Gyrotron Design and Evaluation using New Particle-in-Cell Capability
ITER will depend on high power CW gyrotrons to deliver power to the plasma at ECR frequencies. However, gyrotrons can suffer from undesirable low frequency oscillations (LFOs) which are known to interfere with the gun-region diagnostics and data collection, and are also expected to produce undesirable energy and velocity spread in the beam. The origins and processes leading to these oscillations are poorly understood, and existing gyrotron R & amp;D tools, such as static gun solvers and interaction region models, are not designed to look at time-dependent oscillatory behavior. We are proposing the application of a conformal finite-difference time-domain (CFDTD) particle-in-cell (PIC) method to address the LFO problem. Our company is at the forefront of smooth-curved-boundary treatment of the electromagnetic fields and particle emission surfaces, and such methods are necessary to simulate the adiabatically trapped and reflected electrons thought to be driving the oscillations. This approach provides the means for understanding, in microscopic detail, the underlying physical processes driving the low-frequency oscillations. The basic tools, diagnostics, and templates have been developed and demonstrated for simulate a MIG in Gyrotrons. It is demonstrated that our strategies are appropriate and LFOs in a MIG electron gun have been simulated and reproduced using the 3D CFDTD PIC simulations for the first time. The outcome is promising and close to experiments. The tools, algorithms, diagnostics, and templates will be developed and used to investigate the trapped and reflected electron population behind the LFO phenomenon. Several candidates for the origin of the electron population will be evaluated, in turn, including emission temperature spread, both physical, and geometrically (surface roughness) induced, cavity and capacitive probe effects, and other emitting surfaces. Particle trapping times, life-cycle time, and loss mechanisms will be evaluated, with the goal of being able to reproduce observed experimental variations with gun voltage, current, and LFO mitigation schemes, and predict LFO behavior in future gyrotrons. Commercial Applications and Other Benefits: The new gyrotron models will be implemented in Tech-Xs VORPAL software which is commercially available, and should have an immediate commercial market with gyrotron manufacturers seeking mitigation of the LFO problem. The successful application of this capability will also help fill an important gap in the gyrotron R & amp;D process, where this important type of particle-based time-dependant R & amp;D tool is not yet established.
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