Non-local efficiency in wave driven tokamaks
The current response induced by suprathermal Landau and cyclotron absorption in a tokamak plasma is shown to be non-local. This effect stems from the coupling between space and velocity diffusions. Up to now, for a given wave spectrum, a local response has been used to describe the result of the competition between the collisional relaxation and the quasi-linear excitation in terms of the current, the absorbed power and the slowing down time t(s). Then, on an anomalous diffusion time scale t(d), this suprathermal current is transported away through radial diffusion. However, because of the intricate coupling between the various kinetic processes this picture is revised. Three regimes are considered, depending on whether t(s) is much larger than, much less than or equal to t(d) respectively, as follows: - the current carrying electrons are deconfined before they are significantly slowed down by the plasma (a fast global loss term in the kinetic equation may account for this situation); - the suprathermal electrons are excited and relax nearly on the same drift surface and a local equation is relevant to describe the current response, and - the pertinent physical tool to describe the suprathermal current dynamics arising from the competition between radial diffusion and momentum slowing down is a non-local response. Due to the competition between the slowing down and the anomalous diffusion, a fast electron excited by the waves will relax and drive a significant current. With absorbing boundary condition at the plasma edge and constant radial diffusion, the ratio of the associated current width to the plasma minor radius scales as a function of t(s)/t(d). The impact of this anomalous broadening on current profile scenarios is considered.
Bibliographic Reference: Paper presented: Sherwood Theory Meeting, Williamsburg (US), April 23-25, 1990
Availability: Available from (1) as Paper EN 35450 ORA
Record Number: 199110867 / Last updated on: 1994-12-02
Original language: en
Available languages: en