Gyrotron Design and Evaluation Using New Particle-in-Cell Capability

Fusion experiments such as ITER depend on high power CW gyrotrons to deliver power to the plasma at Electron Cyclotron Resonance (ECR) frequencies. For example, the DIII-D experiment utilizes six 1 MW 110 GHz CPI gyrotrons. However, gyrotrons can suffer from undesirable low frequency oscillations (LFOs), which are known to interfere with gun-region diagnostics and data collection systems. These LFOs also are expected to produce energy and velocity spread in the beam. It is not well known or understood what contribution these spreads may have in terms of efficiency reduction, especially in higher power regimes where little experimental data exists. Furthermore, the origins and processes leading to these oscillations are not fully understood. Existing gyrotron research tools, such as static gun solvers and interaction region models, are not designed to look at this type of time-dependant oscillatory behavior, making it difficult to test theoretical concepts and suggest ameliorations. To address the LFO problem, this project will apply a time-domain particle-in-cell method, which uses a smooth-curved-boundary treatment of the electromagnetic fields. Such methods can provide the necessary self-consistent time-dependant space-charge capability necessary to treat the adiabatically trapped and reflected electrons thought to be driving the oscillations. The new modeling capability will provide the means for understanding, in microscopic detail, the underlying physical processes driving the low-frequency oscillations, and thus lead to an ability to predict their behavior and mitigate their ill effects. Commercial Applications and other Benefits as described by the awardee: The new gyrotron models should have an immediate commercial market with gyrotron manufacturers seeking mitigation of the LFO problem. The successful application of this capability also should help fill an important gap in the gyrotron design and evaluation process, thus perhaps opening the field to additional applications beyond the LFO problem.