes from both diagnostics are in close agreement. A poloidal coherence length of about 2-4 cm was observed. Radial propagation of modes was not seen, but a lab-frame poloidal phase velocity at r/a=0.77 of about 7 × 10[sup 5] cm/s in the electron diamagnetic direction was observed, corresponding to m=8-75 kHz. [ABSTRACT FROM AUTHOR]

Phoma spp. form a group of ubiquitous phytopathogens found in soil and organic matter. (b) Mycological direct examination of cystic injury showing hyphae, suggesting fungal infection. (d) The microculture of the recovered fungus revealed pycnidia and dematiaceous hyphae, all typical aspects of Phoma (Peyronellaea) spp. [Extracted from the article]