Branko Popovic

Nanoscale Surface Roughness Effects on THz Vacuum Electron Device Performance

Vacuum electron devices are the most promising solution for the generation of watt-level power at millimeter wave and terahertz frequencies. However, the three-dimensional nature of metal structures required to provide an effective interaction between an electron beam and THz signal poses significant fabrication challenges. At increasing frequency, losses present a serious detrimental effect on performance. In particular, the skin depth, on the order of one hundred nanometers or less, constrains the maximum acceptable surface roughness of the metal surfaces to be below those values. Microfabrication techniques have proven, in principle, to achieve values of surface roughness at the nanometer scale; however, the use of different metals and affordable microfabrication techniques requires further investigation for a repeatable quality of the metal surfaces. This paper compares, for the first time, the nanoscale surface roughness of metal THz waveguides realized by the main microfabrication techniques. In particular, two significant examples are considered: a 0.346-THz backward wave tube oscillator and a 0.263-THz traveling wave tube.

THz Backward-Wave Oscillators for Plasma Diagnostic in Nuclear Fusion

Understanding of the anomalous transport attributed to short-scale length microturbulence through collective scattering diagnostics is key to the development of nuclear fusion energy. Signals in the subterahertz (THz) range (0.1-0.8 THz) with adequate power are required to map wider wavenumber regions. The progress of a joint international effort devoted to the design and realization of novel backward-wave oscillators at 0.346 THz and above with output power in the 1 W range is reported herein. The novel sources possess desirable characteristics to replace the bulky, high maintenance, optically pumped far-infrared lasers so far utilized in this plasma collective scattering diagnostic. The formidable fabrication challenges are described. The future availability of the THz source here reported will have a significant impact in the field of THz applications both for scientific and industrial applications, to provide the output power at THz so far not available.

Nano-CNC Machining of Sub-THz Vacuum Electron Devices

Nano-computer numerical control (CNC) machining technology is employed for the fabrication of sub-THz (100-1000 GHz) vacuum electron devices. Submicron feature tolerances and placement accuracy have been achieved and surface roughness of a few tens of nanometers has been demonstrated providing high-quality radio frequency (RF) transmission and reflection parameters on the tested circuit structures. Details of the manufacturing approach are reported for the following devices: W-band sheet beam (SB) klystron, two designs of a 220-GHz SB double-staggered grating traveling wave tube (TWT), 263-GHz SB TWT amplifier for an electron paramagnetic resonance spectrometer, 346-GHz SB backward wave oscillator for fusion plasma diagnostics, 346-GHz pencil beam backward wave oscillator, and 270-GHz pencil beam folded waveguide TWT self-driving amplifier. Application of the nano-CNC machining to nanocomposite scandate tungsten cathodes as well as to passive RF components is also discussed.

THz backward-wave oscillators for plasma diagnostic in nuclear fusion

Summary form only given. The understanding of plasma turbulence in nuclear fusion is related to the availability of powerful THz sources and the possibility to map wider plasma regions. A novel approach to realize compact THz sources to be implemented in the plasma diagnostic at NSTX experiment (Princeton Plasma Physics Laboratory, USA) is reported.Two novel 0.346 THz Backward-Wave Oscillators (BWOs) have been designed and are presently in the fabrication phase. One BWO is based on the Double Staggered Grating (DSG) that supports a sheet electron beam to provide a high output power; the second BWO is based on the Double Corrugated Waveguide (DCW) that supports a cylindrical electron beam generated by a conventional Pierce gun. The performance of both the BWOs was computed by Particle-in-cells (PIC) simulations. The DSG-BWO provides about 1W of output power with a beam current of 10 mA and a beam voltage of 16.8 kV. The DCW-BWO provides 0.74W output power with 10 mA beam current and 13 kV beam voltage. The DSG and the DCW have been realized by state of the art prototype nano-CNC milling machine (DMG Mori-Seiki) that permits one to achieve performance, in term of cost and surface finishing, unavailable with any other technology. It is the first time that this technique is applied to structures above 0.3 THz. The high output power of both the BWOs demonstrates the importance of novel approaches in the emerging field of THz vacuum electron devices.

