J.J. Lin

FePt nanoparticle formation with lower phase transition temperature by single shot plasma focus ion irradiation

Uniform FePt nanoparticles were synthesized through nanostructuring of pulsed laser deposited FePt thin films by single shot H+ ion irradiation using a plasma focus device. The annealing temperature required for phase transition from low Ku face-centred cubic to high Ku face-centred tetragonal, for ion irradiated samples, is simultaneously lowered down to 400 °C. The energetic H+ ion irradiation significantly reduces the activation energy for atomic ordering by increasing the number of vacancies. The advantage of using a plasma focus device is that it can achieve nanostructuring in much shorter time, in single shot ion exposure with pulse duration of typically about a few hundreds of nanoseconds, as compared with much longer duration required by continuous ion sources.

Optimization of a plasma focus device as an electron beam source for thin film deposition

Electron beam emission characteristics from neon, argon, hydrogen and helium in an NX2 dense plasma focus (DPF) device were investigated in order to optimize the plasma focus device for deposition of thin films using energetic electron beams. A Rogowski coil and CCD based magnetic spectrometer were used to obtain temporal characteristics, total electron charge and energy distributions of electron emission from the NX2 DPF device. It is found that hydrogen should be the first choice for thin film deposition as it produces the highest electron beam charge and higher energy (from 50 to 200 keV) electrons. Neon is the next best choice as it gives the next highest electron beam charge with mid-energy (from 30 to 70 keV) electrons. The operation of NX2 with helium at voltages above 12 kV produces a mid-energy (from 30 to 70 keV) electron beam with low-electron beam charge, however, argon is not a good electron beam source for our NX2 DPF device. Preliminary results of the first ever thin film deposition using plasma focus assisted pulsed electron deposition using a hydrogen operated NX2 plasma focus device are presented.

Drive Parameter as a Design Consideration for Mather and Filippov Types of Plasma Focus

Experiments show that the performance of the plasma focus (PF) depends on several macroscopic parameters like the energy of the capacitor bank, current, voltage, electrodes dimension, and curvature of current sheath in the axial phase. Recent work (IEEE Trans. Plasma Sci., vol. 24, no. 3, pp. 1101-1105, 1996) shows that overriding the dependence of performance on the individual parameters listed above is the dependence on a combined parameter called the drive or speed parameter S=(Ip/a)/radicrho. This parameter S appears as most fundamental in the process of nondimensionalization of the magnetohydrodynamic equations coupling the highly supersonic motion of the plasma layer with the electromagnetic circuit equation. The drive parameter S is found to directly control the speed of the plasma layer in both axial and radial phases of the PF. A literature survey (IEEE Trans. Plasma Sci., vol. 24, no. 3, pp. 1101-1105, 1996) first pointed out that neutron-optimized Mather-type PF devices with a range of energies from a few kilojoules to hundreds of kilojoules all operate with a remarkably constant drive parameter. This constancy of S has been extended more recently (Plasma Phys. Control. Fusion, vol. 47, pp. A361-A381, 2005) to PF devices over eight orders of magnitude of storage energies, from a fractional of a joule to megajoules. In this paper, experiments on 2-3-kJ dense PF with modified anodes have been conducted to show that this drive parameter remains fairly constant for the different ratios of the anode length to anode radius. It is also suggested that there may be significant differences in the values of drive parameters for Filippov-type focus devices, Mather-type focus devices, and also hybrid-type devices

Nanostructured magnetic CoPt thin films synthesis using dense plasma focus device operating at sub-kilojoule range

A repetitive NX2 dense plasma focus (DPF) device, operating at a low voltage of 8 kV with a stored energy of capacitor bank in the sub-kilojoule range (~880 J), was successfully used to deposit nanostructured magnetic CoPt thin films. The samples were synthesized at different filling hydrogen gas pressures and using different numbers of plasma focus deposition shots. The size of agglomerates/nanoparticles and the thickness of the CoPt thin films depend strongly on the filling gas pressure and the number of plasma focus deposition shots; hence it provides a possibility to control the CoPt agglomerates/nanoparticles size and the deposition rate by simply changing the operating parameters of the DPF system. The typical deposition rate of nanostructured CoPt thin film in the DPF device is much higher as compared with that of conventional PLD. The as-deposited CoPt nanoparticles are in the magnetically soft fcc phase and an annealing temperature of about 600 °C is required for phase transition to the magnetically hard fct phase, which may find possible applications in high density data storage.

