Insulator Sleeve Research Articles

Enhanced and reproducible neutron emission from a plasma focus with pre-ionization induced by depleted uranium (U238)

The effect of pre-ionization induced by depleted uranium (92U238) around the insulator sleeve on the neutron emission of (1.8–3.3 kJ) plasma focus device is investigated by employing time resolved and time integrated neutron detectors. The maximum average neutron yield of 2.5 × 108 is recorded at 3.5 mbar without pre-ionization, which increases up to 3.85 × 108 with pre-ionization. It is found that the pre-ionization enhances neutron yield, about (50 ± 5)%, broadens the neutron emission pressure range and improves shot-to-shot reproducibility of plasma focus operation for neutron emission. The pinhole images of charged particles emitting zones indicate that the pre-ionization increases the high temperature plasma volume.

Catalytic action of β source on x-ray emission from plasma focus

The influence of preionization around the insulator sleeve by a mesh-type β source (Ni6328) for the x-ray emission from a (2.3-3.9 kJ) plasma focus device is investigated. Quantrad Si p-i-n diodes along with suitable filters are employed as time-resolved x-ray detectors and a multipinhole camera with absorption filters is used for time-integrated analysis. X-ray emission in 4π geometry is measured as a function of argon and hydrogen gas filling pressures with and without β source at different charging voltages. It is found that the pressure range for the x-ray emission is broadened, x-ray emission is enhanced, and shot to shot reproducibility is improved with the β source. With argon, the CuKα emission is estimated to be 27.14 J with an efficiency of 0.7% for β source and 21.5 J with an efficiency of 0.55% without β source. The maximum x-ray yield in 4π geometry is found to be about 68.90 J with an efficiency of 1.8% for β source and 54.58 J with an efficiency of 1.4% without β source. With hydrogen, CuKα emission is 11.82 J with an efficiency of 0.32% for β source and 10.07 J with an efficiency of 0.27% without β source. The maximum x-ray yield in 4π geometry is found to be 30.20 J with an efficiency of 0.77% for β source and 25.58 J with an efficiency of 0.6% without β source. The x-ray emission with Pb insert at the anode tip without β source is also investigated and found to be reproducible and significantly high. The maximum x-ray yield is estimated to be 46.6 J in 4π geometry with an efficiency of 1.4% at 23 kV charging voltage. However, degradation of x-ray yield is observed when charging voltage exceeds 23 kV for Pb insert. From pinhole images it is observed that the x-ray emission due to the bombardment of electrons at the anode tip is dominant in both with and without β source.

Current sheath curvature correlation with the neon soft x-ray emission from plasma focus device

The insulator sleeve length is one of the major parameters that can severely affect the neon soft x-ray yield from a plasma focus. The effect of the insulation sleeve length on various characteristic timings of plasma focus discharges and hence the soft x-ray emission characteristics has been investigated using a resistive divider. The pinhole images and laser shadowgraphy are used to explain the observed variation in the average soft x-ray yield (measured using a diode x-ray spectrometer) with variation of the insulator sleeve length. We have found that for a neon filled plasma focus device the change in insulator sleeve length changes the current sheath curvature angle and thus the length of the focused plasma column. The optimized current sheath curvature angle is found to be between 39° and 41°, at the specific axial position of 6.2–9.3 cm from the cathode support plate, for our 3.3 kJ plasma focus device. A strong dependence of the neon soft x-ray yield on the current sheath curvature angle has thus been reported.

Soft X-ray Optimization Studies on a Dense Plasma Focus Device Operated in Neon and Argon in Repetitive Mode

This paper investigates the emission characteristics of a high-performance low-energy (3-kJ) repetitive dense plasma focus device, NX2, operated at up to 1-Hz repetition rate to develop it as an intense source of soft X-rays (SXR) for microlithography and micromachining. Various SXR yield optimization studies with argon and neon as filling gases were performed under different operating conditions (charging voltage, filling pressure, anode length, and insulator sleeve length). The SXR yield was computed using signals obtained from a PIN diode SXR spectrometer with appropriate filters. When operated in neon, the average optimum SXR (/spl lambda//spl sim/1 nm) yield in 4/spl pi/ steradians was found to be up to 140 J/shot, which corresponded to a wall plug efficiency of 5.6%. Operation in argon showed that optimized SXR (/spl lambda//spl sim/0.4 nm) yield was up to 1.3 J/shot. While operating with neon under optimized conditions with a water-cooled anode in repetitive mode, the NX2 device was used as a SXR source to imprint a test lithograph on a highly sensitive chemically-amplified resist SU-8. Test structures showing the effect of a stepper with aspect ratio 3:1 on 10-/spl mu/m-thick SU-8 resist film were obtained.

