Anisullah Baig

System Design Analysis of a 0.22-THz Sheet-Beam Traveling-Wave Tube Amplifier

The primary constituents of a 0.22-terahertz (THz) sheet-beam traveling-wave tube (TWT) amplifier, composed of a staggered double grating array waveguide, have been designed for broadband THz operation (~ 30%) using the fundamental passband (TE-mode). Currently, we are looking into the possibility of a pulsed low-duty test of this device as a proof of principle (POP) and have been making efforts to construct the system. The optimally designed input coupler has ≤ 1 dB insertion loss at 0.22 THz with ~ 75 GHz (34%) 1-dB matching bandwidths. A thin mica RF window provides a coupling bandwidth spanning multiple octaves. The collector is designed to have a jog for collecting the spent beam along the RF path coupled to the output RF window. Computer simulations show that the collector hybridized with a WR-4 window has ~ 60 GHz matching bandwidth with ~ - 0.5 dB insertion loss at 0.22 THz. The hybrid periodic permanent-magnet design combined with the quadrupole magnet (PPM-QM), intended for low-duty pulse operation in a proof-of-concept experiment, allows the elliptical sheet beam from an existing gun (25 : 1 aspect ratio) to unoptimized gun to have 73% beam transmission. The POP pulsed test is designed to be matched to our existing system, which will thereby tolerate beam transmission. However, a proper gun for the sheet-beam tunnel of the designed circuit will provide much better transmission. In our prior works, we successfully proved at W-band that the magnet design provided >; 99% beam transmission of a 10:1 aspect ratio sheet beam. Most of the TWT circuit components have been designed, and currently, a full simulation modeling effort is being conducted.

Modeling Investigation of an Ultrawideband Terahertz Sheet Beam Traveling-Wave Tube Amplifier Circuit

Extensive numerical analysis has demonstrated that a terahertz (H-band) sheet beam traveling-wave tube (TWT) amplifier circuit, composed of a staggered double grating array waveguide, has very broad bandwidth (~30%) of the fundamental passband (TE mode) with a 7:1 aspect ratio sheet beam without excitation of n = 1 space harmonic backward-wave modes. Particle-in-cell (PIC) simulations utilizing MAGIC3D and CST PS predict that the designed circuit produces ~150-300-W output power, corresponding to ~3%-5.5% intrinsic electronic efficiency (~35-38-dB saturated gain from 50-mW input driving power), over ~25% bandwidth, which is in good agreement with CHRISTINE 1-D code predictions. Simulations, using a perfectly matched layer boundary (~ -30-dB return loss), show that the circuit stably operates without noticeable oscillation. With a more realistic matching condition (~ -9.5-dB return loss), it becomes unstable. However, simulations show that the incorporation of an attenuating sever with tapered conductivity suppresses the instability in tube operation.

UV Lithography and Molding Fabrication of Ultrathick Micrometallic Structures Using a KMPR Photoresist

By using a novel negative-tone photoresist, KMPR, we have investigated ultraviolet (UV) lithographic microelectroforming fabrication of ultrathick metallic microstructures ( ¿ 400 ¿m). Scanning coating spin speed together with the film thickness and uniformity has been characterized at low spin speed from 1000 to 200 r/min. Based on the film profile characterization, the single-spin lithography conditions for a 400-¿m-thick electroforming mold are optimized by scanning process parameters of UV exposure energy and bake temperature and time. SEM-measured dimensional accuracy and sidewall verticality of the optimized thick KMPR mold are ¿ ¿ 3 ¿m and 90° ±1°, respectively, which are comparable to those of SU8 molds. The SEM analysis of the patterned film and the electroformed structure has shown that the submillimeter-thick KMPR features have ~ 5-10:1 aspect ratio. The sidewall surface roughness of the copper deposition is locally measured to be about ~ 50-100 nm by atomic force microscopy, which is significantly smoother than that resulting from other mechanical machining approaches. This novel photoresist enables the lithographic molding microfabrication process to mass produce plastic and metallic microcomponents for various microelectromechanical systems applications.

0.22 THz wideband sheet electron beam traveling wave tube amplifier: Cold test measurements and beam wave interaction analysis

In this paper, we describe micro-fabrication, RF measurements, and particle-in-cell (PIC) simulation modeling analysis of the 0.22 THz double-vane half period staggered traveling wave tube amplifier (TWTA) circuit. The TWTA slow wave structure comprised of two sections separated by two sever ports loaded by loss material, with integrated broadband input/output couplers. The micro-metallic structures were fabricated using nano-CNC milling and diffusion bonded in a three layer process. The 3D optical microscopy and SEM analysis showed that the fabrication error was within 2–3 μm and surface roughness was measured within 30–50 nm. The RF measurements were conducted with an Agilent PNA-X network analyzer employing WR5.1 T/R modules with a frequency range of 178-228 GHz. The in-band insertion loss (S21) for both the short section and long section (separated by a sever) was measured as ∼−5 dB while the return loss was generally around ∼−15 dB or better. The measurements matched well with the S-matrix simulation...

