Rickey J. Faehl

Initial operation of a large‐scale plasma source ion implantation experiment

In plasma source ion implantation (PSII), a workpiece to be implanted is immersed in a weakly ionized plasma and pulsed to a high negative voltage. Plasma ions are accelerated toward the workpiece and implanted in its surface. A large‐scale PSII experiment has recently been assembled at Los Alamos, in which stainless steel and aluminum workpieces with surface areas over 4 m2 have been implanted in a 1.5 m diam, 4.6 m length cylindrical vacuum chamber. Initial implants have been performed at 50 kV with 20 μs pulses of 53 A peak current, repeated at 500 Hz, although the pulse modulator will eventually supply 120 kV pulses of 60 A peak current at 2 kHz. A 1000 W, 13.56 MHz capacitively coupled source produces nitrogen plasma densities in the 1015 m−3 range at neutral pressures as low as 0.02 mTorr. A variety of antenna configurations have been tried, with and without axial magnetic fields of up to 60 G. Measurements of sheath expansion, modulator voltage and current, and plasma density fill‐in following a pu...

Recent advances in plasma source ion implantation at Los Alamos National Laboratory

Abstract Plasma source ion implantation (PSII) is an environmentally benign, potentially cost-effective alternative to conventional lineof-sight, accelerator-based implantation and wet-chemical plating processes. PSII offers the potential of producing a high dose of ions in a relatively simple, fast and cost-effective manner, allowing the simultaneous implantation of large surface areas (many square meters), complex shapes and multiple components. The dynamics of the transient plasma sheath present during PSII have been modeled in both 1 1/2-D and 2 1/2-D (one or two spatial dimensions, plus time), and recent results from these efforts are compared with measurements of the uniformity of the implanted ion dose in complex configurations. Ammonia gas (NH 3 ) has been used as a nitrogen source for PSII processing of electroplated hard chromium. A retained dose of 2.2 × 10 17 N atoms cm −2 has been demonstrated to increase the surface hardness of the electroplated Cr by 24%, and decrease the wear rate by a factor of four, without any evidence of increased hydrogen concentration in the bulk material. By adjusting the repetition rate of the applied voltage pulses, and therefore the power input to the target, controlled, elevated temperature implantations have been performed, resulting in enhanced diffusion of the implanted species with a thicker modified surface layer. Experimental work has been performed utilizing cathodic arcs as sources of metallic ions for implantation, and preliminary results of this work are given. The area of ion-beam-assisted deposition (IBAD) has been explored utilizing PSII, with large surface area diamond-like carbon (DLC) layers being generated which can exhibit hardnesses in excess of 20 GPa.

Implosion of solid liner for compression of field reversed configuration

The design and first successful demonstration of an imploding solid liner with height to diameter ratio, radial convergence, and uniformity suitable for compressing a field reversed configuration is discussed. Radiographs indicated a very symmetric implosion with no instability growth, with /spl sim/13x radial compression of the inner liner surface prior to impacting a central measurement unit. The implosion kinetic energy was 1.5 megajoules, 34% of the capacitor stored energy of 4.4 megajoules.

Instability growth in magnetically imploded high-conductivity cylindrical liners with material strength

Magnetically imploded cylindrical metal shells (z-pinch liners) are attractive drivers for experiments exploring hydrodynamics and properties of materials at extreme conditions. As in all z-pinches, the outer surface of a liner is unstable to magneto Rayleigh-Taylor (RT) modes during acceleration, and large-scale distortion arising from RT modes could make such liners unuseable. On the other hand, material strength in the liner should, from first principles, reduce the growth rate of RT modes, and material strength can render some combinations of wavelength and amplitude analytically stable. A series of experiments has been conducted in which high-conductivity, soft, cylindrical aluminum liners were accelerated with 6-MA, 7-/spl mu/s rise-time driving currents. Small perturbations were machined into the outer surface of the liner and perturbation growth monitored. Two-dimensional magneto-hydrodynamic (2-D-MHD) calculations of the growth of the initial perturbations were in good agreement with experimentally observed perturbation growth through the entire course of the implosions. In general, for high-conductivity and soft materials, theory and simulation adequately predicted the behavior of magneto-RT modes in liners where elastic-plastic behavior applies. This is the first direct verification of the growth of magneto-RT in solids with strength known to the authors.

