Plasma startup by electron cyclotron waves

In the startup phase of a fusion device, the neutral hydrogen gas is converted to a plasma predominantly by electron impact ionization. In a usual gas discharge, seed electrons are most likely created by cosmic ray muons. The energy of these electrons has to be above the threshold of ionization so as to further increase the population of electrons in the gas. An appealing mechanism for increasing the energy of electrons is through their interaction with electron cyclotron waves. Importantly, this saves on the volt-seconds that a central solenoid needs to supply for startup. The time needed to transition from a gas with some seed electrons to an ionized plasma depends on an understanding of two key physics points. The first point is related to the mean free path an electron has to traverse prior to an impact ionization event. It sets the minimum time required to complete the startup phase. The dependence of the mean free path on energy of an electron is well documented. The second point has to do with the time required for an electron to gain energy necessary for impact ionization. If this is time is greater than the time needed to negotiate a mean free path, then the startup time is determined by the energization process. We will discuss the physics of startup within the context of tokamaks and mirrors.

Work supported by DoE.