Nathaniel P. Lockwood

Evidence for adsorbate-enhanced field emission from carbon nanotube fibers

We used residual gas analysis (RGA) to identify the species desorbed during field emission (FE) from a carbon nanotube (CNT) fiber. The RGA data show a sharp threshold for H2 desorption at an external field strength that coincides with a breakpoint in the FE data. A comprehensive model for the gradual transition of FE from adsorbate-enhanced CNTs at low bias to FE from CNTs with reduced H2 adsorbate coverage at high bias is developed which accounts for the gradual desorption of the H2 adsorbates, alignment of the CNTs at the fiber tip, and importance of self-heating effects with applied bias.

Morphology dependent field emission of acid-spun carbon nanotube fibers

Acid spun carbon nanotube (CNT) fibers were investigated for their field emission properties and performance was determined to be dependent on fiber morphology. The fibers were fabricated by wet-spinning of pre-made CNTs. Fiber morphology was controlled by a fabrication method and processing conditions, as well as purity, size, and type of the CNT starting material. The internal fiber structure consisted of CNT fibrils held together by van der Waals forces. Alignment and packing density of the CNTs affects the fibers electrical and thermal conductivity. Fibers with similar diameters and differing morphology were compared, and those composed of the most densely packed and well aligned CNTs were the best field emitters as exhibited by a lower turn-on voltage and a larger field enhancement factor. Fibers with higher electrical and thermal conductivity demonstrated higher maximum current before failure and longer lifetimes. A stable emission current at 3 mA was obtained for 10 h at a field strength of <1 V μm(-1). This stable high current operation makes these CNT fibers excellent candidates for use as low voltage electron sources for vacuum electronic devices.

Demonstration of an Acid-Spun Single-Walled Nanotube Fiber Cathode

Field emission dc cold cathodes continue as an important area of research for uses such as electron microscopy, novel X-ray sources, vacuum electronic devices, terahertz sources, and high-power microwave tubes. Each of these applications typically requires high current densities with high-brightness electron beams driven by cathodes exhibiting long lifetime in the presence of deleterious conditions such as ion back bombardment and excessive heating. The Air Force Research Laboratory (AFRL) now investigates cathodes operating in dc mode for use in a terahertz traveling wave tube (TWT). The TWT requires an electron beam of 50 μm in diameter or less, at 10s of kiloelectronvolt energy with energy spreads of less than 10 eV. While AFRL has tested numerous cathodes in this regime, this paper reports on the first demonstration of a dc cathode utilizing a highly aligned carbon nanotube (CNT) rope for the electron emitter. The rope consists of individual single-walled CNTs that have been subjected to a nitrogen-enhanced acid etch and then spun into a rope configuration. Thus, the single rope emitter has an overall diameter of 100 m and a length of 1.5 mm. We report on preliminary results from this cathode, in particular the fabrication of the cathode, the dc cathode test system, and the cathode operation up to a voltage of 5 kV. The cathode operates stably to within 0.6% with a 5-mm anode-cathode gap at 5 keV and 1.0-mA current for hundreds of hours. Finally, we provide estimates of the cathode parameters such as the effective field enhancement factor (βeff) and emitting area (A) through a Fowler-Nordheim plot and comparison of the experimental data with simulations utilizing the particle-in-cell code Improved Concurrent Electromagnetic Particle-in-Cell.

1.5: Development of field emission cathodes, electron gun and a slow wave structure for a terahertz traveling wave tube

High power terahertz (THz) sources and amplifiers hold the potential to greatly improve remote sensing and high bandwidth communication. To enable these applications, a Traveling Wave Tube (TWT) operating at 0.22 THz and a multi-cathode Field Emission (FE) electron gun are developed and characterized using a Particle-in-Cell simulation. Three candidate high current density cathode materials, Halfnium Carbide (HfC), carbon fibers, and Carbon Nanotubes (CNTs) were tested, characterized and their emission properties compared and used to verify simulations. A current of 3.0 mAmps for a single 100 micron diameter single walled nanotube rope was experimentally achieved and used as the basis of the FE gun design. Simulations of the FE gun and THz TWT were coupled and the effects of multiple and single tip FE gun beam characteristics on the TWT gain, bandwidth, and efficiencies are examined for several beam optic configurations.

Virtual prototyping of directed energy weapons

This paper gives an overview of how RF systems, from pulsed power to antennas, may be virtually prototyped with the improved concurrent electromagnetic particle-in-cell (ICEPIC) code. ICEPIC simulates from first principles (Maxwells equations and Lorenzs force law) the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on gigawatt-class sources; the relativistic magnetron is shown as an example. Such simulations require enormous computational resources. These simulations successfully expose undesirable features of these sources and help us to suggest improvements

Virtual Prototyping of Directed Energy Weapons on Thousands of Processors

This paper documents the changes required to permit ICEPIC to scale efficiently to the thousand CPU range. Substantial changes were made to the communication paradigm within the code, so that only one synchronization point is now required. This led to increase of a factor four in the number of processors ICEPIC can productively use on real world problems

Plasma and laser kinetics and field emission from carbon nanotube fibers for an Advanced Noble Gas Laser (ANGL)

A metastable argon laser operating at 912 nm has been demonstrated by optically pumping with a pulsed titanium sapphire laser to investigate the temporal dynamics of an Advanced Noble Gas Laser (ANGL). Metastable argon concentrations on the order of 1011 cm-3 were maintained with the use of a radio frequency (RF) capacitively coupled discharge. The end-pumped laser produced output powers under 2 mW of average power with pulse lengths on the order of 100 ns. A comparison between empirical results and a four level laser model using longitudinally average pump and inter-cavity intensities is made. An alternative, highly-efficient method of argon metastable production for ANGL was explored using carbon nanotube (CNT) fibers.

