Accelerator, Laser, and Coherent Optoelectronics R&D Lab (ALCOL)
High Power Millimeter Wave Sources
Continuing on the Phase-I awarded in 2012, we started Phase-II of DOD (Air Force) SBIR Project in Sept, 2013 (Contract #: FA8650-13-C-1604, “V-Band MPM with a Novel Overmoded V-band TWT”). This project aimed at the development of innovate microwave power module (MPM) (Figure 1) approaches for providing additional SATCOM spectrum for support of Beyond Line of Sight (BLOS) communications for Unmanned Aerial Vehicles (UAVs). A GRA (Andrew Palm, Z1717363) in Master Degree program has been involved in the project since 2014 spring semester and he is currently scheduled with thesis defense in 2015 spring semester. A scope of the project includes electrical design of traveling wave structure, proof of design concept using particle-in-cell simulations, design optimization of sub-components (electron gun, focusing magnet, multistage depressed collector, etc), system level optimization, and mechanical design/thermal modeling. In the project, the main scope of the project tasks was extended to including topics of multi-beam and sheet beam devices and HOM RF structures. Research outcomes accomplished during this period of time were disseminated by being presented in International Vacuum Electronics Conference (IVEC14) and International Conference on Infrared, Millimeter, and THz Waves (IRMMW-THz14).
Figure 1: Sample MPM components where traveling wave tube provides main source of amplification
This project was funded by Bridge12 and the Air Force to design a compact high frequency Traveling Wave Tube amplifier (TWTA) for the upper V-band (73.5 GHz) frequency.
Traveling wave tubes (TWT's) have been around since WW2 predominantly when high power and high frequency RF amplification is needed. TWT's operate by exchanging energy from a DC electron beam and the RF that is to be amplified. Amplification of RF is accomplished by desiging the dispersive waveguide or Slow wave structure (SWS) such that the axial (same direction of electron beam) velocity of the RF is close to the speed of the electrons traveling through the middle of the SWS. The inside of the SWS is vacuum and thus the electrons will be traveling without dispersion through the tube. while the dispersion of the RF depends on the frequency. By correctly designing the dispersion of the SWS, the velocities will be very close and will undergo synchronization. The electrons in the electron beam will experience a static field from the voltage of the RF and begin to bunch in accordance to this electric field essentially impressing the signal from the RF onto the electron beam. When the electrons begin to bunch, however, the electric fields in the tube begin to alter and will create a phase mismatch between the current of the electron beam and the voltage of the RF. This creates acceleration and deceleration regions that the electrons will go into. When the electrons enter acceleration regions, the electrons will speed up and take energy from the RF. When the electrons enter the deceleration region, they will slow down and give energy to the RF. The latter is what condition tube designers want to keep for as long as possible.
Figure 2: Slow wave structure used was Staggered Double Grating Array (SDGA)
The research conducted focused on desiging as small and lightweight TWTA as possible using the SDGA slow wave structure. The project requirements were high power (>50 Watts), broad bandwidth (5 GHz bandwidth), high frequency (71-76 GHz), and high gain (30 dB). Complete design of the tube was needed as well including all the components seen in figure XX. This includes all components to create, focus, and recover the electron beam as well as all components to inject and remove the RF from the SWS.
Project goals was accomplished by extending the velocity synchronization condtions by adjusting how dispersive the circuit in accordance with the lowered electron beam energy. As can be seen in FIgure XX, the output power was dramatically increased by extending the velocity synchronization conditions.
Figure 4: Temporal power distribution of RF and Spatial Energy distribution of Modulated Electron Beam
Figure 2: Complete TWT design along with each component designed
The project achieved all scopes of work necessary as each component of the TWT was designed and optimized in regards to the scope of work. Compact output power was achieved as the Maximum output power was achieved by increasing axial velocity synchronization conditions between the electron beam and the RF. Through optimization, the circuit reached 80 output power (29 dB gain) with 7.5 3dB bandwidth. This proved that high gain, high frequency RF amplification over a wide bandwidth which is necessary for compact coherent radiation sources of the future. This research was completed by Andrew Palm and Professor Young-min Shin
Figure 5: Optimized Output Power and Gain Curve for one and two staged TWT