Alexander Spear

Scandate Dispenser Cathode Fabrication for A High-Aspect-Ratio High-Current-Density Sheet Beam Electron Gun

A high-current-density scandate tungsten dispenser cathode was used for the demonstration of a 25 : 1-aspect-ratio 750-A/cm2 -current-density sheet beam for the Defence Advanced Research Project Agency High-Frequency Integrated Vacuum Electronics (HiFIVE) program intended for the realization of a wideband ( ~ 30%) 220-GHz traveling wave tube. The elliptical cathode with homogeneous microstructure was made from 1-2-μm-size tungsten powder added with nanosized Scandia using the sol-gel method; it has a current density of up to 160 A/cm2 at 1050 °C. A sheet beam gun analyzer was built to test the terahertz sheet beam gun and determine the size and current density of a sheet electron beam produced by the impregnated scandate tungsten dispenser cathode. A sheet electron beam with an aspect ratio of 12.5 : 1 with a current density exceeding 375 A/cm2 has been obtained using a BVERI impregnated scandate dispenser cathode without magnetic compression; further magnetic field compression would give the final current density of 750 A/cm2.

Development of a 220 GHz 50 W sheet beam travelling wave tube amplifier

We report on progress in developing a travelling wave tube amplifier with significant gain and power at 220 GHz. This paper provides an overview of the program, describing fabrication and test of slow-wave structures with bandwidths exceeding 50 GHz centered at 220 GHz, the production of a sheet electron beam, development of a solid state preamplifier delivering 50 mW to the tube with > 17 dB of gain and beam-wave simulation of the entire circuit leading to expected output powers of over 50 W. Two further papers from the group are also submitted to IVEC: from UC Davis describing the interaction structure fabrication and hot test, and from CPI describing the sheet electron beam, TWT design and beam - wave simulations. The tube is currently under test and results will be reported in this paper.

Experimental characterization of LIGA fabricated 0.22 THz TWT circuits

In this paper we report precision MEMS Fabrication using novel LIGA technique for 0.22 THz micro-metallic staggered double-vane TWT circuits. For this high aspect ratio structure negative tone photo-resist KMPR was used. The entire fabrication process starting from spin coating, UV-lithography, electroforming and mold removal processing was fully characterized. Finally, the high tolerance TWT structure with smoothness (50–80 nm) was aligned in a specially engineered fixture for RF measurements in the BWO range 165–270GHz. The experiment showed excellent transmission 5–8 dB in frequency range 210–265 GHz. This result showed a distinct potential of applying this precision fabrication technique for the mass production of THz sources.

8.4: Terahertz sheet beam gun analyzer

Experimentally characterizing actual micron sized sheet beams is essential to the development of terahertz sheet beam guns and devices. We have tested a terahertz sheet beam gun and produced 3D electron beam density plots. The terahertz sheet beam analyzer was designed to automatically collect high resolution scan data using LabVIEW software. The collected data were plotted to show the electron current density profile and size of the sheet beam.

1.3: 220 GHz 50 W sheet beam travelling wave tube amplifier

In this paper we report on a development program aimed at producing a travelling wave tube amplifier with significant gain and power at 220 GHz in a compact and lightweight package. This paper provides an overview of the program, describing development of nanoparticle scandate cathodes with current densities exceeding 100 A/cm2, MEMS fabrication and test of mm-wave waveguides and slow-wave structures with bandwidths exceeding 50 GHz centered at 220 GHz, and the production of a sheet electron beam with a current density exceeding 230 A/cm2 and an aspect ratio of 26∶1. These components may allow us to produce a 220 GHz, 50W high power amplifier with a hot bandwidth of > 20 GHz and a power-bandwidth product > 1000 W-GHz.

220 GHz ultra wide band TWTA: Nano CNC fabrication and RF testing

Precision fabrication and RF testing of a 220 GHz GHz sheet beam TWTA based on the double vane half-period staggered slow wave structure design[1]is reported. NanoCNC Milling technology[2] was employed to precision fabricate the entire TWTA circuit in bulk copper, incorporating input and output couplers, sever ports, and the slow wave structure with matching. To accommodate an overhead input/output coupler design, the TWTA circuit was fabricated in a three layer process with dimensional tolerance of within ~1-2 μm and surface roughness ~50 nm. The TWTA circuit was diffusion bonded (at UC Davis) within an accuracy of less than 10 μm between two circuit halves. Initial TWTA cold tests employing a BWO based scalar network analyzer showed an in-band insert ion loss of ~ -5 dB with a bandwidth exceeding 50 GHz. The in-band return loss was <; -12 dB. These measurements were subsequently confirmed using an Agilent PNA-X VNA. The hot test setup is ready to test the TWTA being baked at CPI and the latest test results will be presented at the conference and added to the abstract.

MEMS fabrications of broadband epsilon negative (ENG) metamaterial electronic circuit for 0.22 THz sheet beam TWT application

In the course of the DARPA HiFIVE (High Frequency Integrated Vacuum Electronics) program, we have investigated various MEMS techniques for micro-fabrication of electronic circuits built in a 0.22 THz traveling wave tube (TWT) amplifier. The so-called “Barnett-Shin” circuit [1] is a TE-mode waveguide structure of ultra wideband epsilon negative (ENG) metamaterial, comprised of a half-period staggered double grating array. Our prior modeling analyses of the device with a 20 keV and 250 mA sheet beam demonstrated ≥ 12 dB/cm growth rate and the saturated conversion efficiency of 3–5.5% (150–275 W) over 25 % (∼ 60 GHz) dynamic bandwidth. Four different MEMS techniques are being developed for circuit fabrication: UV LIGA with (1) KMPR and (2) SU8, (3) high precision CNC nano-machining, and (4) silicon-molded electro-deposition. In the early stages of the program, we performed proof-of-concept experiments at Ka-band, which demonstrated 25 % band response of the ENG metamaterial circuit, and numerically analyzed the potential deleterious effects of deviation of critical dimensions due to fabrication errors on signal response of the fabricated circuits. Currently, the MEMS fabricated 0.22 THz circuits are being investigated by 3D optical microscope and scanning electron microscope (SEM). Also, a scalar network S-parameter analyzing system has been prepared for characterization of the micro-fabricated circuits.

Multidimensional Visualization of MHD and Turbulence in Fusion Plasmas

Quasi-optical imaging at sub-THz frequencies has a major impact on fusion plasma diagnostics. Millimeter-wave imaging reflectometry utilizes microwaves to actively probe fusion plasmas, inferring the local properties of electron density fluctuations. Electron cyclotron emission imaging is a multichannel radiometer that passively measures the spontaneous emission of microwaves from the plasma to infer local properties of electron temperature fluctuations. These imaging diagnostics work together to diagnose the characteristics of turbulence and magnetohydrodynamic activity. Important quantities, such as amplitude and wavenumber of coherent fluctuations, correlation lengths and decorrelation times of turbulence, and poloidal flow velocity of the plasma, are readily inferred.