Magnetic insulation of secondary electrons in plasma source ion implantation

The uncontrolled loss of accelerated secondary electrons in plasma source ion implantation (PSII) can significantly reduce system efficiency and poses a potential x‐ray hazard. This loss might be reduced by a magnetic field applied near the workpiece. The concept of magnetically insulated PSII is proposed, in which secondary electrons are trapped to form a virtual cathode layer near the workpiece surface where the local electric field is substantially reduced. Subsequent electrons that are emitted can then be reabsorbed by the workpiece. Estimates of anomalous electron transport from microinstabilities are made. Insight into the process is gained with multidimensional particle‐in‐cell simulations.

Key issues in plasma-source ion implantation

Plasma-source ion implantation (PSII) is a scaleable, non-line-of-sight method for the surface modification of materials. In this paper, we consider three important issues which should be addressed before wide-scale commercialization of PSII: (1) implant conformality, (2) ion sources, and (3) secondary electron emission. To ensure a uniform implanted dose over complex shapes, the ion sheath thickness must be kept sufficiently small. This criterion places demands on ion sources and pulsed-power supplies. Another limitation to date is the availability of additional ion species beyond B, C, N, and O. Possible solutions are the use of metal arc vaporization sources and plasma discharges in high vapor-pressure organometallic precursors. Finally, secondary electron emission presents a potential efficiency and X-ray hazard issue, since for many metallurgic applications the emission coefficient can be as large as 20. Techniques to suppress secondary electron emission are discussed.

Experimental measurements of a converging flux conserver suitable for compressing a field reversed configuration for magnetized target fusion

Data are presented that are part of a first step in establishing the scientific basis of magnetized target fusion (MTF) as a cost effective approach to fusion energy. A radially converging flux compressor shell with characteristics suitable for MTF is demonstrated to be feasible. The key scientific and engineering question for this experiment is whether the large radial force density required to uniformly pinch this cylindrical shell would do so without buckling or kinking its shape. The time evolution of the shell has been measured with several independent diagnostic methods. The uniformity, height to diameter ratio and radial convergence are all better than required to compress a high density field reversed configuration to fusion relevant temperature and density.

Materials Science Issues of Plasma Source Ion Implantation

Ion beam processing, including ion implantation and ion beam assisted deposition (IBAD), are established surface modification techniques which have been used successfully to synthesize materials for a wide variety of tribological applications. In spite of the flexibility and promise of the technique, ion beam processing has been considered too expensive for mass production applications. However, an emerging technology, Plasma Source Ion Implantation (PSII), has the potential of overcoming these limitations to become an economically viable tool for mass industrial applications. In PSII, targets are placed directly in a plasma and then pulsed-biased to produce a non-line-of-sight process for intricate target geometries without complicated fixturing. If the bias is a relatively high negative potential (20-100kV) ion implantation will result. At lower voltages (50-1200V), deposition occurs. Potential applications for PSII are in low-value-added products such as tools used in manufacturing, orthopedic devices, and the production of wear coatings for hard disk media. This paper will focus on the technology and materials science associated with PSII.

Analysis and simulation of virtual cathode oscillations

A nonlinear mechanism for virtual cathode oscillations is derived from an analysis of a cold plasma fluid model. Frequency scalings and field wave shapes are predicted. These are compared with extensive numerical simulation studies of non‐neutral electron beams using both one‐ and two‐dimensional particle‐in‐cell codes.

RANCHERO explosive pulsed power experiments

The authors are developing the RANCHERO high explosive pulsed power (HEPP) system to power cylindrically imploding solid-density liners for hydrodynamics experiments. Their near-term goal is to conduct experiments in the regime pertinent to the Atlas capacitor bank. That is, they will attempt to implode liners of /spl sim/50 g mass at velocities approaching 15 km/sec. The basic building block of the HEPP system is a coaxial generator with a 304.8 mm diameter stator, and an initial armature diameter of 152 mm. The armature is expanded by a high explosive (HE) charge detonated simultaneously along its axis. The authors have reported a variety of experiments conducted with generator modules 43 cm long and have presented an initial design for hydrodynamic liner experiments. In this paper, they give a synopsis of their first system test, and a status report on the development of a generator module that is 1.4 m long.