Experimental investigation of the effect of insulator sleeve length on the time to pinch and multipinch formation in the plasma focus facility
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The length of insulator sleeve is varied to investigate its effect on the pinch formation in the plasma focus facility. In this paper, the effect of insulator length on the time to pinch at various pressures and working voltages in the 1.15 kJ Mather type plasma focus is investigated. The results show that with 4.5 cm insulator length the time to pinch at all pressures is minimum. Other results also confirm that with increasing of pressure the time to pinch is increased. Moreover, with increasing working voltage the time to pinch is decreased. Pictures, captured using a digital single lens reflex (DSLR) Canon EOS 7D system, show that multipinch phenomenon is formed.
KeywordsPlasma focus Insulator sleeve Multipinch
Plasma focus device (PFD) is an effective device in laboratory for production of high-temperature (~1 keV) and high-density (≈1025–1026 m−3) plasma. PFD was developed in the early 1960s in the former Soviet Union (Filippov type)  and in the USA (Mather type)  independently. The PFD was initially considered as a fast neutron source [3, 4]. It is also a rich source of soft and hard X-rays [5, 6], highly energetic ions [7, 8] and relativistic electrons . The X-ray emission from PFD has been used for defectoscopy, X-ray lithography activation of enzymes, micro-machining and radiography [10, 11, 12, 13]. The energetic ions have been used for material processing such as ion implantation and thin films [14, 15, 16]. PFD has also been used as a pump source for lasers . Plasma produced in PFD can be affected with the plasma focus insulator sleeve length and PFD working voltage and pressure. Recently, experimental studies have been carried out on the effects of insulator sleeve length and pressure on time to pinch and current sheath structure [18, 19]. The effect of insulator sleeve length on X-ray emission has also been reported by Rawat et al. . In [21, 22] Zhang et al. and Zakaullah et al. show that the X-ray and neutron yield is affected by insulator sleeve length. The current sheath formation dynamics and its structure for different insulator lengths in plasma focus device are investigated by Seng et al. . The current sheath dynamics and multipinch phenomena have also been reported by Mohammadi et al. .
In this study, we have shown that the change in insulator sleeve length, pressure and working voltage affects the pinch time. The formation of multipinch phenomena is also reported.
The present investigation was performed on a simple single capacitor DPF device designated at the Shahrood University (SHUPF). It is a Mathertype focus device, energized by a single 16 μF, 12 kV fast discharging capacitor, with a maximum energy storage of 1.15 kJ. In our investigation, the device was operated at a charging voltage ranging between 7 and 9 kV. In our plasma focus, SHUPF, cylindrical anode made of copper has 60 mm in length and 20 mm in diameter. The cathode was built in six brass rods each of 10 mm diameter and 60 mm length and symmetrically located around the anode. The device was evacuated to a vacuum (~0.005 Torr) by a rotary pump and was filled with argon gas to a different pressure (1–1.6 Torr) before the operation.
Results and discussion
This equation shows that with decreasing the modification factor the axial velocity is decreased, so the time to pinch is increased. The optimum insulator sleeve length corresponds to the conditions for uniform discharge development and its take off across the insulator sleeve surface. When the sleeve is too long the increased inductance may cause the current sheath to remain at the sleeve surface for longer period of time. When the sleeve is too short, the rapid current sheath development may cause spoke formation. As a result, when the insulating sleeve is not of appropriate length, the current sheath no longer remains uniform, and the so-called filaments or spokes are developed .
In this equation ρ, b and a are gas density, cathode radius and anode radius, respectively. m f is the mass factor being less than one. This equation explains that the current sheath cannot carry 100% of gas, but some gas is left back near the insulator sleeve. After the first pinch/compression phase, indicated by the first peak in the voltage probe signal, shown in Fig. 2, another discharge on the insulator is produced and a second current sheath is produced which, owing to the low density of gas in front of it moves much faster and collapses at the anode top as the second pinch/compression phase.
Pinch formation time with the various insulator sleeve length is investigated. It was found that at all insulators we have one minimum time to pinch. It was obtained that with insulator with 4.5 cm length the time to pinch at all pressure is minimum. The average pinch time with different pressure shows that with increasing of pressure the time to pinch increased. Experimentally, it was shown that at a higher voltage the time to pinch is decreased. Experiments demonstrate that the multipinch phenomenon is formed.
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