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Till recently, measurements of the magnetic field in gas-puff z-pinch implosions were limited to low density and temperatures typically found at very early times and outside the pinch radius ( r≥ 9 mm and t≤ −90 ns). However, recent, more accurate measurements 1 at higher densities and temperatures at various R and Z-locations on the generator at the Weizmann Institute of Science (WIS) have yielded information close to stagnation and beyond. These measurements seem to be inconsistent with earlier 2D radiation-magneto-hydrodynamics simulations using MACH2-TCRE as well as simple snowplow models when using the inductive current notch, pinch length the pinch radius.

New measurements of the magnetic field at several axial position along the column of an imploding plasma column, as a function of radius and time are presented. The optical configuration, efficient in UV-visible light collection, provided simultaneous radial measurement points, along with revoking the use of Abel-inversion in the analysis. These measurements help to assess the true inductance of the plasma column during the implosion, following our previous studies 1 , where there was no satisfactory explanation to the current turnover (“notch”). 2D MHD simulations are presented for the initial gas flow, including the details of the recessed regions in the cathode- anode structure. Synthetic images and the calculated azimuthal magnetic field distribution at stagnation are compared with the previous and present measurements of the field and current profiles.

We present here the detailed measurements of radial distribution of the magnetic field in a gas-puff z-pinch plasma at the final stages of the implosion phase and at stagnation. While the measurements are chordal, the radial distribution of different charge states was utilized to measure the magnetic field locally for certain radii, so that unlike chordal measurements in general, the magnetic field radial distribution was obtained with no need for the Abel inversion of the data. The distribution was measured using the Zeeman effect via a novel spectroscopic technique, at several axial locations, and demonstrates striking features such as the peak field remaining at a radius much larger than the stagnation radius at all times. Furthermore, while the distribution observed is sometimes monotonic with respect to the radius, it is often not, a behavior that can be linked to 2D features in the plasma column resulting from the Rayleigh–Taylor instability. The current flowing through the stagnating plasma was found to be a small fraction of the total current, resulting in clearly insufficient magnetic pressure to balance the plasma pressure at stagnation. The magnetic field data, taken over several axial positions, are used to obtain the true inductance in the imploding plasma for the first time; it is found that the data cannot explain the current turnover at stagnation. A simulation with the MACH2-Tabular Collisional-Radiative Equilibrium magnetohydrodynamics code in the r–z plane shows that the peak of the magnetic field pinches to a much smaller radius than is observed in the spectroscopic data. Furthermore, the depth of the computed current turnover at stagnation is smaller than the measured one. The two observed features of a radially extended magnetic field at stagnation together with a deep current turnover are a challenge to match in simulations. Various calculations and estimates of the inductive and resistive load voltages are examined to ascertain if they are responsible for the observed current notch. The results demonstrate that the knowledge of the true inductance in the driven load requires such magnetic-field-distribution measurements and that imaging data or electrical measurements are insufficient.We present here the detailed measurements of radial distribution of the magnetic field in a gas-puff z-pinch plasma at the final stages of the implosion phase and at stagnation. While the measurements are chordal, the radial distribution of different charge states was utilized to measure the magnetic field locally for certain radii, so that unlike chordal measurements in general, the magnetic field radial distribution was obtained with no need for the Abel inversion of the data. The distribution was measured using the Zeeman effect via a novel spectroscopic technique, at several axial locations, and demonstrates striking features such as the peak field remaining at a radius much larger than the stagnation radius at all times. Furthermore, while the distribution observed is sometimes monotonic with respect to the radius, it is often not, a behavior that can be linked to 2D features in the plasma column resulting from the Rayleigh–Taylor instability. The current flowing through the stagnating plasma was fou...