Nano CNC milling of two different designs of 0.22 THz TWT circuits

To satisfy the growing interest in high power (∼1–200 W) THz sources, our research group has been working on various technologies to miniaturize vacuum electronic devices while keeping the power levels high and offering wide instantaneous bandwidth operation. We have manufactured two different designs of 0.22 THz circuit structures including input/output couplers to WR4 waveguide. We have used nano CNC milling as our primary approach and we were able to achieve excellent surface finish and meet the high tolerance requirements of the high frequency wide bandwidth traveling wave tube circuit.

Experimental Study of Multichromatic Terahertz Wave Propagation through Planar Micro-Channels

Previous theoretical and numerical studies [Y. M. Shin and L. R. Barnett, Appl. Phys. Lett. 92, 091501 (2008) and Y. M. Shin et al., Appl. Phys. Lett. 93, 221504 (2008)] have reported that a planar micro-channel with an asymmetric corrugation array supports strongly confined propagation of broadband THz plasmonic waves. The highly broad spectral response is experimentally demonstrated in the near-THz regime of 0.19–0.265 THz. Signal reflection and transmission tests on the three designed micro-channels including directional couplers resulted in a full-width-half-maximum bandwidth of ∼50-60 GHz with an insertion loss of approximately −5 dB, which is in good agreement with simulation data. These micro-structures can be utilized for free electron beam and electronic/optic integrated devices.

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.

Beam transport modeling of PPM focused THz sheet beam TWT circuit

We report periodic permanent magnet modeling for an efficiently focused sheet beam (∼7:1) transport achieved in a relatively long (40 mm) drift tube. To perform a quick pulsed device (TWT) test, it is planned for a high aspect ratio electron gun (12:1, designed by CPI1 for solenoidal field focused beam) to be used for currently designed PPM focused ∼7:1 aspect ratio sheet beam, with drift tube to act as an aperture to the incoming planar stream of electrons. Extensive simulations have been conducted to elucidate beam trajectory analysis and parasitic beam interception with drift tube walls. Two different permanent magnet materials were considered (a) SmCo (Br =1.15T) and(b) NdFeB (Br =1.4T). For Case (a), we obtained beam transmission of 80% with spacing between parallel magnet stacks of 2.6 mm and for case (b) we obtained beam transmission of ∼73.35% that includes tuning voltage on Focus Electrode, for an increased stack spacing of 4 mm that is crucial, in order to provide sufficient space for the slow wave structure/couplers fabrication. This corresponds to 190 mA transmitted current. Particle in Cell (PIC) simulations were also conducted to analyze beam-wave interaction at a significantly lower current ∼ 90 mA (approx 50% of what anticipated) to analyze a worst case scenario. The currently designed actual PPM B-Field was employed. The PIC simulation result was very promising, showing ∼ 10 dB gain and maximum output of 16 W for an input drive of 1 W at 0.22THz. For 190 mA transmitted current, the anticipated maximum output power is 75 W with a gain of 17 dB.

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.

Numerical modeling analysis of 0.22 THz sheet beam TWT circuit

Extensive numerical analysis has resulted in the conclusion that the THz (H-band) sheet beam traveling wave tube (TWT) amplifier circuit, comprised of a staggered double grating array waveguide, has the very broad bandwidth (∼ 30 %) of the fundamental passband (TE-mode) with a 7 : 1 aspect ratio sheet beam without excitation of n = 1 space harmonic backward wave modes. Particle-in-cell (PIC) simulations utilizing MAGIC3D and CST PS predict that the designed circuit produces 150 ∼ 300 W output power, corresponding to ∼ 3–5.5 % intrinsic electronic efficiency (35 ∼ 38 dB gain from 50 mW input driving power), over ∼ 25 % bandwidth, which are in good agreement with Christine-1D code simulation results. Simulations, using a perfectly matched layer (PML) boundary (∼ −30 dB return loss), show the circuit stably operates without noticeable oscillation. With a more realistic matching condition with ∼ −9 dB return loss, it becomes unstable. However, simulations show that the incorporation of a sever with tapered conductivity fully suppresses the instability in tube operation.