N. E. Kurmaev

Long time implosion experiments with double gas puffs

Long time implosion experiments with argon double gas puffs have been conducted on the GIT-12 [S. P. Bugaev et al., Izv. Vyssh. Uchebn. Zaved., Fiz. 40, 38 (1997)] generator at the current level of 2.2–2.4 MA. A double gas puff was used as one of the alternative ways to improve implosion stability at implosion times from 230 to 340 ns. The results of these experiments were compared with two-dimensional snowplow simulations. The experiments and the simulations show that the final pinch is sufficiently stable when the inner-to-outer shell mass ratio is greater than 1. The maximum argon K-shell yield obtained in the experiments is 740 J/cm with 220 GW/cm radiation power. At the long implosion times, the K-shell yield obtained in the double gas puff implosion is twice the K-shell yield of a 4-cm-radius single gas puff, with more than an order of magnitude increase in radiation power.

Deuterium gas puff Z-pinch at currents of 2 to 3 mega-ampere

Deuterium gas-puff experiments have been carried out on the GIT-12 generator at the Institute of High Current Electronics in Tomsk. The emphasis was put on the study of plasma dynamics and neutron production in double shell gas puffs. A linear mass density of deuterium (D2) varied between 50 and 85 μg/cm. Somewhat problematic was a spread of the D2 gas at a large diameter in the central anode–cathode region. The generator operated in two regimes, with and without a plasma opening switch (POS). When the POS was used, a current reached a peak of 2.7 MA with a 200 ns rise time. Without the POS, a current rise time approached 1500 ns. The influence of different current rise times on neutron production was researched. Obtained results were important for comparison of fast deuterium Z-pinches with plasma foci. Average DD neutron yields with and without the POS were about 1011. The neutron yield seems to be dependent on a peak voltage at the Z-pinch load. In all shots, the neutron emission started during stagnat...

Characterization of neutron emission from mega-ampere deuterium gas puff Z-pinch at microsecond implosion times

Experiments with deuterium (D2) triple shell gas puffs were carried out on the GIT-12 generator at a 3 MA current level and microsecond implosion times. The outer, middle and inner nozzle diameters were 160 mm, 80 mm and 30 mm, respectively. The influence of the mass of deuterium shells on neutron emission times, neutron yields and neutron energy spectra was studied. The injected linear mass of deuterium varied between 50 and 255 µg cm−1. Gas puffs imploded onto the axis before the peak of generator current at 700–1100 ns. Most of the neutrons were emitted during the second neutron pulse after the development of instabilities. Despite higher currents, heavier gas puffs produced lower neutron yields. Optimal mass and a short time delay between the valve opening and the generator triggering were more important than the better coincidence of stagnation with peak current. The peak neutron yield from D(d, n)3He reactions reached 3 × 1011 at 2.8 MA current, 90 µg cm−1 injected linear mass and 37 mm anode–cathode gap. In the case of lower mass shots, a large number of 10 MeV neutrons were produced either by secondary DT reactions or by DD reactions of deuterons with energies above 7 MeV. The average neutron yield ratio Y>10 MeV/Y2.5 MeV reached (6 ± 3) × 10−4. Such a result can be explained by a power law distribution for deuterons as . The optimization of a D2 gas puff Z-pinch and similarities to a plasma focus and its drive parameter are described.

Study of the current-sheath formation during the implosion of multishell gas puffs

Results are presented from experimental studies of the dynamics of large-diameter multishell gas puffs imploded by microsecond megampere current pulses. The experiments were conducted on the GIT-12 generator in the regime of microsecond implosion (timp = 1.1–1.2 μs, I0 = 3.4–3.7 MA). The influence of the load configuration on the dynamics of current losses and gas-puff radiative characteristics was studied. The correlation between the radial compression ratio (the ratio between the initial and final Z-pinch radii) and the magnitude of the current flowing at the plasma periphery was investigated. The experiments show that, in a multishell gas puff, large-scale instabilities insignificantly affect the gas-puff implosion even over microsecond time intervals and that a compact dense pinch with a relatively high average electron temperature (400–600 eV) forms at the Z-pinch axis. The diameter of the plasma column radiating in the K-shell lines of neon is about 3–4 mm, the K-shell radiation yield being 5–11 kJ/cm. In the final stage of implosion, only a small portion of the current flows through the high-temperature central region of the pinch plasma, whereas the major part of the generator current flows through the residual peripheral plasma.

Formation of tight plasma pinches and generation of high-power soft x-ray radiation pulses in fast Z-pinch implosions

Argon K-shell plasma radiation source experiments were carried out on the GIT-12 generator [Bugaev, S.P. et al., 1997, Russian Phys. Journal, 40, 38] for a long (300 ns) implosion regime. The performance of a shell-on-solid-fill double gas puff was characterized in the experiments with and without an external axial magnetic field. The maximum Ar K-shell radiation yield registered in the experiments without an axial magnetic field was at the level of 1100 J/cm. This yield is consistent with the theoretically predicted yield for a short (100 ns) implosion regime. The experiments showed that the initial magnetic field which provides stabilization of the shell-on-solid-fill double gas puff was lower than that required for stabilization of a single annular gas puff. Satisfactory stabilization of the double gas puff was observed at an initial axial magnetic field of 1.4 kG. The maximum Ar K-shell radiation yield registered in the experiments with the axial magnetic field did not exceed 400 J/cm. A sharp reduction of the K-shell yield cannot be explained only by taking into account the energy losses associated with the compression of the axial magnetic field.

