A technique is described whereby the ion dumps inside the TFTR Neutral Beam Test Stand were used to measure thermal profiles of the fullhyphen;, halfhyphen;, and thirdhyphen;energy ions. 136 thermocouples were installed on the fullhyphen;energy ion dump, allowing full beam contours. Additional linear arrays across the widths of the halfhyphen; and thirdhyphen;energy ion dumps provided a measure of the shape, in the direction parallel to the grid rails, of the halfhyphen; and thirdhyphen;energy ions, and, hence, of the molecular ions extracted from the source. As a result of these measurements, it was found that the magnet was more weakly focusing, by a factor of 2, than expected, explaining past overheating of the fullhyphen;energy ion dump. Hollow profiles on the halfhyphen; and thirdhyphen;energy ion dumps were observed, suggesting that extraction of D+2and D+3is primarily from the edge of the ion source. If extraction of halfhyphen;energy ions is from the edge of the accelerator, a divergence parallel to the grid rails of 0.6deg;plusmn;0.1deg; is deduced. It is postulated that a nonuniform gas profile near the accelerator is the cause of the hollow partialhyphen;energy ion profiles, the pressure being depressed over the accelerator by particles passing through this highly transparent structure. Primary electrons reaching the accelerator produce nonuniform densities of D+2through the ionization of this gas. D+3is created through subsequent D+2hyphen;gas collisions. A technique of rastering the ion beam across the fullhyphen;energy dump was examined as a means of reducing the power density. By unbalancing the currents in the two coils of the magnet, on a shothyphen;byhyphen;shot basis, by up to a 2:1 ratio, it was possible to move the centerline of the fullhyphen;energy ion beam sideways by sim;12.5 cm. The adoption of such a technique, with a ramp of the coil imbalance from 2:1 to 1:2 over a beam pulse, could reduce the fullhyphen;energy ion dump power density by a factor of gsim;1.5.
展开▼
