Field Emitter Array Cathodes Research Articles

Effect of material composition of diamond field emission array cathodes on quality of transversely shaped beams

Diamond field emitter array field emission cathodes (DFEA FECs) are attractive for the next generation of injectors due to their ability to produce transversely shaped beams without the need for complex masking or laser schemes. However, reliability of this cathode technology remains a challenging issue as principal mechanisms guiding and allowing for output beam shaping remained poorly understood. This paper reports the results of testing two DFEA FECs with the same pattern and emitter tip geometry. Although both cathodes were able to sustain gradients of 44 MV/m and produce maximum output integral charge of 0.5 nC per radio frequency pulse, their emission patterns were different. One cathode did not produce a shaped beam, while the other one did. This difference was explained by the extent of the local variations of the diamond material across the arrays as discovered by spatially resolved Raman spectroscopy. The main practical takeaways were (i) tip sharpness was not a prerequisite for producing a shaped beam and instead (ii) material characteristics resulting in different cathode ballast resistance affected emission spatial uniformity across the array and hence the beam shaping.

Demonstration of transport of a patterned electron beam produced by diamond pyramid cathode in an rf gun

We have demonstrated generation and transport of a patterned electron beam from a Diamond Field-Emitter Array (DFEA) cathode in a radio frequency (rf) gun. DFEAs are arrays of micrometer-scale pyramids with nanometer-scale tips. They can be fabricated with base widths ranging from 3 μm to 25 μm and pitches as small as 5 μm. They have an inherent 1:0.7 base to height ratio. DFEAs operate as field-emitter cathodes and potentially produce intrinsically shaped electron beams, which are of interest for a number of accelerator applications. We report on the results of a recent experiment in which a beam, consisting of several beamlets, was produced from a DFEA cathode in an rf gun and transported 2.54 m along a beam line. A macrobunch charge of 60 pC was measured at a cathode field gradient of 15.1 MV/m.

Shaped Beams from Diamond Field-Emitter Array Cathodes

Diamond field-emitter arrays (DFEAs) are arrays of diamond pyramids with exquisitely sharp tips and micrometer-scale bases that produce high current densities. These arrays can be fabricated in arbitrary shapes, ranging from single tips to many millions of tips, so that they produce an inherently shaped electron beam. Each tip emits a modest current, but the large dense array can produce many Amps. We are investigating these cathodes for use in dielectric wakefield accelerators; however, they may also be applicable to vacuum microwave tubes. Recently, shaped beam production and transport have been demonstrated in the 1.3-GHz RF gun at the Argonne Cathode Test Stand at the Argonne National Laboratory. The charge was measured on a Faraday cup and the beam imaged on a YAG screen with peak electric field gradients on the cathode ranging from 12 to 35 MV/m. Three cathode geometries were tested: one 1-mm equilateral triangle with 7- $\mu \text{m}$ base pyramids and 10- $\mu \text{m}$ pitch, one 1-mm equilateral triangle with 10- $\mu \text{m}$ base and 25- $\mu \text{m}$ pitch, and one sparse $5\times 5$ square array with 20- $\mu \text{m}$ base and 400- $\mu \text{m}$ pitch. The two triangular arrays emitted 35 nC in an RF macropulse at 35 MV/m and 13-nC charge at 27 MV/m, respectively, while the sparse array emitted 0.060-nC charge at 15 MV/m. This article presents the results of the triangular array experiments, including damage due to breakdown in the RF gun and initial models of tip-to-tip shielding.

Compact, 3D-Printed Electron Impact Ion Source with Microfabricated, Nanosharp Si Field Emitter Array Cathode

We report the design, fabrication, and characterization of a novel miniature electron impact gas ionizer manufactured via silicon micromachining and high-resolution 3D printing. The ionizer uses an array of 2,500 gated silicon nano-sharp field emitters (20 μm emitter pitch) as cathode and a set of additively manufactured, finely featured polymer and metallic parts as three-dimensional ion-generating structure. Finite element simulation of a gated field emitter tip predicts a start-up bias voltage equal to 64 V –close to the 62 V turn-on bias voltage estimated via experiments. The Si field emitter array emits 124.5 μA of electron current with ~60% gate transmission while operating at 200 V. The ionization efficiency of the gas ionizer is linear with pressure, reaching values as high as ~0.4% at 5×10−4 Torr.

