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Vacuum Microelectronics Conference, 1997. Technical Digest., 1997 10th International; 09/1997
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ABSTRACT: Microfabricated field emitter arrays (FEAs) can provide the very high electron current densities required for rf amplifier applications, typically on the order of 100 A/cm2. Determining the dependence of emission current on gate voltage is important for the prediction of emitter performance for device applications. Field emitters use high applied fields to extract current, and therefore, unlike thermionic emitters, the current densities can exceed 103 A/cm2 when averaged over an array. At such high current densities, space charge effects (i.e., the influence of charge between cathode and collector on emission) affect the emission process or initiate conditions which can lead to failure mechanisms for field emitters. A simple model of a field emitter will be used to calculate the one-dimensional space charge effects on the emission characteristics by examining two components: charge between the gate and anode, which leads to Child’s law, and charge within the FEA unit cell, which gives rise to a field suppression effect which can exist for a single field emitter. The predictions of the analytical model are compared with recent experimental measurements designed to assess space charge effects and predict the onset of gate current. It is shown that negative convexity on a Fowler–Nordheim plot of Ianode(Vgate) data can be explained in terms of field depression at the emitter tip in addition to reflection of electrons by a virtual cathode created when the anode field is insufficient to extract all of the current; in particular, the effects present within the unit cell constitute a newly described effect.
Journal of Applied Physics 07/1997; 82(2):845-854. · 2.17 Impact Factor
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ABSTRACT: Summary form only given. Knowledge of emitter geometry and materials allows prediction of gain, efficiency, power output, and circuit length of an emission gated TWT. The extension of a simple analytical model examines the basic physics involved, and gives a lower limit to the performance from an FEA-based TWT. Previous approaches to modeling an FEA cathode in an emission gated TWT (Twystrode) treated the FEA and tube physics separately, and joined analyses using the Fowler-Nordheim A and B parameters. These parameters were therefore independent. Here, A and B can be predicted on the basis of the geometry, materials, and uniformity of the array, allowing for a seamless relation of fabrication issues to tube performance
Plasma Science, 1997. IEEE Conference Record - Abstracts., 1997 IEEE International Conference on; 06/1997
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ABSTRACT: Summary form only given. Field-emitter arrays (FEAs) are being
fabricated for X-band inductive-output amplifiers, such as Twystrodes
and Klystrodes. The pre-bunched electron beam supplied by a gated FEA
cathode provides smaller size, increased efficiency, and faster turn-on
compared to a thermionic-cathode tube. For this application, the FEAs
must supply beam currents on the order of 80-100 mA and be designed so
that the microwave input drive signal can be efficiently coupled to the
modulating gate of the FEA. The Lincoln FEAs are fabricated using
laser-interferometric lithography, which has provided high yields of
Mo-tip arrays having 0.32-μm tip-to-tip and 0.08-μm gate-to-tip
spacings. These small dimensions reduce the gate modulation voltage that
is required for suitable beam modulation and improve the high-frequency
performance; they represent the smallest dimensions and highest density
yet reported for FEAs. Improvements in fabrication technology, testing
methods, and conditioning techniques are presented that have resulted in
emission currents as high as 22 mA from one quarter of an FEA cathode.
