R. M. Mayo

Optical emission study of ablation plasma plume in the preparation of diamond-like carbon films by KrF excimer laser

Optical emission study of the laser ablation plasma plume during the preparation of diamond-like carbon (DLC) films using KrF excimer (248 nm) pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum (1×10−7 Torr), the emission lines from carbon ions of C+, C2+, and C3+ are observed in addition to atomic carbon emission lines, while no emission from the diatomic carbon molecule (C2) is observed. With increasing background nitrogen pressure up to 500 mTorr, the emission intensities of the C2 Swan band and the carbon nitride (CN) violet band increase. The diamond-like character of deposited DLC film degrades with background nitrogen pressure. The vibrational temperature of C2 and CN molecules decreases with the increasing of nitrogen pressure. The CN vibrational temperature for the first 2 μs after the laser pulse is very high and in agreement with the kinetic energy of monatomic carbon ions. The C2 vibrational ...

Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part I. Optical emission spectroscopy

A comparative study of ablation plasma plumes originated from single crystal graphite (SCG) and amorphous carbon (a-C) targets during the preparation of diamond-like carbon (DLC) films by KrF excimer pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C+, C2+, and C3+ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (EL) increase for both cases. The C2 emission intensity from the SCG plasma plume changes drastically with EL, while that from the a-C plasma plume is almost constant. The C2/C emission intensity ratio for the a-C case decreases with EL increase. As for the SCG case, the C2/C ratio decreases with EL increase up to 3.0 J/cm2, and increases slightly with further EL increase. Nanohardness of the deposited films decreases with the increase of the C2/C emission intensi...

Electrostatic measurement of plasma plume characteristics in pulsed laser evaporated carbon

A triple Langmuir probe measurement has been implemented to investigate plasma plume character in low fluence (∼3.0 J/cm2) pulsed laser evaporation (PLE) discharges and has been found to be an extremely valuable tool. Absolute plasma plume density estimates are found to reside in the range 1.0×1013–2.0×1014 cm−3 for vacuum pulses. A simple heavy particle streaming model for vacuum pulses allows estimates of the plume ionization fraction of ∼10%. This is consistent with typical deposition inventory suggesting that high kinetic energy ions may play an important role in diamond-like carbon (DLC) film deposition. Electron temperature inferred from the electrostatic probe is found to consistently reside in the range 0.5–3.0 eV, and appears to be uninfluenced by operating conditions and large variations in Ar and N2 fill gas pressure. Consistent with strong plume ion and neutral particle coupling to the background fill, constancy of Te suggests expulsion of background gas by the energetic plume. The leading edg...

Plasma plume characteristics and properties of pulsed laser deposited diamond-like carbon films

Pulsed laser deposition is a unique technique for the deposition of hydrogen-free diamond-like carbon films. During deposition, amorphous carbon is evaporated from a solid target by a high-energy KrF laser, ionized, and ejected as a plasma plume. The plume expands outwards and deposits the target material on a substrate. The plasma properties of the plume determine the quality of the thin films deposited on the substrate. These plume properties include ion density, ion flow speed, electron temperature, and plume peaking parameter. In this research, a triple Langmuir probe is used to determine various plasma properties of the plume created from the pulsed laser ablation of amorphous graphite as a function of laser energy density and laser spot size on the target. A thin diamond-like carbon film is deposited and analyzed with electron energy-loss spectroscopy to determine the sp3/sp2 fraction. A special preparation technique was used to prepare the thin film for analysis to prevent the damage that may be ca...

Coaxial plasma thrusters for high specific impulse propulsion

A fundamental basis for coaxial plasma thruster performance is presented and the steady-state, ideal MHD properties of a coaxial thruster using an annular magnetic nozzle are discussed. Formulas for power usage, thrust, mass flow rate, and specific impulse are acquired and employed to assess thruster performance. The performance estimates are compared with the observed properties of an unoptimized coaxial plasma gun. These comparisons support the hypothesis that ideal MHD has an important role in coaxial plasma thruster dynamics.

