Experimental Observation on Enhancement of the Ambipolar Potential by Injection of Neutral Beams in a Tandem Mirror

Neutral hydrogen beams of 13 and 14 keV at equivalent proton currents of 5 and 3 A, respectively, were injected into a target plasma in a tandem mirror. The space potential at plugs increased more than 10 V relative to the space potential at the central cell and was sustained for more than 0.3 ms. The potential increase is due to the heating of electrons with trapped hot ions and is explained by the relation $\varphi =T_e\ln \left(\frac{n_p}{n_c}\right)$.