Experiments on the Z accelerator with deuterium gas-puff implosions have produced up to 3.7×1013 (±20%) neutrons at 2.34MeV (±0.10MeV). Although the mechanism for generating these neutrons was not definitively identified, this neutron output is 100 times more than previously observed from neutron-producing experiments at Z. Dopant gases in the deuterium (argon and chlorine) were used to study implosion characteristics and stagnated plasma conditions through x-ray yield measurements and spectroscopy. Magnetohydrodynamic (MHD) calculations have suggested that the dopants improved the neutron output through better plasma compression, which has been studied in experiments increasing the dopant fraction. Scaling these experiments, and additional MHD calculations, suggest that ∼5×1014 deuterium-deuterium (DD) neutrons could be generated at the 26-MA refurbished Z facility.

http://absimage.aps.org/image/DPP06/MWS_DPP06-2006-000645.pdf

Deuterium Gas-Puff Z-Pinch Implosions on the Z accelerator

Laser produced plasma embedded in a longitudinal magnetic field was studied using a 1 MA pulsed power generator coupled with a 50 TW laser. Half turn coil loads with an internal diameter of 2.5–3.5 mm generate a 50–70 T axial magnetic field near the load. A subpicosecond laser pulse with an intensity of 1018–1019 W/cm2 irradiates a thin Si foil target in the magnetic field of the coil load. A laser produced plasma plume collimates within the longitudinal field to a narrow jet 0.2–0.3 mm in diameter with a length of 3–4 mm and an electron plasma density of (0.2–1) × 1020 cm−3 on the jet axis. The jet propagates with a velocity of 160–200 km/s in general agreement with magnetohydrodynamic simulations. X-ray spectral measurements show an increase in the plasma electron density resulting from the magnetic confinement of the jet.

Study of laser produced plasma in a longitudinal magnetic field

Deuterium Z-pinches are efficient sources of pulsed soft and hard x-rays, fast ions, and neutrons. Many phenomena related to an acceleration of ions and neutron production on the Z-pinches have not yet been explained. Detailed understanding of these phenomena could be multidisciplinary important. Therefore, these phenomena are investigated in joint Czech-Russian experiments on the terawatt class GIT-12 device with the generator output voltage of 600 kV and current at stagnation of about 3 MA. These experiments are interesting since by using the novel experimental load composed of the deuterium gas-puff with outer plasma shell, the neutron yields were significantly increased from the order of 10 to the order of 10. Such relatively high neutron yields were earlier observed on the devices with significantly higher current as the Saturn generator with the pulsed current of about 10 MA. At the same time, in our experiments, hydrogen ions with an energy above 38 MeV were detected. Such results are unique in experiments on the generator with the abovementioned maximum current and voltage. The work reported in this thesis is focused on the diagnostics of Z-pinch plasma by neutron detectors and interpretation of the experimental results. The precise neutron diagnostics is in our experiments necessary because the produced neutron pulses carry the information about the deuterons which produced them. In this thesis, the extensive neutron detection system is presented. This system is based on several principally independent methods: neutron bubble detectors, scintillation neutron time-of-flight diagnostics, neutron activation diagnostics with the moderator (silver activation counter), and fast neutron activation diagnostics with various energy threshold. This diagnostic system was used to evaluate neutron yields, energy spectrum, and neutron fluences at different distances and directions. The influence of non-dd neutrons on the experimental results is discussed.

High Energy Density Plasma Diagnostics Using Neutron and Gamma Detectors

Investigation of wire-array Z-pinches by laser probing diagnostics

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field $B_\mathrm{seed}$ can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, $B_\mathrm{gen} \sim - 2 \pi B_\mathrm{seed} \Delta x/\lambda_0$, where $\Delta x$ is the target thickness and $\lambda_0$ is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a $\mu$m-thick laser-irradiated target with a kilotesla-level seed magnetic field.

https://iopscience.iop.org/article/10.1088/1367-2630/abc496/pdf

Strong surface magnetic field generation in relativistic short pulse laser-plasma interaction with an applied seed magnetic field

An experimental platform for the studying of high-intensity laser plasma interactions in strong magnetic fields has been developed based on the 1 MA Zebra pulsed power generator coupled with the 50-TW Leopard laser. The Zebra generator produces 100-300 T longitudinal and transverse magnetic fields with different types of loads. The Leopard laser creates plasma at an intensity of 1019 W/cm2 in the magnetic field of coil loads. Focusing and targeting systems are integrated in the vacuum chamber of the pulsed power generator and protected from the plasma debris and strong mechanical shock. The first experiments with plasma at laser intensity >2 × 1018 W/cm2 demonstrated collimation of the laser produced plasma in the axial magnetic field strength >100 T.

Experimental platform for investigations of high-intensity laser plasma interactions in the magnetic field of a pulsed power generator.

Deuterium gas puff Z pinches have been used for generation of strong neutron fluxes on the MA class pulse power machines. Due to the low electron density of deuterium Z-pinch plasma, regular laser diagnostics in the visible range cannot be used for observation and study of the pinch. Laser probing at the wavelength of 1064 nm was used for visualization of deuterium plasma. Infrared schlieren and interferometry diagnostics showed the deuterium gas puff plasma dynamics, instabilities, and allowed for the reconstruction of the profile of the plasma density.

K. J. Swanson

Papers

Experimental platform for investigations of high-intensity laser plasma interactions in the magnetic field of a pulsed power generator.

Note: Infrared laser diagnostics for deuterium gas puff Z pinches.