Comparative study of pulsed laser ablated plasma plumes from single crystal graphite and amorphous carbon targets. Part II. Electrostatic probe measurements

In an ongoing effort to investigate plasma plume features yielding high quality diamond-like carbon films, we have applied plasma plume diagnosis and film characterization to examine plume character distinction from KrF laser ablation of both amorphous carbon (a-C) and single crystal graphite (SCG) targets. The advancing plasma plume produced by these structurally different targets are observed to possess quantitatively similar total heavy particle inventory, ionized fraction, and electron thermal content, yet quite different ion kinetic energy, plume profile, C2 formation mechanism, and concentration of complex molecules. Plume electron temperatures are observed to reside in the range 1–3 eV, with those in SCG plumes ∼10%–30% greater than a-C at all spatial positions downstream of the target. For both target cases, we find Te drop off with position away from the target with radiation as the most likely loss mechanism for these noninteracting plumes propagating in vacuum. Electron density is found to be ∼...

Two-dimensional magnetic field evolution measurements and plasma flow speed estimates from the coaxial thruster experiment

Local, time‐dependent magnetic field measurements have been made in the Los Alamos coaxial thruster experiment (CTX) [C. W. Barnes et al., Phys. Fluids B 2, 1871 (1990); J. C. Fernandez et al., Nucl. Fusion 28, 1555 (1988)] using a 24 coil magnetic probe array (eight spatial positions, three axis probes). The CTX is a magnetized, coaxial plasma gun presently being used to investigate the viability of high pulsed power plasma thrusters for advanced electric propulsion. Previous efforts on this device have indicated that high pulsed power plasma guns are attractive candidates for advanced propulsion that employ ideal magnetohydrodynamic (MHD) plasma stream flow through self‐formed magnetic nozzles. Indirect evidence of magnetic nozzle formation was obtained from plasma gun performance and measurements of directed axial velocities up to vz∼107 cm/s. The purpose of this work is to make direct measurement of the time evolving magnetic field topology. The intent is to both identify that applied magnetic field d...

Determination of microturbulence enhanced electron collisionality in magnetized coaxial accelerator channels by direct magnetic field measurement

A miniature magnetic probe array, consisting of 10 spatially separated coils, has been used to obtain profile information on the time varying magnetic field within the 2.54 cm wide flow channel of the coaxial plasma source experiment (CPS-1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)]. The magnetic field data have been used, together with a resistive, Hall magnetohydrodynamic (MHD) model of applied field distortion by the flowing plasma, to obtain estimates of the microturbulent enhancement to electron collisionality within the CPS-1 flow channel. These measurements provide direct experimental evidence of anomalous electron collisionality, a previously predicted effect in these devices. The anomaly parameter, a=νan/νcl, determined both from the distortion of contours of constant magnetic flux, and from local Bθ and Bz measurements scales with the classical electron magnetization parameter (Ωcl=ωce/νecl), indicating that collisionality plays a strong role in determining the level of anom...

Direct magnetic field measurement of flow dynamics in magnetized coaxial accelerator channels

A miniature magnetic probe array, consisting of ten spatially separated coils, has been used to obtain profile information on the time-varying magnetic field within the 2.54 cm wide flow channel of the Coaxial Plasma Source experiment (CPS-1) [R. M. Mayo et al., Plasma Sources Sci. Technol. 4, 47 (1995)] at the North Carolina State University. Two-dimensional (2-D) current profiles within the annular flow channel, which were constructed from the time-varying magnetic field data, reveal several complex features reflecting the influence of gun inductance, the Hall effect, and the applied magnetic field. When an external, electrode linking magnetic field is applied, the evolution of the 2-D current profile shows evidence of an ionizing shock front identified by a narrow current sheet propagating through the channel during the first few microseconds of the discharge. The thickness of this current sheet is on the same order as both the collisional mean-free path and the ion electromagnetic skin depth. In this ...

Initial operation of a 10 ms, quasi‐steady multi‐megawatt, coaxial plasma thruster

The Los Alamos National Laboratory Coaxial Thruster Experiment (CTX) has been upgraded to enable the quasi‐steady operation of MPD type thrusters at power levels from 1 to 40 MW for 10 ms. Initial diagnostics include an 8 position, 3 axis magnetic field probe to measure magnetic field fluctuations during the pulse, a triple Langmuir probe to measure ion density, electron temperature and plasma potential and a time‐of‐flight neutral particle spectrometer to measure specific impulse. Here we report on preliminary investigation of long‐pulse quasi‐steady coaxial thruster performance.