A. Weingarten

Observations of two-dimensional magnetic field evolution in a plasma opening switch

The time dependent magnetic field distribution was studied in a coaxial 100-ns positive-polarity Plasma Opening Switch (POS) by observing the Zeeman effect in ionic line emission. Measurements local in three dimensions are obtained by doping the plasma using laser evaporation techniques. Fast magnetic field penetration with a relatively sharp magnetic field front (⩽1 cm) is observed at the early stages of the pulse (t≲25). Later in the pulse, the magnetic field is observed at the load-side edge of the plasma, leaving “islands” of low magnetic field at the plasma center that last for about 10 ns. The two-dimensional (2-D) structure of the magnetic field in the r,z plane is compared to the results of an analytical model based on electron-magneto-hydrodynamics, that utilizes the measured 2-D plasma density distribution and assumes fast magnetic field penetration along both POS electrodes. The model results provide quantitative explanation for the magnetic field evolution observed.

Spectroscopic investigation of fast (ns) magnetic field penetration in a plasma

The time‐dependent magnetic field spatial distribution in a coaxial positive‐polarity plasma opening switch (POS) carrying a current ≂135 kA during ≂100 ns, was investigated by two methods. In the first, ionic line emission was observed simultaneously for two polarizations to yield the Doppler and Zeeman contributions to the line profiles. In the second method, the axial velocity distribution of ions was determined, giving the magnetic field through the ion equation of motion. This method requires knowledge of the electron density, here obtained from the observed particle ionization times. To this end, a lower bound for the electron kinetic energy was determined using various line intensities and time‐dependent collisional‐radiative calculations. An important necessity for POS studies is the locality of all measurements in r, z, and θ. This was achieved by using laser evaporation to seed the plasma nonperturbingly with the species desired for the various measurements. The Zeeman splitting and the ion moti...

Spectroscopic investigations of the plasma behavior in a plasma opening switch experiment

The electron density, the electron kinetic energy, the particle motion, and electric fields in a coaxial positive‐polarity plasma opening switch (POS) were studied using spectroscopic diagnostics. A gaseous source that injects the plasma radially outward from inside the inner POS electrode was developed. The plasma was locally seeded with various species, desired for the various measurements allowing for axial, radial, and azimuthal resolutions both prior to and during the 180 ns long current pulse. The electron density was determined from particle ionization times and the electron energy from line intensities and time dependent collisional‐radiative calculations. Fluctuating electric fields were studied from Stark broadening. The ion velocity distributions were obtained from emission‐line Doppler broadenings and shifts. The early ion motion, the relatively low ion velocities and the nearly linear velocity dependence on the ion charge‐to‐mass ratio, leads to the conclusion that the magnetic field penetrat...

Plasma dynamics in pulsed strong magnetic fields

Recent investigations of the interaction of fast-rising magnetic fields with multi-species plasmas at densities of 1013–1015 cm−3 are described. The configurations studied are planar or coaxial interelectrode gaps pre-filled with plasmas, known as plasma opening switches. The diagnostics are based on time-dependent, spatially resolved spectroscopic observations. Three-dimensional spatial resolution is obtained by plasma-doping techniques. The measurements include the propagating magnetic field structure, ion velocity distributions, electric field strengths, and non-Maxwellian electron energy distribution across the magnetic field front. It is found that the magnetic field propagation velocity is faster than expected from diffusion. The magnetic field evolution cannot be explained by the available theoretical treatments based on the Hall field (that could, in principle, explain the fast field propagation). Moreover, detailed observations reveal that magnetic field penetration and plasma reflection occur si...

Study of the effects of the prefilled-plasma parameters on the operation of a short-conduction plasma opening switch

Spectroscopic methods are used to determine the density, the temperature, the composition, the injection velocity, and the azimuthal uniformity of the flashboard-produced prefilled plasma in an 85-ns, 200-kA plasma opening switch (POS). The electron density is found to be an order of magnitude higher than that obtained by charge collectors, which are commonly used to determine the density in such POSs, suggesting that the density in short conduction POSs is significantly higher than is usually assumed. We also find that the plasma is mainly composed of protons. The spectroscopically measured plasma parameters are used here to calculate the conduction currents at the time of the opening predicted by various theoretical models for the POS operation. Comparison of these calculated currents to the measured currents indicates that the plasma behavior during conduction is governed either by plasma pushing or by magnetic-field penetration and less by sheath widening near the cathode, as described by existing models. Also, the conduction current mainly depends an the prefilled electron density and less on the plasma flux, which is inconsistent with the predictions of the erosion (four-phase) model for the switch operation. Another finding is that a better azimuthal uniformity of the prefilled plasma density shortens the load-current rise time.