The measured input impedance agrees well with theory and can be matched
by appropriate techniques
Plasma Science, 1997. IEEE Conference Record - Abstracts., 1997 IEEE International Conference on; 06/1997
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ABSTRACT: Summary form only given, as follows. The investigation of a new class of vacuum microelectronic, microwave power amplifiers based on field emitter array (FEA) technology is currently underway at the Naval Research Laboratory. FEAs offer instant activation, higher transconductance and higher current densities than thermionic cathodes and consequently higher performance gridded microwave power tubes should be possible. FEA microwave power amplifiers are attractive for improved vacuum power boosters in the next-generation microwave power module. A twystrode employing FEAs as the cathode is being designed to operate at 10 GHz with 50 W output power and 10 dB gain. FEAs will be used to generate an electron beam of up to 160 mA at 2.5 kV with a magnetic field of 2 to 3 kG. Experiments to characterize this electron beam will be presented. The design of a beam transport experiment, which will simulate the circuit of the twystrode, will be shown. Recent and future FEA experiments to demonstrate the experimental operating parameters of the FEA twystrode will also be discussed
Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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ABSTRACT: Summary form only given. The details of the electron emission from field emitter array structures and trajectories into the acceleration region of the gun structure are required for the design of proposed next generation inductive output amplifiers (IOAs). A model that corresponds well to the actual geometry of the gated field emitter is expected to provide a more accurate comparison with the experimentally measured quantities. The axially symmetric unit cell model selected consists of an anode, a gate with hole, and a base plane with a vertical emitter tip protruding. The small sizes characteristic of the apex of the emitter tips in comparison to the size and distances of the other electrodes suggest the use of a nonuniform discretization of the computational domain. The use of typical finite-difference formulations with uniform mesh is therefore contraindicated. A variable-mesh finite-difference or finite element technique could be used. However, because of the sensitivity of the emission upon the local geometry in the small region near the emitter tip, a boundary-element model was chosen. The boundary element technique discretizes the boundary into sections (in our model, annular ribbons). These sections are taken to be quite small in the vicinity of edges and corners and increase in size in the smooth boundary areas. The list of boundary elements and their physical attributes is developed from a parameterization suggested by experimentally pertinent quantities. These quantities include work function, potential, tip radius, tip height, gate hole radius, base-gate distance, gate-anode distance, gate thickness, and type of tip, i.e., sphere on cone, ellipsoid, tip on post, post height
Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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ABSTRACT: Summary form only given. RF power amplifier performance characteristics can be significantly enhanced if an emission modulated cathode source capable of producing current densities greater than 10 A/cm<sup>2</sup> under gigahertz modulation can be created. Refractory metal Spindt-type field emitter arrays (FEAs) are a promising candidate for the microfabricated emitter. Emission characterization of single- and multiple-tip arrays is imperative for the determination of array performance characteristics in order to design and implement Inductive Output Amplifiers (IOA) or integrable (on-chip) microtriode devices. For IOAs especially, knowledge of the beam spread is paramount in the design of the helix or cavity power extraction region. In this work, we provide a simple analytic model for gated field emitters useful for understanding the spatial dispersion of the emitted electrons and to correlate it with experimental measurements made on a single Spindt-type molybdenum field emitter. The analytical results are based on the “Saturn Model” of a gated field emitter, in which the emitter tip is approximated by a sphere, the gate by a ring of charge, and the anode by an external field. The experimental measurements were made using a nanofabricated anode whose position from the single gated emitter was determined using laser interferometry. The methods used to correlate theory with experiment are explained, and the dependence of the beam profile on tip sharpness, gate diameter, anode distance, and tip work function are examined
Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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M.A. Kodis,
M. Garven,
C.O. Bozler,
D.D. Rathman,
C.T. Harris,
G.A. Lincoln,
R.H. Mathews,
S. Rabe,
R.A. Murphy,
M.A. Hollis,
B. Goplen,
D.N. Smithe
