The Physics of Tokamak Start-up

Abstract

Tokamak start-up on present-day devices usually relies on inductively induced voltage from a central solenoid. In some cases inductive startup is assisted with auxiliary power from electron cyclotron radio frequency heating.ITER, the National Spherical Torus eXperiment Upgrade and JT60, now under construction, will make use of the understanding gained from present-day devices to ensure successful start-up. Design of a spherical tokamak (ST) with DT capability for nuclear component testing would require an alternative to a central solenoid because the small central column in an ST has insufficient space to provide shielding for the insulators in the solenoid. Alternative start-up techniques such as induction using outer poloidal field coils, electron Bernstein wave start-up, coaxial helicity injection and point source helicity injection have been used with success, but require demonstration of scaling to higher plasma current.

Trapped-electron runaway effect

“…Even in the case where primary generation would be small in the center, for example for very peaked density, one could still expect a centrally concentrated runaway electron population under some circumstances. This would be the case if the avalanche growth rate from knock-on collisions decays strongly off-axis owing to magnetic trapping effects as found for toroidal geometries (Nilsson et al 2015). At the same time, the trappedelectron runaways are concentrated near the center, as found in the previous section.…”

“…When the tokamak current is maintained by a DC electric field, it is after all the thermal electrons that carry the toroidal current, and the runaway electrons carry current in the same direction, only they are accelerated to far higher energies. This would also be true during start-up of the tokamak (Mueller 2013), if the start-up relies on an inductive current. In such a case, there is also danger from runaway electrons, since the plasma may not be so dense as to hold back the runaways.…”

“…For an initially Maxwellian distribution function, the fraction of trapped-electron runaways compared to Dreicer runaways would be small, although it increases with the effective charge. However, the relative importance of trapped-electron runaways might increase significantly for three reasons: first, the usual runaways may not be wellconfined, whereas, the trapped-electron runaways, since born nearer the magnetic axis, are very well confined; second, knock-on collisions between existing runaways and thermal electrons produce a non-thermal population of secondary runaways of which a significant number may be trapped due to their high perpendicular velocities (Nilsson et al 2015). Third, tokamak plasmas are typically non-Maxwellian, and a significant population of suprathermal electrons with high perpendicular momentum can be created, for instance, via resonant interaction with electron cyclotron waves.…”

“…They are also sensitive to perturbation of the magnetic field, which lead to enhanced transport (Zeng et al 2013). In addition the runaway population that would arise in a tokamak magnetic field configuration is diminished owing to magnetic trapping effects in a non-uniform magnetic field, since trapped electrons can not immediately contribute to the runaway electron population (Nilsson et al 2015). The fate of the suprathermal electrons, whether trapped or circulating, is determined from the balance of the accelerating electric field and various radiative, convective and diffusive loss mechanisms.…”

Extracting generalized neutron parton distributions from3He data

“…For the present aim, there is no need to use recent, sophisticated GPDs models, such as the ones of Refs. [39,40], which would complicate the description without adding new insights. Once the experiments are planned, more realistic calculations involving phenomenologically motivated models will be easily implemented in our scheme.…”