Direct Measurement of the Ion Distribution Function in a Toroidal Octupole

Abstract

The distribution function was found to be a function only of energy and canonical angular momentum Pθ thus automatically satisfying the Vlasov equation, and being consistent with the assumption of a collisionless plasma. The lifetime of the plasma was inversely proportional to ion velocity and directly proportional to accessible phase space volume. The lifetime of 40 eV ions confined in the field was about 500 μsec which is consistent with the interpretation that the plasma is gradually lost to the supports of the current-carrying hoops although it is known that other losses to walls also occur. Density and current were calculated from the appropriate moments of the measured distribution. Plasma is confined primarily in the low-field central region of the octupole, and the density drops rather sharply in the outer regions of higher field, in the horizontal midplane of the device. The only component of ion current was the azimuthal one, which had a maximum value of about 10−4 A cm−2 and indicated a net mass flow in the positive θ direction.