High-Efficiency RF Amplifiers with Plasma Accommodation for Fusion Plasma Heating

Nuclear fusion power reactors will require low-cost, high-efficiency, reliable, and easy-to-maintain power electronics for plasma heating. Radiofrequencies in the 0.5 to 150 MHz range are relevant to plasma heating techniques including Ion Cyclotron Resonance Heating (ICRH) and electron High Harmonic Frequency Heating (HHFW). Current RF heating systems use linear amplifier vacuum tubes of limited efficiency and require manual tuning and/or feedback control for effective coupling to the plasma. Solid-state RF switching amplifiers that are high-efficiency with circuitry able to handle variable load impedance would improve wall-plug efficiencies of nuclear fusion reactors and reduce the complexity of operation with automatic impedance matching. Our team under ARPA-E GAMOW has proven the capability of building efficient Class-E RF switching amplifiers using wide bandgap (WBG) semiconductor devices with a reactance steering network (RSN) to match variable load impedance. So far, only two Class-E amplifier circuits connected by a single RSN have been tested. In order to scale to high power operation (~MW), the power-combining of multiple boards with RSNs will need to be addressed. The goal of this project is to develop these boards to be power-combinable and operate at high power densities in order to achieve a high-power RF system for fusion plasma heating applications. It is anticipated that the RF boards will achieve high efficiencies due to their Class-E switching amplifier topology and high plasma coupling due to the embedded reactance steering design in the circuitry. The reactance steering network could also allow the amplifiers to follow the time-varying plasma impedance, thus providing plasma accommodation. High board efficiency and plasma coupling are critical to advancing fusion reactors by increasing their net energy efficiency. The frequencies demonstrated so far range from 100ĺs of kHz to 10ĺs of MHz, making this invention relevant to plasma heating techniques including ICRH and HHFW. The testing of power-combining of these boards will ascertain we can scale effectively to many-board systems. During Phase I, we will (1) test the power-combining of two low-power (< 0.5 kW) Class-E boards with reactance steering networks, and (2) design a 30 MHz Class-E RF amplifier board and assembly for ICRH or HHFW, optimized for power density with a representative model of the plasma load and matching networks. Our approach to power combining would be a tree network in which we combine two power amplifiers with a reactance steering network, and then combine two of such structures together with another reactance steering network. This will retire risk for Phase II where we would combine two, and then ~10, high-power boards (the precise number of boards will depend on the maximum achievable power per board, to be determined in Phase I). During Phase II, PFS will prototype and test the optimized high-power-density RF board with reactance steering designed in Phase I. Power combining of two high power-density boards will then be tested, followed by construction and testing of a chassis combining 10ĺs of boards, to achieve power ranges approaching relevance to fusion plasma heating (~1 MW). Testing will be performed on a load representative of an ICRH or HHFW antenna. The technology under this SBIR is applicable to RF-heated fusion systems including those of companies (TAE Technologies and Realta Fusion) and national laboratories (Oak Ridge National Laboratory and Princeton Plasma Physics Laboratory). The proposed innovation extends to more general RF-heated plasma systems relevant to space propulsion and semiconductor processing.