Measurements of the return-current flowing through a solid target irradiated with the sub-nanosecond kJ-class Prague Asterix Laser System is reported. A new inductive target probe was developed which allows us measuring the target current derivative in a kA/ns range. The dependences of the target current on the laser pulse energy for cooper, graphite, and polyethylene targets are reported. The experiment shows that the target current is proportional to the deposited laser energy and is strongly affected by the shot-to-shot fluctuations. The corresponding maximum target charge exceeded a value of 10 μC. A return-current dependence of the electromagnetic pulse produced by the laser-target interaction is presented.

The current balancing the target charging and the emission of transient electromagnetic pulses (EMP) driven by the interaction of a focused 1.315 μm iodine 300 ps PALS laser with metallic and plastic targets were measured with the use of inductive probes. It is experimentally proven that the duration of return target currents and EMPs is much longer than the duration of lasertarget interaction. The laser-produced plasma is active after the laser-target interaction. During this phase, the target acts as a virtual cathode and the plasma-target interface expands. A double exponential function is used in order to obtain the temporal characteristics of EMP. The rise time of EMPs fluctuates in the range up to a few tens of nanoseconds. Frequency spectra of EMP and target currents are modified by resonant frequencies of the interaction chamber.

We describe the characterization of electromagnetic pulses (EMPs) in experiments on solid targets at PALS laser facility in Prague, for energy up to 600 J and intensity up to 10 16 W cm −2 at focus. Measurements of EMPs have been performed by different conductive probes placed inside and outside the experimental chamber. We show results for different targets and probe configurations, and illustrate effects of spurious direct coupling of these transient fields with the read-out apparatus, which are important for high-energy and high-intensity laser-plasma experiments. The related countermeasures are described and demonstrated to be very effective for improving the signal-to-noise ratio, at expenses of measured bandwidths. They allowed us to detect the EMP components due to the intense neutralization currents flowing through the target holder, and those possibly due to wakefields associated with emitted charged particles, which resulted in these experiments to be of the same order of magnitude. It is the first time both discharge current and associated EMP are effectively measured in the same nanosecond-scale experiment, where this EMP contribution is effectively detected by conductive probes. A remarkable agreement was obtained from comparison of the detected EMP profile with measured neutralization current. We also show the results achieved by means of electromagnetic simulations of fields in the modeled experimental chamber, in particular in the regions where the probes were actually placed during the experiments, and compare them with measured signals. It appears that conductive probes have limitations for the measurement of the high-frequency components of the EMP fields. The illustrated results are of primary importance for the hot topic of EMP characterization and minimization in plants for inertial-confinement-fusion (NIF, LMJ, PETAL) as well as for laser-plasma acceleration (PETAL, ELI, ApollonK).

Z-pinch experiments with deuterium gas puffs have been carried out on the GIT-12 generator at 3 MA currents. Recently, a novel configuration of a deuterium gas-puff z-pinch was used to accelerate deuterons and to generate fast neutrons. In order to form a homogeneous, uniformly conducting layer at a large initial radius, an inner deuterium gas puff was surrounded by an outer hollow cylindrical plasma shell. The plasma shell consisting of hydrogen and carbon ions was formed at the diameter of 350 mm by 48 plasma guns. A linear mass of the plasma shell was about 5 µg cm −1 whereas a total linear mass of deuterium gas in single or double shell gas puffs was about 100 µg cm −1 . The implosion lasted 700 ns and seemed to be stable up to a 5 mm radius. During stagnation, m = 0 instabilities became more pronounced. When a disruption of necks occurred, the plasma impedance reached 0.4 Ω and high energy (>2 MeV) bremsstrahlung radiation together with high energy deuterons were produced. Maximum neutron energies of 33 MeV were observed by axial time-of-flight detectors. The observed neutron spectra could be explained by a suprathermal distribution of deuterons with a high energy tail. Neutron yields reached 3.6 × 10 12 at a 2.7 MA current. A high neutron production efficiency of 6 × 10 7 neutrons per one joule of plasma energy resulted from the generation of high energy deuterons and from their magnetization inside plasmas.

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In this paper, the possible evolution of a pinched plasma column is presented from the results of temporally resolved measurements using a magnetic probe, interferometry and neutron diagnostics performed on the plasma focus PF-1000 device with deuterium as the filling gas. Together with the discharge axial current of about 1.5 MA a toroidal current component of the order of 100 kA was estimated in the toroidal, helical and plasmoidal structures formed within the dense plasma column. The mass inside these structures increases due to injection of the plasma from the neighborhood regions with a higher pinching pressure. This injected plasma increases the intensity of the internal magnetic field, probably through turbulent motion and the magnetic dynamo effect. The neutrons from the D-D fusion reaction, produced during the formation and decay of plasmoidal structures and constrictions, are accompanied by changes in the axial component of the magnetic field. Then, the transformation and decay of internal closed currents can contribute to the acceleration of high-energy electrons and ions.

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