Anthony J. Webster JET EFDA Contributors
The generic question is considered: How can we determine the probability of an otherwise quasirandom event, having been triggered by an external influence? A specific problem is the quantification of the success of techniques to trigger, and hence control, plasma instabilities in magnetically confined fusion (MCF) experiments. The successful develo…
PublishedAnthony J. Webster Richard Kemp
The mark-recapture method was devised by Petersen in 1896 to estimate the number of fish migrating into the Limfjord, and independently by Lincoln in 1930 to estimate waterfowl abundance. The technique can be applied to any search for a finite number of items by two or more people or agents, allowing the number of searched-for items to be estimated…
PublishedAnthony J. Webster
This paper reconsiders the problem of calculating the expected set of probabilities hpii, given the observed set of items {mi}, that are distributed among n bins with an (unknown) set of probabilities {pi} for being placed in the ith bin. The problem is often formulated using Bayes theorem and the multinomial distribution, that with a constant prio…
PublishedAnthony J. Webster
During plasma instabilities, “surface currents” can flow at the interface between the plasma and the surrounding vacuum, and in most cases, they are a harmless symptom of the instability that is causing them. Large instabilities can lead to “disruptions,” an abrupt termination of the plasma with the potential to damage the machine in which …
PublishedAnthony Webster
The edge of a Tokamak plasma is interesting due to its geometrical structure that is difficult to model either analytically or computationally, its tendency to form "transport barriers" with increased confinement of energy and momentum, and the edge-localised instabilities associated with transport barriers that threaten the lifetime of components …
PublishedAnthony J. Webster
The surface or “skin” current that can flow at a perturbed interface between plasma and vacuum is considered in the approximation where a surface marks a sharp transition from plasma to vacuum. A short magnetohydrodynamic calculation gives an exact and general expression for the component perpendicular to the average of the magnetic field eithe…
PublishedAnthony J. Webster
This paper describes physically and mathematically how the plasma-vacuum boundary of a tokamak plasma equilibrium can be perturbed to form a separatrix with an X-point, while having an otherwise negligible affect on the plasma equilibrium. A deliberate consequence of the technique is that the radial and poloidal extent of the perturbed region may b…
Published