Energetic electron and ion beam generation in plasma opening switches

In this paper, we present measurements of ion and electron flows in a nanosecond plasma opening switch (NPOS) and a microsecond plasma opening switch (MPOS), performed using charge collectors. In both experiments, an electron flow toward the anode, followed by an ion flow, were observed to propagate downstream toward the load side of the plasma during the plasma opening switch (POS) conduction. In the MPOS, ion acceleration was observed to propagate axially through the entire plasma. These results are in satisfactory agreement with the predictions of the electron magnetohydrodynamics (EHMD) theory and the results of fluid and particle-in-cell (PIC) code simulations. At the beginning of the POS opening, a high-current density (/spl ap/2 kA/cm/sup 2/) short-duration (10-30 ns) axial ion flow downstream toward the load was observed in both experiments, with an electron beam in front of it. These ions are accelerated at the load side of the plasma and are accompanied by comoving electrons. In the NPOS, the ion energy reaches 1.35 MeV, whereas in the MPOS, the ion energy does not exceed 100 keV. We suggest that in the NPOS the dominant mechanism for the axial ion acceleration is collective acceleration by the space charge of the electron beam, while in the MPOS, axial ion acceleration is probably governed by the Hall field in the current carrying plasma.

Visible-light spectroscopy of pulsed-power plasmas (invited)

We describe the investigations of the plasma behavior in three pulsed‐power systems: a magnetically insulated ion diode, and plasma opening switch, and a gas‐puffed Z pinch. Recently developed spectroscopic diagnostic techniques allow for measurements with relatively high spectral, temporal, and spatial resolutions. The particle velocity and density distributions within a few tens of microns from the dielectric‐anode surface are observed using laser spectroscopy. Fluctuating electric fields in the plasma are inferred from anisotropic Stark broadening. For the plasma opening switch experiment, a novel gaseous plasma source was developed which is mounted inside the high‐voltage inner conductor. The properties of this source, together with spectroscopic observations of the electron density and particle velocities of the injected plasma, are described. Emission line intensities during the switch operation are discussed. In the Z‐pinch experiment, spectral emission‐line profiles of various charge‐state ions ar...

The time-dependent electron density and magnetic field distributions in a 70-ns plasma opening switch

The time-dependent two-dimensional electron density distribution is determined for the first time during the operation of a short-conduction plasma opening switch. The electron density, resolved in 3-D, is determined from the line intensities of different ions doped into the plasma. A rise in the electron density followed by a drop is observed to propagate from the generator towards the load at the proton Alfven velocity. Based on spectroscopic magnetic field and ion velocity measurements, the density evolution can be explained as pushing of the protons ahead of the propagating magnetic piston, followed by magnetic field penetration into the rest of the plasma composed of the lower-density heavier ions.

Spectroscopic Investigations Of The Plasma Behavior In A Plasma Opening Switch Experiment

Abstracts IEEE hztemational Conference on Plasma Science, Tampa, Florida, 1992 (The Institute of Electrical and Electronics Engineers, New York, 1992), p. 166. 32M. Sarfaty, Ya. E. Krasik, R. Arad, A. Weingarten, S. Shkolnikov, and Y Maron, in Ref. 27, p. 633; A. Weingxten, M.Sc. thesis, Feinberg Graduate School, Weizmann Institute of Science, Israel, 1993. 33M. Sarfaty, R. Arad, Ya. E. Krasik, Y. Maron. B. Peryaslovetz, S. Shkolni- kov, R. Shpitalnik, and A. Weingarten, Bull. Am. Phys. Sot. 38, 1895 (1993). 34Yu. Ralchenko (private communication, 1993). 35H. J. Kunze and H. R. Griem, Phys. Rev. Lett. 21, 1048 (1968). 36G. V. Sholin and E. A. Oks, Sov. Phys. Dokl. 18, 254 (1973). 37H G. Griem, Spectral Line Broadening by Plasmas (Academic, New York 1674). “G Bekefi C. Deutch, and B. Yakobi, in Principles of Laser Plasmas, edited by b. Beketi (Wiley, New York 1976). “E. Sarid. Y. Maron, and L. Troyansky, Phys. Rev. E 48, 1364 (1993). ‘ @H. Tawara, T. Kato, and M. Ohnishi, At. Data Nucl. TabIes 36, 167 (1987);

Interaction of rapid magnetic fields with plasmas and implications to pulsed-power systems

The behavior of rapidly rising magnetic fields decisively affects the operation of plasma opening switches, high-power transmission lines, and particle-beam diodes. Here, we describe recently observed novel phenomena in the interaction of magnetic fields with plasmas in such systems. The understanding of the underlying physics is used to design and optimize plasma opening switches and their coupling to loads. High-resolution spectroscopic methods, with the aid of plasma doping techniques, were used to obtain the time-dependent 2-D distributions of the magnetic field, ion velocities, electron density, and electron energy distribution. The results showed simultaneous rapid magnetic field penetration and plasma reflection accompanied by ion-species separation [A. Weingarten et. al., Phys. Rev. Lett. 87, 115004 (2001)]. These phenomena strongly modify the plasma dynamics, thereby influencing the effective switch impedance and the resultant coupling of the power to the load. The current conduction and switch operation are also found to be highly influenced by the electron density distribution and plasma composition. Possible ways to improve the system operation, brought about by these findings, are suggested.