Divergence study and emittance measurements for the electron beam emitted from a diamond pyramid

This paper reports the results of a divergence study and emittance measurements for an electron beam emitted from a single diamond pyramid in a sparse diamond field-emitter array (DFEA) cathode. DFEA cathodes are arrays of micrometer scale diamond pyramids with nanometer scale tips. A single diamond pyramid produces currents as high as 10 μA from a small surface area. DFEA cathodes are expected to be a good beam source for compact dielectric laser accelerators. For the electron beam divergence experiment, we have designed and assembled a test stand consisting of a DFEA cathode, a mesh anode, and a screen. We measured current and the size of the beam on the screen as functions of the distance between the cathode and the anode, the size of a pyramid’s base, and applied voltage on the cathode. In order to perform beam emittance measurements, we designed and fabricated a magnetic lens to be placed between the mesh and the screen. By measuring the size of the beam for different distances between the lens and the screen, we estimate the beam’s emittance. We also simulated the electron beam dynamics with Computer Simulation Technology Studio and General Particle Tracer codes. Experimentally measured divergence angles and normalized transverse emittance show good agreement with simulations.

Tunnel Emission from Nanostructured Field-Emission Array Cathodes with a Fluorine–Carbon Coating

Tunnel Emission from Nanostructured Field-Emission Array Cathodes with a Fluorine–Carbon Coating

Carbon Nanotube Fiber Field Emission Array Cathodes

Field emission cathodes made from single bulk carbon nanotube (CNT) fibers have demonstrated high emission currents and long lifetimes. This paper investigates the goal of achieving even higher current levels from field emission array (FEA) cathodes comprised of multiple CNT fibers arranged in various array configurations. Arrays were fabricated with 25- $\mu \text{m}$ -diameter CNT fibers in 2-fiber ( $1 \times 2$ ), 4-fiber ( $2 \times 2$ ), and 25-fiber ( $5 \times 5$ ) configurations, and their field emission properties were measured. The $1 \times 2$ and $2 \times 2$ FEA cathodes achieved the maximum current values that scaled to greater than 2 mA per emitter. The $5 \times 5$ FEA cathode achieved a high maximum current value of 22 mA and exhibited stable emission for 10 h at ~9 mA at an applied field strength of 0.11 V/ $\mu \text{m}$ . The Fowler–Nordheim theory was used to calculate the effective field enhancement factor ( $\beta _{\mathrm {eff}}$ ) values for all of the arrays. Electrostatic simulations were performed using COMSOL Multiphysics modeling software to model the field enhancement of a perfectly uniform $5 \times 5$ array. The $\beta _{\mathrm {eff}}$ value for the total array was compared to the individual $\beta $ values at the fiber tips predicted by a surface potential model that predicted the field amplification of all 25 CNT fibers in the array.

Measurement of transverse emittance and coherence of double-gate field emitter array cathodes.

Achieving small transverse beam emittance is important for high brightness cathodes for free electron lasers and electron diffraction and imaging experiments. Double-gate field emitter arrays with on-chip focussing electrode, operating with electrical switching or near infrared laser excitation, have been studied as cathodes that are competitive with photocathodes excited by ultraviolet lasers, but the experimental demonstration of the low emittance has been elusive. Here we demonstrate this for a field emitter array with an optimized double-gate structure by directly measuring the beam characteristics. Further we show the successful application of the double-gate field emitter array to observe the low-energy electron beam diffraction from suspended graphene in minimal setup. The observed low emittance and long coherence length are in good agreement with theory. These results demonstrate that our all-metal double-gate field emitters are highly promising for applications that demand extremely low-electron bunch-phase space volume and large transverse coherence.

Modelling of field emitter array characteristics in diode system

In terms of a mathematical model, this paper studies features of the current characteristics of a thin-film matrix field emission array cathode with vertical blade-shaped emitters. Microscopical distribution of electric field strength was calculated and its dependence on geometric parameters of the structures was defined.

Operation of an ungated diamond field-emission array cathode in a L-band radiofrequency electron source

We report on the operation of a field-emitter-array cathode in a conventional L-band radio-frequency electron source. The cathode consisted of an array of ∼106 diamond tips on pyramids. Maximum current on the order of 15 mA was reached and the cathode did not show appreciable signs of fatigue after weeks of operation. The measured Fowler-Nordheim characteristics, transverse beam density, and current stability are discussed.