Effect of insulator sleeve length on soft x-ray emission from a neon-filled plasma focus device

Insulator sleeves of different lengths were used at the closed end of the coaxial electrode assembly of the 3.3 kJ United Nation University-International Centre for Theoretical Physics (UNU-ICTP) plasma focus device to investigate soft x-ray (SXR) (900–1550 eV) yield for different filling gas pressures of neon. The time and space resolved SXR emission characteristics were investigated using a multi-channel diode x-ray spectrometer and a CCD based pinhole imaging camera. At pressures below 6 mbar, the average SXR yield was also found to increase with increasing sleeve length from 19 to 22 mm. A further increase in insulator sleeve length to 25 mm, however, decreased the SXR yield. At pressures above 6 mbar, the SXR yields were almost the same for insulator sleeves of different lengths. The UNU-ICTP plasma focus device is found to be best optimized for neon SXR production with a 22 mm insulator sleeve at 4 mbar filling gas pressure, with the highest average yield of 8.2 J per shot, for which the conversion efficiency is about 0.25%. For this sleeve length, even the average yield at 2 or 6 mbar pressure was comparable with the maximum yield of a 25 mm sleeve.

Study of neutron emission in a low-energy plasma focus with β-source-assisted breakdown

The influence of pre-ionization around the insulator sleeve of plasma focus, by a mesh-type β-source 28Ni63 on neutron emission is investigated. The experiment is conducted on two low-energy (1.15 and 0.58 kJ) Mather-type plasma focus devices. The neutron emission is found to be increased by upto 25%. The results of this experiment suggest that pre-ionization may be helpful in designing a device as a sealed neutron source for different field applications.

Comparative study of low energy Mather-type plasma focus devices

The characteristics of three different low energy Mather-type plasma focus devices are investigated and compared. The effects of anode length and insulator sleeve length on neutron yield and on fluence anisotropy are studied. It is found that proper tuning of the focus tube inductance with that of the driver enhances the neutron emission from the focus region, increases the neutron pulse width and broadens the deuterium pressure range for high neutron yield. The insulator contamination which occurs due to Cu evaporated from the electrodes drastically changes the device characteristic. The possibility of using the device as a multi-pulse neutron generator is also examined.

Pressure range broadening for a plasma focus operation

The effects of insulator sleeve length and anode length variation on the pressure range of neutron emission in a low energy plasma focus are investigated. It is found that the proper choice of the two parameters broadens the pressure range of the high neutron yield and hence improves the shot to shot reproducibility of the system. The experiment also demonstrates that the bulk of the neutrons are emitted from a region within 2 cm above the anode top and that, in turn, reflects on the nature of the dominant production mechanism.

Effect of insulator sleeve contamination on the low energy plasma focus performance

The characteristics of a low energy plasma focus of copper electrodes operated by a single 32 μF, 15 kV (3.6 kJ) capacitor with an insulator sleeve contamination are studied. When the plasma focus is operated, the insulator sleeve is contaminated due to the deposition of copper evaporated from the electrodes. A slight contamination improves the system performance. When the cumulative discharge energy over successive shots across the insulator sleeve exceeds ∼ 1 MJ, the copper deposition on the sleeve surface makes it rough with a grain-type structure, with as result that the system becomes less reproducible and shot-to-shot variations in neutron yield are pronounced. In addition, a high voltage probe records multiple foci formation, giving rise to multiple neutron pulses, multiple X-ray pulses as well as multiple ion beams. When the cumulative discharge energy approaches 1.6 MJ, the neutron yield starts deteriorating, and the resistor divider signal begins to indicate less compression. It is suggested that the neutron yield degradation occurs due to copper coating with grain structure on the sleeve surface, which decreases the resistance of the sleeve surface and may therefore increase the current partition and eventually lower the snowplow efficiency. The situation may improve if low-sputtering-rate conductors are employed for the electrodes of the device or the truncated end of the anode is lined with low sputtering rate material like molybdenum.

Influence of insulator contamination by copper evaporation on neutron yield in a low-energy plasma focus

Deterioration of neutron yield in a low-energy plasma focus operated by a single 32 mu F, 15 V, (3.6 kJ) capacitor is observed. When the cumulative discharge energy over successive shots across an insulator sleeve approaches 1.6 MJ, the neutron yield from the device starts deteriorating. The insulator sleeve, when examined, is found to have a approximately 3 mu m thick layer of copper deposit. The contaminated sleeve surface appears rough with a grain-type structure. It is therefore concluded that the degradation of neutron yield in the low-energy device occurs due to Cu deposit on the insulator sleeve surface. The situation may improve if low-sputtering-rate conductors are employed for the electrodes of the device.

Effect of insulator sleeve material on neutron emission from a plasma focus

Neutron yield from a small (2kJ) plasma focus using insulator sleeves of different materials is investigated. Experimental results indicate that the neutron yield from plasma foci may be enhanced several times if an insulator sleeve of higher dielectric constant is employed.

Effect of insulator sleeve length on neutron emission in a plasma focus

Neutron emission from a small Mather-type plasma focus facility operated by a single 32 μF, 15 kV capacitor is studied. Specifically, the effects of insulator sleeve length and a “ceiling” introduced in front of the central electrode on the neutron yield and on the fluence anisotropy are investigated.