Ohmic Heating in the B-1 Stellarator

A detailed comparison between experimental data and the predictions of ohmic heating theory is made over a wide range of axial magnetic confining field, electric heating field, and helium pressure. It is found that, in the early stages of the discharge, agreement between theory and experiment is good, and the predicted scaling laws are valid. However, experimentally it is found that it takes about 5 times longer to complete first ionization than predicted. Also, the current is seen to level off at a plateau at a time when theory predicts a continuous heating. These discrepancies are attributed to three energy sinks: cold neutral particles entering the discharge, runaway electrons leaving the discharge, and radiation from impurities. In addition, the electron density is observed to decrease during the length of the discharge (pump-out). For the early current plateau observed at low ratios of heating field to pressure, it appears that the plasma density is about 15% less at 18 000 gauss than at 27 000 gauss as a result of this pump-out. The hydromagnetic instability predicted by Kruskal and previously demonstrated to exist is given the physical characteristics of an increased rate of diffusion.