TU‐E‐108‐09: Development of An Einzel Electron Focusing Lens for Microbeam Radiation Therapy Device

Purpose: The purpose of this work is to develop a novel Einzel electron focusing lens for carbon nanotube (CNT) field emission based microbeam radiation therapy (MRT). The CNT cathode used for the proposed microbeam radiation source has a unique linear cathode profile so that the focusing structure needs to be optimized in order to achieve the desired focal spot size for efficient generation of microbeam radiation. Methods: The proposed design of the MRT device has a square system configuration with four independent linear x‐ray tube modules. Each tube module includes a linear carbon nanotube field emission cathode array, which is the core component of the MRT device. The CNT cathode is comprised of 5 individually controlled CNT cathode segments, each of which has a dimension of 2.5mm×30mm. A commercial software package (Opera 3D, Cobham plc) was used to simulate and design the optimal focusing structure. A full scale computer model covering the entire electron beam path was constructed for the electron optical simulation. Results: Based on the simulation, the two‐stage electrostatic focusing lens is capable of providing the 102um focal spot size required for the proposed microbeam radiation therapy treatment with 100um beam width. Based on our simulation results, we are able to achieve over 65% electron beam transmission rate. Both results were verified experimentally using a small scale testing x‐ray chamber. Conclusion: We have designed a novel Einzel electron focusing lens to achieve the desired focal spot in order to improve the efficiency of microbeam generation, thus the dose rate. The simulation and experiment show the two‐stage electrostatic focusing lens is capable of providing a 102um effective focal spot size. Meanwhile we have also demonstrated reasonable electron beam transmission rate (>65%), which is critical in terms of achieving high x‐ray output.

Emittance measurements of electron beams from diamond field emitter arrays

Electron injector technology is presently dominated by a variety of photo- and thermionic electron injectors. Although new electron injectors based on field emission appear promising, their success is predicated on the development of reliable, high current density, low emittance, and spatially uniform, field emitter array cathodes. The authors report recent results of transverse-emittance measurements on a particularly promising cathode, the diamond field emitter array. A simple pepperpot technique is used to measure the divergence of the beam emitted from one such cathode at low current density. Based on these measurements, a 1 mm diameter uniformly emitting cathode will have a normalized transverse emittance of ∼1 mm mrad. Our results suggest that the beam quality of these cathodes is satisfactory for use in a variety of applications.

Five picocoulomb electron bunch generation by ultrafast laser-induced field emission from metallic nano-tip arrays

Laser-induced field emission from metallic field emitter array cathodes excited by femtosecond near infrared laser pulses is explored. When 50 fs laser pulses irradiated a 1.2 × 105-tip emitter array under a DC field emission bias, electron bunches with bunch charge up to 5.2 pC were observed. The variation of the bunch charge at different laser intensities and polarizations indicated that electrons were produced from the field emitters by a photofield emission process. The result demonstrates the feasibility of metallic field emitter array cathodes for high-charge short-pulse electron source applications.

Sub-nanosecond switching and acceleration to relativistic energies of field emission electron bunches from metallic nano-tips

The authors explore the generation of short electron bunches from a field-emitter array cathode consisting of nanometer-sized metallic tips that is compatible with an acceleration electric field above 10 MV/m. Sub-nanosecond field emission electron bunches were generated by applying fast electrical pulses to an on-chip electron extraction gate electrode of the cathode. The subsequent acceleration of the field emission electron bunches to 5 MeV was demonstrated using the combined diode-RF cavity SwissFEL electron gun test facility.

Pulse deposition of large area, patterned manganese oxide nanowires in variable aspect ratios without templates

Manganese oxide nanowires with β-MnOOH in core and Mn 3O 4 in shell were successfully plated onto various conductive substrates from a Mn(CH 3COO) 2 solution by anodic deposition under a two-electrode, pulse-rest mode. The aspect ratio of uniform nanowire morphologies in cm 2 scale is controllable by varying the deposition variables. Patterned MnO x nanowire arrays were obtained by combining lithographic and electroplating techniques demonstrated to be a powerful method for preparing MnO x nanowires in the field emission (FE) array cathodes with a low turn-on voltage (∼3.4 V/μm at 1 μA/cm 2).

Preparation of a Low Cost Carbon Nanotubes Field Electron Emission Thin Film and its Initial Application Research

A large area perfect aligned carbon nanotubes (CNTs) thin film was grown directly on a stainless steel substrates by a low cost bias-assisted hot-filament chemical-vapor deposition (BA HF-CVD) technology. The CNT’s / stainless steel field emission array (FEA) cathode (CNT’s/FEA) has been fabricated. The field electron emission behavior of the CNT’s/FEA cathode was characterized in a vacuum condition, a high electron emission of about 10mA/cm2 at a low electric field of 4V/μm implies the promising the application as a cathode of the flat panel display device, so further assembled this cathode in a kind of vacuum device, and the possibility of the operation from a flat panel character (Pictures) display tube FCT has been tested initially.