Experimental study of an argon-hydrogen Z pinch plasma radiation source

Experiments with Ar-H/sub 2/ double gas puffs have been conducted on the GIT-12 generator at the current level of 2.1-2.4 MA and 250-350 ns implosion times. The argon-hydrogen mixture was used as a working medium in the outer shell of a double gas puff. The goal of the experiments was to verify whether the use of the argon-hydrogen mixture in the outer shell can improve the stability of a double gas puff implosion and provide increased argon K-shell radiation yields. The experiments showed that hydrogen admixture results in the change of implosion dynamics: decreased implosion velocities and compression ratios. The experimental data does not allow a conclusion that the use of an Ar-H/sub 2/ mixture in the outer shell of a double gas puff significantly improves the implosion stability. An increase in the hydrogen percentage in the outer shell leads to a decrease in plasma density and temperature, and as a result, reduced K-shell radiation yields and powers.

Investigation of the mechanism for current switching from the outer gas puff to the inner wire array in a structured Z-pinch

Experiments are reported on the implosion of structured loads with outer argon, krypton, and xenon gas puffs and an inner tungsten multiwire array. Experiments were carried out in the GIT-12 generator with a current of 2.6 MA and a current rise time of 270 ns. It is shown that the current successfully switches to the wire array only when the gas puff is sufficiently light. The total implosion time is 300 ns, and the implosion time of a wire array, determined from streak camera images, is 50–70 ns. It is suggested that the switching is efficient only when the active impedance of the gas puff is higher than the transitional resistance of the electrically exploded wires.

Design and Experimental Validation of Two Current Multiplier Configurations on a Microsecond MA Generator

Summary form only given. The current multiplier (CM) concept was proposed to increase the driver-to-load energy transfer efficiency. The suggested CM requires additional volumes with high self-inductance (magnetic flux extruders) connected through vacuum convolutes prior to the load and they extrude the magnetic flux toward the load magnifying the load current. We present the design criteria allowing to achieve high extruder self-inductance at low parasitic inductances added to generator and load in the modified circuit. Two configurations of this new device with one extruder are considered for GIT 12 microsecond MA generator having inductance of ~100 nH. The extruder inductance was either a large vacuum volume or a smaller volume with magnetic core. The discussed design procedure allowed to define optimum coreless and cored CM hardware configurations at conservative values of the AK gaps in CM vacuum lines (15-25 mm). The optimum coreless CM had 80 cm height and 170 cm diameter. Operating on a 8 nH inductive load in experimental tests on GIT 12 it allowed to increase the load current from 4.7 MA @ 1.7 mus without CM to ~6 MA @ 1.5 mus2. A more compact cored CM configuration with 36 cm height and 85 cm diameter operating at a 4.6 nH inductive load allowed further load current increase up to ~8 MA @ 1.7 mus. Further experimental tests with a Ne gas-puff z-pinch load showed that the peak load current at ~1 mus was increased from 3.5 MA (no CM) to 5.2 MA and that the energy-delivered to load at implosion was increased from ~170 kJ to ~330 kJ. No considerable energy losses in the CM vacuum gaps and CM convolute were recorded. Therefore, it is experimentally confirmed that the proposed new device is applicable for improving characteristics of existing and future pulse-power generators.

K-shell radiation of multiple shell gas puff microsecond implosions

This paper presents the results of experiments with a multiple shell gas puff z-pinch at microsecond implosion times performed on the GIT-12 generator. Two concentric annular gas jets had diameters of 16 cm (outer shell) and 8 cm (middle shell). The inner on-axis solid fill had a diameter of 2.2 cm. The plasma stability and the neon and argon K-shell yields are discussed.

Deuterium z-pinch as a powerful source of multi-MeV ions and neutrons for advanced applications

A novel configuration of a deuterium z-pinch has been used to generate a nanosecond pulse of fast ions and neutrons. At a 3 MA current, the peak neutron yield of (3.6 ± 0.5) × 1012 was emitted within 20 ns implying the production rate of 1020 neutrons/s. High neutron yields resulted from the magnetization of MeV deuterons inside plasmas. Whereas deuterons were trapped in the radial direction, a lot of fast ions escaped the z-pinch along the z-axis. A large number of >25 MeV ions were emitted into a 250 mrad cone. The cut-off energy of broad energy spectra of hydrogen ions approached 40 MeV. The total number of >1 MeV and >25 MeV deuterons were 1016 and 1013, respectively. Utilizing these ions offers a real possibility of various applications, including the increase of neutron yields or the production of short-lived isotopes in samples placed in ion paths. On the basis of our experiments with various samples, we concluded that a single shot would have been sufficient to obtain GBq positron activity of 13N ...