One of the principal advantages of a solar thermal propulsion system as compared to a conventional chemical propulsion one is high specific impulse which is proportional to the square root of a propellant temperature. Obviously, next generation solar propulsion and bimodal systems must take advantage of high and ultra-high temperatures. This requires use of an appropriate energy conversion system capable to take advantage of high temperature potentially achievable in a solar receiver. High efficiency and power density of a high temperature thermionic