CURRENT-VOLTAGE CHARACTERISTICS OF THE END LOSS ION FLUX FROM A TANDEM MIRROR COLLECTED ON A GRIDDED ENERGY ANALYZER

Examination is made of the relation between the distribution function of the ions confined in a tandem mirror and the current-voltage (I-V) characteristics of the end loss ions collected on a gridded energy ana lyzer. To do this, a model ion distribution function characterized by two parameters, an anisotropy parameter chi and a decay parameter Y is used. The decay parameter represents depletion of ion distribution in the loss cone. A Maxwellian distribution as the limit of Y=infinity i s shown not to reproduce the measured I-V curve as an energy spectrum of the ions lost from the central cell of the GAMMA 10 tandem mirror [ Phys. Rev. Lett. 55, 939 (1985)]. The ions in the central cell are sub ject to strong ICRH and have a highly anisotropic velocity distributio n. These ions are deeply trapped in the central cell and the loss cone is not fully occupied. The model distribution with a small value of Y represents such features of the ion distribution. From comparison bet ween a measured loss current and calculated one, the decay parameter o f the model distribution function is roughly determined. The model dis tribution with this parameter yields I-V characteristics similar to th e measured one. The present article gives the relation between the app arent ion temperature T-i simply determined from the slope of the log plot of the measured I-V curve and the real parallel ion temperature T -i parallel to. When the ion distribution is of a loss cone type, T-i is almost equal to T-i parallel to. On the other hand, T-i can be more than two times as high as T-i parallel to if the ion distribution is Maxwellian-like. (C) 1996 American Institute of Physics.