Backward plume deposition as a novel technique for high deposition rate Fe nanoclusters synthesis

Fe nanoclusters with a much higher deposition rate and significantly less laser droplets were successfully synthesized by laser ablation of an Fe target along a circular strip which encircled the Si substrate mounted on the target itself. This method is coined as backward plume deposition (BPD). The mechanism of higher deposition rate for BPD was investigated by characterizing the ablated plume dynamics using time-resolved ICCD imaging. Detailed nanocluster morphology, deposition rates and magnetic properties were studied by SEM, surface profiler and VSM. This method opens up a new paradigm for industrial applications of pulsed laser deposition due to appreciably enhanced deposition rates with reduced laser droplets.

Magnetic trapping induced low temperature phase transition from fcc to fct in pulsed laser deposition of FePt:Al2O3 nanocomposite thin films

FePt:Al2O3 nanocomposite thin films have been synthesized by magnetic trapping assisted pulsed laser deposition. The annealing temperature required for phase transition from low Ku fcc-FePt to high Ku fct-FePt is lowered down from about 600°C for conventional pulsed laser deposition (PLD) to 300°C for magnetic trapping assisted PLD. For the sample annealed at 300°C with fct phase FePt nanoparticles, the average nanoparticle size is estimated to be about 8.7±2.1nm, and the magnetic properties are improved which makes it a potentially good candidate for possible applications in high density data storage.

FePt: Al2O3 nanocomposite thin films synthesized by magnetic trapping assisted pulsed laser deposition with reduced intergranular exchange coupling

FePt : Al2O3 nanocomposite thin films synthesized by magnetic trapping (MT) assisted pulsed laser deposition (PLD) were found to have lower transition temperature for L10 face-centred-tetragonal (fct) phase due to higher concentration of defects. The low phase transition temperature together with non-magnetic matrix materials helps to reduce grain growth and agglomeration during annealing. Small remanence ratio and coercive squareness for nanocomposite thin films annealed at 300 °C to fct phase confirm that the main intergranular interaction is magnetostatic interaction rather than exchange coupling. The MT assisted PLD can synthesize fct-FePt : Al2O3 nanocomposite thin films with reduced intergranular exchange coupling.

Effect of Anode Shapes on Neutron Emission from a Repetitive Plasma Focus Device

The conventional tapered anode of the NX2 repetitive plasma focus device has been changed to two other shapes (straight and spherical) to investigate the effect of anode shape on neutron emission characteristics of the device. The newly designed beryllium counter and 3He proportional counter are used for estimation of neutron yield and time resolved neutron emission characteristics are measured using two PMT based plastic scintillator detectors located at different radial distances from the anode axis. It is found that the straight anode mostly produces multiple neutron peaks, the spherical anode on the other hand essentially has a single neutron peak in its signal while the tapered anode has a mixed behavior. It is found that there is no significant difference in the average neutron yield (typically of the order of 107 neutrons per shot) for different anode shapes. However the pressure at which the maximum neutron yield occurs depends on the anode shape. The anode with spherical tip shows the most stable neutron emission with consistently good neutron yield over a much wider pressure range than the other anode shapes.

Drive parameter as a design consideration for mather and filippov types of plasma focus

Summary form only given. The operating conditions of the plasma focus depend strongly on several macroscopic parameters like energy of cap bank, current, voltage, electrodes dimension, and curvature of current sheath in axial phase. The drive parameter (Ip/a)/radicrho which is proportional to the speed of the plasma in both axial and radial phases is one of the most important parameter which determines the performance of a DPF as a source of fusion neutron. Experiments on 2-3 kJ DPF have been done to show that this parameter is important over a wide range of other parameters like anode length, insulator sleeve length and operating gas pressures. The NX2 machine was modified to have different aspect ratios ranging from A = L/2a ap 0.2 (NX2-F) to 1.52 (NX2). Here 2a, the diameter of the anode is taken from the flat top of the anode and L is taken as the free length of the anode, i.e. between the tip of anode to the insulator sleeve. In the case of neutron yield the speed is critical as the highest temperature ~1 keV needs to be achieved as the cross section is strongly dependent on temperature. A between high speed which implies high and low speeds which implies longer time gives the optimum yield for drive parameter 89 kA/cm/torr1/2 for Mather type focus and a drive parameter of 50 kA/cm/torr1/2 for Filippov type focus. The NX2-F with its mushroom shaped anode has a drive parameter intermediate between the Filippov and Mather type plasma focus is due to the shape intermediate between the Filippov and Mather type plasma focus devices. The drive parameter for NX2-F is found to keep constant at different voltages. In addition, neutron yields from the modified plasma focus tube without original cathode rods were also found to have the same order magnitude as the original electrode systems and reach the highest at the same gas pressure. This confirms that the drive parameter has an importance over cathodes at a big range of gas pressures