Laser stimulated grain growth in 304 stainless steel anodes for reduced hydrogen outgassing (Erratum)

Metal anodes in high power microwave (HPM) devices erode during operation due to hydrogen outgassing and plasma formation; both of which are thermally driven phenomena generated by the electron beam impacting the anode’s surface. This limits the lowest achievable pressure in an HPM device, which reduces its efficiency. Laser surface melting the 304 stainless steel anodes by a continuous wave fiber laser showed a reduction in hydrogen outgassing by a factor of ~4 under 50 keV electron bombardment, compared to that from untreated stainless steel. This is attributed to an increase in the grain size (from 40 - 3516 μm2), which effectively reduces the number of characterized grain boundaries that serve as hydrogen trapping sites, making such laser treated metals excellent candidates for use in HPM applications.

Field emission excitation of a high pressure noble gas

Electric Hybrid Lasers (EHL) combines the benefits of a solid state laser (SSL) and a gas phase system. EHLs have the electrical capacity of an SSL and the thermal management and beam quality of a gaseous lasing medium. Researchers at Emory University have developed a novel EHL.1 A three-level EHL is being developed that utilizes a capacitively coupled RF discharge to produce metastable excited states of a Noble gas to and from the ground state of the laser. The lowest meta-stable state is optically pumped by employing diodes resonant with the highest energy state and then is spin mixed to transition to the lasing state. The atom then lases back to the beginning meta-stable state. To improve upon the efficiency of the capacitively coupled RF discharge for producing the meta-stable ground state, a new approach for producing meta-stables is investigated utilizing field emission into a high pressure Noble gas. If the electric field to pressure (E/P) ratio is kept sufficiently low, ions and electrons produced via ionization is negligible. The low E/P ratio is achieved due to the low turn-on electric field for the field emitters, thus the majority of the electrons in the gas are due to field emission, resulting in a highly non-neutral plasma. Experimental results have shown that individual field emission fibers can produce relatively high current of greater than a micro-Amp at extremely low electric fields (160 kV/m). In addition, experimental results show that at lower currents, the current-voltage characteristic is consistent with Fowler-Nordheim emission. At higher current levels, the current-voltage characteristic enters into a space charge limited regime where current increases as the square of the voltage. Excitation of the Argon gas using field emission was accomplished and spectroscopic measurements of the optical emission were made showing the lasing state was excited and relaxed to the ground meta-stable state. PIC simulations were able to reproduce the same trends observed in the experimental results. Experimental results showed that Argon meta-stables could be produced at E/P ratios well below what could be used to sustain a standard plasma discharge.

Noble gas meta-stable state excitation using carbon nanotube fiber cathodes

Summary form only given. Electric Hybrid Lasers (EHL) are of great interest for commercial and government application due to their ability to combine the benefits of a solid state laser (SSL) with the benefits of a gas phase system. EHLs have the electrical capacity of an SSL and the thermal management and beam quality of a gaseous lasing medium. Recently, researchers at Emory University have developed a novel EHL.1 The Discharge Assisted Noble Gas Laser (DANGL), is a three-level laser that utilizes a mild electrical discharge to produce metastable excited states of a Noble to form the ground state of the laser. The meta-stables are optically pumped by employing diodes resonant with the highest energy state. After excitation, relaxation via collisions with helium from the highest excited state to the lasing state occurs. The atom then lases back to the metastable state. To improve upon the efficiency of the mild electrical discharge of the original DANGL, a new approach for producing meta-stables is investigated utilizing field emission from Carbon-Nanotube (CNT) fibers into a high pressure Noble gas. If the electric field to pressure (E/P) ratio is kept sufficiently low and pulse widths are short, ionization is significantly reduced. Not allowing for full sustained breakdown allows the majority of the electrons in the gas to result from field emission from the CNT fiber, thus creating a non-neutral plasma. Modeling of the DANGL meta-stable excitation was accomplished with a combined 3-D electromagnetic Particle-in-Cell (PIC) and Monte Carlo Collision (MCC) model. Modeling was performed to optimize the geometry of field emission from the CNT fibers in order to maximize the yield of meta-stable states. Model results showed high yields of Ar meta-stables could be achieved at E/P ratios that could not sustain a standard plasma discharge. Model results enabled the development of optimized experimental set-up and interpretation of the experimental current-voltage characteristics. Experimental results have also shown that CNT fibers can produce relatively high current pulses for extremely low electric field (160 kV/m) at a 5 nanosecond pulse width, thus enabling Noble gas meta-stable excitation without neutral plasma production.