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Stellarator transport deals with the issue of how rapidly the plasma leaks out of a magnetic confinement configuration. This is ultimately important in determining whether a particular stellarator device can be extrapolated to a fusion reactor. Losses of plasma energy in a reactor must be kept to a low enough level so that net energy is realized from the fusion reactions (i.e., if too much energy is required to offset the plasma losses, then the fusion reactor may either produce no net power or produce insufficient net power to be economically interesting).
Transport losses can be categorized into several types. First there are:
neoclassical losses
- based on the movement of classical particle orbits in the presence of collisions with other particles as they follow their trajectories through the 3-dimensional stellarator magnetic field
- quantitative calculations depend on following the particle orbit trajectories -> these depend on the 3-D structure of the magnetic field
- can be analyzed either by directly solving the plasma kinetic equation or by using Monte Carlo simulation techniques
anomalous losses
- caused by low levels of plasma instabilities and micro-turbulence that are generally present in all magnetic confinement systems
- the simplest models for anomalous transport are based on scaling laws obtained from experimental observations over a wide range of conditions and devices
- can also be analyzed by various fluid and particle based simulation methods
Under the neoclassical losses there are at least two components of the plasma whose confinement is of interest:
thermal Maxwellian plasma
- collisionally coupled, electron, ion, and impurity species
- diffusive transport of the plasma produces electric fields and currents
energetic particles
- 3.5 Mev alphas generated by fusion reaction (good confinement of these is necessary in a reactor to achieve a self-sustained state)
- energetic "tails" from heating populations (RF and neutral beam heating tend to produce particles with energies well above the thermal Maxwellian plasma temperature)
Plasma and energetic particle confinement is analyzed using a set of Hamiltonian equations for the guiding center position of the particle (i.e., the rapid cyclotron motion around the magnetic field lines is averaged over)
Due to the variation of the magnetic field strength as a function of position, a variety of particle trajectories are possible
- If the particle velocity is nearly parallel to the magnetic field, it will follow a streaming motion along the magnetic field (passing orbit)
- If the particle velocity is across or perpendicular to the magnetic field, the particle orbit can become "mirrored" and bounce back and forth in a local region in space (trapped orbit)
