Abstract
Recently, there has been intense interest in small dense plasma focus (DPF) devices for use as pulsed neutron and X-ray sources. Although DPFs have been studied for decades and scaling laws for neutron yield versus system discharge current and energy have been established (Milanese, M. et al., Eur. Phys. J. D 2003, 27, 77–81), there are notable deviations at low energies due to contributions from both thermonuclear and beam-target interactions (Schmidt, A. et al., Phys. Rev. Lett. 2012, 109, 1–4). For low energy DPFs (100 s of Joules), other empirical scaling laws have been found (Bures, B.L. et al., Phys. Plasmas 2012, 112702, 1–9). Although theoretical mechanisms to explain this change have been proposed, the cause of this reduced efficiency is not well understood. A new apparatus with advanced diagnostic capabilities allows us to probe this regime, including variants in which a piston gas is employed. Several complementary diagnostics of the pinch dynamics and resulting X-ray neutron production are employed to understand the underlying mechanisms involved. This apparatus is unique in its employment of a 50 fs laser-based shadowgraphy system that possesses unprecedented spatio-temporal resolution.
Highlights
A dense plasma focus (DPF) is a compact device, often termed a pinch, which produces high-density, energetic plasmas, used for production of radiation ranging from hard X-rays to fusion neutrons
The green and red traces are the outputs from scintillator-photomultiplier tubes (PMT) sensors at distances of 0.3 m and 1.6 m from the DPF
The whole solid angle leading to these detectors was blocked by 7 mm of lead, which attenuates any X-rays below 100 keV to a negligible level while only slightly attenuating the fast neutrons
Summary
A dense plasma focus (DPF) is a compact device, often termed a pinch, which produces high-density, energetic plasmas, used for production of radiation ranging from hard X-rays to fusion neutrons. This is achieved with a coaxial set of electrodes in a relatively low-pressure environment, connected to a pulsed, high voltage power source (Figure 1). During the first phase of operation, a fast switch is closed, energizing the DPF and causing a plasma sheath to form between the anode and cathode, initially along the surface of the insulator This plasma sheath layer carries current in the radial direction, while the currents on the surface inner electrode that feed this radial current travel in the axial direction. The resultant electromagnetic pressure-induced travel of the discharge sheath along the electrode is termed “run down”, and as it moves, the sheath collects the gas in front of it in a snowplow-like mechanism [1]
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