Plasma Initiation by Neutral Beam Injection

A unique and reliable method of plasma initiation has been established in the Large Helical Device (LHD) by using neutral beam (NB) injection into vacuum. Since LHD is a superconducting machine, the confining magnetic field exists unrelated to plasma. Under these circumstances it is demonstrated that the NB can initiate plasma by itself. A small fraction of injected NB is ionized by collision with the background neutral gas and is confined by the magnetic field. Although these high-energy ions are lost quickly by charge exchange, they work as the energy source for ionizing the background neutral particles and heating the produced plasma. As a result, very thin but hot "seed" plasma is generated, which ionizes puffed gas and makes dense target plasma that is sufficient for NB absorption. This process is simulated numerically and the results agree well with the experimental observations for both absolute values and temporal behavior of plasma parameters. The method does not depend on magnetic field strength strongly, and plasma can be initiated at the magnetic field strength as low as 0.4 T, although standard field strength of LHD is 2.75 T. The progress of high-beta studies in LHD owes this plasma production method much.