Microstructure characteristics and interface morphology evolvement of Si-TaSi 2 eutectic in situ composite for field emission

As one of the semiconductor-metal eutectic (SME) composites, Si-TaSi 2 eutectic composite has many characters such as the high melting point of TaSi 2 material, the large density of TaSi 2 fibers incorporated into the Si matrix, three-dimensional array of Schottky junctions grown in the composite spontaneously. So it is an ideal candidate for field emission array cathodes. In this paper, the directionally solidified Si-TaSi 2 eutectic in situ composite for field emission is prepared by means of the electron beam floating zone melting (EBFZM) technique on the basis of Czochralski (CZ) method. The Si-TaSi 2 eutectic in situ composite, which has high-aligned and uniformly distributed TaSi 2 fibers in the Si matrix, can be obtained when the solidification rate changes from 0.3 to 9.0 mm/min. As the solidification rate is increased, both the fibers’ diameter and inter-rod spacing are decreased, while the fibers’ density and the volume fraction are increased. Moreover, the transition from a planar interface to cellular interface and then to planar interface morphologies with increasing velocity is observed with the zero power method.

A novel high resolution micro‐radiotherapy system for small animal irradiation for cancer research

Micro-radiotherapy (micro-RT) system is specially designed for small animal (cancer cell) irradiation for basic and translational cancer research. We use carbon nanotube (CNT) field emission technology to develop a novel micro-RT system for image-guided high precision irradiation that is similar to the state of the art radiotherapy which our cancer patients receive today at mouse scale. Through the field emission control of its individually addressable x-ray pixel beams the micro-RT system electronically shapes the radiation field and forms intensity modulation pattern. In this paper, we present the development of a carbon nanotube field emission cathode array chip--a key component for our novel micro-RT system. The prototype micro-RT CNT field emission cathode array chip has 5 x 5 individually addressable cathode pixels that are 1 mm in diameter and 2 mm in pitch. An individual CNT cathode pixel is predicted to generate a dose rate in the order of 100 cGy/min at the center of the irradiated mouse based on our Monte Carlo simulation. The temporal and spatial resolutions of the micro-RT system are expected to be approximately ms level and < 2 mm respectively.

Fabrication and Characterization of Individually Controlled Multi-Pixel Carbon Nanotube Cathode Array Chip for Micro-RT Application for Cancer Research

We report here the development of a new carbon nanotube (CNT) field emission multi-pixel cathode array chip, a vital component for the multi-pixel beam x-ray micro-radiotherapy (micro-RT) system under development in our group for cancer research. The CNT field emission cathode array chip has up to 25 (5 × 5) individually addressable cathode pixels, each 1 mm in diameter and with center-to-center distance of 2 mm. The fabrication is a two-step process: first a Cr/Cu electrical contact was fabricated on Si substrates with a 5 μm SiO(2) dielectric layer using photolithography; and second the CNTs were selectively deposited on 1 mm-diameter predefined Cr/Cu contact dots by using a combined photolithography/electrophoresis deposition technique. The electron pixel beams produced from the multi-pixel array chips are uniform and individually controllable. Each pixel beam is expected to generate a dose rate in the order of 100 cGy/min based on our Monte Carlo simulations.

MO‐D‐330A‐07: A Stationary Scanning X‐Ay Imaging System Based On Carbon Nanotube Field Emitters

Purpose: Most tomographic imaging systems available today use a single x‐ray source and multiple projection images are obtained by rotating the x‐ray source around the object. Therefore the data acquisition rate is limited by the gantry rotation speed, which is approaching the physical limit. We proposed to develop a novel stationary scanning x‐ray imaging system based on carbon nanotube field emission x‐ray (FEX) technology. Instead of a single x‐ray source the proposed system is based on a multi‐pixel FEX source. The new scanner promises a dramatically faster data acquisition rate by reducing or totally eliminating the mechanical motion. Method and Materials: We have constructed a prototype stationary scanning x‐ray imaging system with an array of 9 individually addressable x‐ray source pixels, each of which can produce a different projection image of the object. The core of this novel x‐ray imaging technology is a gated carbon nanotube field emission cathode array. By programming the gate voltage of the cathode array, the multi‐pixel x‐ray source can generate an electronically triggered scanning x‐ray beam and produce multiple projection images from different viewing angles without mechanical motion. A Hamamatsu C7921 flat panel x‐ray sensor was used to collect all 9 projection images. Results: Tomosynthesis images of a mouse and a standard breast‐imaging phantom (Stereotactic Needle‐biopsy Tissue Equivalent Phantom, Nuclear Associates, NY) using the prototype stationary scanning x‐ray imaging system are acquired. Tomosynthesis reconstructions were applied to the breast phantom. The slice images reconstructed using an iterative reconstruction algorithm clearly show the internal structures of the breast‐imaging phantom at different depths. Conclusion: We have developed a stationary scanning x‐ray imaging system using a carbon nanotube based multi‐pixel FEX source. The mechanical motion free approach can lead to a faster and simplified tomographic imaging system.Conflict of Interest: Research partially supported by Xintek Inc.