Design of a compact and high performance 263 GHz SB-TWT circuit

A compact, high performance staggered double grating (SDG) slow wave structure (SWS) circuit has been designed for a 263 GHz sheet beam traveling wave tube amplifier (SB-TWTA) for use in an electron paramagnetic resonance (EPR) spectrometer system being developed at UC Davis under National Science Foundation (NSF) Multidisciplinary Research Initiative (MRI) funding. By adopting a phase matching choke, the input/output structure exhibits superior low frequency band edge transmission characteristics. A compact sever is employed to broaden the bandwidth and reduce the overall circuit length thereby benefiting the S-parameters and the electron optical system design. The circuit was fabricated using Nano-CNC milling technology and the transmission and reflection parameters of the circuit were measured prior to diffusion bonding. In order to extend the operating bandwidth of the device, a hybrid mode transmission pill-box CVD window is employed. The Computer Simulation Technology (CST) Microwave Studio (MWS) results indicate the maximum reflection losses (S11) for the circuit over 245-285 GHz are -15 dB, while the transmission coefficient (S21) exceeds -0.5 dB. The maximum VSWR for the pill-box window within a 36 GHz bandwidth is less than 1.25.

Electron Beam Transport System for 263-GHz Sheet Beam TWT

The efficient transport of high current density electron beams is essential for the success of millimeter wave and terahertz vacuum devices. To achieve high current density sheet electron beam transmission, an advanced periodic cusped magnet-tunable quadrupolar magnet (PCM-TQM) is proposed to improve the transmission factor of the sheet beam. Through numerical a nalysis, the TQM is shown to provide the ability to be compatible with a larger initial transverse velocity spread and match the current density increasing caused by beam bunching. This new PCM-TQM focusing system is employed to transport a 19 kV, 0.15 A, ~8.5:1 aspect ratio sheet beam with an elliptical cross section of 0.60 mm (width) × 0.07 mm (height), whose current density exceeds 400 A/cm2, through a 45-mm long circuit, whose beam tunnel is 0.7 mm × 0.12 mm. CST PARTICLE STUDIO is used to model the beam transport system and verify the analytical results. With the tunable feature of the QM poles, the “no RF” and “with RF” beam transmission factors under the focusing magnetic field of the advanced PCM-TQM are predicted to achieve 99.3% and 97.6%, respectively.

Performance of a Nano-CNC Machined 220-GHz Traveling Wave Tube Amplifier

We report on hot test measurements of a wide-bandwidth, 220-GHz sheet beam traveling wave tube amplifier developed under the Defense advanced research projects agency (DARPA) HiFIVE program. Nano-computer numerical control (CNC) milling techniques were employed for the precision fabrication of double vane, half-period staggered interaction structures achieving submicrometer tolerances and nanoscale surface roughness. A multilayer diffusion bonding technique was implemented to complete the structure demonstrating wide bandwidth (>50 GHz) with an insertion loss of about −5 dB achieved during transmission measurements of the circuit. The sheet beam electron gun utilized nanocomposite scandate tungsten cathodes that provided over 438-A/cm2 current density in the 12.5:1 ratio sheet beam. An InP HBT-based monolithic microwave integrated circuit preamplifier was employed for TWT gain measurements in the stable amplifier operation region. In the wide-bandwidth operation mode (for gun voltage of 20.9 kV), a gain of over 24 dB was measured over the frequency range of 207–221 GHz. In the high-gain operation mode (for gun voltage of 21.8 kV), over 30 dB of gain was measured over the frequency range of 197–202 GHz. High-power tests were conducted employing an extended interaction klystron.

Design and fabrication of a sheet beam BWO at 346 GHz

Applications such as fusion diagnostics, imaging and security systems require high frequency sources. As part of a joint international effort regarding novel THz BWOs, a double staggered grating sheet beam BWO at 346 GHz is underdevelopment and being fabricated. Design work has been done on various components, with nano machining and cold testing of the slow wave structure completed.

Development of nano machining techniques to bridge the terahertz gap

Research conducted in parallel with the construction of various vacuum electronic devices, including a 220 GHz Sheet Beam Traveling Wave Tube (SBTWT), a 263 GHz SBTWT, and two 346 GHz Backward Wave Oscillators (BWOs), has demonstrated that data garnered from the manufacturing process helps improve the performance, reduce the time to build, and lower the cost of subsequent devices. The data collected from metrology, microscopy, and manufacturing control are also critical to improving future device builds and provide powerful insight into the nature of fabricating high performance vacuum electronics.

Magnetic fusion energy plasma diagnostic needs novel THz BWOs

The development of collective scattering diagnostics is essential for understanding of the anomalous transport attributed to short scale length microturbulence which poses a threat to the development of nuclear fusion reactors. Signals in the sub-THz range (0.1 - 0.8 THz) with adequate power are required to probe the plasma. A joint international effort is therefore devoted to the design and realization of novel backward wave oscillators at 0.346 THz and above with output power in the 1 Watt range to replace the bulky, high maintenance optically pumped FIR lasers so far utilized for this plasma diagnostic.