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ABSTRACT: Summary form only given. Inductive output amplifiers (IOAs) such as twystrodes and klystrodes use an emission-gated beam in which the emission current density is modulated. Although emission gating offers clear advantages in efficiency and compactness, it makes stringent demands on the cathode structures where the gating occurs. Recent improvements in gated field-emitter array (FEA) technology provide a new alternative to thermionic electron sources for emission gating at frequencies above UHF; this new opportunity, however, is subject to the integration of FEA technology into the vacuum-tube environment. The gain, efficiency, and power output of an inductive output amplifier are contingent upon the current-voltage characteristic and high-frequency impedance of the gated cathode. A model, called the “end-to-end analysis”, has been developed to examine quantitatively the design specifications for such cathodes. We will present a specific set of design curves for an IOA (a twystrode) having field-emission cathode, to recommendations for mutual optimization of the FEA and the output leading circuit of the IOA. Progress in the realization of an experimental twystrode based on these guidelines will be described, including perveance, focusing, and beam quality considerations. The cathodes being fabricated for this tube are gated, low-voltage FEA cathodes having record densities of Mo field-emission tips. These cathodes will be discussed, with specific emphasis on how requirements for emission-current density, current/tip, transconductance, capacitance, and cathode layout are being satisfied. Scaling and optimization of key parameters such as cathode transconductance and device reliability will be described. An important issue for the twystrode is the realization of a broadband (~one octave) cathode input matching circuit. S-parameter measurements of the input impedance of the cathodes will be presented, together with the design of a cathode input circuit having an octave bandwidth
Plasma Science, 1996. IEEE Conference Record - Abstracts., 1996 IEEE International Conference on; 07/1996
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ABSTRACT: In an inductive output amplifier, an emission-gated electron beam
induces high-frequency fields in an output circuit via displacement
current, not convection current. Emission-gated electron beams
experience strong interactions when traversing a resonant or synchronous
electromagnetic field, and this strong interaction is responsible for
both the interesting nonlinear physics and the attractive efficiency and
compactness of emission-gated amplifiers. Field emission cathodes, due
to their extremely low electron transit time and high transconductance,
offer the opportunity to extend the advantages of emission gating into C
and X band. This paper presents design criteria for the joint
optimization of the field emission array (FEA) structure and the RF
input and output circuits of inductive output amplifiers. We find that
while output circuits yielding net efficiencies of 50% or greater are
well within the state of the art, the gain is likely to be moderate
(10-20 dB). With today's FEA performance, a desirable operating regime
is achievable, yielding a new class of compact, highly efficient, and
moderate gain power booster amplifiers
IEEE Transactions on Plasma Science 07/1996; · 1.17 Impact Factor
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ABSTRACT: A linearized relativistic field theory of a helix traveling wave
tube (TWT) is presented for a configuration where either a thin annular
beam or a solid beam propagates through a sheath helix enclosed within a
loss-free wall in which the gap between the helix and the outer wall is
filled with a dielectric. A linear analysis of the interaction is solved
subject to the boundary conditions imposed by the beam, helix, and wall.
In the case of the annular beam, the electrons are assumed to be
strongly magnetized. In contrast, the effect of variations in the axial
magnetic field are included in the electron dynamics for the solid beam
analysis. Determinantal dispersion equations are obtained for the
azimuthally symmetric modes which implicitly includes beam space-charge
effects without recourse to a heuristic model of the space-charge field.
Numerical solutions of the dispersion equations are discussed and
compared with experiments
IEEE Transactions on Plasma Science 07/1996; · 1.17 Impact Factor
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ABSTRACT: A simple analytic approach for gated field emission array unit cell modeling, the ‘‘Saturn’’ model, has been developed and serves to show how those parameters which most affect inductive output amplifier performance are dependent on the material and geometry of the unit cell. The Saturn model replaces the field emission tip with a sphere and the gate with a charged ring. A relation then exists between the gate radius, the gate voltage, the sphere radius, the field, and its angular variation on the sphere, from which the total emitted current may be given analytically, and from which the geometric and material dependence of those quantities which most affect amplifier performance (Fowler Nordheim A and B parameters, tip field, capacitance, and field enhancement factor) can be qualitatively determined. The qualitative predictions of this model may be made quantitative (the ‘‘seminumerical model’’) by the inclusion of a subset of parameters normally provided in a full boundary element numerical simulation which accounts for how the field varies along the surface of the emitter. The manner in which a statistical distribution of tip radii affects the least‐squares estimate of the A and B parameters is shown.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/1996; · 1.34 Impact Factor
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ABSTRACT: The utility of analytic and seminumerical models for a gated field emitter array (FEA) unit cell is judged by comparing the predictions of the Saturn and seminumerical models against the numerically simulated behavior for FEA parameters deemed most important from the standpoint of rf amplifier applications. The numerical simulation algorithm based on the Boundary Element method, used to validate the seminumerical model and to test the qualitative predictions of the Saturn model, is briefly described. Further, by varying the distribution of emitters, work function, and tip radius, we match experimental data given in the literature for molybdenum FEAs: the manner in which the relevant geometrical and material parameters are chosen is explained and shown to be compatible with physically based expectations about emitter geometry and materials.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/1996; · 1.34 Impact Factor
