A model for periodic pellet ablation in toroidal plasma confinement experiments
A model is presented for the ablation of a frozen hydrogen pellet moving across field lines into a magnetically confined plasma, with emphasis on the stability of the self-controlled evaporation process. The basic mechanism investigated is the non-locality of the self-shielding as a consequence of the pellet motion relative to the magnetically captured ablation material. Assuming the shielding cloud profile to be given, a one-dimensional model equation is derived which contains the essential physics of the problem, but also allows for realistic plasma background profiles. The stability of the linearised equation is analytically investigated and the fully nonlinear equation is solved numerically for various scenarios. For small amplitude, an exponentially growing sinusoidal ablation instability is obtained, if there is significant attenuation of the heat influx along field lines and if, at the same time, the shielding cloud profile is sufficiently hollow. The typical spatial wavelength is of the order of the ionisation radius. In a homogeneous plasma the instability increases until nonlinear saturation of the amplitude occurs and a quasi-stationary periodic ablation is obtained. For an inhomogeneous background plasma, relevant for tokamaks like ASDEX, an ablation signature qualitatively similar to the one experimentally observed is found. The non-local shielding mechanism considered is a viable candidate for explanation of the regular striations observed along the pellet path.
Bibliographic Reference: Report: IPP5/30 EN (1989) 19 pp.
Availability: Available from Max-Planck-Institut für Plasmaphysik, 8046 Garching bei München (DE)
Record Number: 199010702 / Last updated on: 1994-12-01
Original language: en
Available languages: en