Vacuum Electron Device

The millimeter wave to THz region (0.1 - 1 THz) of the electromagnetic spectrum possesses unique properties which lead to potential applications ranging from non-intrusive security, high data rate communications, medical imaging, and industrial quality control. However, the unavailability of oscillators/amplifiers with adequate power in a compact package has led to the expression 'THz Gap'. Microwave Vacuum Electron Device (MVED) technology is considered by many to be the most promising approach for a THz source.

Vacuum Electronics Devices are devices can transfer kinetic energy of the collision-less ballistic motion of electrons in vacuum to electromagnetic (RF/Microwave) energy. The heart of a Microwave Vacuum Electron Devices (MVEDs) is a high current emission cathode. A thermionic cathode emits a high current density beam of electrons when heated. The electron beam is accelerated by the applied high voltage between the cathode and anode. Then, the electrons go into the high frequency structure, where the interaction between the electrons and the electromagnetic field takes place. After the interaction between the electrons and the electromagnetic field, the excited/amplified electromagnetic energy can be exported by the coupler, at the same time, the spent electron beam goes to a copper block called collector. There are various kinds of MVEDs: Klystrons, Traveling Wave Tube Amplifiers (TWTAs), Backward Wave Oscillators (BWOs), Crossed-Field Amplifiers and Oscillators, Electron Masers and Free Electron Lasers (FELs).

The electron gun and periodically cusped magnet stack of the original Stanford Linear Accelerator Center designed W-band sheet beam klystron circuit, which exhibited poor beam transmission (~55%), have been carefully investigated through theoretical and numerical analyses taking advantage of three-dimensional particle tracking solvers. The redesigned transport system is predicted to exhibit 99.76% cold and 97.38% thermal beam transmission, respectively, under space-charge-limited emission simulations. The optimized design produces the required high aspect ratio 10:1 sheet beam with 3.2 A emission current with highly stable propagation. In the completely redesigned model containing all the circuit elements, more than 99% beam transmission is experimentally observed at the collector located about 160 mm distant from the cathode surface. Currently, we are designing more devices based on this magnet.

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). 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 un-optimized gun to have 73% beam transmission. The POP pulsed test is designed to be matched to our existing system. However, a proper gun for the sheet-beam tunnel of the designed circuit will provide much better transmission. In our prior work, 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.

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 analysis, 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 0.15 A, ~8.5:1 aspect ratio sheet beam with elliptical cross section of 0.60 mm (width) by 0.07 mm (height), whose current density exceeds 400 A/cm², through a 45 mm long circuit, whose beam tunnel is 0.7 mm by 0.12 mm. CST PARTICLE STUDIO is used to model the beam transport system and verify the analytic 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.

Backward wave oscillators (BWOs) are still the best devices to provide local oscillator power or to be used as power sources at terahertz frequencies. In particular, the BWO is the sought after solution for LO power to extend the investigation region in plasma diagnostic applications. The inherent low efficiency is balanced by a simple structure and wide tuning range. However, at frequencies in the sub-millimeter range, the small dimensions pose significant fabrication difficulties and long assembly time. Novel effective solutions of BWOs are designed with performance up to 1 Watt at 0.346 THz based on the double staggered grating (DSG) and on the double corrugated waveguide (DCW).