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ABSTRACT: In an inductive output amplifier, an emission gated electron beam induces high‐frequency fields in an output circuit via displacement currents, not convection current. Emission gated electron beams experience strong interactions when traversing a resonant or synchronous electromagnetic field, and this strong interaction is responsible for both the interesting nonlinear physics and the attractive efficiency and compactness of emission gated amplifiers. Field emission cathodes, due to their extremely low electron transit time and high transconductance, offer the opportunity to extend the advantages of emission gating into C and X bands. Design criteria for the joint optimization of the field emission arrays (FEAs) structure and the rf input and output circuits of inductive output amplifiers are presented. It is found that while output circuits yielding net efficiencies of 50% or greater are well within the state of the art, the gain is likely to be moderate (10–20 dB). With today’s FEA performance, a desirable operating regime is achievable, yielding a new class of compact, highly efficient, and moderate gain power booster amplifiers. © 1996 American Vacuum Society
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/1996; · 1.34 Impact Factor
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ABSTRACT: A method for predicting the Fowler–Nordheim parameters A and B, and the capacitance for a unit cell of a gated field emission array is developed, applied, and integrated into end‐to‐end rf amplifier design. Density modulated rf amplifiers require a high frequency, well characterized electron emission source for efficient operation; a cathode constructed of field emission arrays is a potential candidate for insertion into the next generation of twystode and klystrode amplifiers. The A, B, and C parameterization of the emission from model unit cell geometries is done by a boundary element technique. The behavior of A, B, and C with variation in device material and geometry is illustrated with two sample case sets. Finally, the utility of A, B, and C in predicting rf device performance requirements is illustrated.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/1996; · 1.34 Impact Factor
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ABSTRACT: An inductive output amplifier is an RF device in which an emission-gated electron beam induces RF fields in an output circuit by means of displacement currents, not convection current. Emission-gated electron beams experience strong interactions when traversing a resonant or synchronous electromagnetic field, and this strong interaction is responsible for both the interesting nonlinear physics and the attractive efficiency and compactness of emission-gated amplifiers. Field emission cathodes, due to their extremely low electron transit time and high transconductance, offer the opportunity to extend the advantages of spatiotemporal modulation into C and X bands. This paper presents design criteria for the joint optimization of the field emission array structure and the ltF input and output circuits of inductive output amplifiers. We find that while output circuits yielding net efficiencies of 50% or greater are well within the state of the art, the gain is likely to be moderate (10 to 20 (13). With today's FEA performance, a desirable operating regime is achievable, yielding a new class of compact, highly efficient, and moderate-gain power booster amplifiers.
10/1995;
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ABSTRACT: Field emitter array structures are under consideration as the gated electron source in inductive-output amplifiers currently being designed, due to the potential for spatio-temporal modulation at the cathode surface. Emission gating of electron beams places stringent demands on the cathode structures where modulation occurs. In spite of the intense interest generated by these structures, a simple analytical treatment of their properties has not been forthcoming. In this work, we present a simplified theory of a gated FEA triode geometry, describe a numerically intensive calculation of current-voltage characteristics and other properties, and describe a semi-numerical model that combines the features of both. The semi-numerical model is being developed as a more comprehensive device analysis tool to predict RF amplifier performance in determining FEA properties that best meet IOA objectives. (AN)
10/1995;
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ABSTRACT: A time‐dependent nonlinear formulation of the interaction in the helix traveling wave tube is presented for a configuration in which an electron beam propagates through a sheath helix surrounded by a conducting wall. In order to describe both the variation in the wave dispersion and in the transverse inhomogeneity of the electromagnetic field with wave number, the field is represented as a superposition of waves in a vacuum sheath helix. An overall explicit sinusoidal variation of the form exp(ikz−iωt) is assumed (where ω denotes the angular frequency corresponding to the wave number k in the vacuum sheath helix), and the polarization and radial variation of each wave is determined by the boundary conditions in a vacuum sheath helix. Thus, while the field is three‐dimensional in nature, it is azimuthally symmetric. The propagation of each wave in vacuo as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. A dynamical equation for the field amplitudes is derived analogously to Poynting’s equation, and solved in conjunction with the three‐dimensional Lorentz force equations for an ensemble of electrons. Numerical examples are presented corresponding to both single‐ and multiwave interactions. © 1995 American Institute of Physics.