Gas-puff implosions on the generators at the Weizmann Institute of Science (WIS) are unique for understanding the basic plasma dynamics in the high energy density regime because of the extensive diagnostic capability. Key results are based on spatial and temporal measurements of the magnetic field during the gas-puff implosions1. The data from the Zeeman polarization measurements indicate that the azimuthal magnetic field is not as large as one would expect from the current of the pulsed-power generator and the pinch radius. Recent simulations of such pinches on the WIS generators are presented using the $2\mathrm{D}$ radiation-magnetohydrodynamic code, MACH2- TCRE. This code incorporates radiation using the Tabular Collisional-Radiative Equilibrium. It is well known that initial density profile plays a critical role in regulating the performance of Z-pinch implosions. In the first step, simulations of the initial density profiles will be presented. Neutral gas-flow of subsonic oxygen or argon through De-Laval nozzles are simulated using MACH2 including viscosity. In the second step, the shell-on jet initial density profile obtained in the previous step will be used as an initial condition for investigating the magnetic field during the gas-puff implosions. In the third and final step, the effect of the presence of both azimuthal and axial magnetic fields is investigated by including the effect of the Hall term. Comparisons will be made with existing data.

The experimental results of a promising pulsed plasma source, producing electron beams with a current density of up to 10 ${\rm kA}/{\rm cm}^{2}$ are presented. The beam duration was up to ${\sim}{\rm 1}~\mu{\rm s}$ , in an accelerating voltage of up to ${\sim}{\rm 350}~{\rm kV}$ , without shorting of the cathode–anode gap by the cathode plasma. This plasma electron source is proven to sustain hundreds of pulses without degradation in its emission properties. The cathode plasma electron density $n_{e}\,\leq\,10^{15}~{\rm cm}^{-3}$ , electron $T_{e}\leq 13~{\rm eV}$ and ion $T_{i}\leq 4~{\rm eV}$ temperatures, and expansion velocity $V_{\rm pl}\leq 1.6\times 10^{6}~{\rm cm}/{\rm s}$ were determined using time- and space-resolved spectroscopy and light emission.

The results of investigations of a double-gap vircator driven by a 20 Ω and 500-ns generator operating in the output voltage range 400-600 kV are presented. The vircator generated microwave pulses with a peak power of up to 200 MW at ~5% efficiency and the frequency varied from 2.0 to 2.3 GHz depending on the cavity geometry. The limitations on the microwave pulse duration not related to the cathode plasma expansion are addressed. On the one hand, the microwave generation is terminated because of the plasma formation at the foil separating the cavity sections, so that the virtual cathode (VC) electron space charge is neutralized by the plasma ion flux. On the other hand, if the electron beam energy deposition into the foil is reduced, a substantial delay in the start time of the microwave generation appears, which has been studied in detail. With these limiting factors, the microwave pulse full duration varied from 100 to 350 ns; the maximal full width at half maximum duration achieved in the experiments was ~180 ns. Measurements of the current transmitted through the vircator cavity indicated the existence of a VC in spite of the absence of microwave generation during the delay. The experimental dependence of the microwave generation starting current on the diode voltage is presented, and possible mechanisms behind the generation delay are discussed. Simplified numerical simulations emphasize the role of the portion of electrons that are reflected from the VC, the number of which must be sufficient for the microwave generation to occur.

This study focuses on the performance of a bimodal carbon fiber (CF) cathode and a carbon-epoxy multicapillary (CEM) cathode with microwaves at 50kV/m at 2m lasting 100 ns in a compact reflex triode virtual cathode oscillator (vircator). It was previously revealed that the CEM cathode was able to produce uniform emission distribution for a long duration1. To further uncover the location(s) where plasma is forming, an intensified CCD camera is used to image both cathodes with high spatial and temporal resolution. A titanium grade 1 (TiG1) anode that is 70% transparent is used to minimize outgassing in the system resulting in the cathode being the primary outgassing constituent2. One data set for each cathode was taken. Each data set contains diode current, voltage, and microwave fields over the course of 10,000 shots. A performance baseline is shown by comparing the evolution of these data over the course of the experiments. An 8 stage, 168 J pulse forming network (PFN) based Marx generator serves to drive both cathodes at 250 kV, 4 kA with ∼175 ns pulsewidth, achieving current densities of ∼200 A/cm3. The operating frequency of interest is set in the range of 1–2 GHz, where the tunability is achieved by varying the size of the anode-cathode (A-K) gap. In all experiments the gap is fired at a 10 Hz pulse repetition rate. The characteristics of one type of cathode over the other, depending on the operating conditions, are discussed in detail.

