Plasma startup by electron cyclotron waves in magnetic mirrors

The creation of a plasma from neutral hydrogen gas in a fusion device is primarily through electron impact ionization. Of the various techniques available for the startup phase, the utility of electron cyclotron waves in magnetized plasma has garnered substantial interest. In transitioning from a few electrons, generated by events such as cosmic rays, to about 10% ionization of the gas in a device, we consider two necessary aspects related to the underlying physics. The first is the mean free path length an electron has to traverse prior to an impact ionization event. This physics is well understood, both experimentally and theoretically, and sets the minimum time required for the startup phase. The second aspect is the time needed to energize an electron to energies meaningful for impact ionization. If this is greater than the time needed to negotiate a mean free path length, then the startup time is determined by the energization process. We examine these two aspects within the context of a magnetic mirror, in which the electrons interact with a model Gaussian beam in the electron cyclotron range of frequencies. The decorrelation induced by the mirror magnetic field is effective in heating the electrons and the startup time is determined by the physics of the mean free path.