Physics of Plasmas 09/1995; 2(10):3871-3879. · 2.15 Impact Factor
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ABSTRACT: Summary form only given. We present a three-dimensional nonlinear formulation and simulation of a helix travelling wave tube (TWT) using a sheath helix model. The simulation is capable of treating both DC and pulsed electron beams as well as single-frequency or multi-tone operation. The model relies upon a spectral decomposition of the electromagnetic fields in terms of the vacuum sheath helix polarizations. A Poynting-like equation describes the energy balance. The electron orbits are treated using the complete Lorentz force equations. The field equations are integrated on a grid and advanced in time using a MacCormack predictor-corrector scheme, and the electron orbit equations are integrated using a fourth order Runge-Kutta algorithm. Charge is accumulated on the grid and the field is interpolated to the particle location by a linear map. Several numerical cases are considered. Simulation of the injection, of a DC beam and a signal at a single frequency is compared with a linear field theory of the helix TWT interaction, and good agreement is found. Simulation of a prebunched beam is also discussed
Plasma Science, 1995. IEEE Conference Record - Abstracts., 1995 IEEE International Conference on; 07/1995
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ABSTRACT: A series of experiments has been performed at NRL to determine the
saturation behavior and maximum conversion efficiency of emission gated
electron beams in slow wave circuits. Simulations suggest that
single-pass conversion efficiencies exceeding 50% to the fundamental
harmonic may be achieved with moderately tight bunches in a properly
tapered helix. Such an efficiency, achieved in a circuit only two to
three slow-wavelengths long, points the way toward significant advances
of microwave power tubes into new markets and applications. Emission
gated devices, including klystrodes and twystrodes, have potential as
compact and efficient power booster amplifiers. The NRL Twystrode
Experiment is a flexible instrumented amplifier operating in a wide
parameter space, allowing thorough verification of analytical and
simulation models. The electron beam (from a gridded thermionic cathode)
can be modulated in any degree, from DC, to 100% amplitude modulation,
to discrete electron bunches only 90° wide. The beam voltage is
variable from below to well above synchronism, and the beam current from
the ballistic to the spacecharge regimes. We find that moderate bunching
(270° or greater width) and velocity tapering of the output circuit
improve beam conversion efficiency at the cost of an increased
saturation circuit length
Electron Devices Meeting, 1994. IEDM '94. Technical Digest., International; 01/1995
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ABSTRACT: A method is described for the design of helix traveling wave tubes
(TWT) which is based on the linear field analysis of the coupled
beam-wave system. The dispersion relations are obtained by matching of
radial admittances at boundaries instead of the individual field
components. This approach provides flexibility in modeling various beam
and circuit configurations with relative ease by choosing the
appropriate admittance functions for each case. The method is
illustrated for the case of a solid beam inside a sheath helix which is
loaded externally by lossy dielectric material, a conducting cylinder,
and axial vanes. Extension of the analysis to include a thin tape helix
model is anticipated in the near future. The TWT model may be divided
into axial regions to include velocity tapers, lossy materials and
severs, with the helix geometry in each region varied arbitrarily. The
relations between the AC velocities, current densities, and axial
electric fields are used to derive a general expression for the new
amplitudes of the three forward waves at each axial boundary. The sum of
the fields for the three forward waves (two waves in a drift region) is
followed to the circuit output. Numerical results of the field analysis
are compared with the coupled-mode Pierce theory. A method is suggested
for applying the field analysis to accurate design of practical TWT's
that have a more complex circuit geometry, which starts with a simple
measurement of the dispersion of the helix circuit. The field analysis
may then be used to generate a circuit having properties very nearly
equivalent to those of the actual circuit
IEEE Transactions on Electron Devices 08/1994; · 2.32 Impact Factor