Summary form only given. The results of experiments on the reproducible generation of electron beams in a vacuum diode supplied by a high-voltage pulse with amplitude of a -300 kV and duration of -600 ns are presented. A cathode based on carbon-epoxy capillary structure was used for generation of electron beams with current density of up to 600 A/cm2 and a satisfactorily uniform cross-sectional distribution of current density. It was found that the source of the electrons is the plasma formed as a result of flashover inside the capillaries at an electric field -15 kV/cm. The cathode sustained thousands of pulses without degradation in its emission properties. Time- and space-resolved visible light observation and spectroscopy analyses were used to determine the cathode plasma's density, temperature, and expansion velocity. It was found that the density of the cathode plasma density decreases rapidly versus the distance from the cathode. In addition, it was found that the main reason for the short-circuiting of the accelerating gap is the formation and expansion of the anode plasma.

Time-resolved optical-emission spectroscopy measurements are used to evaluate plasma density in an interference switch during the extraction of a nanosecond output pulse from a high-power microwave compressor. The compressor represents a resonant cavity connected to an H-plane waveguide tee with a shorted side arm filled with helium at a pressure of ; the plasma discharge in the tee side arm is triggered by a Surelite laser. A nanosecond-scale dynamics of the plasma density is obtained by analyzing the shape of the helium spectral lines. The analysis of the experimental data evidences a correlation between the rise time of the plasma density and the peak power of the microwave output pulse. Numerical simulations of the microwave energy release from the cavity with the appearance of the plasma yield results in good agreement with the measured output pulse peak power and waveform.

The results of reproducibly generating an electron beam with a current density of up to 5 kA/cm2, without the cathode-anode gap being shorted by the plasma formed inside the cathode carbon-epoxy capillaries, in a ∼350 kV, ∼600 ns diode, with and without an external guiding magnetic field, are presented. The cathode sustained hundreds of pulses without degradation of its emission properties. Time- and space-resolved emissions of the plasma and spectroscopy analyses were used to determine the cathode plasma's density, temperature, and expansion velocity.

Results of time- and space-resolved spectroscopic studies of cathode plasma during a S-band relativistic magnetron operation and a magnetically insulated diode having an identical interelectrode gap are presented. It was shown that in the case of the magnetron operation, one obtains an earlier, more uniform plasma formation due to energetic electrons' interaction with the cathode surface and ionization of desorbed surface monolayers. No differences were detected in the cathode's plasma temperature between the magnetron and the magnetically insulated diode operation, and no anomalous fast cathode plasma expansion was observed in the magnetron at rf power up to 350 MW.

The results of experiments on the reproducible generation of an electron beam having a high current density of up to 300 A/cm2 and a satisfactorily uniform cross-sectional distribution of current density in a ∼200 kV, ∼450 ns vacuum diode with a carbon-epoxy capillary cathode are presented. It was found that the source of the electrons is the plasma formed as a result of flashover inside the capillaries. It is shown that the plasma formation occurs at an electric field ≤15 kV/cm and that the cathode sustains thousands of pulses without degradation in its emission properties. Time- and space-resolved visible light observation and spectroscopy analyses were used to determine the cathode plasma’s density, temperature, and expansion velocity. It was found that the density of the cathode plasma decreases rapidly in relation to the distance from the cathode. In addition, it was found that the main reason for the short-circuiting of the accelerating gap is the formation and expansion of the anode plasma. Finally...

Time-resolved light emission imaging was used to observe the plasma formation within the cavity of the double-gap vircator powered by a sub-microsecond generator (∼500 kV, ∼10 kA, ∼500 ns). The vircator generated well reproducible S-band microwave pulses of ∼200 MW peak power and up to 200 ns full duration. The plasma light emission was observed ∼30 ns prior to the ending of the generated microwave pulses at the surface of the aluminum foil separating the vircator cavity gaps, in the gap where the virtual cathode is formed. Estimations showed that the energy deposition into the foil by the high-current electron beam is sufficient for the surface plasma formation. The plasma ions accelerated toward the virtual cathode neutralize its electron space charge. The latter was confirmed by the increase in the electron current transmitted through the vircator cavity. In addition, the time of the plasma appearance was determined by comparing the measured transmitted current with that following from the one-dimensio...