Radio-Frequency Heating & Current Drive

Electron Cyclotron Heating (ECH) technology research focuses on the high-power microwave sources and transmission lines needed for EC plasma heating. The electron cyclotron frequency (fc = 28 GHz /T) requires microwave sources in the range of 80 to170 GHz. Research is conducted on 110 to 170 GHz gyrotrons that are capable of producing continuous output power at the megawatt level.  Research is also conducted on low-loss, corrugated metallic waveguides needed for transmission to the plasma.

A 6 MW, 110 GHz gyrotron system is currently in operation at General Atomics (GA) using gyrotrons built by Communications and Power Industries (CPI) and transmission lines built by GA. The US will supply the transmission lines for the 24 MW ITER ECH system operating at 170 GHz. A test stand for ITER transmission line components is under construction at Oak Ridge National Laboratory (ORNL). Additional research is conducted by universities (MIT, Univ. MD; Univ. WI) and small businesses.

Challenges in ECH technology include increasing the power level of gyrotrons from the megawatt level to the 1.5-2.0 MW range; increasing the efficiency of gyrotrons to over 55%; making frequency tunable gyrotrons for use over a range of operating magnetic fields in the plasma; reducing losses in transmission lines; and developing advanced, efficient solid-state power supplies. Gyrotrons have many spinoff applications, including radar, industrial heating, materials processing, and Terahertz spectroscopy.

Radiofrequency (RF) antenna systems that excite compressional Alfven or “fast” waves, typically operating in the frequency range of 10-120 MHz, are used for both cyclotron magnetic resonance heating of ions and non-resonant heating of electrons. In addition to heating plasmas to the high internal temperatures needed to initiate fusion reactions, RF waves can be used to drive plasma currents, tailor internal pressure profiles, improve energy confinement, and stabilize plasmas. RF systems on present major U.S. experiments are capable of supplying 3-6 MW of power to their plasmas; future reactors such as ITER may need an order of magnitude more power.

There are technological challenges to the reliable operation of RF wave launchers with hundreds of amps of current flowing at tens of thousands of volts a few cm from a hot, electrically conducting plasma. Deleterious interactions between the plasma and antenna must be minimized, large electromagnetic forces operating on the antenna structure during plasma disruptions must be withstood, techniques to maintain power delivery to the plasma’s rapidly varying electrical load must be developed, and components compatible with the high flux neutron environment of future reactors must be deployed. Although antenna component development and qualification on dedicated test facilities is followed as much as possible, the intimate close coupling between the antenna and plasma ultimately demands that final technology assessment and improvement be carried out on operating plasma experiments. We have designed, fabricated, and operated multimegawatt RF antennas on major U.S. and international experiments.